Post on 21-Aug-2020
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.