The science of global warming: Often confused but actually clear

2011 MinnTS Lecture

Katsumi MatsumotoGeology and Geophysics, University of Minnesota

24 March, 2011

The science of global warming: Often confused but actually clear

2011 MinnTS Lecture

Katsumi MatsumotoGeology and Geophysics, University of Minnesota

24 March, 2011

Do you believe that global warming is occurring?

Do you believe that scientists are divided over global warming?

Do you believe that global warming is caused by human activities?

Outline

• The big picture: Energy in vs. energy out

• Detection of global warming

• Attribution of global warming

• Paleoclimate perspective

• Scientists on climate change

What determines the temperatures of the terrestrial planets?

Sun

Venus Earth MarsMercury

179 ºC 477 ºC 15 ºC -47 ºC(relative sizes are to scale)

Hot House Ice HouseJust Right

What determines the surface temperature?(energy “in” versus energy “out”)

Houghton (2009)

Analogy: bank account balance is determined by income and expense

Charles (2009)

Infrared radiation from a building(depends on temperature)

Infrared radiation from Earth (again depends on temperature)

(Sverdrup, 2006)

Northern hemisphere winter

CO2 in atmosphere will absorb thermal radiation

Houghton (2009)

Human emissions of CO2 increases the greenhouse effect and reduces energy “out”

In > Out, so warms, but when will it stop?

Houghton (2009)Earth’s surface

Global warming is a natural response of the planet to restore radiative balance...something we’ve known for a long time

Earth will stop warming when it is warm enough to radiate out as much energy as before

A long history

• 1681 : greenhouse effect of glass (Mariotte)

• 1824 : greenhouse effect of atmosphere (Fourier)

• 1861 : laboratory confirmation of the greenhouse properties of CO2 and H2O (Tyndall)

• 1895 : prediction of CO2-induced warming (Arrhenius)

All this history lead to the Keeling Curve

Hansen (2004)2 W/m2

Today’s radiative imbalance: 2 X-mas light bulbs/m2

Physics predicts that there must be global warming!

Detection of global warming

Instrumental records

•1653 - First meteorological network in Italy

•1873 - IMO (later WMO) to standardize temperature data

Figure 1.3, IPCC AR4

550 million temp readings!

Ann

ually

ave

rage

d, g

loba

l sur

face

tem

pera

ture

IPCC TAR(Fig. TS6, IPCC AR4)

2005 - 1979

Satellite data

Historical data

NSIDC

Shrinking arctic sea ice

Northwest Passage

Hansen (2004)

Greenland is melting…

JakobshavnIce Stream

Satellite altimetry data net loss of ice mass

(NASA)

Glacial earthquakes

(Ekström, Tsai, Nettles, 2006)

(Fig 5.4, IPCC AR4)

2003-19612003-1993

Global ocean heat content“The smoking gun” of GW

• About 90% of heat from GW into the ocean...but small temp change

• Reason for delay in climate response

1022 J

Wikipedia – data from Douglas (1997)

~1 mm/yr globally

Period of ice cover in Bayfield, WI

Howk (2009)

Detection – winter in Lake Superior

Last boatFirst boat

Root et al., 2003

Other organisms noticing the change:

(Analysis of ~1500 species on migration, flowering,

reproduction...)

Attribution of global warming

Attribution of global warming

How to show this?Earth science – no control experimentUse global climate models

What do we need to show?(1) Anthropogenic forcings are doing it(2) Natural forcings are insufficient

• Orbital variations• Tectonic activity• Solar variability• Internal variability

IPCC TARForcings

How reliable are the global climate models?(show movies)

1) Models based on established physical laws

2) Ability to model key aspects of current climate

large scale temperature, precipitation, radiation, wind, ocean temperatures, currents, ice cover, seasonality of monsoons and storm tracks…

3) Examine model predictions of:

past climate, larger warming of nighttime temperatures, larger NH warming, short-term cooling following volcanic eruptions…

Uncertainties in tropical precipitation, El Nino, representation of clouds, small scale projections

IPCC AR4 FAQ8.1

1991 Mt Pinatubo

(Ed Wolfe, USGS, 1991)

In 1988 Hansen predicted the radiative effect of a volcanic eruption – later proved correct (-1 W m-2)

(Fig. 9.5 & 9.14, IPCC AR4)

Models correctly predict impacts of volcanic eruptions

obs

obs

model mean

Height of lower atmosphere

Temperature

IPCC TAR(Fig. TS6, IPCC AR4)

2005 - 1979

More NH warming

Other “fingerprinting”of anthropogenic warming: faster night time warming etc.

