Climate History Tour Through Time

Earth’s Climate HistoryDuring its long history, Earth has experienced wide variations in climate from Ice Ages to Hothouse. Fortunately for us, the range has never been large enough to exterminate or incinerate life. What has caused the climate variations? There are several culprits. First, is Solar Radiation (assuming Earth’s distance to the Sun has not changed). Theory indicates that the Sun’s Irradiance has gradually been increasing over its long history and was about 25 to 30% lower some 4.5 billion years ago. If there have been short term variations, we know little about that. Today, solar irradiance varies by about 0.1% and peaks when the Sunspot Cycle is at its maximum.

But if the young Sun was weaker, why wasn’t the young Earth encased in Ice? This is called the Faint Young Sun Paradox. The Second Major Cause of Climate variations, is due to changes in atmospheric composition via the Greenhouse Effect. Climate can vary widely if the amount of Greenhouse gases varies. And there is plentiful evidence that the amount of Greenhouse gases, most notably CO2 and CH4, have varied widely.

The third cause of Climate Change is due to changes in Earth’s geography, via Plate Tectonics and Continental Drift. Land that moves away from the Equator gets drier at first as it approaches the Subtropics and then markedly colder as it approaches the Poles. The continents move so slowly that these changes take tens of millions of years.

The fourth cause of major climate change results from cyclic variations in Earth’s Orbit and occurs on the time scale of tens to hundreds of thousands of years. This alters the distribution of solar radiation over the seasons and with latitude.

Fifth is the impact of albedo. Lighter surfaces reflect more light and make climate cooler.

Permo-Carboniferous Ice AgeMajor climate changes on Earth have occurred as continents moved from the equator to the poles or vice – versa. Climate is warm and rainy near the equator where the Sun is high in the sky and coldest near the poles where the Sun is low in the sky. Climate is dry in the subtropical latitudes (between about 20° and 30° latitude) where the air sinks. Therefore, tropical rainforests are located near the equator, most deserts in the subtropics and ice sheets near the poles.

A great Ice Age occurred during Carboniferous and Permian Periods – from about 320 to 240 MBP - long before the Age of Dinosaurs. At the time the continents were largely clustered together in a giant supercontinent called Pangaea. For the most part this landmass was located much further south than today. Antarctica was close to where it is now, centered over the South Pole, but South America, Africa, India and Australia were all attached to it, and therefore much further south and nearer the Pole than now.

Late in the Devonian, another event combined with Continental Drift to start the Permo-Carboniferous Ice Age. Giant forests spread over much of the land. During the Carboniferous, wholesale burial of fallen trees in swamps accumulated to form the world’s greatest coal deposits and remove vast quantities of CO2 from the atmosphere. This weakened the Greenhouse Effect and led to the Ice Age.

The long Ice Age finally ended at the end of the Permian. A great outpouring of lava and CO2 from rifts (and resulting forest and coal fires) in China (260 MBP) and Siberia (251 MBP) produced a great warming that melted all the ice. It also ended deep ocean circulation, depriving the oceans of O2, which led to the greatest mass extinction in Earth history. This Extinction paved the way for the Mesozoic Age of Dinosaurs.

http://en.wikipedia.org/wiki/Paleoclimate

Ice Sheets and their Movement During the Carboniferous and Permian

Plate Tectonics

Animation of the Moving Continents over the past 200 million years

A map of the world early in the Permian, about 300 million years ago. Much land in the South Hemisphere was covered by glaciers, which spread northward. Coal was produced in Equatorial rainforests and in Temperate forests during the warmer "Interglacial" periods. Deserts occurred in Canada, Scandinavia, and the Sahara, which were located between about 20 and 30° latitude.

South America

North America

Greenland

Europe

Africa

Antarctica India Australia

Asia and Siberia

China

Arabia

Tropical Rainforest

Subtropical Desert

Subtropical Desert

Polar Ice Cap

Madagascar

Mid-Cretaceous Climate and Geography: A Hothouse

Mid-Cretaceous HothouseDuring the Mid-Cretaceous, the tropics were about 5ºC warmer than today but the high latitudes were much warmer. This was not a time of major mountain building, so erosion and burial of carbon was reduced. Rapid sea floor spreading increased volcanism, raising CO2 levels to 4 - 10 times higher than now and widening the mid ocean ridges so that the sea became shallower. This plus the absence of ice sheets raised sea level so high that much land including the Great Plains was inundated and transformed into broad, shallow seas such as the Tethys. The low albedo and high heat storage of the widespread waters added to the warming, especially in winter. Dinosaurs migrated far south to the polar regions of Antarctica and Australia or Canada. Tropical plants grew and alligators cruised around Greenland and tropical reefs extended to at least 40º latitude. Currents were weak in the warm oceans, so oxygen rich water did not sink, and

http://www.soest.hawaii.edu/GG/FACULTY/POPP/Lecture9.ppt

oxygen-poor black muds and shales rich in organic matter accumulated. The buried carbon taken from the atmosphere helped cool the climate late in the Cretaceous, but now provides the world with much of its oil and gas. The warm waters also helped deposit in Europe’s shallow seas the huge chalk deposits that gave the Cretaceous its name.

