The Science of Climate Change:  What have we done? What can we expect?What can we do?

Michael J. PratherFred Kavli Professor of Earth System ScienceUC Irvine

26 Jan 2008

Thanks to the IPCC

Bert Bolin (d. 30 Dec 2007)

Sir John Houghton

IPCC TAR 2001:Third Assessment Report

IPCC AR4 2007:Fourth Assessment Report

The Earth's climate is now clearly out of balance and is warming.

Many components of the climate system—including the temperatures of the atmosphere, land and ocean, the extent of sea ice and mountain glaciers, the sea level, the distribution of precipitation, and the length of seasons—are now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century. . . .Evidence from most oceans and all continents except Antarctica shows warming attributable to human activities. Recent changes in many physical and biological systems are linked with this regional climate change. . . .

American Geophysical Union statement re Human Impacts on Climate, adopted Dec 2007

so, what about climate?

CLIMATE = statistics of atmospheric weather + oceans + ecosystems + ...

Both weather and climate have inherent dynamical variability like El Nino.

This precludes a definitive prediction, and leads naturally to a statistical approach in forecasts (e.g., 80% chance of rain tomorrow, 84% chance that global warming over this century will be greater than 2°C).

How has climate varied in the past ?How are we changing the atmosphere ?Is climate change a response to human activities ?What can we expect this century ?

“We have sun, earth, and atmosphere, so we have weather!” Harris Telemacher (Steve Martin) in LA Story

Ice Age Climates: the Pleistocene

Current warm period

Last Ice Age

Last interglacial

N2O

CO2

CH4

Temperature proxy

Ice Volume proxy

Thousands of Years before 2005

measurements from ice cores show correlations between the greenhouse gases and temperatures and size of glaciers

Ice Ages: the Pleistocene

2007 IPCC AR4

The Holocene

the last 11,000 yr of relatively stable climate

11,000 yr

2007 IPCC AR4

COOLING WARMING2001 IPCC TAR

The Anthropocene: the atmosphere

Agriculture, Industry & Land-use change have dramatically changed change the global atmosphere: increasing bothgreenhouse gases (CO2, CH4, N2O)and sulfate aerosols.

CO

OL

ING

W

AR

MIN

G

2001 IPCC TAR

The Anthropocene: the radiative imbalance

Greenhouse gases trap the Earth’s heat and warm the planet.Aerosols and their interactions with clouds reflect sunlight and cool.

The net heat imbalance is called radiative forcing (RF, in W m-2)

2007 IPCC AR4

The Anthropocene: the heat is going into the ocean

The RF imbalance is heating the oceans.

softwarebug in floats

2007 IPCC AR4

The Anthropocene: mean surface temperatures warm

but not everywhere, and not uniformly

2001 IPCC TAR

ICE-AGE Radiative Forcing

2007 IPCC AR4

Note: CO2 and CH4 are positive feedbacks here.

Note: CO2 and CH4 are the driving forces here.

Global mean surface temperature anomalies (°C) from 1900 to 2005

from observations (black)

from AOGCM* simulations forced with natural forcings only (blue, 19 simulations w/ 5 models)

from AOGCM simulations forced with both anthropogenic and natural forcings (red, 58 simulations w/ 14 models)

*AOGCM = atmosphere+ocean climate models

Detection & Attribution: How do we know we caused it?

2007 IPCC AR4

Greenhouse gas signal seen on every continent except Antarctica

PINK matches BLACK (obs.)

2007 IPCC AR4

What else is happening to the climate?

Global average sea level rose at an average rate of 1.8 mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 mm per year. Whether the faster rate for 1993 to 2003 reflects decadal variability or an increase in the longer term trend is unclear.

2007 IPCC AR4

During recent millennia of relatively stable climate, . . . populations have grown rapidly. In the next 50 years, even the lower limit of impending climate change (an additional global mean warming of 1°C above the last decade) is far beyond the range of climate variability experienced during the past thousand years and poses global problems in planning for and adapting to it.

