effects of changing climate on weather and human ......304.2’5–dc21 00-023978 printed in the...

G L O B A L C H A N G E I N S T R U C T I O N P R O G R A M

Kevin E. Trenberth, Kathleen Miller,Linda Mearns and Steven Rhodes

EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES

EFFECTS OF CHANGING CLIMATE

ON WEATHER AND HUMAN ACTIVITIES

Kevin E. Trenberth, Kathleen Miller,Linda Mearns and Steven Rhodes

National Center for Atmospheric ResearchBoulder, Colorado

UNIVERSITY SCIENCE BOOKSSAUSALITO, CALIFORNIA

EFFECTS OF CHANGING CLIMATE

ON WEATHER AND HUMAN ACTIVITIES

University Science Books55D Gate Five RoadSausalito, CA 94965Fax: (415) 332-5393www.uscibooks.com

Scientific director: Tom M.L. WigleyManaging editor: Lucy WarnerEditor: Carol RasmussenArt and design: NCAR Image and Design ServicesCover design and composition: Craig MaloneCover photo by Mickey Glantz

The cover photograph of the effects of drought on a farm in eastern Colorado in 1977 isprototypical of scenes in the 1930s during the “dust bowl” era. The risk of such droughtswith global warming increases owing to increased drying of the landscape.

This book is printed on acid-free paper.

Copyright © 2000 by University Corporation for Atmospheric Research.All rights reserved

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to UCARCommunications, Box 3000, Boulder, CO 80307-3000.

Library of Congress Cataloging-in-Publication Data

Effects of changing climate on weather and human activities / Kevin Trenberth ... [et al.].p. cm. – (The global change instruction program)

Includes bibliographical references and index.ISBN 1-891389-14-9 (softcover : alk. paper)1. Climatic changes. 2. Weather. 3. Human beings–Effect of climate on. I. Trenberth,

Kevin E. II. Series.

QC981.8.C5 E44 2000304.2’5–dc21

00-023978

Printed in the United State of America10 9 8 7 6 5 4 3 2 1

V

A Note on the Global Change Instruction Program

This series has been designed by college professors to fill an urgent needfor interdisciplinary materials on global change. These materials areaimed at undergraduate students not majoring in science. The modularmaterials can be integrated into a number of existing courses—in earthsciences, biology, physics, astronomy, chemistry, meteorology, and thesocial sciences. They are written to capture the interest of the student whohas little grounding in math and the technical aspects of science butwhose intellectual curiosity is piqued by concern for the environment.

For a complete list of modules available in the Global Change Instruc-tion Program, contact University Science Books, Sausalito, California, [email protected]. Information is also available on the World Wide Webat http://www/uscibooks.com/globdir.htm orhttp://home.ucar.edu/ucargen/education/gcmod/contents.html.

Contents

Preface ixIntroduction 1

I. Climate 4The Climate System 4The Driving Forces of Climate 6The Spatial Structure of Climate 7

II. The Weather Machine 9

III. Climate Change 14Human-Caused Climate Change 14The Enhanced Greenhouse Effect 14Effects of Aerosols 15The Climate Response and Feedbacks 15

IV. Observed Weather and Climate Change 17Observed Climate Variations 17Interannual Variability 19

V. Prediction and Modeling of Climate Change 21Climate Models 21Climate Predictions 21Interpretation of Climate Change in Terms of Weather 23

VI. Impacts of Weather and Climate Changes on Human Activities 25Weather Sequences 25Location, Location, Location 25Severe Weather Events 26Societal Responses 26Managing Risk 27Impacts on Agriculture 27Planning for Local Weather Changes 31

VII. The Need for More Research 33

Glossary 34Suggested Readings 38Discussion Questions 39Index 40

IX

Preface

It is now widely recognized that human activities are transforming the globalenvironment. In the time it has taken for this book to come to fruition and bepublished, the evidence for climate change and its disruption of societal activi-ties has become stronger. In the first 11 months of 1998, there were major floodsin China, Peru, and California, enormous damage from Hurricane Mitch inCentral America, record-breaking heat waves in Texas, and extensive droughtand fires in Indonesia; weather-related property losses were estimated at over$89 billion, tens of thousands of lives were lost, and hundreds of thousands ofpeople were displaced. This greatly exceeds damage estimates for any otheryear. The environment was ravaged in many parts of the globe. Many of theselosses were caused by weird weather associated with the biggest El Niño onrecord in 1997–98, and they were probably exacerbated by global warming: thehuman-induced climate change arising from increasing carbon dioxide andother heat-trapping gasses in the atmosphere. The climate is changing, andhuman activities are now part of the cause. But how does a climate changemanifest itself in day-to-day weather?

This book approaches the topic by explaining distinctions between weatherand climate and how the rich natural variety of weather phenomena can be sys-tematically influenced by climate. Appreciating how the atmosphere, where theweather occurs, interacts with the oceans, the land surface and its vegetation,and land and sea ice within the climate system is a key to understanding howinfluences external to this system can cause change. One of those influences isthe effect of human activities, especially those that change the atmospheric com-position with long-lived greenhouse gases.

Climate fluctuates naturally on very long time scales (thousands of years),and it is the rapidity of the projected changes that are a major source of concern.The possible impacts of the projected changes and how society has respondedin the past and can in the future are also described. Everyone will be affectedone way or another. So this is an important topic, yet it is one about which acertain amount of disinformation exists. Therefore it is as well to understandthe issues in climate change and how these may affect each and every one of us.What we should do about the threats, given the uncertainties, is very much achoice that depends upon values, such as how much we should be stewards forthe planet and its finite resources for the future generations. Many people favora precautionary principle, “better safe than sorry,” and err on the side of takingactions to prevent a problem that might not be as bad as feared. This book helpsprovide the knowledge and enlightenment desirable to ensure that the debateabout this can be a public one and carried out by people who are well informed.

Kevin E Trenberth

Acknowledgments

This instructional module has been produced by the GlobalChange Instruction Program of the University Corporationfor Atmospheric Research, with support from the NationalScience Foundation. Any opinions, findings, conclusions, orrecommendations expressed in this publication are those ofthe authors and do not necessarily reflect the views of theNational Science Foundation.

This project was supported, in part, by the

National Science FoundationOpinions expressed are those of the authorsand not necessarily those of the Foundation

We experience weather every day in all its won-derful variety. Most of the time it is familiar, yetit never repeats exactly. We also experience thechanging seasons and associated changes in thekinds of weather. In summer, fine sunny daysare interrupted by outbreaks of thunderstorms,which can be violent. Outside the tropics, aswinter approaches the days get shorter, it getscolder, and the weather typically fluctuates fromwarm, fine spells to cooler and snowy condi-tions. These seasonal changes are the largestchanges we experience at any given location.Because they arise in a well-understood wayfrom the regular orbit of the Earth around theSun, we expect them, we plan for them, and weeven look forward to them. We readily and will-ingly plan (and possibly adapt) summer swim-ming outings or winter ski trips. Farmers plantheir crops and harvests around their expecta-tion of the seasonal cycle.

By comparison with this cycle, variations inthe average weather from one year to the nextare quite modest, as they are over decades orhuman lifetimes. Nevertheless, these variationscan be very disruptive and expensive if we donot expect them and plan for them. For exam-ple, in summer in the central United States, themajor drought in 1988 and the extensive heavyrainfalls and flooding in 1993 were at theextremes for summer weather in this region. (Inthe upper Mississippi Basin, rainfalls in May,June, and July changed from about 150 millime-ters in 1988 to over 500 mm in 1993.) These twovery different summers were the result of verydifferent weather patterns. We assumed, beforetheir occurrence, that the usual summertimemix of rain and sun would occur and that farm-ers’ crops would flourish. Because this assump-

tion was wrong, major economic losses occurredin both years and lives were disrupted.

These weather patterns and kinds of weatherconstitute a short-term climate variation or fluc-tuation. If they repeat or persist over prolongedperiods, then they become a climate change. Forinstance, in parts of the Sahara Desert we nowexpect hot and dry conditions, unsuitable forhuman habitation, where we know that civiliza-tions once flourished thousands of years ago.This is an example of a climate change.

How has the climate changed? What are thefactors contributing to climate and therefore topossible change? How might climate change inthe future? How does a change in climate alterthe weather that we actually experience? Howmuch certainty can we attach to any predic-tions? What do we do in the absence of pre-dictability? Why are climate change and associ-ated weather events important? What are thelikely impacts on human endeavors and societyand on natural-resource-based economic activi-ties, such as agriculture? These are some of thequestions we address in this module. Our dis-cussion of impacts will focus on human activi-ties. Although very important, the impacts ofclimate change on the natural environment andthe unmanaged biosphere are not dealt withhere. Some of these consequences are discussedfurther in the Global Change Instruction Pro-gram module Biological Consequences of GlobalClimate Change.

Many of these questions, although of con-siderable importance, unfortunately do not havesimple answers. Also, many of the answers arenot very satisfying. Because of the nature of thephenomena involved, many outcomes can onlybe stated in a statistical or probabilistic way.

1

Introduction

We first need to distinguish between weath-er and climate. An important concept to grasp ishow weather patterns and the kinds of weatherthat occur relate to climate. We refer to this rela-tionship as the “weather machine” because ofthe way the weather helps drive the climate sys-tem. It is the sum of many weather phenomenathat determines how the large-scale general cir-culation of the atmosphere (that is, the averagethree-dimensional structure of atmosphericmotion) actually works; and it is the circulationthat essentially defines climate. This intimatelink between weather and climate provides abasis for understanding how weather eventsmay change as the climate changes.

There are many very different weather phe-nomena that can take place under an unchang-ing climate, so a wide range of conditionsoccurs naturally. Consequently, even with amodest change in climate, many if not most ofthe same weather phenomena will still occur.Because of this large overlap between theweather events experienced before and aftersome climate change, it may be difficult to per-ceive such a change. Our perceptions are mostlikely to be colored not by the more commonweather events but by extreme events. As cli-mate changes, the frequencies of differentweather events, particularly extremes, willchange. It is these changes in extreme condi-tions that are most likely to be noticed.

We normally (and correctly) think of thefluctuations in the atmosphere from hour tohour or day to day as weather. Weather isdescribed by such elements as temperature, airpressure, humidity, cloudiness, precipitation ofvarious kinds, and winds. Weather occurs as awide variety of phenomena ranging from smallcumulus clouds to giant thunderstorms, fromclear skies to extensive cloud decks, from gentlebreezes to gales, from small wind gusts to torna-does, from frost to heat waves, and from snowflurries to torrential rain. Many such phenome-na occur as part of much larger-scale organizedweather systems which consist, in middle lati-tudes, of cyclones (low pressure areas or sys-tems) and anticyclones (high pressure systems),

and their associated warm and cold fronts.Tropical storms are organized, large-scale sys-tems of intense low pressure that occur in lowlatitudes. If sufficiently intense these becomehurricanes, which are also known as typhoonsor tropical cyclones in other parts of the world.Weather systems develop, evolve, mature, anddecay over periods of days to weeks. From asatellite’s viewpoint, they appear as very largeeddies, similar to the turbulent eddies thatoccur in streams and rivers, but on a muchgreater scale. Technically, they are indeed formsof turbulence in the atmosphere. They occur ingreat variety, but within certain bounds andover fairly short time frames.

Climate, on the other hand, can be thoughtof as the average or prevailing weather. Theword is used more generally to encompass notonly the average, but also the range andextremes of weather conditions, and where andhow frequently various phenomena occur. Cli-mate extends over a much longer period of timethan weather and is usually specified for a cer-tain geographical region. It has been said thatclimate is what we expect, but weather is whatwe get! Climate involves variations in which theatmosphere is influenced by and interacts withother parts of the climate system, the oceans, theland surface, and ice cover. Climate can changebecause of changes in any of these factors or iffactors outside the Earth or beyond the climatesystem force it to change.

The Earth’s climate has changed in the pastand is expected to change in the future. We willexperience these changes through the day-to-day weather. It is natural to want to ascribe acause to any perceived unusual weather, and“climate change” is often espoused by the pop-ular press as a possible cause. In some cases thisinference may be correct—but proving it to becorrect is exceedingly difficult. More often,extremes of weather occur simply as a manifes-tation of various interacting atmosphericprocesses. In other words, extremes are general-ly nothing more than examples of the tremen-dous natural variability that characterizes theatmosphere.

