Aerosols and Climate V. Ramaswamy (“Ram”) U.S. National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory Princeton University [USA]

Human and Natural Drivers of Climate Change IPCC (2007)

Lecture # 3 Diagnosing the role of aerosols in the 20 th century climate change using models and observations. Uncertainties associated with aerosols in past and future climate changes, including hints of nonlinearity, and “non-straightforward” climate impacts due to aerosol additions/ removals.

Unfortunately, we don’t have a twin planet earth that we can use to perform large-scale climate laboratory experiments. What is a State-of-the-Art Global Climate Model?

The computer is our lab. The computer model is our research tool.

Attribution Asks whether observed changes are consistent with  expected responses to forcings  inconsistent with alternative explanations Most of the observed increase in globally averaged temperatures since mid-20th century is very likely (>90% certainty) due to the observed increase in anthropogenic GHG concentrations TS-23 Anthro+ Nat forcing IPCC (2007)

Radiative Forcing (All forcing agents) Radiative Forcing (BC+OC) Radiative Forcing (Anthro. Aerosol  BC,OC,Sulfate)

If the 20 th century had been “driven” by LLGHGs and Ozone only…………

The 20 th century climate “driven” by all known forcing agents

Schwarzkopf and Ramaswamy (2008)

Delworth et al. (2005)

Simulated North Atlantic AMOC Index Aerosol only forcing All forcings Greenhouse gas only forcing

Uncertainties Dust aerosols’ role Sensitivity to aerosol microphysics Aerosol-Cloud interaction

Global Dust - the last 50 Years ? Dust concentration at Barbados (Prospero and Lamb, 2003) Sahel drought Factor 4 increase Sahel Precipitation Index (previous year) Barbados Dust Since 1970ies dust concentration in Caribbean (Prospero and Lamb, 2003) and dust deposition in French Alps (De Angelis and Gaudichet, 1991) have increased by a factor 4-5 Correlation at Barbados (Prospero and Lamb, 2003) Fuyu Li et al. (2008) {GFDL AM2n}

Clean/Maritime Polluted/Continental Aerosol Indirect Effects (1 st and 2 nd ) Ramanathan et al. (2001) Aerosol vs. Dynamics

Atmosphere + MIxed-Layer Ocean Equilibrium simulations to compare Greenhouse gas and Aerosol effects WMGG  well-mixed gases d  direct s  semi-direct i  total indirect A new paradigm  Treating the ‘direct’ and ‘indirect’ aerosol effect as a TOTAL AEROSOL EFFECT (TAE) How large?? Ming-Ramaswamy (JC, in press)

Ming and Ramaswamy (J. Climate, in press)

Differing roles of Scattering AND Absorbing Aerosols Experiments with the GFDL Atmospheric Model {a la “Menon-ic” investigations}: - prescribed SSTs - consider aerosol increases over India and China from the 1950 to 1990s (guided by available observations) - consider the uncertainty of the scattering and absorbing components

Scattering and Absorbing Aerosol over Asia The SeaWiFS Project and GeoEye, Scientific Visualization Studio NASA Goddard Jeff Schmaltz/Moderate Resolution Imaging Spectroradiometer Land Rapid Response Team NASA Goddard Estimated fossil-fuel black carbon emissions since 1875, in GT per year [Chen, Berkeley Lab Science Beat, July 14, 2004]. Uncertainties remain regarding: 1. The amount of extinction due to increasing amounts of aerosols. 2. The amount of aerosol absorption. IndiaChina Changes  1950 to 1990s Sensitivity of climate to changes in aerosol extinction and absorption? Impacts on precipitation and circulation in the Asian region?

Precipitation Rate Change (∆P [mm d -1 ]) Low ω o High Absorption High ω o Low Absorption ω o = 0.99ω o = 0.85 ω o = 0.99 Change in JJA precipitation rate [mm d -1 ] between the BASE and experiments; change in aerosols is the only external forcing. Land-area average given in figure for India (green) and China (red) Contour interval is 1 mm d -1. XCaXChW

Change in Surface Pressure Low ω o High Absorption High ω o Low Absorption ω o = 0.85ω o = 0.99 Contour interval 0.4 hPa. Change in JJA surface pressure (∆P sfc ) [hPa] between the BASE and experiments; change in aerosols is the only external forcing. ω o = 0.85ω o = 0.99 XCaXChW

Low ω o High Absorption High ω o Low Absorption Zonally averaged change in vertical velocity [hPa s -1 × ] relative to the BASE. Red is increased upward motion. Blue is relative subsidence. Contour interval 5 hPa s -1 × ΔCloud Amount [%] over Land ω o = 0.99ω o = 0.85 ω o = 0.99 XCa XChW India China ω o = 0.99 ω o = 0.85 XCa XChW LandOcean LandOcean Land OceanLand Cloud Amount & Vertical Velocity Change

Increases (decreases) in cloud amount can reduce (enhance) the surface solar flux reduction to the surface associated with high aerosol extinction optical depths and thus can increase (decrease) surface radiative cooling. Increased (decreased) aerosol absorption optical depth can enhance (spin-down) the hydrological cycle over the Asian land-mass. Summary table (right) shows sign change relative to BASE case over land areas. XCa XChW Low  High 

The Future What will be the impacts of changes in Greenhouse Gases and Aerosols?

Key Points: Most CO2 emission scenarios level off or decrease by 2100 Most sulfate emissions decrease by 2030

A1B Warming (CM 2.1) 2020s ~ s2090s

Summer Surface Air Temperature Change AllForce GHGs only Aerosol Reduction effect A1B Scenario 2090s – 2000s Levy et al., (JGR, 2008)

The “Great Spatial Scaling” problem in Climate and impacts Clouds Aero- sols Human Socio-Economic Scales

Principal Sources Atmospheric Radiation lectures [Boulder, 1986] “Global Physical Climatology” by D. HARTMANN Intergovernmental Panel on Climate Change, 2001 and 2007, Working Group I (The Physical Science Basis) Ming and Ramaswamy (Journal of Climate, in press) C. Randles Ph. D. thesis (Princeton University, 2007); JGR (2008, in press) Fuyu Li’s Graduate Research (Princeton University); JGR (2008)

The END Thank you for your attention !