Climate Change: A Quantitative Question Stephen E. Schwartz New York University Graduate School of Journalism Science, Health and Environmental Reporting Program (SHERP) October 2, 2008 http://www.ecd.bnl.gov/steve
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