AEROSOL INFLUENCES ON CLIMATERADIATIVE FORCING AND BEYOND

Stephen E. Schwartz

Symposium on

Impacts of Clouds and Aerosols on Terrestrial Carbon

and Hydrological Cycles (ICATCH)

American Geophysical Union Fall Meeting, December 5-9, 2005, San Francisco

These viewgraphs available at

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

OUTLINEAttribution of excess atmospheric CO2 to fossil fuel emissions

and land use changes

Time constant of CO2 emissions and CO2 residence time

Time constant of climate change

Aerosol influences on climate change

Residence time of tropospheric aerosols

Integrated forcing of greenhouse gases and aerosols

Concluding remarks

INCREASES IN CO2 OVER THEINDUSTRIAL PERIOD

ATMOSPHERIC CARBON DIOXIDETime series 1700 - 2003

500

450

400

350

300

Car

bon

diox

ide

mix

ing

ratio

, ppm

2000195019001850180017501700

Law Dome (Antarctica) Siple (Antarctica)Mauna Loa (Hawaii)

Law - Etheridge et al.Siple - Friedli et al.

Mauna Loa - Keeling

ATMOSPHERIC CO2 EMISSIONSTime series 1700 - 2003

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Car

bon

diox

ide

emis

sion

s, p

pm/y

r

2000195019001850180017501700

6

5

4

3

2

1

0

Carbon dioxide em

issions, Pg C

/yr

Fossil Fuel Emissions

Fossil Fuel - Marland

ATMOSPHERIC CARBON DIOXIDETime series 1700 - 2003

500

450

400

350

300

Car

bon

diox

ide

mix

ing

ratio

, ppm

2000195019001850180017501700

Fossil Fuel Cumulative Emissions

Law Dome (Antarctica) Siple (Antarctica)Mauna Loa (Hawaii)

Law - Etheridge et al.Siple - Friedli et al.

Mauna Loa - KeelingFossil Fuel - Marland

LAND USE CARBON EMISSIONS BY SOURCE REGION

1000 Tg = 1 Pg = 1015 g,Equivalent to 0.47 ppm

Carbon flux estimated as land area times carbon emissions associated withdeforestation (or uptake associated with afforestation).

United States dominates emissions before 1900 and uptake after 1940.

ATMOSPHERIC CO2 EMISSIONSTime series 1700 - 2003

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Car

bon

diox

ide

emis

sion

s, p

pm/y

r

2000195019001850180017501700

6

5

4

3

2

1

0

Carbon dioxide em

issions, Pg C

/yr

Land Use Emissions Fossil Fuel Emissions

Fossil Fuel - MarlandLand Use - Houghton

Prior to 1910 CO2 emissions from land use changes were dominant.

Subsequently fossil fuel CO2 has been dominant and rapidly increasing!

ATTRIBUTION OF INCREASE INATMOSPHERIC CO2

Comparison of cumulative CO2 emissions from fossil fuel combustion andland use changes with measured increases in atmospheric CO2.

500

450

400

350

300

Car

bon

diox

ide

mix

ing

ratio

, ppm

2000195019001850180017501700

400

300

200

100

0

Cum

ulative carbon dioxide emissions, P

g C

Fossil Fuel Cumulative Emissions

Fossil + Land UseCumulative Emissions

Land Use ChangeCumulative Emissions

Law Dome (Antarctica) - Etheridge et al Siple (Antarctica) - Law et alMauna Loa (Hawaii) - Keeling

Prior to 1970 the increase in atmospheric CO2 was dominated byemissions from land use changes, not fossil fuel combustion.

ATTRIBUTION OF ATMOSPHERIC CO2Comparison of CO2 mixing ratio and forcing from

fossil fuel combustion and land use changes400

380

360

340

320

300

280

260

Car

bon

diox

ide

mix

ing

ratio

, ppm

20001950190018501800

250

200

150

100

50

0

Carbon dioxide atm

ospheric burden, Pg C

2.5

2.0

1.5

1.0

0.5

0.0

ForcingW m-2

% CO2 in Atmosphere CO2 Residence Time, yr

40 ∞ 50 240 60 120 70 75

Partition-decay model: d

dtf Q e t∆ ∆CO

CO2A 2= − − /τ

ATTRIBUTION OF ATMOSPHERIC CO2Comparison of CO2 mixing ratio and forcing from

fossil fuel combustion and land use changes400

380

360

340

320

300

280

260

Car

bon

diox

ide

mix

ing

ratio

, ppm

20001950190018501800

250

200

150

100

50

0

Carbon dioxide atm

ospheric burden, Pg C

2.5

2.0

1.5

1.0

0.5

0.0

ForcingW m-2

(50, 240)

Landuse

(50, 240)

% CO2 in Atmosphere CO2 Residence Time, yr

40 ∞ 50 240 60 120 70 75

(60, 120)

(60, 120)

Fossil

CO2 from land use emissions – not fossil fuel combustion – was the dominant contribution to atmospheric CO2 and forcing overthe 20th century. This conclusion is not sensitive to the parameters.