Warming greater than 2°C above 19th century levels is projected to be disruptive, reducing global agricultural productivity, causing widespread loss of biodiversity, and—if sustained over centuries—melting much of the Greenland ice sheet with ensuing rise in sea level of several meters. . . .

American Geophysical Union statement re Human Impacts on Climate, adopted Dec 2007

2001 IPCC TAR

The Anthropocene: what about the future?

2001 IPCC TAR

The Anthropocene: what about the future?

impacts

2007 IPCC AR4

1971-2000 U.S. Normals ~8,000 stations

Climate Prediction: “Climate Normals” are thedaily, monthly & annual 30-year climate averages

• Main Parameters– Temperature (max, min, mean)– Heating & Cooling Degree Days– Spring/Fall Freeze dates; growing season– Precipitation (usually liquid equivalent;

sometimes snow)– Occasionally: clouds, sea level pressure, RH,

others

• WMO Recommendation– Countries update normals every 10 years– Current period 1971-2000– Next scheduled period:

1981-2010 for use in 2010sT. Karl, 2007

but climate is changing:

• January Min. Temperature– Most areas in continental U.S are now 3 – 7

degrees warmer during winters nights – Florida , S. Georgia now 2 – 4 degrees cooler

• July Max. Temperature– Most of Western U.S. now 3 - 7 degrees

warmer during summer days– Some Parts of South U.S. cooler

-7 °C to +7 °C

2001-2006 ACTUALS minus 1971-2000 NORMALS (°C)

T. Karl, 2007

predicting the next decade from the climate normals

predict

able incre

ases a

bove cl

imate norm

als

unpredictable

2007 IPCC AR4

Katrina 28 Aug 2005 extreme climate

Rita 25 Sep 2005

+22 more paragraphs + ½-page graphic!

October 9, 2003-9 Copyright © 2003 Earth Policy Institute

RECORD HEAT WAVE IN EUROPE TAKES 35,000 LIVES Far Greater Losses May Lie Ahead

Janet Larsen

A record heat wave scorched Europe in August 2003, claiming an estimated 35,000 lives. In France alone, 14,802 people died from the searing temperatures—more than 19 times the death toll from the SARS epidemic worldwide. In the worst heat spellin decades, temperatures in France soared to 104 degrees Fahrenheit (40 degrees Celsius) and remained unusually high for two weeks.

Heat Waves: 2003

climatology of heat waves

2003 was an extremely rare event in terms of climate statistics, a 5.4-sigma extreme,

a one-in-a-thousand year event.

2007 IPCC AR4

predicting the next heat waves

without anthropogenic forcing: every 1000 yrs

with anthropogenic forcing: every 250 yrs

2007 IPCC AR4

GFDL, 2001

translating global warming

2 x CO2 = about 3°C global mean warming

= about 10°F warmer summers in Southeastern US

American Geophysical Union statement re Human Impacts on Climate, adopted Dec 2007

With climate change, as with ozone depletion, the human footprint on Earth is apparent.

The cause of disruptive climate change, unlike ozone depletion, is tied to energy use and runs through modern society. Solutions will necessarily involve all aspects of society.

Mitigation strategies and adaptation responses will call for collaborations across science, technology, industry, and government. Members of the . . . scientific community, collectively have special responsibilities:

to pursue research needed to understand it; to educate the public on the causes, risks, and hazards; and to communicate clearly and objectively with those who can implement policies to shape future climate.

Greenhouse Gas Emissions

Sector GHG Emissions

2007 IPCC AR4

size of the problem

plus 7 more

Two views of current emissions of the Kyoto Greenhouse Gases

2007 IPCC AR4

2007 IPCC AR4

A Climate Forecast:

uncertainty in positive climate feedbacks in a warmer world

2007 IPCC AR4

Our future climate

We can do little to alter the next 25 years of climate change.

We can choose the scenario for the next 90 years.

2007 IPCC AR4