INTRODUCTION

2

These considerations make it essential tounderstand and deal with the natural variabilityin the climate system. One way of thinkingabout the variability in the atmosphere is to con-sider the inherent natural variability as being inthe realm of “weather,” while systematicchanges in the atmosphere that can be linked toa cause, such as interactions with the ocean orchanges in atmospheric composition, are in therealm of climate.

For example, interactions between the atmo-sphere and the tropical Pacific Ocean result inthe phenomenon known as El Niño, which isresponsible for disruptions in weather patternsall over the world. Technically, El Niño is awarming of the eastern equatorial Pacific thatoccurs every two to six years and lasts for sever-al seasons; it is a natural phenomenon and hasoccurred for thousands of years at least. It caus-es heavy rainfall along the western South Amer-ican coast and southern part of the UnitedStates; drought or dry conditions in Australia,Indonesia, southeastern Asia (including theIndian subcontinent), parts of Africa, and north-east Brazil and Colombia; and unusual weatherpatterns in other parts of the world. It can bethought of as a short-term climatic phenome-non.

Other climate perturbations are more subtleand their effects on weather less obvious.Increases in heat-retaining gases called green-house gases, the best known of which is carbondioxide, are currently causing the climate towarm because of human activities. In this case,the climate change is very gradual and shouldbe noticeable only when the weather from onedecade is compared with that of another. Eventhen, because of the background natural vari-ability of the climate system, weather variationsspecifically attributable to human influencesmay be extremely difficult to identify.

While increasing greenhouse-gas concentra-tions cause global-mean warming, this does not

mean that the globe will warm everywhere atonce. An example is the Northern Hemispherewinter of December 1993 to February 1994. Thiswinter was very cold and snowy, with many-more-than-normal winter storms in thenortheastern part of the United States. Howdoes this jibe with expectations of global warm-ing?

The pattern of exceptionally wintry weathercontinued for several months, long enough toheighten interest in its apparent climate implica-tions. However, as part of this pattern, therewere often mild and sunny conditions in thewestern half of the United States and Canada,with above-average temperatures. Temperatureswere substantially above average in parts ofsoutheast Asia, northern Africa, the Mediter-ranean, and the Caribbean. The Northern Hemi-sphere as a whole was 0.2°C above the averagefor 1951 to 1980.

Extensive regions of above and below nor-mal temperatures are the rule, not the exception,even in the presence of overall warmer condi-tions. A bout of below-average temperaturesregionally may not be inconsistent with globalwarming, just as a bout of above-average tem-peratures may not indicate global warming.

In the following pages, a discussion is pre-sented of how the climate may change and thereasons for possible changes. The primary rea-son for particular future climate change is thecontinuing influence of humans, especiallythrough changes in atmospheric compositionsuch as increases in greenhouse gases (notablycarbon dioxide). We therefore pay particularattention to these effects and attempt to trans-late them into weather changes. A further issueis how these changes may in turn affect humanactivities. Accordingly, we consider how possi-ble changes in climate and weather affect vari-ous economic sectors and human activities, andwe discuss some steps that can be taken to soft-en the possible impacts.


3

The Climate System

The Earth’s climate involves variations in acomplex system in which the atmosphere inter-acts with many other parts (Figure 1). The othercomponents of this climate system include theoceans, sea ice, and the land and its features.Important characteristics on land include vege-tation, ecosystems, the total amount of livingmatter (or biomass), and the reflectivity of the

surface (or albedo). Water is a central element ofthe climate system, and it appears in manyforms: snow cover, land ice (including glaciersand the large ice sheets of Antarctica and Green-land), rivers, lakes, and surface and subsurfacewater.

Climate is also affected by forces outsidethis system: radiation from the Sun, the Earth’srotation, Sun-Earth geometry, and the Earth’sslowly changing orbit (Figure 2). Over long

4

IClimate

Figure 1. Simplified schematic view of the components of the global climate system and their interactions. Components of theclimate system are indicated in bold type in boxes. Larger boxes at the top and bottom indicate the potential changes. Interac-tions are shown by the arrows.

periods of time, the physical and chemicalmakeup of the Earth’s surface also changes.Continents drift, mountains develop and erode,the ocean floor and its basins shift, and, in addi-tion to water vapor changes, the composition ofthe dry atmosphere also changes. These alter-ations, in turn, change the climate.

Radiation is measured in Watts or per unitarea in Watts per meter squared (W/m2). Aver-aged over day and night, as well as over allparts of the world, the solar radiation receivedat the top of the atmosphere is 342 W/m2 or 175PetaWatts (175,000,000,000,000,000 Watts). Forcomparison, a typical light bulb puts out 100Watts, and a one-bar electric heater is 1,000Watts.

Atmospheric composition is fundamental tothe climate. Most of the atmosphere consists ofnitrogen and oxygen (99% of dry air). Sunlightpasses through these gases without beingabsorbed or reflected, so the gases have no cli-matic influence. The climate-relevant gasesreside in the remaining 1% of dry air, togetherwith water vapor. Some of these gases absorb aportion of the radiation leaving the Earth’s sur-face and re-emit it from much higher and colderlevels out to space. Such gases are known asgreenhouse gases, because they trap heat andmake the atmosphere substantially warmer thanit would otherwise be, somewhat analogous tothe effects of a greenhouse. This blanketing isknown as the natural greenhouse effect. Themain greenhouse gases are water vapor, whichvaries in amount from about 0 to 2%; carbondioxide, which is about 0.04% of the atmos-phere; and some other minor gases present inthe atmosphere in much smaller quantities.

The greatest changes in the composition ofthe atmosphere are entirely natural and involvewater in various phases in the atmosphere: aswater vapor, clouds of liquid water and/or icecrystal clouds, and rain, snow, and hail. Otherconstituents of the atmosphere and the oceanscan also change. A change in any of the climatesystem components, whether it is initiatedinside or outside of the system, causes theEarth’s climate to change.


5

Figure 2. Top: The Earth’s orbit around the Sun, illustrat-ing the seasons in both current times and 9,000 years ago.Today the Earth is nearest the Sun in northern winter, andhas an axial tilt of 23 1/2 degrees; in the past, the Earth wasnearest the Sun in northern summer and tilted by 24degrees. Bottom: Changes in average Northern Hemispheresolar radiation, in Watts per square meter, from 9,000 yearsago (ka) to the present over the annual cycle.

The Driving Forces of Climate

The source of energy that drives the climateis solar radiation (Figure 3). The Sun’s energytravels across space as electromagnetic radiationto the Earth and determines the energy avail-able for climate. Infrared (or “thermal”) radia-tion, radio waves, visible light, and ultravioletrays are all forms of electromagnetic radiation.

The Earth’s atmosphere interferes with theincoming solar radiation (Figure 4, see page 7).About 31% of the radiation is reflected away bythe atmosphere itself, by clouds, and by the

surface. (The fraction of solar radiation a planetreflects back into space, and that therefore doesnot contribute to the planet’s warming, is calledits albedo. So the albedo of the Earth is about31%.) Another 20% is absorbed by the atmos-phere and clouds, leaving 49% to be absorbed bythe Earth’s surface.

To balance the incoming energy, the planetand its atmosphere must radiate, on average,the same amount of energy back to space (Fig-ure 4). It does this by emitting infrared radia-tion. If the balance is upset in any way, forexample, by a change in solar radiation, then

CLIMATE

6

Figure 3. The incoming solar radiation (right) illuminates only part of the Earth while the outgoing longwave radiation isdistributed more evenly. As the panel at left shows on an annual mean basis, the result is an excess (hatched) of absorbedsolar radiation over the outgoing longwave radiation in the tropics, while there is a deficit (stippled) at middle to high lati-tudes. Thus there is a requirement for a poleward heat transport in each hemisphere (broad arrows, left) by the atmosphereand the oceans. This radiation distribution results in warm conditions in the tropics but cold at high latitudes, and the tem-perature contrast results in a broad band of westerlies in the extratropics of each hemisphere in which there is an embeddedjet stream (shown by the banded arrows) at about 10 km above the Earth’s surface. The flow of the jet stream over the differ-ent underlying surfaces (ocean, land, mountains) produces planetary waves in the atmosphere and geographic spatial struc-ture to climate.

Outgoing Longwave Radiation

90°N60

30

0

30

6090°S

Net

Rad

iatio

n

Hea

t Tra

nspo

rt

Solar

RadiationNight Day

the Earth either warms or cools until a new bal-ance is achieved. (Solar radiation, the electro-magnetic spectrum, and the entire process ofenergy transfer between Sun and Earth are dis-cussed in greater detail in the GCIP module TheSun-Earth System.) Most of the radiation emittedfrom the Earth’s surface does not escape imme-diately into space because of the presence of theatmosphere and, in particular, because of thegreenhouse gases and clouds in the atmospherethat absorb and re-emit infrared radiation.

Clouds play a complicated role in the plan-et’s energy balance. They absorb and emit ther-mal radiation and have a blanketing effect simi-lar to that of the greenhouse gases. They alsoreflect incoming sunlight back to space and thus

act to cool the surface. While the two opposingeffects almost cancel each other out, the netglobal effect of clouds in our current climate, asdetermined by space-based measurements, is tocool the surface slightly relative to what wouldoccur in the absence of clouds. Consequently,the bulk of the radiation that escapes to space isemitted either from the tops of clouds or by thegreenhouse gases, not from the Earth’s surface.

The Spatial Structure of Climate

Some parts of the Earth’s surface receivemore radiation than others (Figure 3). Thetropics get the most, and actually gain more


7

Figure 4. The Earth’s radiation balance. The net incoming solar radiation of 342 W/m2 (top center) is partially reflected byclouds and the atmosphere or by the Earth’s surface (a total of 107 W/m2, shown on the left-hand side of the figure). Of theremainder, 168 W/m2 (49%) is absorbed by the surface. Some of that heat is returned to the atmosphere as sensible heating(indicated by thermals, bottom center) and some as evapotranspiration that is realized as latent heat in precipitation. Therest is radiated as thermal infrared radiation, and most of that is absorbed by the atmosphere and reemitted both up anddown, producing the greenhouse effect (bottom right). The radiation lost to space comes from three sources. Some of it isemitted directly from the surface at certain wavelengths (40 W/m2); this region of the electromagnetic spectrum is calledthe “atmospheric window.” Additional radiation is reflected to space from cloud tops (30 W/m2). The largest fraction (165W/m2) comes from parts of the atmosphere that are much colder than the Earth’s surface.

energy than they lose to space. The midlatitudesget less. The poles receive the least of all, emit-ting more energy than they receive from theSun. This imbalance sets up an equator-to-poletemperature difference or “gradient” thatresults, when coupled with the influence of theEarth’s rotation, in a broad band of westerlywinds in each hemisphere in the lower part ofthe atmosphere. Embedded within these pre-vailing westerlies are the large-scale weathersystems and winds from all directions (see Fig-ure 6). These in turn, along with the ocean, actto transport heat poleward to offset the radia-tion imbalance (Figure 3). These weather sys-tems are the familiar events that we see everyday on television weather forecasts: eastward-migrating cyclones and anticyclones (i.e., low-and high-pressure systems) and their associatedcold and warm fronts. Because they carry warmair toward the poles and cool air toward the

equator, they are recognized as a vital part ofthe weather machine.

The continental land-ocean differences andobstacles such as mountain ranges also play arole by creating geographically anchored plane-tary-scale waves in the westerlies (Figure 3).These are the reasons why climate varies from,for instance, the west coast of the United Statesto the east coast. These waves are only semiper-manent features of the climate system: they areevident in average conditions in any given year,but may vary considerably in their locationsand general character from year to year. Specifi-cally, changes in heating patterns can alter thesewaves and cause substantial regions of bothabove- and below-average temperatures in dif-ferent places during any given season, such asthe example given earlier for the winter of1993–94.

CLIMATE

8

Weather phenomena such as sunshine, clouds ofall sorts, precipitation (ranging from light driz-zle to rain to hail and snow), fog, lightning,wind, humidity, and hot and cold conditionscan all be part of much-larger-scale weather sys-tems. The weather systems are cyclones (low-pressure systems) and anticyclones (high-pres-sure systems) and the associated warm and coldfronts. Figure 5 gives a satellite image of a majorstorm system on the east coast of the UnitedStates. Accompanying panels show the tempera-tures that delineate the cold front (see below)and the sea-level pressure contours. It is sys-tems like these, and their associated weatherphenomena, that make up the weather machine.

