Climate Change:A Quantitative Question

Stephen E. Schwartz

New York UniversityGraduate School of Journalism

Science, Health and EnvironmentalReporting Program (SHERP)

October 2, 2008http://www.ecd.bnl.gov/steve

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

Schwartz, 1996, modified from Ramanathan, 1987

ATMOSPHERICRADIATION

Energy per area pertime

Power per area

Unit:Watt per square meterW m-2

STEFAN - BOLTZMANN RADIATION LAWEmitted thermal radiative flux from a black body

F T= σ 4

F = Emitted flux, W m-2

T = Absolute temperature, K

σ = Stefan-Boltzmannconstant, W m-2 K-4

600

500

400

300

200

100

0

Em

itted

Infr

ared

Rad

iatio

n, W

m-2

-40 -20 0 20 40Temperature, °C

300280260240Temperature, °K

Top of Atmosphere

Global MeanSurface Temperature

Stefan-Boltzmann law “converts” temperature to radiative flux.

370360350340330320310

20001990198019701960

C. D. Keeling

Year

CO

2 co

ncen

trat

ion

(ppm

)

180

200

220

240

260

280

300

320

340

360

380

800 1000 1200 1400 1600 1800 2000

Law Dome Adelie LandSipleSouth Pole

Mauna Loa Hawaii

ATMOSPHERIC CARBON DIOXIDE IS INCREASING

Global carbon dioxide concentration and infrared radiative forcing over the last thousand years

Polar ice cores

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Forcing, W

m-2

RADIATIVE FORCING

A change in a radiative flux term in Earth’s radiationbudget, ∆F, W m-2.

Working hypothesis:On a global basis radiative forcings are additive andfungible.

• This hypothesis is fundamental to the radiativeforcing concept.

• This hypothesis underlies much of the assessment ofclimate change over the industrial period.

CHANGE IN GLOBAL MEAN SURFACETEMPERATURE 1855-2004

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

Tem

pera

ture

Ano

mal

y, K

2000

2000

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

Climate Research Unit, University of East Anglia, UK

CLIMATE RESPONSEThe change in global and annual mean temperature,∆T, K, resulting from a given radiative forcing.

Working hypothesis:The change in global mean temperature isproportional to the forcing, but independent of itsnature and spatial distribution.

∆T = S ∆F

CLIMATE SENSITIVITYThe change in global and annual mean temperature perunit forcing, S, K/(W m-2),

S = ∆T/∆F.

Climate sensitivity is not known and is the objective ofmuch current research on climate change.

Climate sensitivity is often expressed as thetemperature for doubled CO2 concentration ∆T2×.

∆T2× = S∆F2×

∆F2× ≈ 3.7 W m-2

CLIMATE SENSITIVITY ESTIMATESTHROUGH THE AGES

Estimates of central value and uncertainty range from major

6

5

4

3

2

1

0

ΔT 2×

, Sen

sitiv

ity to

2 ×

CO

2, °C

190018901880

Arrhenius

Stefan-Boltzmann

2010200019901980

CharneyNRC – – – – IPCC

> 66%"Likely"

1.5

1.0

0.5

0.0

Sensitivity, K/(W m

-2)

Despite extensive research, climate sensitivity remains highly uncertain.

– – ––1 sigma

national and international assessments

IMPLICATIONS OF UNCERTAINTY INCLIMATE SENSITIVITY

Uncertainty in climate sensitivity translates directlyinto . . .

• Uncertainty in the amount of incrementalatmospheric CO2 that would result in a givenincrease in global mean surface temperature.

• Uncertainty in the amount of fossil fuel carbon thatcan be combusted consonant with a given climateeffect.

At present this uncertainty is at least a factor of 2.

EQUILIBRIUM SENSITIVITIES IN CURRENTCLIMATE MODELS

20 Models employed in IPCC AR4 simulations

543210

Equilibrium sensitivity to doubled CO2 ∆T2×, K

IPSL-CM4

UKMO-HadGEM1

MIROC3.2(hires)

MIROC3.2(medres)

CGCM3.11(T47)

CGCM3.11(T63)

ECHAM5/MPI-OM

GFDL-CM2.1

UKMO-HadCM3

ECHO-G

MRI-CCGCM2.3.2

CSIRO-MMK3.0

GFDL-CM2.0

CCSM3

GISS-EH

GISS-ER

FGOALS-g1.00

INM-CCM3.0

PCM

Sensitivity varies by more than a factor of 2.

ZONAL MONTHLY MEAN ALBEDO20 GCMs – Difference vs. ERBE Satellite

Modified from Bender et al., Tellus, 2006

GLOBAL-MEAN RADIATIVE FORCINGS (RF)Pre-industrial to present (Intergovernmental Panel on Climate Change, 2007)

LOSU denotes level of scientific understanding.

TOO ROSY A PICTURE?Ensemble of 58 model runs with 14 global climate models

“ Simulations that incorporate anthropogenic forcings, including increasinggreenhouse gas concentrations and the effects of aerosols, and that alsoincorporate natural external forcings provide a consistent explanation of theobserved temperature record.

“ These simulations used models with different climate sensitivities, rates ofocean heat uptake and magnitudes and types of forcings.

TOO ROSY A PICTURE?Ensemble of 58 model runs with 14 global climate models

Factor of 4

Factor of 2

Schwartz, Charlson & Rodhe, Nature Reports – Climate Change, 2007

Uncertainty in modeled temperature increase – less than a factor of 2, red –is well less than uncertainty in forcing – a factor of 4, green.The models did not span the full range of the uncertainty and/or . . .The forcings used in the model runs were anticorrelated with the

sensitivities of the models.

Looking to theFuture . . .

Prediction is difficult, especially about the future.

– Niels Bohr

Unknown

PROJECTIONS OF FUTURE CO2 EMISSIONS

Unknown

PROJECTIONS OF FUTURE CO2 CONCENTRATIONS

Unknown

PROJECTIONS OF FUTURE TEMPERATURE CHANGE

stepheneschwartz

stepheneschwartz