CLIMATE CHANGE: A REALLY TOUGH SCIENTIFIC PROBLEM

Stephen E. Schwartz

Upton NY USA

Physics 311: Connections in Science “Energy Problems”

Stony Brook University

September 26, 2013

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

GLOBAL TEMPERATURE CHANGE SINCE 1850

0.8

0.6

0.4

0.2

0.0

-0.2

Tem

pera

ture

ano

mal

y vs

. 185

0-19

00, K

20001980196019401920190018801860

0.8 K

Climatic Research Unit, East Anglia, UK

CO

2co

ncen

tratio

n (p

pm)

180200220240260280300320340360380

800 1000 1200 1400 1600 1800 2000

Law DomeAdelie LandSipleSouth Pole

ATMOSPHERIC CARBON DIOXIDE IS INCREASING

Global carbon dioxide concentration over the last thousand years

Antarctic ice cores

400

370360350340330320310 20001990198019701960

C. D. KeelingMauna Loa Hawaii

2010

380390400

OBSERVED EXPECTED AND TEMPERATURE CHANGE OVER THE TWENTIETH CENTURYExpected warming for forcing by long-lived greenhouse gases only

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Tem

pera

ture

ano

mal

y re

lativ

e to

185

0, K

20001980196019401920190018801860Observations: Climatic Research Unit, East Anglia, UK

Expected increase substantially exceeds observed.

Expected

Range

Observed

Externally imposed change in Earth radiation budget

Decades to centuries: CO , CH , N O, CFCs2 24

2009 COPENHAGEN ACCORD AGREES ON 2˚C MAXIMUM TEMPERATURE RISE

The Heads of State, Heads of Government, Ministers . . . present at the United Nations Climate Change Conference 2009 in Copenhagen:

Albania, Algeria, Armenia, Australia, Austria, . . . [106 countries] . . . , United States of America, Uruguay and Zambia, have agreed on this Copenhagen Accord. . . .

We underline that climate change is one of the greatest challenges of our time. We emphasise our strong political will to urgently combat climate change. . . .

To . . . stabilize greenhouse gas concentration in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system, we shall, recognizing the scientific view that the increase in global temperature should be below 2 degrees Celsius . . . enhance our long-term cooperative action to combat climate change.

DEGREES OF SEPARATION

-6

-5

-4

-3

-2

-1

0

1

2

Tem

pera

ture

Diff

eren

ce to

Pre

indu

stria

l, K 1970195019301910189018701850

Last Glacial Maximum (21 000 years before present)

Best estimate Uncertainty range

Observed

Expected increase equals or exceeds 2 degree threshold.

Expected

Range

20101990

4

KEY QUESTION • How much more CO2 can be emitted

without committing Earth to a temperature increase of 2 ˚C above preindustrial?

ATMOSPHERICRADIATION

Power per area

Unit:Watt per square meterW m-2

Photo: S. E. Schwartz

THE SOLAR SPECTRUM Outside Earth’s atmosphere – Compare Planck spectrum at 255 K

600

500

400

300

200

100

0TOA

Spec

tral ir

radia

nce,

W m

-2 (lo

g 10 !

/m)

-1

0.1 1 10 100Wavelength, m

100100010000100000Wavenumber, cm-1

Planck function at 5770 K normalized to 340 W m-2 (1/4) Solar spectral irradiance at 1 AU

Planck function at 255 K normalized to 240 W m-2

5770 K

255 K

Data source: Gueymard, Solar Energy, 2004

Short- and longwave spectra are nearly non-overlapping.

H2 2O, CO , CH4. . .

Atmosphere

Shortwaveabsorbed

Longwaveemitted

Rayleigh 36Rayleigh 36Aerosol 4Aerosol 4Aerosol 4

Latent heat 88

Sensible heat 21

1/4 solarconstant

70.6% = 1-α340

302≈ 287 K

385

100α = 29.4%

162162

78

240

255 K240

4731

13

Albedo

Stefan-BoltzmannRadiation law

Radiative Fluxesin W m -2

=

EARTH’S RADIATION BUDGET AND THE GREENHOUSE EFFECT

J = σ T4Stefan-BoltzmannRadiation law

T = (J/ )T = (J/ )T = (J/ )σT = (J/ )1/413601360

≈

Solar Solar constantconstant

Greenhouse effect

WHAT IT REALLY LOOKS LIKE Measurements for a single day, March 10, 2012, W m-2

-200 -150 200150100500-50-10067 99 387335323291259227195163131

16 540 644 749 854 958 1063435330225121 101087974861748635522493-38-169-300

Shortwave upwelling

Longwave upwelling

Net daytime, positive downward

Net 24-hr, positive downward

NASA CERES Program, courtesy Norman Loeb

RADIATIVE FORCING An externally imposed change in Earth’s radiation

budget, W m-2.

Working hypothesis:

On a global basis radiative forcings are additive and interchangeable.

Global temperature change is proportional to forcing.

CO

2co

ncen

tratio

n (p

pm)

180200220240260280300320340360380

800 1000 1200 1400 1600 1800 2000

Law DomeAdelie LandSipleSouth Pole

ATMOSPHERIC CARBON DIOXIDE IS INCREASING

Global carbon dioxide concentration over the last thousand years

Antarctic ice cores

400

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Forcing, W

m-2

CLIMATE FORCINGS OVER THEINDUSTRIAL PERIODExtracted from IPCC AR4 (2007)

3210-1-2Forcing, W m-2

CO2 CH4CFCs

N2OLong Lived

Greenhouse Gases

Greenhouse gas forcing is considered accurately known.Gases are uniformly distributed; radiation transfer is well understood.

H2 2O, CO , CH4. . .