Weather systems exist in a broad band bothseparating and linking warm tropical and sub-tropical air and cold polar air. They not onlydivide these regions but also act as an efficientmechanism for carrying warmer air toward thepoles and cold air toward the equator. Thus, inthe Northern Hemisphere, southerlies (windsfrom the south) are typically warm and norther-lies (winds from the north) are cold. Within aweather system, the boundary of a region wherewarm tropical or subtropical air advances pole-ward is necessarily a region of strong temperaturecontrast. This boundary is called a warm front. Asthe warm air pushes cooler air aside, it tends alsoto rise, because warm air is less dense. Becausethe rising air also moves to regions of lower pres-sure it expands and cools, so that moisture con-denses and produces clouds and rain.

The advancement equatorward of cold airoccurs similarly along a cold front, but in thiscase, the colder and therefore denser air pushesunder the somewhat warmer air in its path,forcing it to rise, often causing convective

clouds, such as thunderstorm clouds, to form.Note that the movement poleward of warm airand the movement equatorward of cold air usu-ally go together as part of the same systembecause otherwise air would pile up in someplaces, leaving holes elsewhere.

The process of warm air rising and cold airsinking is pervasive in the atmosphere and isalso a vital part of the weather machine. Warmair is less dense than cold air and is thus natu-rally buoyant. As seen in Figure 4, warmth isgenerally transferred from the surface to higherlevels in the atmosphere, where the heat iseventually radiated to space. The process oftransferring heat upward is called convection. Itgives rise to a vast array of weather phenomena,depending on the geographic location, the timeof year, and the weather system in which thephenomena are embedded. Clouds that resultfrom convection are called convective clouds.These range from small puffy cumulus clouds,to multicelled cumulus that produce rain show-ers, up to large cumulonimbus clouds that mayproduce severe thunderstorms.

Weather systems over the oceans have asomewhat different character from those overland because of the abundant moisture over theoceans which more readily allows clouds andrain to form. Over land, storms are often moreviolent, in part because the land can heat andcool much more rapidly than the ocean and alsobecause mountain ranges can create strongwinds and wind direction changes (called windshear) that can help facilitate the developmentof intense thunderstorms and even tornadoes.These conditions often occur in the UnitedStates in spring to the east of the Rocky Moun-tains, where northward-moving air has an

9

IIThe Weather Machine

THE WEATHER MACHINE

10

Figure 5. Satellite imageryof a major storm system onthe East Coast of the Unit-ed States (Panel 3). Panel1 shows the temperaturesthat delineate the coldfront, and Panel 2 givesthe sea-level pressure con-tours in millibars. In Panel1, cold air over the UnitedStates is pushing south and east, carriedby strong northwesterly winds. Panel 2shows the low-pressure cyclone systemover the East Coast, which has a coldfront attached, indicating the leadingedge of the cold air. High pressures andan anticyclone exist over the northernGreat Plains, accompanied by clear skies(Panel 3). The cloud associated with thecold front is also shown in Panel 3,along with many other weather phenom-ena typical in such cases, as marked onthe figure. From Gedzelman (1980).

1

2

3

abundant supply of moisture (a prerequisite forcloud development) from the Gulf of Mexico.Weather phenomena and the larger weather sys-tems develop, evolve, mature, and decay largelyas turbulent instabilities in the flow of theatmosphere. Some of these instabilities arisefrom the equator-to-pole (i.e., horizontal) tem-perature contrast (Figure 6). If, for some reason,the contrast becomes too large, the situationbecomes unstable, and any disturbance can setoff the development of a weather system. Othertypes of instability occur as a result of verticaltemperature gradients—often associated withwarm air rising and cold air sinking (convectiveinstability). These types of instability may berelated to the warming of the surface air frombelow, or the pushing of warm and cold airmasses against one another as part of a weathersystem developing. They may also occur as partof the cycle of night and day. Many other weath-er phenomena arise from other instabilities orfrom breezes set up by interactions of the atmo-sphere with complex surface topography.

Weather phenomena and weather systemsmostly arise from tiny initial perturbations thatgrow into major events. The atmosphere, likeany other system, is averse to unstable situa-tions. This is why many triggering mechanismsexist that will push the atmosphere back towarda more stable state in which temperature con-trasts are removed. In general, therefore, oncethe atmosphere has become unstable, some formof atmospheric turbulence will take place andgrow to alleviate the unstable state by mixing upthe atmosphere. It is not always possible to saywhich initial disturbance in the atmosphere willgrow, only that one will grow. There is, there-fore, a large component of unpredictable behav-ior in the atmosphere, an unpredictability that isexacerbated by and related to the underlyingrandom component of atmospheric motions. Theprocesses giving rise to this randomness are nowreferred to in mathematics as chaos. Because ofthe above factors, weather cannot be accuratelyforecast beyond about ten days.

The processes and interactions in the atmo-sphere are also very involved and complicated.

This aspect of atmospheric behavior is referredto as “nonlinear,” meaning that the relationshipsare not strictly proportional. They cannot becharted by straight lines on a graph. The rela-tionships in nonlinear systems change in dispro-portionate (and sometimes unpredictable) waysin response to a simple change. A gust of windmay be part of a developing cloud that is


11

COLD

WARM

COLD

COLD

COLD

CLOUD

WARM

WARM

WARM

L

L

L

Figure 6. Baroclinic instability is manifested as the develop-ment of a storm from a small perturbation in the NorthernHemisphere with associated cold fronts (triangles) andwarm fronts (semicircles). The arrows indicate the directionof wind. The shading on the bottom panel indicates theextensive cloud cover and rain or snow region in themature stage.

embedded in a big thunderstorm as part of acold front, which is attached to a low-pressuresystem that is carried along by the overall west-erly winds and the jet stream (an example isgiven in Figure 5). All these phenomena interactand their evolution depends somewhat on justhow the other features evolve.

Nevertheless, on average, we know thatweather systems must behave in certain ways.There are distinct patterns related to the climate.So, while we may not be able to predict the

exact timing, location, and intensity of a singleweather event more than ten days in advance,because they are a part of the weather machine,we should be able to predict the average statis-tics, which we consider to be the climate. Thestatistics include not only averages but alsomeasures of variability and sequences as well ascovariability (the way several factors varytogether). These aspects are important, forinstance, for water resources, as described inWeather Sequences (see page 22).

THE WEATHER MACHINE

12

Figure 7. Many variables, such as temperature, have adistribution or frequency of occurrence that is close to a“normal” distribution, given by the bell-shaped curvesshown here. The center of the distribution is the mean(average). The variability (horizontal spread) is measured by the standard deviation. The values lie within one standard deviation 68% of the time and within two standard deviations 95% of the time. Panel 1 (right) shows the distribution of temperature for a hypothetical location. The axis shows the departures from the mean in units of standard deviation (vertical lines) and the temperature in °F, with a mean of 50°F and a standard deviation of 9°F.

Panel 2 (left) shows, in addition, the distributionif there is both an increase in mean temperatureof 5°F and a decrease in variability in the stan-dard deviation from 9 to 7°F. Because of thedecreased variability, extremely high tempera-tures do not increase in spite of the overallwarmer conditions, but note the decrease in inci-dence of temperatures below 45°F.

To consider a more concrete example, sup-pose that the average temperature in a month is50°F. In addition to this fact, it is also useful toknow that the standard deviation of daily val-ues is 9°F (Figure 7). This is the statistician’sway of saying that 68% of the time the tempera-tures fall within 50 plus or minus 9, or between41° and 59°F, and 95% of the time the values are

expected to fall within 50 plus or minus 18, orbetween 32° and 68°F. We may also wish toknow that the lowest value recorded in thatmonth is 22°F and the highest 79°F. Moreover, ifthe temperature is above 60°F one day, we canquantify the likelihood that it will also be above60°F the next day. And so on.


13

We have shown how climate and weather areintimately linked and explained how climatemay be considered as the average of weathertogether with information about its variabilityand extremes. Climate, however, may be forcedto change, not through internal weather effects,but due to the influence of external factors. And,if the climate changes in this way, so too will itsunderlying statistical nature, as characterized bythe weather we experience from day to day. Wenow address this possibility.

Human-Caused Climate Change

The climate can shift because of naturalchanges either within the climate system (such asin the oceans or atmosphere) or outside of it (suchas in the amount of solar energy reaching theEarth). Volcanic activity is an Earth-based eventthat is considered outside of the climate systembut that can have a pronounced effect on it.

An additional emerging factor is the effectof human activities on climate. Many of theseactivities are producing effects comparable tothe natural forces that influence the climate.Changes in land use through activities such asdeforestation, the building of cities, the storageand use of water, and the use of energy are allimportant factors locally. The urban heat islandis an example of very local climate change. Inurban areas, the so-called concrete jungle ofbuildings and streets stores up heat from theSun during the day and slowly releases it atnight, making the nighttime warmer (by sever-al degrees F in major cities) than in neighbor-ing rural regions. Appliances, lights, air condi-tioners, and furnaces all generate heat. Rainfall

on buildings and roads quickly runs off intogutters and drains, and so the ground is notmoist, as it would be if it were an open field.By contrast, when the Sun shines on a farmer’sfield, heat usually goes into evaporating sur-face moisture rather than increasing the tem-perature; the presence of water acts as an airconditioner. In fact, in some places a reverse ofurban warming, a suburban cooling effect, hasbeen found because of lawns and golf coursesthat are excessively watered. Changes in theproperties of the surface because of changes inland use give rise to these aforementioned cli-mate changes. Nevertheless, these effects aremostly rather limited in the areas they influ-ence.

The Enhanced Greenhouse Effect

Of most concern globally is the graduallychanging composition of the atmosphere causedby human activities, particularly changes aris-ing from the burning of fossil fuels and defor-estation. These lead to a gradual buildup of sev-eral greenhouse gases in the atmosphere, withcarbon dioxide being the most significant. Theyalso produce small airborne particulates—aerosols—that pollute the air and interfere withradiation. Because of the relentless increases inseveral greenhouse gases, significant climatechange will occur—sooner or later. The green-house-gas component of this change in climateis called the enhanced greenhouse effect. Whilethis effect has already been substantial, it isextremely difficult to identify in the past record.This is because of the large natural variability inthe climate system, which is large enough to

14

IIIClimate Change

have appreciably masked the slow human-pro-duced climate change.

The amount of carbon dioxide in the atmo-sphere has increased by more than 30% (Figure8) since the beginning of the industrial revolu-tion, due to industry and the removal of forests.In the absence of controlling factors, projectionsare that concentrations will double from pre-industrial values within the next 60 to 100 years.Carbon dioxide is not the only greenhouse gaswhose concentrations are observed to beincreasing in the atmosphere from human activi-ties. The most important other gases aremethane, nitrous oxide, and the chlorofluoro-carbons (CFCs).

Effects of Aerosols

Human activities also put other pollutioninto the atmosphere and affect the amount ofaerosols, which, in turn, influences climate inseveral ways. From a climate viewpoint, themost important aerosols are extremely small: inthe range of one ten-millionth to one millionthof a meter in diameter. The larger particles (e.g.,dust) quickly fall back to the surface.

Aerosols reflect some solar radiation back tospace, which tends to cool the Earth’s surface.They can also directly absorb solar radiation,leading to local heating of the atmosphere and,to a lesser extent, contributing to an enhancedgreenhouse effect. Some can act as nuclei onwhich cloud droplets condense. Their presencetherefore tends to affect the number and size ofdroplets in a cloud and hence alters the reflec-tion and absorption of solar radiation by thecloud.

Aerosols occur in the atmosphere from nat-ural causes; for instance, they are blown off thesurface of deserts or dry regions. The eruptionof Mt. Pinatubo in the Philippines in June 1991added considerable amounts of aerosol to thestratosphere, which scattered solar radiation,leading to a global cooling for about two years.Human activities that produce aerosols includebiomass burning and the operation of power

plants. The latter inject sulfur dioxide into theatmosphere, a molecule that is oxidized to formtiny droplets of sulfuric acid. In terms of theirclimate impact, these sulfate aerosols arethought to be extremely important; they formthe pervasive milky haze often seen from air-craft windows as one travels across NorthAmerica. Because aerosols are readily washedout of the atmosphere by rain, their lifetimes areshort—typically a few days up to a week or so.Thus, human-produced aerosols tend to be con-centrated near industrial regions.

