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)

Outline

• What are aerosols?

• Importance in present day atmosphere

• Estimates of past climate impact

• Uncertainties

• Estimates of future changes

• What next?

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

Sources - Natural

• sea salt• volcanic aerosols• mineral dust• Biomass burning

Sources - Man-made

• Fossil fuel burning (produces several different types)

• Biomass burning• Mineral dust

Importance: Direct solar effect• Aerosols scatter and absorb solar radiation

No aerosol Scattering aerosol Absorbing aerosol

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

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

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

e.g. seasalt

GCM(no aerosols) - ERBE GCM (aerosols) - ERBE

GCM (Aerosols + sea salt) - ERBE

e.g. radiative effect of Saharan dust outbreaks

Figure courtesy of SeaWiFs and OrbiImage

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

AV

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R C

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AV

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R C

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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

Change in SST (K) from AVHRR data when dust is present September 2000.

The SST anomaly over the Cape Verde Islands reaches -3.6K.

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-3.6 Figure courtesy of J.M. Haywood, Met. Office

Estimating climate change due to changes in aerosols

• Emission sources and time history

• Chemistry and transport model

• Radiation code

• Climate model

Radiative forcing

Global and annual mean radiative forcing can be related to a global and annual mean change in surface temperature using:

T = F

e.g. F over past 250 years

From IPCC TAR (2001)

Greenhouse gases

Dust

Sulphates

IndirectFrom Shine and Forster, 1999

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.

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

Radiation code

Other components

Optical propertiesDistribution

Uncertainties

Uncertainty in forcing

EmissionsProcessingChemistryTransport

BackgroundNatural aerosols

Chemical compositionMixingSize Distribution

WavelengthsTransfer scheme

CLOUDSRelative humiditySurface albedo

Distribution: sulphates

• Formed from gases SO2 (from fossil fuel or volcanoes) and DMS (from ocean algae)

Distribution: carbonaceous from anthropogenic sources

• Fossil fuel burning

• Inventories have an uncertainty of a factor of 2.

Distributions: Biomass burning

•Some biomass burning is natural.

•Episodic and regional in nature

Distribution: Mineral dust

50% of dust burden due to anthropogenic sources due to land use change, overgrazing etc.

Past Trends

From ice cores: very uncertain. (From IPCC 2001)

1st indirect effectIncrease in aerosol

Increase in cloud

droplet number

Change in reflectivity

(albedo)

From Brenguier et al (2000)

2nd indirect effect

• Aerosols affect precipitation efficiency and therefore cloud lifetime.

• Also affect cloud reflectivity?

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.

Uncertainties

Climate response?

Uncertainty in forcing

EmissionsProcessingChemistryTransportBackgroundNatural aerosols

Distribution

Chemical compositionMixingSize Distribution

Optical properties

WavelengthsTransfer scheme

CLOUDSRelative humiditySurface albedo

Other components

Radiation code

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)

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Fixed cloud All feedbacks

Adapted from Hansen (1997)

Climate response 2

Reader and Boer (1998): large scale responses surprisingly similar

Modelling climate change over past 250 years

Global Mean Temperature (Anomaly from 1961-1990)

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Global Mean Temperature (Anomaly from 1961-1990)

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Pink - observations, blue - model

No aerosol + aerosol

Future changes in aerosols

From IPCC (2001)

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

“Real knowledge is to know the extent of one’s ignorance”

Confucius