Acceleration of warming

Paleoclimate perspective

Glaciation and Moraines in Minnesota

(Pictures from www.winona.edu)

Glacial(20,000 yrs ago)

CLIMAP (1981)

Interglacial(today)

G-I cycles

18,000 YEARS AGO

Note the ice, lakes and coasts

14,000 YEARS AGO

13,000 YEARS AGO

12,000 YEARS AGO

11,000 YEARS AGO

10,000 YEARS AGO

9,000 YEARS AGO

8,000 YEARS AGO

7,000 YEARS AGO

6,000 YEARS AGO

We can reconstruct past ice by examining isostatic rebound

Houghton (2009)

Ice age viewed from ice and ocean sediment cores(climate and CO2 are highly correlated)

Houghton (2009)

The last glacial cycle

Past sea level vs. global temperature

Archer (2007)

Climate change deniers

Scientists on the problem

Jim Hansen in 1988 testified to US Senate that he was certain that record warmth was not natural

Got the ball rolling (UNFCCC in ‘92)

An almost complete unanimity among climate scientists on the reality of global warming

Complete disconnect from public perception...

Scientific consensus

Experts read peer-reviewed publications, have informal discussions in the hallways and conferences...usually hard for the public to assess

What is consensus? And how do we know it exists?

What is peer-review?

Climate science consensus – unusually publicIPCC assessment of the state of climate science on the basis of peer-reviewed publications

Careful, highly critical examination of the work being proposed for publication; very toug

Anyone can say anything, but not everyone can get research results published in peer-reviewed journals

Oreskes’ survey of peer-reviewed publications

Searched 8500 journal publications between 1993 and 2003 w/ “global climate change.” Is global warming occurring and are humans partly responsible?

Who’s arguing that global warming is here?

Intergovernmental Panel on Climate Change

• 1990 – qualitative persuasion of human interference • 1995 – “discernable human influence on global climate”• 2001 – “most of the warming over the last 50 yrs is

likely attributable to human activities• 2007 – “very likely” (>90% probability)

National science academies

Professional societies whose membership expertise bears on global climate change

Reports and statements by IPCC, academies, and societies draftedthrough a careful process involving many opportunities to comment, criticize, and revise won’t deviate much from membership opinions

Who are the deniers?

Works with Exxon, American Petroleum Institute: “proposed a $5 million campaign… to convince the public that the science of global warming is riddled with controversy and uncertainty”

– Newsweek 2007

Fred Singer, electrical engineer

Paid at least $100,000 by companies involved in coal-fired power production to make the public case against climate change

Richard Lindzen, MIT professor in meteorology

Patrick Michaels – fellow of the Cato Institute

Michael Crichton! (invited by Congress to testify)

gets funding from OPEC, $2500/day “consulting” fee

Myths and skepticisms – look these up yourself

1. Scientists can’t even predict next week’s weather

2. Climate model predictions have never been tested

3. Hansen has been wrong before

5. Global warming + global dimming -> Southern warming

6. GCMs don’t have clouds

7. Climate models can’t explain the past

8. Climate is chaotic and thus not predictable

9. Regional and local climate predictions are bogus

Summary• Physics predicts global warming when incoming solar

radiation is greater than outgoing terrestrial radiation

• Detection by observations - warming of atmosphere, ocean, Arctic sea ice, Greenland melting, sea level rise

• Attribution to humans - use of global climate models, fingerprinting predictions validated, observed changes cannot be explained with natural causes alone

• Paleoclimate perspective – CO2 and climate are highly correlated; so are sea level and global temperature

• Complete unanimity among active climate scientists