Aerosols can help offset, at least temporari-ly, global warming arising from the increasedgreenhouse gases. However, their influence isregional and they do not cancel the global-scaleeffects of the much longer-lived greenhousegases. Significant climate changes can still bepresent.

The Climate Response and Feedbacks

Some climate changes intensify the initialeffect of greenhouse gases and some diminish it.These are called, respectively, positive and nega-tive feedbacks, and they complicate the way theclimate responds. For example, water vapor is a


15

Figure 8. Annual carbon dioxide concentrations in partsper million by volume (ppmv). The total values are given atleft, and the departures from the 1961–90 average (calledanomalies) are given at right. The solid line is from meas-urements at Mauna Loa, Hawaii, and the dashed line isfrom bubbles of air in ice cores.

powerful greenhouse gas and therefore absorbsinfrared radiation, so when a warmer climatecauses more moisture to evaporate, the resultingwater vapor increase will make the temperatureeven warmer. Clouds can either warm or coolthe atmosphere, depending on their height, type,and geographic location. Hence they may con-tribute either positive or negative feedbackeffects regionally; their net global effect in awarmer climate is quite uncertain as it is not

clear just how clouds may change with changingclimate. Other important feedbacks occurthrough atmospheric interactions with snow andice, the oceans, and the biosphere. Quantifyingthese various feedbacks is perhaps the greatestchallenge in climate science, and the uncertain-ties in their magnitude are the primary source ofuncertainty in attempts to predict the large-scaleeffects of future human-induced climate change.

CLIMATE CHANGE

16

Observed Climate Variations

Scientists expect climate change, but whatchanges have they observed? Analysis of globalobservations of surface temperature show thatthere has been a warming of about 0.6°C overthe past hundred years (Figure 9). The trend is

toward a larger increase in minimum than inmaximum daily temperatures. The reason forthis difference is apparently linked to associatedincreases in low cloudiness and to aerosoleffects as well as the enhanced greenhouseeffect. Changes in precipitation and other com-ponents of the hydrological cycle are deter-mined more by changes in the weather systemsand their tracks than by changes in temperature.Because such weather systems are so variable inboth space and time, patterns of change in pre-cipitation are much more complicated than pat-terns of temperature change. Precipitation hasincreased over land in the high latitudes of theNorthern Hemisphere, especially during thecold season.

Figure 10 shows changes observed in theUnited States over the past century. Note espe-cially the trend for wetter conditions after aboutthe mid-1970s in the first panel (a). Panel breveals that the main times of drought in theUnited States were in the 1930s and the 1950s. Inthe 1930s there was extensive drying in theGreat Plains, referred to as the Dust Bowlbecause of the blowing dust and dust stormscharacteristic of that time. In part, the Dust Bowlwas exacerbated by poor farming practices.

Naturally, times of moisture surplus tend toalternate with times of extensive drought. Panel creveals the increasing tendency for rainfall tooccur in extreme events of more than two inchesof rain per day over more of the country. Thus,heavy rainfalls tend to occur more often or overmore regions than previously, a steady and sig-nificant trend of about a 10% increase in suchevents. Temperatures have also increased in gen-eral (Panel d), but the warmest years tend to bethose associated with the big droughts, which

17

IVObserved Weather and Climate Change

Figure 9. Average annual mean temperatures, expressed asanomalies from the 1961–90 average, over the Northern andSouthern Hemispheres (middle and bottom panels) and forthe globe from 1860 to 1998. Mean temperatures for1961–90 are 14°C for the globe, 14.6°C for the NorthernHemisphere, and 13.4°C for the Southern Hemisphere.Based on Jones et al. (1999).

18

Figure 10. (a) The variations in U.S. average annual precipitation from the long-term average (mm), (b) the incidence ofdroughts and floods expressed as percentages of the U.S. land area, (c) the percentage of the United States that receives morethan 2 inches (50.8 mm) of rainfall in one day, (d) the variation of the average annual U.S. temperature from the long-termaverage (°C), (e) the incidence of much-above and much-below normal temperatures expressed as percentage of U.S. landarea, and (f) the number of hurricanes making landfall. In panels b, c, and e, the definitions of drought, flood, much abovenormal, and much below normal all correspond to the top or bottom 10% of all values on average. In panels b and e, theextents of the much-above and much-below normal areas are plotted opposite one another as they tend to vary inversely. Thisis not guaranteed, however, as wet (warm) conditions in one part of the country can be and often are experienced at the sametime as dry (cold) conditions elsewhere (see Figure 11). From Karl et al. (1995).

(a) U.S. average annual precipitation (b) Dry (drought) and wet (flood) conditions in the U.S.

(c) Percent U.S. much above normal rainfall from 1day extreme events (>2”)

(d) U.S. Temperatures

(e) Much above and much below normal U.S. temper-atures

(f) Number of hurricanes making landfall


19

contribute to many heat waves (because water isno longer present to act as a natural air condi-tioner). In the United States, some of the warmestyears occurred in the 1930s. The warmth of the1980s and 1990s, especially compared with the1900 to 1920 period, cannot simply be explainedby heat waves and changes in drought, however.In Panel e, we see that most of the temperaturesmuch below average occurred in the early part ofthis century, while most of the temperatures wellabove average occurred either in the past 15years or in the 1930s. Hurricanes naturally varyconsiderably in number from year to year (Panelf). Since they are so variable, and relatively rare,no clear trends emerge.

Figure 11 shows a consolidation of these fac-tors into a U.S. climatic extremes index (CEI). Itis made up of the annual average of whetherseveral indicators are much-above or much-below normal, where these categories corre-spond to the top and bottom 10% of values. Avalue of 0% for the index would mean no por-tion of the country experienced extreme condi-tions in any category. A value of 100% wouldmean the entire country was under extremeconditions throughout the year under all cate-gories. The average value, because of the waythe index is defined, must be around 10%, and

the variations about this value indicate theextent to which the country was experiencingan unusual number of extremes of one sort oranother. The major droughts of the 1930s and1950s again are evident in this figure. In morerecent decades, the increase in extremes comesfrom the increases in much-above normal tem-peratures and the increase in extreme one-dayrainfall events exceeding two inches.

Interannual Variability

A major source of variability from one yearto the next is El Niño. The term El Niño (Spanishfor the Christ child) was originally used alongthe coasts of Ecuador and Peru to refer to awarm ocean current that typically appearsaround Christmas and lasts for several months.Fish yields are closely related to these currents,which determine the availability of nutrients, sothe fishing industry is particularly sensitive tothem. Over the years, the term has come to bereserved for those exceptionally strong warmintervals that not only disrupt the fishing indus-try but also bring heavy rains.

El Niño events are associated with muchlarger-scale changes across most of the Pacific

Figure 11. The CEI is thesum of two numbers. Thefirst reflects the percentageof the United States, byarea, where maximum andminimum temperatures,moisture, and days of pre-cipitation were much-aboveor much-below normal. Thesecond number is twice thepercentage of the UnitedStates, by area, where thenumber of days of veryheavy precipitation (morethan two inches) was muchgreater than normal. FromKarl et al. (1995).

OBSERVED WEATHER AND CLIMATE CHANGE

20

Ocean. These changes in turn alter weatherpatterns around the globe through changes inthe atmospheric circulation. They can alter theatmospheric waves (Figure 3) and thus thetracks of storms across North America and else-where. The major floods in the summer of 1993in the upper Mississippi River basin were partlycaused by El Niño. Recent floods in California(winters of 1994–95 and 1997–98) were alsolinked to El Niño as the storm track continuallybrought weather systems onto the west coast ofthe United States. (El Niño is discussed more

extensively in the module El Niño and the Peru-vian Anchovy Fishery.)

Because the magnitude of El Niño events isrelatively large compared with climate changeon the slower decadal time scale, El Niño ismanifested much more readily than globalwarming in the weather we experience and inthe regional climate variations. This is a primeexample of interannual variability of climate,which, in general, tends to mask the climatechange associated with global warming.

In general, climate changes cannot be predictedsimply by using observations and statistics.They are too complex or go well beyond condi-tions ever experienced before. For the mostdetailed and complicated projections, scientistsuse computer models of the climate systemcalled numerical models. These models arebased on physical principles, expressed asmathematical formulas and evaluated usingcomputers.

Climate Models

Global climate models attempt to include theatmospheric circulation, oceanic circulation, landsurface processes, sea ice, and all other processesindicated in Figure 1. They divide the globe intothree-dimensional grids and perform calculationsto represent what is typical within each grid cell.For climate models, owing to limitations intoday’s computers, these grid cells are quitelarge—typically 250 kilometers in the horizontaldimension and a kilometer in the vertical dimen-sion. As a result, many physical processes canonly be crudely represented by their averageeffects.

One method used to predict climate is tofirst run a model for several simulated decadeswithout perturbations to the system. The qualityof the simulation can then be assessed by com-paring the average, the annual cycle, and thevariability statistics on different time scales withobservations. If the model seems realisticenough, it can then be run including perturba-tions such as an increase in greenhouse-gas con-centrations. The differences between the climatestatistics in the two simulations provide an esti-

mate of the accompanying climate change.To make a true prediction of future climate

it is necessary to include all the human and nat-ural influences known to affect climate (cf. Fig-ures 1 and 12). Because future changes in sever-al external factors, such as solar activity andvolcanism, are not known, these must beassumed to be constant until such time as weare able to predict their changes.

Climate Predictions

The climate is expected to change becauseof the increases in greenhouse gases andaerosols, but exactly how it will change dependsa lot on our assumptions concerning futurehuman actions. When developing countriesindustrialize, they burn more fossil fuels, gener-ate more electricity, and create industries, mostof which produce some form of pollution.Developed countries are currently the largestsources of pollution and greenhouse gases.Because future changes are not certain, climatemodels are used to depict various possible “sce-narios.” These are not really predictions but pro-jections of what could happen. If a projectionindicates that very adverse conditions couldhappen, policy actions could be taken to try tochange the outcome. The following are somefeatures of possible future climate changes cre-ated by human activities. Greatest confidenceexists on global scales; regional climate changesare more uncertain.

1. The models indicate warming of 1.5 to4.5°C for a climate with atmospheric CO2 con-centrations doubled from preindustrial times,when they were 280 parts per million by vol-

21

VPrediction and Modeling ofClimate Changes

ume. An effective doubling of CO2, taking intoaccount aerosols and other greenhouse gases, islikely to occur around the middle of the 21stcentury. Corresponding manifestations of North-ern Hemisphere climate change will take placesome 20 to 50 years later because it takes theoceans at least that long to respond. The lag islikely to be greater over the Southern Hemi-sphere because of the influence of the largerocean area. Aerosols are also expected toincrease in areas undergoing industrialization(such as China) and to decrease in North Ameri-ca and Europe, where steps are being taken todecrease acid rain by decreasing sulfur emis-sions. The effects of aerosols will complicate cli-mate change and will most likely change theregional distribution of the temperature increase.When effects of aerosols and greenhouse gasesare combined, one estimate puts the average rateof temperature increase in the next century atabout 0.15 to 0.25°C per decade. Such a warming

is expected to lead to an increase in extremelyhot days and a decrease in extremely cold days.

So far, over the past century, during whichtime carbon dioxide has increased from 290–300to 360 parts per million by volume (roughly a20% increase), the observed temperature increasehas been fairly modest, about 0.5°C (see Figure9). This temperature increase is reasonably con-sistent with model predictions when effects ofaerosols are included. But large uncertaintiesremain, particularly because of questions abouthow clouds might change.

2. The hydrological cycle is likely to speedup by about 10% with CO2 doubling, bringingincreased evaporation and increased rainfall ingeneral. With warming, more precipitation is aptto fall as rain in winter instead of snow, and,with faster snowmelt in spring, there is likely tobe less soil moisture at the onset of summer overmidlatitude continents. When this change iscombined with increased evaporation in sum-

PREDICTION AND MODELING OF CLIMATE CHANGES

22

Figure 12. Schematic model of the fluid and biological Earth that shows global change on a time scale of decades to cen-turies. A notable feature is the presence of human activity as a major inducer of change; humanity must also live with theresults of change from both anthropogenic and natural factors. From Trenberth (1992).

mer, any natural tendency for a drought to occuris likely to be enhanced. However, there is notgood agreement among the models on thisaspect. An enhanced hydrological cycle alsoimplies increased intensity of rainfall, such ashas been found for the United States (Figure 10).Increases in rainfall in winter but drier condi-tions in summer would challenge future watermanagers to avoid flood damage and keep upwith the demand for fresh water.