IMPULSE RESPONSE PROFILES OFATMOSPHERIC CO2

1.0

0.8

0.6

0.4

0.2

0.0

Impu

lse

Res

pons

e F

unct

ion

200150100500

Time, yr

IP90

IINIT

% CO2 in Atmosphere CO2 Residence Time, yr

40 ∞ 50 240 60 120 70 75

(40, ∞)

(50, 240)

(60, 120)

(70, 75)

Preindustrial (IINIT) and IPCC-1990 (IP90) profiles from Entinget al., 1994.

ATTRIBUTION OF ATMOSPHERIC CO2Comparison of CO2 mixing ratio and forcing from

fossil fuel combustion and land use changes400

380

360

340

320

300

280

260

Car

bon

diox

ide

mix

ing

ratio

, ppm

20001950190018501800

250

200

150

100

50

0

Carbon dioxide atm

ospheric burden, Pg C

2.5

2.0

1.5

1.0

0.5

0.0

ForcingW m-2

Fossil

Landuse

IP90 IINIT

CO2 from land use emissions – not fossil fuel combustion – hasbeen the dominant contribution to atmospheric CO2 and forcing overthe 20th century. This conclusion is not sensitive to the model.

INCREASE OF CO2 EMISSIONS IS ROUGHLYEXPONENTIAL

0.001

0.01

0.1

1

Car

bon

diox

ide

emis

sion

s, p

pm/y

r

2000195019001850180017501700

0.01

0.1

1

Carbon dioxide em

issions, Pg C

/yr

Fossil Fuel CO2 Emissions

e-folding time = 40 yr

The time constant for emissions growth is well less than time constant fordecrease of excess CO2.

The mean age of fossil fuel CO2 in the atmosphere is ~ 40 years.The climate influence of excess fossil fuel CO2 already in the atmosphere

will continue well into the future.

OBSERVATIONS ABOUT CO2

The residence time of excess atmospheric CO2 >∼ 100 years.

CO2 from land use emissions was the dominant contribution toexcess CO2 and its climate forcing over the 20th century.

CO2 from fossil fuel combustion now the dominantcontribution to excess CO2 and its climate forcing.

Fossil fuel CO2 emissions are increasing with time constant of~40 years.

Excess CO2 now in the atmosphere is 100% of ~40 years’emissions.

The forcing of present excess CO2 will remain for a long time.

TIME CONSTANTS OF EARTH’SCLIMATE SYSTEM

Consider a perturbation to the climate system

How long does it take for the system to adjust to the new state?

There are many time constants:

Minutes. It gets cooler when the sun goes “behind a cloud.”

Hours. It is cooler at night than during the day; but there is alag.

Months. It is colder in winter than in summer, but there is a lag.

Years. Thermal buffering of the ocean mixed layer.

Thousands of years. The deep oceans.

Millions of years. Thermal mass of the whole planet (Kelvinand the age of Earth)

TIME CONSTANT OF THE CLIMATE SYSTEMFor the relevant climate system consisting of the atmosphereand the mixed layer of the ocean, the time constant forrelaxation of a perturbation is τ β= C0 / ,

where β α ε = 0

0

41

1

4

1 1

40 0

J

T

d

d T

d

d T−

−+

ln( )

ln

ln

ln,

C0 is the thermal mass of the system, J0 is the emittedlongwave flux at the TOA, and T0 is temperature at the TOA.

For taken as unity and C0 given by the thermal mass of the

ocean mixed layer (100 m), τ ≈ 4 yr.

Climate response is essentially instantaneous! Warming dueto excess CO2 will diminish as the excess CO2 decays.

AEROSOL INFLUENCES ON RADIATIONAND CLIMATE

Direct Effect (Clear sky)Light scattering → Cooling influenceLight absorption → Warming influence, depending on surface

Indirect Effects (Aerosols influence cloud properties)More droplets → Brighter clouds (Twomey)More droplets → Enhanced cloud lifetime (Albrecht)More droplets → Broadening of drop distribution → warming (Liu)

Semi-Direct Shortwave Radiative EffectAbsorbing aerosol → Cloud evaporation (Hansen)

Longwave Radiative Effect (Clear sky)Greenhouse effect of aerosol particles (Vogelmann)

Hydrological EffectsSuppressed surface evaporation → Spinning down the water cycleLonger-lived clouds → Displaced precipitation (Rosenfeld)

Ecological EffectsDecreased surface irradiance affects primary productivityIncrease in diffuse/direct ratio affects primary productivity

ELEMENTS OF AEROSOL FORCING

Forcing depends on amount of material present and on aerosolmicrophysical and optical properties (size, single scatteringalbedo, ability to nucleate cloud drops).

