Mayurakshi Dutta Department of Atmospheric Sciences March 20, 2003 Impact of Aerosols on the Global Climate and Hydrological Cycle The INDOEX endeavor Mayurakshi Dutta Department of Atmospheric Sciences March 20, 2003

Acknowledgements Ramanathan,V, P.J. Crutzen,J.T Kiehl and D. Rosenfeld, 2001, Aerosols, Climate, and the Hydrological Cycle, Science, 294, 2119 pp. Ramanathan et.al., 2001,The Indian Ocean Experiment: An Integrated Analysis of the Climate Forcing and Effects of the Great Indo-Asian Haze., J. Geophys. Res.-Atmos.,106, 2837 pp. Lelieveld,J.et al.,The Indian Ocean Experiment: Widespread Air pollution from South and Southeast Asia,Science, 291, 1031 pp

What are AEROSOLS ? Suspended particles in the atmosphere which range in size from about 10-3 µm to 20 µm Anthropogenic - inorganic /organic sulphates carbonaceous aerosols [black carbon(BC)] and organic carbon (OC) Dust Sea-salt Natural sources Volcanic dust Sea-spray Wind-generated dust Forest fire smoke

Contribution to Aerosol Optical Depth - AOD AOD is an index of the attenuation of radiation as it passes through the atmosphere due to the presence of suspended particles.

Global distribution of Aerosols Biomass burning + fossil fuel combustion – S/SE Asia Biomass burning – Africa + S. America Fossil fuel combustion- mid and high latitude

Effects of aerosols on climate Direct radiative forcing (RF) - increase the reflection of solar radiation to space. Global estimates of TOA RF due to anthropogenic aerosols ~ -0.5 to –2.5 Wm-2. The reflected solar flux due to anthropogenic sulphate averaged over NH ~ -1.1 Wm-2 ~ 50% (but opposite sign) of the RF due to anthropogenic CO2 (Jayaraman et. Al. 1998) Both the surface and the atmosphere will cool BUT

Effects of aerosols on climate Carbonaceous aerosols (BC and organic) absorb and scatter solar radiation…..thus decreases the solar radiation reaching the surface. In addition, carbonaceous aerosols absorb the upward solar radiation reflected by the surface and clouds At the TOA - BC effect opposes the cooling effect of sulphates At the surface - all aerosols reduce solar radiation

Effects of aerosols on climate Indirect radiative forcing- Aerosols serve as Cloud Condensation Nuclei (CCN) and ice nuclei  aerosols   [droplet number]   reflection to space of solar radiation from clouds  cooling effect  in precipitation efficiency due to microphysical effect Increase in cloud lifetime Semi direct effect - solar heating of the boundary layer by BC evaporate some clouds  more solar radiation reach the surface

Effects of aerosols on Radiative Forcing INDOEX Global

INDOEX Endeavor International field experiment over North Indian Ocean Intense field phase b/w Jan-Mar,1999 First in situ observation of aerosols + air chemistry Location unique

South Asian Brown Haze

Regional Distribution of Anthropogenic Aerosol Climate Forcing (Wm-2) January- March 1999

Impact on Global Surface Temperature TOA forcing determines global average surface temperature change….. Global surface warming due to GHGs  global cooling effect of the aerosols Climate models suggest – global surface would warm by about 0.5 to 0.8 K per 1 Wm-2 2.4 Wm-2 = ~ 0.4 Wm-2  2.0 Wm-2 20th century Stored in should have anthropogenic GHG oceans caused a 1-1.6 K forcing warming But the observed warming between 1900 – 2000 is only about 0.6 K . The estimated global mean TOA aerosol forcing of –10.5 Wm-2 sufficient for the missing cooling

Impact on Global Hydrological Cycle 80% of the net radiative heating of the tropical oceans is balanced by evaporation So  in RF   in evaporation   tropical rainfall  thus perturbation of the water budget  in rainfall   latent heat released to the middle and upper troposphere  implications for the lapse rate and Hadley and Walker circulations Again

Impact on Global Hydrological Cycle Between 0-3 km the low level air temperature increased where the aerosol heating was imposed Response to this low level heating   in moist convection + strengthen rainfall along the ITCZ Subsidence  away from the region of enhanced precipitation   in rainfall north and south of ITCZ

Future Implications…….. Important to differentiate the decadal to centennial time scales of the GHG warming from the time scale of aerosol lifetimes Regional aerosol effects will continue to play a major role as long as air pollution continues

……..and Future Plans A reliable global inventory of aerosol emission rates, lifetimes and concentrations needed Understanding BC transport and how carbonaceous aerosols regulate microphysical processes in clouds More reliable measurements of AOD over land needed Careful designing of OC-GCM studies to better determine how hydrological cycle responds to the microphysical and radiative effects of aerosols

Now we can have Questions Thanks very much Now we can have Questions