3. Because warming causes the ocean toexpand and snow and glacial ice to melt, onereal threat is a rise in sea level. There may besome compensation through increased snowfallon top of the major ice sheets (Greenland andAntarctica) so that they could increase in heighteven as they melt around the edges. Currently,sea level is observed to be rising by 1 to 2mm/year, and this rate should increase, so thatthere are prospects for about a 50-cm rise in sealevel by 2100, but the main impacts are not like-ly to be felt until the 22nd century.

4. With warming, increases in water vapor(a greenhouse gas) and decreases in snow coverand sea ice (lower albedo) provide positivefeedbacks that should enhance the warming astime goes on. The land should warm more thanthe oceans, and the largest warming shouldoccur in the Arctic in winter.

5. Stratospheric cooling is another likelyeffect of increased greenhouse gases. This cool-ing has important implications for ozone deple-tion, because the chemistry responsible for theAntarctic ozone hole is more effective at lowertemperatures. The loss of ozone also increasesstratospheric cooling.

6. Because of increased sea-surface tempera-tures, there may be changes in tropical stormsand hurricanes. Hurricanes sustain themselvesat temperatures above 27°C, feeding on theextra water vapor and latent heat those temper-atures create. However, natural variability ofhurricanes is large (Figure 10), so any effectfrom climate change will be hard to detect formany decades.

7. Coupled ocean-atmosphere general circu-lation models have only very recently been able

to simulate rudimentary El Niño cycles. It seemslikely that El Niño will continue to exist in awarmer world. Because El Niño and its coolcounterpart La Niña create droughts and floodsin different parts of the world, and becauseglobal warming tends to enhance the hydrologi-cal cycle, there is a real prospect that future suchevents will be accompanied by more severedroughts and floods. In the tropics, in particular,because of the great dependence on thunder-storm rainfall and its tendency to fall at certaintimes of the year (during the wet or monsoonseason) the main prospect that looms is one oflarger variability and larger extremes in weatherevents.

Interpretation of Climate Change in Termsof Weather

For assessing impacts, what is most neededare projections of local climate change. However,producing such projections represents a consider-able challenge. Climate predictions are especiallydifficult regionally because of the large inherentnatural variability on regional scales. We havediscussed changes in climate mostly in terms ofchanges in average conditions. But we experiencethose changes mainly through changes in the fre-quency of extreme weather events, e.g., how hotit gets on a daily basis, or how frequent and vio-lent thunderstorms become. An average monthlychange in temperature of 3°C (5°F) may notsound like very much, but it has a very dramaticeffect on the daily frequency of extreme tempera-tures, e.g., see Figure 7. For example, currently inDes Moines, Iowa, the likelihood that the maxi-mum temperature on any day in July will exceed35°C (95°F) is about 11%. However, with anincrease in the average monthly maximum tem-perature of 3°C the likelihood almost triples, toabout 30%. Small changes in the average canbring about relatively large changes in frequen-cies of extremes.

In addition to a change in the average cli-mate, the variability itself could also change. Ifthe daily variability of temperature increases in


23

THE SUN-EARTH SYSTEM

24

Des Moines, then an even greater portion ofdays would exceed 35°C. If, on the other hand,the variability decreases, the temperature fromone day to another would be more similar thanbefore. Changes in variability affect changes inthe frequency of extremes and have more effectthan changes in averages (see Figure 7). There issome evidence that with climate warming, dailyvariability of temperature might decrease sothat there might be fewer cold extremes in win-ter. Variability of temperature could decrease insome seasons (e.g., winter) but increase in oth-ers.

Changes in variability of precipitation arealso anticipated and will tend to be associatedwith changes in the average precipitation. Varia-bility generally increases as average precipitationincreases. In the United States, precipitationextremes have been found to increase in the pastfew decades (see Figure 10 Panel c). The picturefor precipitation is more complicated, however.For example, climate change is likely to alter thejet stream and associated location of storm tracks,so that some places will experience an increase instorminess while others, not very far away, willexperience a decrease. Such opposite changesover short distances should be expected and arean inherent part of climate for rainfall, but this islikely to be confusing to many people.

It is likely that most people in developed

countries will continue to experience weathermuch as they have before. In some places theymay notice that the time between major snowstorms is longer, heat waves are more frequentand debilitating, the intensity and frequency ofthunderstorms are changed, coastal damage tobeaches is more common, prices of some com-modities increase while others decrease, waterconserving practices in certain communities areintensified, and so on. Areas where the cumula-tive effects of weather are important, such aswater resources and agriculture, may be more atrisk.

Many of the effects may be rather subtlemost of the time, and the actual impact mayoriginate through other pressures (increasingpopulation, as an example) and may only beexacerbated by the changes in climate. But thereare also likely to be dramatic effects. As anexample, during a drought a string of wide-spread heat waves may put increased demandon air conditioning, causing brownouts andeven blackouts as the electricity demand exceedsavailable capacity; or there may be more medicalemergencies, such as heat stroke, involving thosewho do not have or cannot afford air condition-ing. Ironically, the extra use of air conditioningleads to increased fossil fuel use and hence agreater emission of greenhouse gases.

Human activities and many sectors of eco-nomic activity depend on weather and climatein different ways. Some rely on average condi-tions. Others are sensitive to extremes. Yet oth-ers depend upon variety and so weathersequences can be important. Aside from choos-ing the climate by selecting the right location,there are other ways we can attempt to copewith climate change and its consequences foragriculture, fisheries, and so forth.

Weather Sequences

Conditions may be altered not only by indi-vidual weather events but also by sequences ofweather events. Weather sequences, for exam-ple, play a big role in determining stream runoffand soil moisture, and can result in prolongedperiods of abnormal temperatures and sun-shine. These are important determinants of agri-cultural yields, and the responsiveness of yieldsto such other inputs as fertilizer depends on thegrowing conditions supplied by a sequence ofweather events.

Runoff to surface streams and groundwaterrecharge, or replenishment, depend on extendedsequences of weather events so that the contribu-tion of individual rainstorms to runoff dependson whether previous conditions were wet or dry.In addition, the timing of runoff in mountainousriver basins is strongly dependent on snowpackaccumulation and rate of melt. Mountain runoff,thus, is quite sensitive to temperature variations.The quantity and timing of runoff, in turn, deter-mine the availability of water for competing agri-cultural, municipal, industrial, hydropower,recreational, and ecological uses.

As an example, suppose place A has 0.5inches of gentle rain every three days, for amonthly average of 5 inches, and place B has 2.5inches of rain on two consecutive days of themonth but with all other days dry, again for amonthly total of 5 inches. The monthly total isthe same, but the sequence differs greatly andthe climates would be quite different. At place A,the rain would replace the evaporation and useof moisture by plants; there would be few pud-dles, so there would be no runoff into streams.As a rule of thumb, anytime there is more than 3inches of rain in a day, there will be fairly exten-sive flooding. So at place B it is likely that low-lying parts of roads would be flooded, culvertswould overflow, basements would flood, andthere would be substantial damage from all therunoff during the two rainy days. But then therest of the month, the ground would dry out andplants would become stressed and wilt unlessthey had very deep and extensive roots. The dif-ferent sequences of weather make for very dif-ferent impacts.

Location, Location, Location

Climate and weather contribute to personalsatisfaction. For example, the satisfaction provid-ed by a walk in the park varies according towhether conditions are balmy or blustery. A sim-ple economic model of the allocation of timebetween walks in the park and other activitiespredicts that parks will become more crowded asthe weather improves. Casual observations con-firm that prediction. Many people also express awillingness to pay to live where they can expectto enjoy particular climatic characteristics, such

25

VIImpacts of Weather and ClimateChanges on Human Activities

as frequent mild, sunny weather. Their valua-tions of those characteristics may be expressed asa willingness to accept a somewhat lower realwage or to pay more for housing of comparablequality in order to live in a preferred climate.

Climates are tied to particular locations, sothat when individuals decide to move them-selves and their productive activities to a certainplace, they are also choosing the climate inwhich they will live and operate. For most eco-nomic activities, climate is only one of manyfactors influencing choice of location. For someactivities, the characteristics of climate are a cen-tral factor in location decisions. The expectedavailability of snow is an important concern forthe location of ski resorts. A sufficiently low riskof severe freezes is a critical consideration in thelocation of orange groves, and crop selectiondecisions and farm management strategies areheavily influenced by probable growing-seasonconditions.

The location of other industries is tied to theavailability of particular natural resources. Thelumber and paper industries require trees.Hydropower dams are located where streamgradients and rates of flow offer significantpotential generation. Fishing fleets and process-ing capacity are based to allow access to expect-ed concentrations of commercially valuable fish.Such resources are themselves tied to climate.The connections are obvious for hydropower,where drought conditions can quickly lead toreduced generation. The impacts of climaticvariations on the timber industry are less imme-diate, although prolonged droughts can signifi-cantly reduce the stock of healthy standing treesand often create favorable conditions for forestfires.

Severe Weather Events

The most dramatic impact of weather onhuman endeavors is often through severeweather events that may alter as the climatechanges. Severe weather has always affectedhuman activities and settlements as well as the

physical environment. It can damage property,cause loss of life and population displacement,destroy or sharply reduce agricultural cropyields, and temporarily disrupt essential servic-es such as transportation, telecommunications,and energy and water supplies. Society hasdeveloped various methods to avoid or mini-mize adverse impacts of weather and has alsodeveloped means to facilitate recovery fromextreme weather phenomena. Yet, becausesevere weather events repeatedly disruptsocioeconomic activities and cause damage,society continues to search for new ways to pro-tect lives and property. Some of these involvebehavioral adjustments based on past societalexperience, such as educating citizens aboutwhat to do in the event of a tornado warning.Others involve the application of new meteoro-logical research findings for improving the pre-diction of where and when severe weather willoccur (see page 28).

Societal Responses

One way of reducing vulnerability toweather is to reduce damage to property,through such strategies as stricter constructionstandards, tighter building codes, and restric-tions on development in floodplains and oncoastal barrier islands. The construction ofstorm sewers can help minimize short-termflood damage in highly developed areas wherethere is substantial impermeable surface such aspavement. The casualty and hazard insuranceindustry in more developed countries helpsinsured parties rebuild and replace propertydamaged by severe weather. Of course, insur-ance does not physically protect property fromweather-related damage, but it does facilitaterecovery and replacement in the aftermath ofextreme weather events such as tornadoes, hur-ricanes, and floods. The insurance industryitself has been altered by perceptions of climatechange, such as rates for coastal insurance inFlorida. Reservoirs increase resilience to short-term fluctuations in streamflows and thus pro-

IMPACTS OF WEATHER AND CLIMATE CHANGES ON HUMAN ACTIVITIES

26

tect the water supply and hydropower produc-tion. Electric utilities also increase resilience tovariable hydropower output and variabledemand by maintaining backup generationcapacity (e.g., coal-fired plants) and by buyingor selling power over interconnected transmis-sion grids.

A second way of reducing vulnerability toweather is through technology. Many technolo-gies are so common that they have become partof society’s everyday affairs and activities. Forexample, modern tires, windshield wipers, andfog lights have helped reduce the hazard ofdriving in bad weather conditions. Indoor heat-ing and air conditioning provide comfort andprotection from extreme temperatures in winterand summer. The invention of shelter itself wasprobably prompted by human desires to haveprotection from the extremes of weather and cli-mate as well as from predators and human ene-mies.

Modern weather forecasting, which has pro-gressed rapidly over the past half-century, cangive advance warning of possibly dangerousweather conditions. Forecasters can frequentlyprovide information minutes to several daysahead of possible severe weather conditions. Inmany cases, decisions may be made based onforecasts to reduce or eliminate potential vulnera-bility to severe weather. For example, on a con-struction site, concrete deliveries may be resched-uled to ensure that snow, ice, and cold tempera-tures do not interfere with its proper curing. Busi-nesses may alter trucking schedules and routes inresponse to anticipated foul weather. In certaincircumstances, farmers may be able to harvest allor part of their crops in advance of what could bedestructive weather. The usefulness of weatherforecast information varies among economic sec-tors. While a reliable weather forecast may help afarmer to efficiently schedule crop irrigation, forexample, it cannot help that farmer protect a cropfrom imminent hail damage. Other coping mech-anisms, such as crop insurance, preparedness,and routine maintenance of flood levees andstorm sewers, also help society manage its vul-nerability to extreme weather events.