Atmosphere

Shortwaveabsorbed

Longwaveemitted

Rayleigh 36Rayleigh 36Aerosol 4Aerosol 4Aerosol 4

Latent heat 88

Sensible heat 21

1/4 solarconstant

70.6% = 1-α340

302≈ 287 K

385

100α = 29.4%

162162

78

240

255 K240

4731

13

Albedo

Stefan-BoltzmannRadiation law

Radiative Fluxesin W m -2

=

EARTH’S RADIATION BUDGET AND THE GREENHOUSE EFFECT

J = σ T4Stefan-BoltzmannRadiation law

T = (J/ )T = (J/ )T = (J/ )σT = (J/ )1/413601360

≈

Solar Solar constantconstant

+ 2.8 Forcing

HOW MUCH WARMING IS EXPECTED?Steady-state change

in global meansurface temperature

= Climatesensitivity × Forcing

ΔT S F= ×

S is “equilibrium” sensitivity. Units: K/(W m-2)

Sensitivity is commonly expressed as

ΔT S F2 2× ×≡ ×

where F2× is the “CO2 doubling forcing” ca. 3.7 W m-2.

2 doubling temperature”“CO

H2 2O, CO , CH4. . .

Atmosphere

Shortwaveabsorbed

Longwaveemitted

Rayleigh 36Rayleigh 36Aerosol 4Aerosol 4Aerosol 4

Latent heat 88

Sensible heat 21

1/4 solarconstant

70.6% = 1-α340

302≈ 287 K

385

100α = 29.4%

162162

78

240

255 K240

4731

13

Albedo

Stefan-BoltzmannRadiation law

Radiative Fluxesin W m -2

=

EARTH’S RADIATION BUDGET AND THE GREENHOUSE EFFECT

J = σ T4Stefan-BoltzmannRadiation law

T = (J/ )T = (J/ )T = (J/ )σT = (J/ )1/413601360

≈

Solar Solar constantconstant

Greenhouse effect

Emissivityε = 240/385 = 0.61

Co-albedoγ = 1- = 0.706α

ENERGY BALANCE MODEL OFEARTH’S CLIMATE SYSTEM

Global energy balance: dH

dtQ E

JT= − = −γ εσS

s4

4

Ts is global mean surface temperature H is global heat content

Q is absorbed solar energy E is emitted longwave flux

JS is solar constant γ is planetary co-albedo

σ is Stefan-Boltzmann constant ε is effective emissivity

At radiative steady state:

γ α= − ≈1 0 7. ; ε γσ

= J

TS

s4/ 4; for Ts = 288 K, ε ≈ 0 61.

γ εσJTS

s4

4=

NO FEEDBACK CLIMATE SENSITIVITY

In absence of feedbacks

€

γ and

€

ε do not depend on

€

Ts Change in emitted flux per change in temperature:

€

dEdTs

=d(εσTs

4 )dTs

= 4εσTs3 =

4TsE =

4TsγJS4

=γJSTs

No-feedback sensitivity: SNF ≡dTsdQ

=dTsdE

=dEdTs

⎛

⎝⎜

⎞

⎠⎟

−1=TsγJS

€

JS =1360

€

Wm-2;

€

Ts = 287 K;

€

γ = 0.7;

€

SNF = 0.30

€

K / (Wm-2)

€

ΔT2× = F2×SNF = 3.7 Wm-2 × 0.30 K / (Wm-2) = 1.1 K

Water Vapor Feedback: Pretty Well Understood

Higher temperature, More water vapor. More infrared is absorbed

Positive FeedbackHigher Sensitivity

Higher temperature, Clouds evaporate. More sunlight is absorbed

Higher temperature, More water vapor, More clouds. Less sunlight is absorbed

Cloud Feedbacks: A Big Mystery in Climate Sensitivity

Positive FeedbackHigher Sensitivity

Negative FeedbackLower Sensitivity

CLIMATE SENSITIVITY ESTIMATESTHROUGH THE AGES

Estimates of central value and uncertainty range from major

6543210 190018901880

Arrhenius

Stefan-Boltzmann

2010200019901980

CharneyNRC – – – – IPCC

> 66%"Likely"

Despite extensive research, climate sensitivity remains highly uncertain.

– – ––

national and international assessments

1 sigma

ΔT 2

×Se

nsiti

vity

, K

?2013

ESTIMATES OF EARTH’S CLIMATE SENSITIVITYAND ASSOCIATED UNCERTAINTY

Major national and international assessments and current climate models

19 IPCC AR4 Models6

5

4

3

2

1

02010200019901980

NRC – – IPCC – –

1 σ

> 66%

"Likely"

Charney

Current estimates of Earth’s climate sensitivity are centered about a CO2doubling temperature ΔT2× = 3 K, but with substantial uncertainty.

Range of sensitivities of current models roughly coincides with IPCC“likely” range.

CO

2Δ

T 2×, K

Dou

blin

g Te

mpe

ratu

re

?? QUESTION ??• Why is there such a large range of

sensitivities in current climate models and why hasn’t this situation improved much in thirty years?

ANSWER• This is a really tough scientific problem!

A REALLY TOUGH SCIENTIFIC PROBLEM

• Determine the consequences of a systematic change of less than 1% in a quantity that is highly variable in time and space applied to a noisy dynamic system.

• Right now we do not even know the sign of how much more CO2 can be added to the atmosphere without committing Earth to a temperature increase ≥ 2˚ C.

• Even at the low end of the climate sensitivity range, expected temperature increase from long-lived greenhouse gases ~ 1.4 ˚C, the consequences would be severe.

• Why has Earth not warmed up the expected amount, 1.4 to 3.2 ˚C? We don’t know, but that’s another lecture.