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 aerosolScattering aerosolAbsorbing 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

AVHRR Ch4AVHRR Ch5 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 The SST anomaly over the Cape Verde Islands reaches -3.6K 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 Indirect From 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  Emissions  Processing  Chemistry  Transport  Background  Natural aerosols  Chemical composition  Mixing  Size Distribution  Wavelengths  Transfer scheme  CLOUDS  Relative humidity  Surface albedo

Distribution: sulphates Formed from gases SO 2 (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 effect Increase 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  Emissions  Processing  Chemistry  Transport  Background  Natural aerosols Distribution  Chemical composition  Mixing  Size Distribution Optical properties  Wavelengths  Transfer scheme  CLOUDS  Relative humidity  Surface albedo Other components Radiation code

Climate response 1 Is climate response to changes in aerosol the same as for changes in CO 2 or solar constant? Adapted from Hansen (1997)

Climate response 2 Reader and Boer (1998): large scale responses surprisingly similar

Modelling climate change over past 250 years 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