Managing Risk

Climate and day-to-day weather variationsaffect a wide variety of economic activities. Cli-mate influences the spatial distributions of pop-ulation and of industrial, agricultural, andresource-based production activities, whileweather can affect levels of production andproduction costs. In addition, severe weathercan damage or destroy property.

In gambling, even the most astute playerswill occasionally lose. In economics, if climate-induced loss reveals new information on thenature of the climatic risk or on the vulnerabili-ty of affected activities, or if it alters people’sperceptions of the risk, then they will readjusttheir risk-management strategies. If not, theywill go back to the status quo. For example,towns that are hit by tornadoes are usuallyrebuilt in the same location because one hit doesnot signal any change in the long-term risk. Aseries of extreme events, on the other hand, maybe taken as a signal that previously availableinformation provided an inaccurate picture ofthe true risk, or that the climate has changed. Inthat situation, a town might not rebuild in thesame location.

Impacts on Agriculture

Humans have been interested in under-standing and predicting the effects of climate oncrop production since the rise of agriculture,because food production is critical to humansurvival. A classic Biblical example is in Gene-sis, where Joseph interprets a dream of thePharaoh’s as a portent of seven coming years ofgood grain harvests followed by seven years ofcrop failure.

Crop yields are strongly affected by changesin technological inputs such as fertilizer, pesti-cides, irrigation, plant breeding, and manage-ment practices, but the major cause of year-to-year fluctuations in crop yield is weather fluctu-ations. Agricultural crops are mainly sensitive tofluctuations in temperature and precipitation,


27

although solar radiation, wind, and humidityare also important. In general a crop grows bestand produces maximum yield for some opti-mum value of the relevant climate variable; asconditions depart from the optimum, the plantssuffer stress. The responsiveness of yields, andtherefore the financial return, to such inputs asfertilizer and pesticides varies with weatherconditions, so that it is prudent for farmers tomake adjustments depending on the weather.

Effects of Temperature and Precipitation onCrop Yield. The temperature regime of aparticular locale will affect the timing ofplanting and harvesting and the rate at whichthe crop develops. With adequate moisture, thepotential growing season is largely determinedby temperature; in temperate mid-latituderegions this generally extends from the last frostin the spring to the first frost in the fall. The rateat which plants develop and move through their


28

In the case of private production and invest-ment decisions, the climate-related risks falllargely on the parties making the decisionsunless they have chosen to purchase someform of insurance, allowing the sharing of therisk with others. To the extent that the deci-sion makers bear the risk, they have theincentive to engage in appropriate risk-man-agement strategies and to make efficient useof available climate- and weather-relatedinformation.

Many climate-sensitive natural resourcesare managed as public property, and decisionsregarding their use are made by governmentagencies, often with considerable input fromthe interested public. In such cases, the effectsof climatic variability often complicate thealready difficult task of balancing the conflict-ing demands of competing interests. Fisheriesare sensitive to climatic variations, but thetrue impacts of climate are often complex anddifficult to separate from the impacts of otherfactors (such as fishery management, over-fishing, spawning habitat degradation, waterdiversions, building of dams, and pollution)influencing the survival, growth, and spatialdistribution of fish populations.

The Pacific salmon fishery provides anexample. Since the mid-1970s, warmer sea-

surface temperatures along the Pacific coast ofNorth America and changes in near-shore cur-rents associated with more frequent and per-sistent El Niño events appear to have con-tributed to remarkable increases in the pro-ductivity of Alaskan salmon stocks and todeclining runs of some salmon spawning inWashington, Oregon, and California. In theearly 1990s, these trends culminated in aseries of record Alaskan salmon harvests andsevere declines in once-thriving Coho andChinook fisheries in Washington and Oregon.These fluctuations in northern and southernsalmon stocks contributed to the breakdownof international cooperation under the PacificSalmon Treaty. Under pressure from commer-cial, sport, and Indian fishing interests withintheir respective jurisdictions, British Colum-bia, Alaska, and the West Coast states wereunable to come to a consensus over a fair andbiologically sound division of the harvest forsix years. The resulting inability to controlAlaskan and Canadian exploitation of deplet-ed stocks migrating to the southern spawningareas contributed to their further decline.Finally, in June 1999, the governmentsresponded to the imperiled state of the stocksby implementing a new agreement thatadjusts harvests to changes in abundance.

Pacific Salmon

growth stages (crop phenology) is regulated bytemperature. The thermal requirements of cropsare often determined by adding up thetemperatures over time and determining thetotal thermal units, often referred to as growingdegree days, that are required to completeparticular growth stages. Temperature alsoaffects the rate of plant respiration and partiallydetermines the plant’s need for water bydetermining the evapotranspiration rate.

Growing plants can be damaged by temper-ature extremes that interfere with their metabol-ic processes and may be especially sensitiveduring particular stages of growth. For example,in growing corn, severe high temperature stressfor a ten-day period during silking (a criticalphenological stage when the numbers of kernelson the corn ear is determined) can result incomplete crop failure.

Water is necessary for plant growth, and soprecipitation is also extremely important. Allimportant physiological processes such as pho-tosynthesis, respiration, and grain formationrequire moisture. Drought is certainly theweather extreme that has been most studied interms of its impact on agriculture. Crops areparticularly sensitive to moisture stress duringcertain phenological stages. For example, mois-ture stress is especially harmful to corn, wheat,soybean, and sorghum during the periods offlowering, pollination, and grain-filling. Inade-quate moisture causes reduced crop yield.

Agriculture and Climate Fluctuations of thePast. In the 20th century there have beenvarious periods of drought in North America,but the most serious was the prolonged droughtof the 1930s in the Great Plains of the UnitedStates and Canada. The extremely lowprecipitation and relatively high temperatures(Figure 10) resulted in drastic reductions ingrain yields. Wheat yields in Saskatchewanprovince in Canada for the years 1933–37 wereless than half the yields obtained in the 1920s. Inthe south-central United States (Oklahoma,Kansas, Colorado, Texas, and New Mexico)rainfalls about 100 mm below normal for these

years and poor farming practices combined toproduce a lot of blowing dirt and many severedust storms, creating the Dust Bowl.

Another example is the prolonged cool peri-od during the 16th and 17th centuries inEurope, known as the Little Ice Age. In its cold-est phase, the average annual temperature inEngland was approximately 1.5°C less than thatof the 20th century, resulting in widespread andfrequent crop failure because of the greatlyreduced growing season and cold damage tocrops. In the hill country of southeast Scotlandbetween 1600 and 1700 the oat crop failed onaverage one year out of every three.

Short-lived extreme temperatures can alsoseverely affect crops. In the Corn Belt of theUnited States in 1983, substantial lossesoccurred because of blistering hot temperaturesin July, including a week when maximum tem-peratures remained above 35°C, when the cornwas flowering. This example indicates that evennow, when farming is technologically advanced,extreme weather can result in serious losses.

Future Climate Change and Agriculture.Although there are many uncertaintiesregarding how climate may change due toincreased greenhouse gases, there are somelikely changes that would affect agriculture inspecific ways. Generally, increasedtemperatures would bring about longerpotential growing seasons, which would allowfor multiple cropping in some areas (i.e., raisingmore than one crop per season). Also, cropswould reach maturity more quickly; however,this could result in declining yields, since thecrop would have less time to form grain.Higher temperatures would increase therespiration rates of plants (the process by whichplants break down organic substances), thusreducing the amount of biomass available foryield formation. More-frequent hightemperatures could also result in crop damageby increasing evaporation and hence moisturestress and wilting in plants even if there wereno changes in precipitation.

Increased greenhouse warming is likely to


29


30

An instructive example is provided by theexperience of Florida citrus growers with aseries of devastating freezes during the 1980s.At the beginning of the 1980s, Florida’s orangegroves were concentrated in the center of thestate, along the north-south central ridge. Nearthe northern edge of the citrus belt, freezedamage to fruit and occasional loss of treeswere a common occurrence, but growers hadlearned to manage those risks: by balancingtheir investments in orange groves againstother sources of income, by avoiding plantingin known cold pockets, and by engaging inmitigative actions, such as turning on sprin-klers as freezing temperatures approached.

Then January 1981 brought the first in aseries of five tree-killing freezes that pro-foundly altered the central Florida landscape.The two most damaging freezes, in December1983 and January 1985, together killedapproximately one-third of the state’s com-mercial citrus trees, virtually eliminatinggroves in several counties close to the formerheart of the citrus belt. Lake County, whichhad the second-largest citrus acreage in thestate at the beginning of 1980, lost more than90% of its orange trees to the 1983 and 1985freezes. The final freeze in the series, inDecember 1989, killed the majority of the treesthat had managed to survive the earlierfreezes as well as 61% of Lake County’s newlyreplanted trees. Statewide, the 1989 freezekilled far fewer trees than did the 1983 and1985 events, in part because of a major shift innew citrus planting to more southerly areas.

Why, you might ask, were the groves notlocated in those southern relatively freeze-safe

counties to begin with? Because the heavy,wet soils there require expensive preparationand drainage before citrus can be planted. Inaddition, yields were traditionally lower, andtrees were more prone to diseases in thoseareas. So growers had weighed the risk offreeze-related losses against the expected dif-ferences in net returns in their original deci-sions about where to locate groves. Theirexperiences during the 1980s increased theirapparent wariness of the freeze risk, tiltingthe balance in favor of the southern growingareas, where millions of new citrus trees havebeen planted since the mid-1980s.

Such adjustments to long-term climaticvariations and to new information regardingweather-related risks can be made more easilyfor some types of activities than for others. Itis relatively easy to alter the mix of annualcrops to be planted or the proportion of fal-low to planted acreage if new informationbecomes available before the beginning of theplanting cycle. A forecast of unusually hot,dry conditions over the growing season mightinduce farmers to leave a larger proportion oftheir land fallow and increase the proportionof the remainder devoted to drought-tolerantvarieties.

Rapid adjustments are more difficultwhere the production process is not resilientto climatic variations and depends on rela-tively immobile capital assets. In the Floridacitrus example, the trees were expected to belong-lived, immobile capital assets. Theirdestruction provided growers with the neces-sity and opportunity to rethink their invest-ment strategies.

Florida Citrus

result in both increases and decreases in precipi-tation in different areas. Increased droughtwould bring about reduced yields in generaland a greater likelihood of complete crop fail-ure, particularly in areas of the world that arecurrently vulnerable to environmental changesand where water is already limited for agricul-ture, such as in the semiarid areas of Africa bor-dering the Sahara Desert.

The Effects of Nonclimatic Factors. Severalfactors could limit loss in agriculturalproductivity due to climate change. One ismitigation by the direct physiological effects ofincreased CO2 on plants. If CO2 were increasingwithout any change in climate, agriculturalproductivity worldwide would likely increase.Experiments indicate that some plants grown inatmospheres enriched in CO2 show increasedrates of photosynthesis and of netphotosynthesis (total photosynthesis minusrespiration). They tend to use water moreefficiently and thus require less. These plants(the so-called C3 class), include wheat, rice, andsoybeans. Other crops, such as corn andsorghum (in the C4 group), do not benefit fromincreased concentrations. If CO2 increases, thesespecies—which are particularly important indeveloping countries—may even be at acompetitive disadvantage compared to weedsbelonging to the C3 group.

Another factor is technological adaptationsto environmental changes. Agriculture is anecosystem managed by humans. Possible adap-tations to climate change include increasing (ordecreasing) irrigation, changing crop type toone more adapted to the new climate, breedinghybrids of the original crop that can better copewith the new climate (e.g., breeding for droughttolerance), and adjusting fertilizer, herbicide,and pesticide use. While many adaptations arepossible, their success is difficult to determine,because the ultimate value of agricultural cropsor changes in productivity can only be deter-mined when considering the interactions ofglobal economies. For example, let us say thatwith climate change, sorghum grows better in

the central Great Plains because it is moredrought tolerant than wheat. Farmers may beable to switch crops, but the profitabilitydepends on the demand for sorghum in domes-tic and international markets.

In recent years, climate change assessmentsof agriculture have become sophisticatedenough to analyze the impact of climate changeon agriculture, direct physiological effects, pos-sible technological adaptations, and changes inthe global economy. Such “integrated assess-ments” make numerous assumptions about thefuture, above and beyond how the climate maychange. These studies are highly complex andrife with uncertainties. It seems that on a globalbasis developed countries may be able to adaptfairly well to climate change, but there is con-cern that some developing countries could suf-fer serious economic and human hardships.

