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 20th 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 atwin planet earth that we can use

to perform large-scale climatelaboratory experiments.

What is a State-of-the-Art What is a State-of-the-Art Global Climate Model?Global Climate Model?

What is a State-of-the-Art What is a State-of-the-Art Global Climate Model?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)

RadiativeForcing

(All forcing agents)

Radiative Forcing(BC+OC)

RadiativeForcing

(Anthro. Aerosol BC,OC,Sulfate)

1860

2000

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

1860

2000

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

Schwarzkopf and Ramaswamy (2008)

Delworth et al. (2005)

Delworth et al. (2005)

Simulated North Atlantic AMOC Index

Aerosol only forcingAll 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)

Ba

rba

do

s D

ust

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 (1st and 2nd)

Ramanathan et al. (2001)

Aerosol vs. Dynamics

Atmosphere + MIxed-Layer OceanEquilibrium simulations to compareGreenhouse gas and Aerosol effects

WMGG well-mixed

gases

d directs semi-direct i total indirect

A newparadigm

Treating the ‘direct’and ‘indirect’

aerosol effect as aTOTAL 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 AsiaThe SeaWiFS Project and GeoEye, Scientific Visualization StudioNASA 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.

India China

Changes 1950 to 1990sSensitivity 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 ωo

High AbsorptionHigh ωo

Low Absorptionωo = 0.99ωo = 0.85

ωo = 0.85 ωo = 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).

-0.2 -0.6-0.2 -1.1

Contour interval is 1 mm d-1.

XCa XChW

Change in Surface Pressure

Low ωo

High AbsorptionHigh ωo

Low Absorption

ωo = 0.85 ωo = 0.99

Contour interval 0.4 hPa.

Change in JJA surface pressure (∆Psfc) [hPa] between the BASE and experiments; change in aerosols is the only external forcing.

ωo = 0.85 ωo = 0.99

XCa XChW

Low ωo

High AbsorptionHigh ωo

Low Absorption

Zonally averaged change in vertical velocity [hPa s-1× 10-5] relative to the BASE. Red is increased upward motion. Blue is relative subsidence. Contour interval 5 hPa s-1× 10-5.

ΔCloud Amount [%] over Land

ωo = 0.99ωo = 0.85

ωo = 0.85 ωo = 0.85ωo = 0.99ωo = 0.99XCa XCaXChW XChW

Ind

iaC

hin

aωo = 0.99

ωo = 0.99

ωo = 0.85

ωo = 0.85

XCa

XCa

XChW

XChW

LandOcean

LandOcean

Ocean Land

Ocean Land

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

XCa XChW

XChW

Low High

The Future

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

Key Points:

Most CO2emissionscenarioslevel off or decrease by 2100

Most sulfate emissions decrease by 2030

A1B Warming (CM 2.1)

2020s ~2050

2070s 2090s

Summer Surface Air Temperature Change

AllForce

GHGs only

Aerosol Reduction

effect

A1B Scenario2090s – 2000s

Levy et al.,(JGR, 2008)

The “Great Spatial Scaling” problem in Climate and impacts

CloudsAero-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 !