Zachos, et al. Trends, Rhythms in Cenozoic Climate, Science 292, pp 686-693 (2004)

Cenozoic Climate Overview

The Cenozoic – Recent Ice AgesA long period of global cooling preceded the recent Ice Ages. A permanent ice sheet covered part of Antarctica by 34 MBP and ultimately covered East Antarctica by 11 MBP. Ice cover in West Antarctica experienced large oscillations from perhaps 5.5 to 2.8 MBP. Frigid Antarctica chilled the nearby waters which sunk to fill the deep oceans. Deep ocean water cooled from about 15ºC to about 3ºC. This cooling set the stage for the North Hemisphere ice sheets which began growing by 2.8 MBP. Ice Ages then became somewhat periodic events, timed to cycles in Earth’s Orbit (see ATM_CLIM_CYCLE.) Before 0.8 MBP they matched the 41,000 year cycle of obliquity. Since then, there have been about 8 major ice ages that have more or less matched the 100,000 year cycle of eccentricity. At the peak of these Ice Ages, ice sheets between 2 and 3 km deep covered Canada, parts of the northern United States and large sections of north Europe and Asia. Sea level dipped to 120 m lower than now. T fell by less than 5ºC in the tropics but more than 10 or 15ºC in regions covered by ice. Rapid and irregular swings of T occurred between warm and ice age conditions, often within a few years. Warming and melting after each Ice Age was so rapid that incredible flooding occurred and sea level rose up to 30 mm yr-1.

During Ice Age Peaks, the regime of Polar Climate expanded. This squeezed all other climate zones toward the Equator. The subtropical deserts expanded and encroached on much smaller tropical rain forests. In the frigid world, most climates were drier, but because the jet stream and storm belt were centered over the Western United States, precipitation and lake levels rose while temperature fell. The snow line lowered so that many mountain glaciers expanded in the Himalayas, Alps, Andes, Rockies, etc.

Dome C (elevation 3222 m), one of the coldest places on Earth. T hardly rises above −25°C in summer and can fall below −80°C in winter. Annual average T = −54.5°C.

Scenic Variety on the Antarctic Ice Sheet

Year Round Ice Cover: Today

Today, despite the spectre of Global Warming, there is still much ice on Earth.

Year Round Ice Cover: 20 KBP

But there was a time when ice covered much more of the Earth.

http

://w

ww

.mus

eum

.sta

te.il

.us/

exhi

bits

/ice_

ages

/inde

x.ht

ml

Melting of the North American Ice Sheet

Postglacial Warming and the Younger Dryas Cold SpellAs the world began to emerge from the ice age, warming and melting accelerated to a breathtaking pace about 15,000 years ago. Sea level rose more than 1 m (3.3 ft) per century on average, with bursts above 4 m (13.3 ft) per century. As the climate warmed, snowfall actually increased on the ice caps. Most other parts of the world also experienced wetter conditions. Lake Bonneville (now the Great Salt Lake) expanded rapidly and by 15,000 years ago, was ten times larger and 825 feet (250 m) higher than now. At that point it filled the Great Basin and overflowed into the Snake River. At the same time, other lakes in the western United States such as Lake Lahonton in Nevada also expanded and overflowed.

Around 14,500 years ago, colder and drier conditions returned. For the fifth time in 30,000 years, huge masses of ice calved off the eastern end of the North American ice sheet around Labrador, freshening the Atlantic and diverting its warm currents from the Arctic. But the ice sheets were soon retreating again. By about 12,500 years ago almost half of Scandinavia was free of ice. Then, a second event in North America gave the ice age its last hurrah.

Several enormous lakes were impounded at the southern fringe of the retreating North American ice sheet. At first, overflow from these lakes poured down the Mississippi River into the Gulf of Mexico. Around 12,500 years ago the ice sheet retreated far enough to reopen the St. Lawrence River and the huge postglacial lakes quickly drained into the North Atlantic. This diverted the North Atlantic’s warm currents from the Arctic once again and set off a 1000-year long cold, dry spell called the Younger Dryas. As sea ice spread southwards, Greenland returned to a deep freeze and the Scandinavian ice sheet advanced to the southern tip of Sweden. Water levels at Lakes Magadi and Natron in Africa’s East Rift Valley, which had risen about 60 m (200 ft) in the wake of the ice age, fell by 50 m (167 ft) and remained low throughout the Younger Dryas.