Planning for Local Weather Changes

Society should develop appropriateresponses to help manage and reduce vulnera-bility to extreme meteorological events. It islikely that increasing frequency and/or intensi-ty of severe weather as a result of climatechange will put more lives and property at risk,particularly in coastal and inland areas close tocoastlines. Around the world, these areas havealready become very vulnerable over the pastseveral decades as human populations anddevelopment have grown dramatically incoastal and near-coastal regions. As an illustra-tion, the population of U.S. coastal counties(Atlantic, Great Lakes, Gulf of Mexico, andPacific) grew by nearly 32 million residentsbetween 1960 and 1990 (from about 75 millionto 107 million), according to the U.S. Bureau ofthe Census. Similar or even higher rates ofcoastal-zone population growth occurred inmany other countries during the same period,making them more vulnerable to coastal stormsand wave surges as well.

In response to more frequent and/or moreintense weather events, societal rules governing


31

such matters as building codes and land usecould be modified or strengthened to helpreduce vulnerability. Alterations in the insur-ance industry (e.g., changes in government reg-ulation of the industry) could help restructurefinancial instruments that help protect the valueof property in the event of damaging weather.Government policies that encourage settlementin vulnerable areas such as floodplains (e.g.,government-subsidized flood insurance) couldbe amended to deter such settlement. Govern-ment regulations and insurance programs couldalso be used to deter activities (e.g., by eliminat-ing subsidies) such as farming in vulnerablelocales like floodplains, areas prone to freezedamage, or areas where scarce water resourcesmay have a higher value in the future than ifthey are used for irrigation of low-value crops.

In some circumstances, preventive or pro-tective structures could be built to guard certainareas from flood, storm surge, or hurricanelandfall. Coastal barrier walls could be con-structed in some valued areas, flood levees maybe built to protect low-lying farmland, andother structures such as bridges and causewaysreinforced so as to protect them from a widerrange of severe weather impacts. Note, howev-er, that there are examples where human tinker-ing has backfired, such as some levees which,

once broached, exacerbated the Mississippiflooding in the summer of 1993. Municipal andregional water distribution systems could berebuilt to reduce and/or eliminate leakage,which would conserve scarce water suppliesduring periods of drought. Changes in waterpricing for domestic and industrial users couldalso encourage water conservation duringdrought, as demonstrated by pricing conserva-tion measures already adopted in many regionssuch as the southwestern United States. In areasdependent on groundwater for municipaland/or agricultural uses, regulations for waterextraction could promote conservation and evenreuse of finite groundwater supplies.

While society may not be able to insulateitself completely from changes in local andregional weather that may accompany a futureclimate change, human populations do possessintellectual, economic, and physical capacities tomanage their vulnerability to severe weatherevents. Nonetheless, it is important to search forhistorical lessons as to how well (or how poorly)society may adjust to possible climate-change–induced alterations in regional weather.Society can either continue to repeat mistakes, orlearn how and where behavioral and physicaladjustments can help manage societal vulnera-bility to weather phenomena.


32

33

The complex interactions and feedbacks thatoccur within Earth’s climate system make it dif-ficult to establish just how large the human-induced effects will be or how soon we may beable to detect the climate change unequivocally.It is critical to increase our understanding of thenatural variability of the climate system, tobuild better climate models that more explicitlyand more accurately represent weather phe-nomena, and to reduce uncertainties in predic-

tions of what human activities are contributingto the climate system. In turn there is a greatneed to be able to better translate what changesin climate might mean in terms of the weather,weather sequences, and extremes that mayoccur, so that these in turn can be translated intoimpacts on various sectors of society andhuman endeavor. In this way, improved strate-gies for dealing with Earth’s ever-changingenvironment might be effected.

VIIThe Need for More Research

Aerosol—Microscopic particles suspended inthe atmosphere, originating from either anatural source (e.g., volcanoes) or humanactivity (e.g., coal burning).

Albedo—The reflectivity of the Earth.Anaerobic—Occurring in the absence of free

oxygen; an example of an anaerobic processis digestion in cattle.

Annual cycle—The sequence of seasons over afull year.

Anthropogenic climate change—Climatechange arising from human influences.

Anticyclone—A high-pressure weather system.The wind rotates clockwise around these inthe Northern Hemisphere and counterclock-wise in the Southern Hemisphere. Theyusually give rise to fine, settled weather.

Atmospheric chemistry—The science of thechemical composition of the atmosphere.

Atmospheric instability—The growth of smalldisturbances into large disturbancesthrough internal processes.

Baroclinic instability—An atmospheric instabil-ity associated with horizontal temperaturegradients such as between the equator andthe poles.

Biomass burning—The burning of organic mat-ter from plants, animals, and other organ-isms.

Carbon dioxide (CO2)—A naturally occurring,colorless atmospheric greenhouse gas. Itarises in part from decay of organic matter.Plants take up carbon dioxide during photo-synthesis. Animals breathe it out during res-piration. Humans contribute to carbon diox-ide concentrations in the atmosphere byburning fossil fuels and plants.

Chaos—In a technical sense, a process whosevariations look random even though theirbehavior is governed by precise physicallaws.

Chlorofluorocarbon (CFC)—One of a family ofgreenhouse gas compounds containingchlorine, fluorine, and carbon. CFCs do notoccur naturally; all are made by humans.They are generally used as propellants,refrigerants, blowing agents (for producingfoam), and solvents.

Climate—The average weather together withthe variability of weather conditions for aspecified area during a specified time inter-val (usually decades).

Climate change—Long-term (decadal or longer)changes in climate, whether from natural orhuman influences.

Climate model—A computer model that usesthe physical laws of nature to predict theevolution of the climate system.

Climate system—The interconnected atmosphere-ocean-land-biosphere-ice components of the Earth involved in climateprocesses.

Climate variation—A fluctuation in climatelasting for a specified time interval, usuallymany years.

Cold front—A transition zone where a cold airmass advances, pushing warmer air out ofthe way. Warm air is forced to rise, com-monly creating convection and thunder-storms, so that a period of “bad weather”occurs as the temperatures drop.

Composition of the atmosphere—The makeupof the atmosphere, including gases andaerosols.

34

GLOSSARY

Convection—In weather, the process of warmair’s rising rapidly while cooler air sub-sides, usually more gradually, over broaderregions elsewhere to take its place. Thisprocess often produces cumulus clouds andmay result in rain.

Cumulus cloud—A puffy, often cauliflower-like,white cloud that forms as a result of convec-tion.

Cyclone— A low-pressure weather system. Thewind rotates around cyclones in a counter-clockwise direction in the Northern Hemi-sphere and clockwise in the Southern Hemi-sphere. Cyclones are usually associated withrainy, unsettled weather and may includewarm and cold fronts.

Dust Bowl era—The period during the 1930swhen prolonged drought and dust stormsarose in the central Great Plains of the Unit-ed States.

Dynamics—In climate, the study of the actionof forces on the atmospheric and oceanicfluids and their response in terms of windsand currents.

Ecosystem—A system involving a living com-munity and its nonliving environment, con-sidered as a unit.

El Niño—The occasional warming of the tropi-cal Pacific Ocean off South America. Associ-ated warming from the west coast of SouthAmerica to the central Pacific typically lastsa year or so and alters weather patternsaround the world.

Electromagnetic spectrum—The spectrum ofradiation at different wavelengths, includ-ing ultraviolet, visible, and infrared rays.

Enhanced greenhouse effect—The increase inthe greenhouse effect from human activities.

Evapotranspiration—The evaporation of mois-ture from the surface together with transpi-ration, the release of moisture from withinplants.

Feedback—The transfer of information on asystem’s behavior across the system thatmodifies behavior. A positive feedbackintensifies the effect; a negative feedbackreduces the effect.

Fossil fuel—A fuel derived from living matterof a previous era; fossil fuels include coal,petroleum, and natural gas.

General circulation model—A computer model,usually of the global atmosphere or theoceans; GCMs are often used as part of evenmore complex climate models.

Glacier—A mass of ice, commonly originatingin mountainous snow fields and flowingslowly down-slope.

Global warming—The increasing heating of theatmosphere caused by increases in green-house gases from human activities and their“entrapment” of heat. It produces increasesin global mean temperatures and anincreased hydrological cycle. This phenome-non is also popularly known as the green-house effect.

Greenhouse effect—The effect produced as cer-tain atmospheric gases allow incoming solarradiation to pass through to the Earth’s sur-face but reduce the escape of outgoing(infrared) radiation into outer space. Theeffect is responsible for warming the planet.

Greenhouse gas—Any gas that absorbs infraredradiation in the atmosphere.

Groundwater—Water residing underground inporous rock strata and soils.

Hydrological cycle—The cycle by which watermoves and changes state through theatmosphere, oceans, and Earth. Evaporationand transpiration of moisture producewater vapor, which is moved by winds andfalls out as precipitation to become ground-water, which in turn may run off in streamsor in glaciers into the seas or become storedbelow ground.

Infrared radiation—The longwave part of theelectromagnetic spectrum, corresponding towavelengths of 0.8 microns to 1,000microns. For the Earth, it also correspondsto the wavelengths of thermal emitted radi-ation. Also known as longwave radiation.

Jet stream—The strong core of the midlatitudewesterly winds, typically at about 8 to 10km above the surface of the Earth, in eachhemisphere.


35

Land surface exchange—An exchange of gasesfrom the land surface into the atmosphereor vice versa. The most common is evapora-tion of water into water vapor.

Little Ice Age—A prolonged cool period, espe-cially in Europe, occurring primarily in the16th and 17th centuries.

Longwave radiation—See infrared radiation.Mean—The average of a set of values.Methane (CH4)—A naturally occurring green-

house gas in the atmosphere produced fromanaerobic decay of organisms. Commonsources include marshes (thus the name“marsh gas”), coal deposits, petroleumfields, and natural gas deposits. Humanactivities contribute to increased amounts ofmethane, which can come from the diges-tive system of domestic animals (such ascows), from rice paddies, and from landfills.

Natural greenhouse effect—The part of thegreenhouse effect that does not result fromhuman activities.

Negative feedback—See feedback.Net radiation— The sum of all the shortwave

and longwave radiation passing through alevel in the atmosphere.

Nitrous oxide (N2O)—A naturally occurringgreenhouse gas in the atmosphere producedby microbes in the soil and ocean. Humanscontribute to concentrations through burn-ing wood, using fertilizers, and manufactur-ing nylon.

Nonlinear—Not linear. Linear relationshipsbetween two variables can be plotted as astraight line on a graph. Nonlinear relation-ships involve curved or more complex lines.

Normal distribution—A bell-shaped curve ofthe distribution of the frequency with whichvalues occur, defined by the mean and thestandard deviation.

Ozone (O3)—A molecule consisting of threebound atoms of oxygen. Most oxygen in theatmosphere, consists of only two oxygenatoms (O2). Ozone is a greenhouse gas. It ismostly located in the stratosphere, where itprotects the biosphere from harmful ultravi-

olet radiation. Human activities contributeto near-surface ozone through car exhaustand coal-burning power plants; ozone in thelower atmosphere has adverse affects ontrees, crops, and human health.

Phenology—The study of natural phenomenathat occur in a cycle, such as growth stagesin crops.

Photosynthesis—The process by which greenplants make sugar and other carbohydratesfrom carbon dioxide and water in the pres-ence of light.

Positive feedback—See feedback.Runoff—Excess rainfall that flows into creeks,

rivers, lakes, and the sea.Scattering radiation—The dispersion of incom-

ing radiation into many different directionsby molecules or particles in the atmosphere.Radiation scattered backwards is equivalentto reflected radiation.

Solar radiation—Radiation from the sun, mostof which occurs at wavelengths shorter thanthe infrared.

Southern Oscillation—A global-scale variationin the atmosphere associated with El Niñoevents.

Stability—In meteorology, a property of theatmosphere, making it resistant to displace-ments. The atmosphere is stable if a pertur-bation decays and it returns to its formerstate. It is unstable if the perturbation grows.

Standard deviation—A measure of the spreadof a distribution. For a normal distribution,68% of the values lie within one standarddeviation of the average.

Stratosphere—The zone of the atmospherebetween about 10–15 and 50 kilometersabove the Earth’s surface. Most of the ozonein the atmosphere is in the stratosphere. Thestratosphere is separated from the tropo-sphere below by the tropopause.

