The Role of Aerosols in Climate Change
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Transcript of The Role of Aerosols in Climate Change
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The Role of Aerosols in Climate Change
Eleanor J. Highwood
Department of Meteorology,
With thanks to all the IPCC scientists, Keith Shine (Reading) and James Haywood (Met. Office)
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Outline
• What are aerosols?
• Importance in present day atmosphere
• Estimates of past climate impact
• Uncertainties
• Estimates of future changes
• What next?
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What are aerosols?
• Small particles or droplets suspended in the atmosphere
• Radius is 0.01 to 10 microns
• Many different types and sources
• Natural and man-made sources
• Important for both present day climate and climate change
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Sources - Natural
• sea salt• volcanic aerosols• mineral dust• Biomass burning
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Sources - Man-made
• Fossil fuel burning (produces several different types)
• Biomass burning• Mineral dust
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Importance: Direct solar effect• Aerosols scatter and absorb solar radiation
No aerosol Scattering aerosol Absorbing aerosol
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Importance: Direct terrestrial effect
• Large aerosols (e.g. dust or sulphuric acid in the stratosphere) behave like greenhouse gases.
No aerosol: ground emits to space
Aerosol absorbs radiation from ground and re-emits a smaller amount up and down
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Importance: Indirect effects
• Some aerosols can alter the properties of clouds, changing their reflectivity or lifetime
• Some aerosols can allow chemical reactions between atmospheric constituents to take place very rapidly
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Measuring aerosol effects on climate
• Measure effect on radiation at top of atmosphere and surface.
• “Radiative effect” : effect of having aerosol in the present day atmosphere
• “Radiative forcing”: effect of changes in aerosol on radiation budget over a given period of time
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e.g. seasalt
GCM(no aerosols) - ERBE GCM (aerosols) - ERBE
GCM (Aerosols + sea salt) - ERBE
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e.g. radiative effect of Saharan dust outbreaks
Figure courtesy of SeaWiFs and OrbiImage
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The solar radiative effect of Saharan dust
can be very large - measurements from
SHADE on 25th September 2000 between Sal and
Dakar show:
3 times more solar radiation being
scattered back to space than in clear sky (so a big reduction in
the amount of radiation that reaches
the surface).Figure courtesy of J.M. Haywood, Met. Office
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AV
HR
R C
h4
AV
HR
R C
h5
Dust also affects our knowledge of other climate variables like sea surface temperature because it absorbs outgoing terrestrial radiation.
Figure courtesy of J.M Haywood, Met. Office
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Change in SST (K) from AVHRR data when dust is present September 2000.
The SST anomaly over the Cape Verde Islands reaches -3.6K.
+2.4
+1.8
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0
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-1.2
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-3.6 Figure courtesy of J.M. Haywood, Met. Office
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Estimating climate change due to changes in aerosols
• Emission sources and time history
• Chemistry and transport model
• Radiation code
• Climate model
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Radiative forcing
Global and annual mean radiative forcing can be related to a global and annual mean change in surface temperature using:
T = F
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e.g. F over past 250 years
From IPCC TAR (2001)
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Greenhouse gases
Dust
Sulphates
IndirectFrom Shine and Forster, 1999
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Summary of issues
• Aerosols all much more uncertain than greenhouse gases
• Can’t add up aerosols to cancel out greenhouse gases
• Total aerosol forcing is unlikely to be a linear combination of individual contributions
• Indirect is holding us up.
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What do we need to know about aerosols?
5 key parameters to give us radiative forcing
– mass light scattering efficiency– dependence of scattering on relative humidity– Single scattering albedo (absorption vs
scattering)– Asymmetry parameter– change in mass burden over time
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Radiation code
Other components
Optical propertiesDistribution
Uncertainties
Uncertainty in forcing
EmissionsProcessingChemistryTransport
BackgroundNatural aerosols
Chemical compositionMixingSize Distribution
WavelengthsTransfer scheme
CLOUDSRelative humiditySurface albedo
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Distribution: sulphates
• Formed from gases SO2 (from fossil fuel or volcanoes) and DMS (from ocean algae)
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Distribution: carbonaceous from anthropogenic sources
• Fossil fuel burning
• Inventories have an uncertainty of a factor of 2.
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Distributions: Biomass burning
•Some biomass burning is natural.
•Episodic and regional in nature
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Distribution: Mineral dust
50% of dust burden due to anthropogenic sources due to land use change, overgrazing etc.
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Past Trends
From ice cores: very uncertain. (From IPCC 2001)
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1st indirect effectIncrease in aerosol
Increase in cloud
droplet number
Change in reflectivity
(albedo)
From Brenguier et al (2000)
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2nd indirect effect
• Aerosols affect precipitation efficiency and therefore cloud lifetime.
• Also affect cloud reflectivity?
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Semi-direct effect
Aerosol such as black carbon absorbs solar radiation
Layer heats up
Cloud burns off or atmosphere is stabilised
and cloud prevented from forming.
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Uncertainties
Climate response?
Uncertainty in forcing
EmissionsProcessingChemistryTransportBackgroundNatural aerosols
Distribution
Chemical compositionMixingSize Distribution
Optical properties
WavelengthsTransfer scheme
CLOUDSRelative humiditySurface albedo
Other components
Radiation code
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Climate response 1
Is climate response to changes in aerosol the same as for changes in CO2 or solar constant?
Climate sensitivity (Hansen et al 1997)
-30
-25
-20
-15
-10
-5
0
5
2xCO2
+2% S
o
-2%
So O3
S. w=1.
0
S. w=0.
85
T. w=1
.0
T. w=0
.85
Sen
siti
vity
Fixed cloud All feedbacks
Adapted from Hansen (1997)
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Climate response 2
Reader and Boer (1998): large scale responses surprisingly similar
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Modelling climate change over past 250 years
Global Mean Temperature (Anomaly from 1961-1990)
-1.50
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0.00
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1.00
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1850
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1950
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1995Year
Tem
p A
no
mal
y (d
eg C
)
Global Mean Temperature (Anomaly from 1961-1990)
-0.80
-0.60
-0.40
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0.00
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0.60
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1.00
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Year
Tem
p A
no
mal
y (d
eg C
)
Pink - observations, blue - model
No aerosol + aerosol
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Future changes in aerosols
From IPCC (2001)
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Future areas of research
• Mixing of aerosol types
• Remote sensing of aerosol properties and amount using satellites, combination with in-situ data
• Long term and consistent modelling of aerosol profiles across globe
• Regional climate modelling
• Indirect effect and semi-direct effect
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“Real knowledge is to know the extent of one’s ignorance”
Confucius