CLIMATE AND CLIMATE CHANGE

CERTAINTIES AND UNCERTAINTIES

Stephen E. Schwartz

http://www.ecd.bnl.gov/steve/schwartz.html

December 4, 2001

Updated, March 4, 2002

OUTLINE

• Overview of the Earth climate system

• Increased concentrations of “greenhouse gases”

• Radiative forcing of climate change

• Climate system response: Observations oftemperature change on various time scales

• Climate system sensitivity: Models and Observations

• Sources of uncertainty: Aerosols

• Decision making in the face of uncertainty

GLOBAL ENERGY BALANCEGlobal and annual average energy fluxes in watts per square meter

343

237

237≈ 254K

390≈288K

106 68

169

327 9016

Rayleigh

Aerosol

α = 31%

69% = 1 -α

1/4 S0 1/4 S0 =(1-α ) σT4

Shortwave Longwave

H2O, CO2, CH4...

Atmosphere

LS

Schwartz, 1996, modified from Ramanathan, 1987

WATT PER SQUARE METER

STEFAN-BOLTZMANN RADIATION LAW

F T= εσ 4

F = Emitted Flux (W m-2)ε = Emissivity (approximately 1)σ = Stefan-Boltzmann constant (5.67 × 10-8 W m-2 K-4)T = Absolute temperature (K)

600

500

400

300

200

100

0

Em

itted

Infr

ared

Rad

iatio

n, W

m-2

-40 -20 0 20 40Temperature, °C

300280260240

Temperature, °K

Emissivity ε = 1

For T = 288 K (15 °C or 59 °F) F = 390 W m-2.

GLOBAL CARBON DIOXIDE OVER THE INDUSTRIAL PERIOD

360

340

320

300

280

CO

2 M

ixin

g R

atio

, µm

ol/m

ol(a

ir) [p

pm]

200019501900185018001750

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Forcing, W

m-2

370

360

350

340

330

320

31020001990198019701960

C. D. Keeling

ICE CORESSIPLE STATION ANTARCTICA

← MAUNA LOA HAWAII →

GREENHOUSE GAS MIXING RATIOS OVER THE INDUSTRIAL PERIOD

280

300

320

340

360

ppm

CO2

ice core

in situ↓ ← →

800

1200

1600

ppb

CH4

ice core insitu

Antarctica

Greenland↓ ↑

↑

↓

280

300

320

ppb

N2O

ice core in situ↓

↑

1850 1875 1900 1925 1950 1975 2000 0

200

400

600

CFC-11CFC-12other trace gas forcingpp

t

CFCs

converted to CFC-11 amount

Hansen et al., PNAS. 1998

GREENHOUSE GAS FORCINGS OVER THE INDUSTRIAL PERIOD2.5

2.0

1.5

1.0

0.5

0.0

For

cing

rel

ativ

e to

185

0, W

m-2

2000197519501925190018751850

Year

Total

CO2

CH4

N2OCFC-12CFC-11

Other

Data: GISS

NORTHERN HEMISPHERE TEMPERATURE TREND (1000-1998)From tree-ring, coral, and ice-core proxy records

As calibrated by instrumental measurements

1000 1200 1400 1600 1800 2000YEAR

-1.0

-0.5

0.0

0.5

1.0

TE

MP

ER

AT

UR

E A

NO

MA

LY (

o C)

Reconstruction (AD 1000-1980)Instrumental data (AD 1902-1998)Calibration period (AD 1902-1980) meanReconstruction (40 year smoothed)Linear trend (AD 1000-1850)

1998

Mann et al., GRL, 1999

GLOBAL TEMPERATURE TREND OVER THE INDUSTRIAL PERIOD

-0.6

-0.4

-0.2

0.0

0.2

0.4

Tem

pera

ture

Ano

mal

y, K

1990

1990

1980

1980

1970

1970

1960

1960

1950

1950

1940

1940

1930

1930

1920

1920

1910

1910

1900

1900

1890

1890

1880

1880

1870

1870

1860

1860

GISS East Anglia

BILLIARD BALL TEMPERATURE SENSITIVITYClimate Sensitivity Evaluated according to the

Stefan-Boltzmann Radiation Law

F T= εσ 4

dF

dTT= 4 3εσ

Sensitivity λ = = −dT

dFT( )4 3 1εσ

For T = 288 K (15 °C or 59 °F) λ = 0.18 K / (W m-2).

THE “BIBLE” OF CLIMATE CHANGE RESEARCH

Cambridge University Press, 2001

CLIMATE CHANGE SENSITIVITYSummary of Current Models

Units: K / (W m-2)

Number ofModels

Mean StandardDeviation

Range

15 0.87 0.23 0.5 - 1.25

Climate Change 2001, Cambridge University Press, 2001

EMPIRICAL TEMPERATURE SENSITIVITY

Greenhouse forcing over the industrial period is 2.5 W m-2

Temperature increase over the industrial period is 0.6 K.

Empirical Sensitivity: λ = = =dT

dF

0 60 24

..

K

2.5 W m K / (W m )-2

-2

This value is much lower than model predictions.

WHY?