Temperature gradient—The differences in tem-perature across a specified region.

Thermal—A rising pocket of warm air.Thermal radiation—Longwave (infrared) radia-

tion from the Earth.

GLOSSARY

36


37

Transpiration—The giving off of water vaporthrough the leaves of plants.

Troposphere—The part of the atmosphere inwhich we live, ascending to about 15 kmabove the Earth’s surface, in which temper-atures generally decrease with height. Theatmospheric dynamics we know as weathertake place within the troposphere.

Urban heat island—The region of warm airover built-up cities associated with the pres-ence of city structures, roads, etc.

Visible radiation—Electromagnetic radiation,lying between wavelengths of 0.4 and 0.7microns, to which the human eye is sensi-tive.

Warm front—A transition zone where a warmair mass pushes cooler air out of the wayover a broad region. The warm air tends torise, often creating stratiform clouds andrain as the temperatures rise.

Weather—The condition of the atmosphere at agiven time and place, usually expressed interms of pressure, temperature, humidity,wind, etc. Also, the various phenomena inthe atmosphere occurring from minutes tomonths.

Weather systems—Cyclones and anticyclonesand their accompanying warm and coldfronts.

Wind shear—Large differences in wind speedand/or direction over short distances.

Adams, R.M., R.A. Fleming, and C. Rosenzweig,1995: Reassessment of the economic effectsof global climate change on U.S. agriculture.Climatic Change 30, 147–168.

Andrews, W.A., 1995: Understanding GlobalWarming. D.C. Heath Canada Ltd., Toronto,Canada.

Dotto, L., 1999: Storm warning: Gambling with theclimate of our planet. Doubleday, Toronto,Canada.

Gedzelmen, S.D., 1980: The Science and Wondersof the Atmosphere. John Wiley and Sons, NewYork, New York.

Hartmann, D.L., 1994: Global PhysicalClimatology. Academic Press, San Diego,California.

Jones, P.D., M. New, D.E. Parker, S. Martin, andI.G. Rigor, 1999: Surface air temperature andits changes over the past 150 years. Review ofGeophysics 37, 173–199.

Karl, T.R., R.W. Knight, D.R. Easterling, and R.G. Quayle, 1995: Trends in the U.S. climateduring the twentieth century. Consequences1, 2–12.

Karl, T., and K. Trenberth, 1999: The humanimpact on climate. Scientific American.December, 100–105.

IPCC (Intergovernmental Panel on ClimateChange), 1996: Climate Change 1995: The Sci-ence of Climate Change. J.T. Houghton, F.G.Meira Filho, B.A. Callander, N. Harris, A.Kattenberg, and K. Maskell, eds. CambridgeUniversity Press, Cambridge, U.K.

Lamb, H.H., 1982: Climate, History, and the Mod-ern World. Cambridge University Press,Cambridge, U.K.

Mearns, L.O., 1993: Implications of globalwarming on climate variability and theoccurrence of extreme climatic events. InDrought Assessment, Management, and Plan-ning: Theory and Case Studies. D. A. Wilhite,ed. Kluwer Publishers, Boston, Massachu-setts, 109–130.

Mooney, H.A., E.R. Fuentes, and B. I. Kronberg,eds., 1993: Earth System Responses to GlobalChange: Contrasts between North and SouthAmerica. Academic Press, San Diego, Cali-fornia.

Parry, M.L., 1978: Climatic Change, Agriculture,and Settlement. Dawson and Sons, Ltd.,Folkestone, U.K.

Raper, C.D., and P.J. Kramer, eds., 1983: CropReactions to Water and Temperature Stresses inHumid, Temperate Climates. Westview Press,Boulder, Colorado.

Rosenzweig, C., and D. Hillel, 1993: Agriculturein a greenhouse world. Research and Explo-ration 9 (2), 208–221.

Trenberth, K.E., ed., 1992: Climate System Model-ing. Cambridge University Press, Cam-bridge, U.K.

Trenberth, K.E., 1996: Coupled climate systemmodeling. In Climate Change: DevelopingSouthern Hemisphere Perspectives. T. Giambel-luca and A. Henderson-Sellers, eds. JohnWiley & Sons, New York, New York, 63-88.

Trenberth, K.E., 1999: The extreme weatherevents of 1997 and 1998. Consequences, Vol 5,1, 2–15.

Williams, J., 1992: The Weather Book. VintageBooks, New York, New York.

38

SUGGESTED READINGS

1. Given the large seasonal changes in climate,why are relatively modest changes in cli-mate from one year to the next so disrup-tive?

2. Weather patterns never repeat exactly, so thevariations are apparently endless, yet therange of patterns is limited. Explain thisapparent contradiction.

3. People like to blame weather disasters onsome cause. Although a disaster can belinked to various weather phenomena, theremay not be a single true cause. Explain why.

4. List possible human influences on climate.Note which ones are likely to be global,which are more likely to just be regional,and why.

5. What kinds of weather events most affecthuman activities? What can be done to pre-pare for these or ameliorate the effects?

6. What do you think is the value of a climateforecast of the expected weather for the sea-son ahead? How accurate would it have tobe to be useful? Given forecasts that mightbe classed as having (a) some but not muchaccuracy, (b) reasonable accuracy, or (c) com-plete accuracy, discuss how this informationmight be used to economic advantage.

7. Climate changes are under way but are notyet large. People disagree over whether totake action to try to stop or slow humaninfluences on climate. Their views seem tobe related to their values and perspectiveson what is important now versus how muchweight should be given to the future. In agroup, put forward your own views onwhat you think should be done and discussthe range of different views and what fac-tors influence them.

8. People offset risks of natural disasters bytaking out insurance. Discuss other ways tomitigate the effects of weather and climatechange on various activities.

9. If the climate warms, it is sometimes sug-gested that crops and plants should just begrown in locations farther north. Explainwhy this may not be possible (consider espe-cially sunlight, soil conditions, and disease).

10. What does “climate is what we expect, butweather is what we get” mean?

39

DISCUSSION QUESTIONS

aerosols 14, 15, 21–22agricultural yields 25–31agriculture 1, 24, 25, 27–31albedo 6–7, 23anticyclones 2, 7, 8, 9atmospheric composition 5–6,14atmospheric window 7–8

bell curve 12–13building codes 26, 31–32

carbon dioxide 3, 5, 14, 21–22, 31chaos 11chlorofluorocarbons 15circulation, general 1citrus 30climate 1–5, 7–8, 14–19, 25–26, 29climate change 2–5,14–24, 29–31climate change, human–caused 14–15, 19climate models see modelingclimate scenarios 21climate system 2–5, 14, 21–22, 33climate, U.S. 17–18climatic extremes index 17–19clouds 2, 6–7, 9–11, 15, 22corn 27–29covariability 12crop yields see agricultural yieldscropping, multiple 29–31cyclones 2, 7, 8–9

deforestation 14drought 1, 3, 17–18, 22, 24, 28–31drought, 1988 1Dust Bowl 17, 29

economic activities 1, 2, 25–32El Niño 3, 19–20, 23, 28evaporation 14, 21–22, 25, 31evapotranspiration 7–8, 28–29

extreme events 17, 19, 28see also drought, severe storms

feedbacks 15, 23, 33fertilizer 25, 28–31fire 26fishing 19, 20, 25–28floodplain 26, 31–32floods 1, 17–19, 25, 26, 27, 31–32floods, 1993 1, 20floods, Mississippi 20, 31–32food production 27–29fossil fuel 14, 21freezes 25, 30fronts 2, 7, 8, 9, 11

global warming 3, 15, 20–23greenhouse effect 5–6, 14greenhouse effect, natural 5–6greenhouse gases 3, 5–8, 14–17, 21–24, 29, 31groundwater 31–32growing degree days 28–29growing season 27–29

harvesting 1, 27–29heat transport 6–7heat waves 2, 7, 24heat, latent 7, 23hurricanes 2, 18–19, 23, 26hydrological cycle 15, 22, 23hydropower 25, 26

ice cores 15ice sheets 4, 23industrialization 21instability, baroclinic 11instability, convective 11insurance 26, 27, 31–32irrigation 27–31

40

INDEX

jet stream 6, 11

La Niña 23levees 27, 31–32Little Ice Age 29

Mauna Loa 15methane 15modeling 19–23, 25, 33Mt. Pinatubo 15

nitrous oxide 15nonlinear dynamics 11 normal distribution 12

observations 17–20, 21–25orange groves 25, 30orbital changes 5ozone hole 23

pesticide 29–31phenology 28–29photosynthesis 29–31planetary waves 6–7planning 31–32plants 25, 29, 30, 31pollution 15, 21, 27precipitation 2, 7, 8, 9, 17, 18, 22, 24, 30, 31, 32 prediction 1, 21–24, 25, 26, 33projections 14, 21, 23property damage 25–30

radiation 4–7, 14–17radiation, infrared 5–7, 15 radiation, solar 4–6, 7, 15, 28–29rainfall 1, 3, 5, 9, 14, 17–19, 22–25, 28–29randomness 11regulations, government 31–32reservoirs 26respiration 29–31risk 24, 25–30runoff 25

Sahara Desert 1salmon 28satellite images 9, 10sea level 9, 10, 23sequences 12, 25, 33shelter 27societal responses 26–32sorghum 28, 29soybean 28, 29standard deviation 12statistics 12, 21stratosphere 15, 23storms, severe 9, 26–31subsidies 32sulfur dioxide 15summer 1, 22, 23sun 1, 5–6

temperature 2, 3, 6, 7, 9–13, 14, 15, 18, 22, 23,25, 28–31

temperature change 17, 19, 21–22, 23, 28temperature, maximum 17, 30-31temperature, minimum 17thermals 7trees 26, 30storms, tropical 2, 18, 23turbulence 2, 11

urban heat island 14

volcanic activity 14vulnerability 26–30, 32

water vapor 4–5, 15, 22–23weather 1–2, 7–12, 17, 19, 23–29, 32weather, severe 9, 23, 26–32weather forecasting 10, 11, 27weather machine 1, 7–12weather phenomena 2, 9–11, 26weather systems 2, 7, 9–11, 17, 20westerlies 6, 7wheat 28, 29winter 1, 3, 22–23

yields see agricultural yields


41

EFFECTS OF CHANGING CLIMATE ONWEATHER AND HUMAN ACTIVITIES

The global climate is changing, and human activities are now part of the cause. Buthow does a climate change manifest itself in day-to-day weather? This moduleapproaches the topic by explaining distinctions between weather and climate.Readers will gain an understanding of how the rich natural variety of weather can besystematically influenced by climate, and how external influences — such as humanactivities — can cause change. Impacts of climate variations and societal strategiesfor coping with them are also discussed. The book includes topics for discussion anda glossary.

GLOBAL CHANGE INSTRUCTION PROGRAM

• STRATOSPHERIC OZONE DEPLETIONby Ann M. Middlebrook and Margaret A. Tolbert

• SYSTEM BEHAVIOR AND SYSTEM MODELING, by Arthur A. Few Winner of the EDUCOM Award, includes STELLA® II demo CD for Macs and Windows

• THE SUN-EARTH SYSTEM, by John Streete

• CLOUDS AND CLIMATE CHANGE, by Glenn E. Shaw

• POPULATION GROWTH, by Judith Jacobsen

• BIOLOGICAL CONSEQUENCES OF GLOBAL CLIMATE CHANGEby Christine A. Ennis and Nancy H. Marcus

• CLIMATIC VARIATION IN EARTH HISTORY, by Eric J. Barron

• EL NIÑO AND THE PERUVIAN ANCHOVY FISHERY, by Edward A. Laws

NEW FROM UNIVERSITY SCIENCE BOOKS:

CONSIDER A CYLINDRICAL COW, by John Harte

The cow is back. This time she is cylindrical, not spherical. Featuring a new core set of25 fully worked-out problems, this book is organized according to five thematic sectionson probability, optimization, scaling, differential equations, and stability & feedback.Following in the hooves of Consider a Spherical Cow, the Cylindrical Cow will help stu-dents achieve a whole new level of environmental modeling and problem solving.

UNIVERSITY SCIENCE BOOKS55 D Gate Five Road, Sausalito, CA 94965Fax (415) 332-5393/www.uscibooks.com

EARTH AND ENVIRONMENTAL SCIENCE

Cover design by Craig Malone, cover photograph by Mickey Glantz

effects of changing climate on weather and human ......304.2’5–dc21 00-023978 printed in the...

Documents