Then, in an abrupt turnaround, the climate warmed over a twenty year period from about 11,660 to 11,640 BP, and the Younger Dryas came to an end. Warming and melting continued for the next 1500 years, after which global temperatures approached today’s values.

In the past 10,000 years most climate variations have been small compared to the wild ice-age gyrations. But one more cold, dry episode that lasted about 400 years swept across the middle latitudes about 8200 years ago, likely when some remaining ice age waters impounded in North American lakes were released. At this time the Black Sea was separated from the ocean and partly evaporated. When warming resumed, so did the rising of sea level. About 7600 years ago water from the Mediterranean Sea poured into the Black Sea, causing an inundation that may have given birth to the Biblical account of Noah’s flood.

The Altithermal and Wet Subtropics

On the Tassili plateau in the center of the Sahara desert, ancient paintings and etchings on the rock walls of people herding cattle, raising grain and hunting hippopotamus from canoes speak of much wetter times. Human testimony is not the only evidence that the subtropical deserts were once much wetter. Fossils of fish abound, ancient beach terraces stand well above the level of today’s shriveled or dried out desert lakes, and radar has probed beneath the shifting sands to reveal buried drainage patterns of forgotten streams and rivers.

For most of the time from the end of the Younger Dryas until about 5000 years, the Sahara and much of today’s subtropical deserts were fertile grasslands, which averaged about 4 inches of rain in each of the summer months. A difference in the Earth’s orbit was the cause. Now, we are closest to the sun on January 4, but 11,000 years ago we were closest at the beginning of July, and the summer sunshine was 7% more intense than it is today. Ironically, as hot as the Sahara is

today, it is not quite hot enough for the air to rise and produce adequate rain, but up to about 5000 years ago it was. Urged by the stronger summer sun, the tropical rain belt and the summer monsoon of Africa and Asia wandered further north than they do today, watering lands it has since abandoned.

But by 5000 years ago the Earth did not approach the sun until after the summer monsoon had ended. The monsoon weakened and the tropical rainbelt no longer moved quite as far north. Rains began to fail across the Sahara, which began reverting to desert. Lake levels fell precipitously and many lakes and smaller rivers dried up. The dessication forced most Saharan residents to flee to places like Egypt with a guaranteed water supply from large rivers. It is no coincidence that in the drying subtropical millenium from about 5000 to 4000 years ago Egypt consolidated into a nation, the Akkadians and Sumerians clustered along the fertile floodplain of the Tigris and Euprates rivers, and the highly urbanized and sophisticated Harappan civilization sprang up on the fertile Indus River basin.

As the drying continued, droughts became more frequent and severe. A severe drought about 2200 BC may have caused the sudden collapse of the Akkadian Empire. A few centuries later the Harappan civilization disappeared without any apparent violence other than drought and a change of the course of the Indus River. The Bible also records how famine drove first Abraham and then the sons of Jacob to Egypt for grain. By the time of Christ, perihelion had advanced to early December, far too late to rouse summer rains. From the Atlas Mountains of Morocco to the Indus River the desert had conquered.

The wetter period up to 5000 years ago was also the warmest time during the past 125,000 years. The world of the early civilizations grew more than 1°C warmer than it is today. The tree line across Europe and in the western United States was almost 200 m (660 ft) higher than now.

An

Atla

s of

Ice

Age

Ear

th

http

://w

ww

.esd

.orn

l.gov

/pro

ject

s/qe

n/ne

rc.h

tml#

map

s

The Horned GoddessPetroglyph

Aouanrhet, Tassili, Central Sahara Desert ca 3500 BC

This painting describes a wetter time in the middle of the

Sahara Desert from about 9000 to 5500 BP when wheat was grown, cattle were raised,

and rain fell (note rainbow). This wet period occurred

because the summer sun was strong enough to make air in the Sahara rise to produce

rain.

The recent, dramatic increase of CO2 is not matched by anything in the last several million years on Earth. This has caused a similar sharp rise in global T.

Changes of CO2 and CH4 over the past 20,000 years

When did human impact on climate begin? Deforestation from the Agricultural Revolution changed albedo and released CO2 and CH4 in the atmosphere, increasing the Greenhouse Effect and raising global T. Did this prevent the Ice Age from returning?