Amount of material present depends on emissions, atmosphericchemistry, and removal.

Anthropogenic emissions are associated largely with fossil fuelcombustion (sulfate, soot, secondary organics), biomassburning (organics and soot), mineral dust from disturbed soils.

Removal occurs mainly by precipitation with residence time ofabout a week.

Level of Scientific Understanding

�-2

�-1

0

1

2

3

Rad

iativ

e fo

rcin

g (

Wat

ts p

er s

quar

e m

etre

)

Coo

ling

War

min

g

High Medium Medium Low VeryLow

VeryLow

VeryLow

VeryLow

VeryLow

VeryLow

CO2

VeryLow

CH4

N2OHalocarbons

Stratosphericozone

Troposphericozone

Sulphate

Blackcarbon from

fossilfuel

burning

Organic carbon

from fossilfuel

burning

Biomassburning

Contrails

SolarMineral

Dust

Aerosolindirect effect

Land-use

(albedo) only

Aviation-induced

Cirrus

VeryLow

Aerosols

The global mean radiative forcing of the climate system for the year 2000, relative to 1750

Summary for Pol icymakers A Repor t o f Work ing Group I o f the In tergovernmenta l Pane l on Cl imate Change

RADIATIVE FORCING OVER THE INDUSTRIAL PERIODIPCC (2001)

IMPLICATIONS OF AEROSOL FORCING• Aerosol negative (cooling) forcing is likely offsetting and

masking a substantial fraction of positive (warming) forcingby greenhouse gases.

• A substantial fraction of the forcing of 40 years of CO2emissions is being offset by a week’s worth of aerosol.

• The global warming due to CO2 and other GHGs is almostcertainly substantially greater than has been experienced thusfar.

• Quantifying aerosol forcing is essential because theuncertainty in aerosol forcing precludes meaningfulempirical inference of climate sensitivity and evaluation ofclimate models.

• Aerosols influence hydrological cycles, vertical heatingprofiles, surface heating, vegetation, etc., and these influencesmust also be understood and quantified.

INTEGRATED FORCINGA MEASURE OF THE IMPACT OF INCREMENTAL

GREENHOUSE GASES OR AEROSOLS

Φ( ) ( ) ( ) ( ) ( )T F t dt c c t dt c t dt cT T T

= = ≈ =∫ ∫ ∫0 0 0 0α α α II t dtT

( )0∫

Where

Φ( )T = Integrated forcing over time horizon T [Unit: W m-2 yr]

F t( ) = Time dependent forcing of incremental gas or aerosol species

α( ) ( ) /c F c c≡ = Forcing intensity of incremental gas or aerosol species(may depend on concentration c)

c0 = Initial incremental concentration of gas or aerosol species

I t( ) = Impulse response function of gas or aerosol species

Integrated forcing is akin to absolute greenhouse warming potential AGWP.

Φ = c AGWP T0 ( )

COMMITTED INTEGRATED FORCING OF ANTHROPOGENICGASES AND AEROSOLS IN PRESENT ATMOSPHERE

Evaluated for 100-year time horizon

100

80

60

40

20

0Inte

grat

ed F

orci

ng (

Wat

ts p

er s

quar

e m

etre

yea

rs)

CO2

CH4

N2OCFC-11

CFC-12CFC-113

0.04

0.02

0

0.02

0.04

Inte

grat

ed fo

rcin

g (

Wat

ts p

er s

quar

e m

etre

yea

rs)

Coo

ling

War

min

g

Sulphate

Blackcarbon from

fossilfuel

burning

Organic carbon

from fossilfuel

burning

Biomassburning

MineralDust

Aerosolindirect effect

Aerosols

Note vastly different scales, 1000 ×, greenhouse gases >> aerosols.

Despite comparable forcings, greenhouse gases exert much greaterintegrated forcings because of long atmospheric lifetimes.

SOME CONCLUDING OBSERVATIONS• GHG concentrations and forcing are increasing. GHGs persist

in the atmosphere for decades to centuries.

• Tropospheric aerosols remain in the atmosphere for about aweek.

• Increasing scattering aerosols is not a viable strategy formitigating greenhouse warming.

• Decreasing absorbing aerosols would be of little long-termavail.

• Integrated forcing is potentially useful for policymakers, e.g.,Integrated Forcing Impact Assessment for new power plantsand the like.