CLIMATE FORCING BY SULFATE AEROSOL

HUDSON RIVER 1998 - 11 - 09

BIOMASS BURNING AND WIDESPREAD AEROSOL,Western Arkansas, 2000-12-01

SEAWIFS IMAGE OF NORTH AMERICAN AEROSOL

GSFC/seaWIFS_2001062712Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGEhttp://www.nrlmry.navy.mil/aerosol/satellite/seawifs/conus/200106/2001062712_conus.jpg

MONTHLY AVERAGE AEROSOL JUNE 1997

Optical Thickness at 865 nm

Ångström Exponent

Polder/Adeos CNES/NASDA LOA/LSCE

AbsNo CV

RSSCV

Abs

TOTAL

Global-and-Annual-Mean Radiative Forcing[ 1750-present ]

IPCC, 2001

IPCC-2001 COMPARISONS OF MEASURED ANDMODELED SURFACE TEMPERATURE TRENDS

IPCC-2001 STATEMENT ON CONFIDENCE IN ABILITYOF MODELS TO PROJECT FUTURE CLIMATE

• Simulations that include estimates of natural andanthropogenic forcing reproduce the observed large-scalechanges in surface temperature over the 20th century(Figure 4). However, contributions from some additionalprocesses and forcings may not have been included in themodels. Nevertheless, the large-scale consistency betweenmodels and observations can be used to provide anindependent check on projected warming rates over the nextfew decades under a given emissions scenario.

IPCC-2001 STATEMENTS ON DETECTION ANDATTRIBUTION OF CLIMATE CHANGE

• Detection and attribution studies comparing modelsimulated changes with the observed record can now takeinto account uncertainty in the magnitude of modelledresponse to external forcing, in particular that due touncertainty in climate sensitivity.

• Most of these studies find that, over the last 50 years, theestimated rate and magnitude of warming due to increasingconcentrations of greenhouse gases alone are comparablewith, or larger than, the observed warming. Furthermore,most model estimates that take into account bothgreenhouse gases and sulphate aerosols are consistent withobservations over this period.

IPCC-2001 STATEMENTS ON DETECTION ANDATTRIBUTION OF CLIMATE CHANGE (cont'd)

• The best agreement between model simulations andobservations over the last 140 years has been found when allthe above anthropogenic and natural forcing factors arecombined, as shown in Figure 4 (c). These results show thatthe forcings included are sufficient to explain the observedchanges, but do not exclude the possibility that otherforcings may also have contributed.

REMARKS BY THE PRESIDENTON GLOBAL CLIMATE CHANGE

June 11, 2001

“Our useful efforts to reduce sulfur emissionsmay have actually increased warming, becausesulfate particles reflect sunlight, bouncing itback into space.”

“Kyoto also failed to address two majorpollutants that have an impact on warming:black soot and tropospheric ozone. Both areproven health hazards. Reducing both wouldnot only address climate change, but alsodramatically improve people's health.”

RECOMMENDED READING

NATURE VOL 414 |8 NOVEMBER 2001 149

FROM THE NATURE REVIEW

“ This survey of global environmental problems — food, forests,energy, water, pollution, biodiversity, global warming — reads like acompilation of term papers from one of those classes from hell whereone has to fail all the students. It is a mass of poorly digested material,deeply flawed in its selection of examples and analysis.”

“ Like bad term papers, Lomborg’s text relies heavily on secondarysources. Out of around 2,000 references, about 5% come from newssources and 30% from web downloads — readily accessible,therefore, but frequently not peer reviewed. A mere 1% are originalpapers in Nature...”

“ The text employs the strategy of those who, for example, argue thatgay men aren’t dying of AIDS, that Jews weren’t singled out by theNazis for extermination, and so on. “Name those who have died!”demands a hypothetical critic, who then scorns the discrepancybetween those few we know by name and the unnamed millions weinfer.”NATURE VOL 414 |8 NOVEMBER 2001 149

FROM THE BOOK

“ This chapter accepts the reality of man-made global warming butquestions the way in which future scenarios have been arrived at andfinds that forecasts of climate change of 6 degrees by the end of thecentury are not plausible.

“ I shall argue that the limitations of computer modeling, the unrealisticnature of the basic assumptions made about future technologicalchange and political value judgments have distorted the scenariosbeing presented to the public.

“ I shall further argue that an economic analysis of the costs andbenefits of an immediate reduction in CO2 emissions clearly showsthat the world was a whole would benefit more from investing intackling problems of poverty in the developing world and in researchand development of renewable energy than in policies focused onclimate change. ”

Lomborg, Skeptical Environmentalist, 2001, p. 259

MORE FROM THE BOOK

“ Temperatures have increased 0.6 ˚C over the past century and it isunlikely that this is not in part due to an anthropogenic greenhouseeffect...

“ The central climate sensitivity of 1.5 - 4.5 ˚C has not changed over thepast 25 years, indicating a fundamental lack of model adequacy,because we still do not know whether we live in a world wheredoubling the CO2 concentrations will mean a rather small (1.5 ˚C) or adramatic (4.5 ˚C) temperature increase.

“ All the IPCC predictions are based on GCMs, but there are still crucialproblems with the representation of aerosols, water vapor feedback,and clouds. ”

Lomborg, Skeptical Environmentalist, 2001, p. 317

SOME CONCLUDING OBSERVATIONS• Greenhouse gases persist in the atmosphere for

decades to centuries. (Aerosols last about a week.)

• Actions we take today will affect future generations.

• The sensitivity of the climate system remains highlyuncertain.

• Decisions must be made in an uncertain world. (Lackof controls on emissions is also a decision).

• How do we make decisions in the face ofuncertainty?