Reforestation may have been one of the causes of the Little Ice Age. The Spaniards brought diseases to the New World that reduced the population by 90 to 99%. That destroyed agricultural societies and allowed the forests to grow back, reducing atmospheric CO2. Solar irradiance was also lower as indicated by the absence of sunspots.

Global Temperature for the Last 1000 Years

A relatively warm period of Medieval Warmth was followed by a cold period called the Little Ice Age. Since the decade of 1810 – 1820, and especially since 1980, the world has experienced significant warming in conjunction with the increase of CO2 and CH4.

GLOBAL DIMMING

Global Dimming, the reduction of solar irradiance at the Earth’s surface due to the high albedo of human-produced aerosols has acted to cool climate. But as coal burning has become cleaner, soot has been reduced (except in China) so that there is now less dimming. Ironically, this has increased the rate of global warming.

Now look at China’s haze and polluted waters

MODIS AQUA 4/7/2007

Over the past century, global T has risen 0.8C on average – about 0.35C from 1900 to 1940, followed by a decrease of 0.1C from 1940 to 1970 and since then a further increase of 0.55C. Nightly minimum T has increased more sharply, so that the daily range of T has decreased. T over land has increased somewhat more than over the oceans, and especially at higher latitudes. T in the lowest 10 km has decreased even more than at the surface but T in the lower stratosphere has decreased.

Accompanying observed increases in surface T, have been decreases in the length of river and lake ice seasons and glacial mass and extent, melting of the Greenland Ice Sheet, decreases of snow cover in many Northern Hemisphere regions, and of both ice thickness and extent in the Arctic, especially in spring and summer; the oceans are warming; and sea level is rising.

Relative humidity seems to be about the same, so that the temperature increase is accompanied by an increase in the amount of water vapor in the atmosphere. No noticeable changes have taken place in total global precipitation, but the distribution of precipitation has changed. It appears to be concentrated in more intense storm events and the mid-latitude storm belt has migrated poleward as the realm of polar air has shrunk. This means that places on the poleward margins of the subtropical deserts have dried while higher latitudes have become wetter and even snowier. Drought has become more prevalent because while temperature and hence potential evaporation has increased precipitation has not.

Several natural oscillations of atmospheric and oceanic temperatures and circulation including El Niño, and the North Pacific and North Atlantic Oscillations are superimposed on the secular trends and reduce our degree of certainty regarding the global changes.

Sea Level Rise and Polar Sea IceOver the past several decades the sea ice covering the Arctic Ocean has been melting. It has thinned from an average thickness of 3 m to about 2 m and its area, particularly in Summer has decreased markedly so that the famous Northwest Passage has finally opened. The summer of 2008 has had the most dramatic melting by far. This greatly exceeded any prediction and shocked the scientific community.

One of the gravest consequences of global warming is the rise of Sea Level. Sea level has been rising, and at an increasing rate. The present rate is just over 3 mm per year. Although sea level does not change when floating sea ice melts, sea level rises when water gets warmer (because it expands) and when ice perched on land melts. About half the rise of sea level is due to thermal expansion and half to melting of ice sheets. The total possible sea level rise from melting of the Greenland and Antarctic Ice Sheets is 64 m. Increasing T by 1C causes water to expand by 0.008%. When this occurs through the 4000 m depth of the ocean, it will raise sea level by 0.32 m.

While floating sea ice does not raise sea level when it melts, it may indirectly allow a rise of sea level because it may no longer hold back perched ice from sliding into the Sea. One concern is that the melt water of the perched ice sheets, which forms shallow lakes on the ice surface will lubricate the ice and allow large chunks to slide into the sea. Many of these lakes (that form in summer) suddenly disappear by draining when they melt their way clear through to the bottom of the ice sheet. No cataclysmic breakoff event has occurred in the past century, but it remains a grave concern because of the sudden rise of sea level such an event would produce.

http://data.giss.nasa.gov/efficacy/#table1

Into the Future: Predicted Temperature ChangesNASA GISS Climate Model Simulation for 2xCO2.

Computer Simulation of Melting of Greenland Ice Sheet with 4 times Preindustrial Level of Atmospheric CO2.

Here is a prediction of the almost unthinkable. If we continue rampantly burning fossil fuels, CO2 and global temperatures will increase enough to melt the Greenland Ice Sheet, although it will take almost 2000 years. That will submerge many places now just barely above sea level. And melting in Antarctica will raise sea level even more.

So, as sea level rises, watch Florida disappear beneath the waves.

What will rising sea level do to Florida?

00.0 m

10.0 m10.0 m

20.0 m

30.0 m

At the present rate of sea level rise (3 mm/yr) this would take 10,000 years