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Here comes the sun:Here comes the sun:Aerosols and Solar Aerosols and Solar

DimmingDimmingLelia HawkinsESP Seminar

November 17, 2006

An aerosol is . . .

. . . a liquid or solid particle, suspended in a gas.

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Aerosol Sources

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Dust

VOCs

Nitrates

and Soot

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Black Carbon

Sea SaltAsh and Sulfate

Aerosols and Climate:the Direct Effects

Sulfate aerosols Black carbon and soot

Aerosols and Climate:the Indirect Effects

And longer lasting . . .

Clouds are more reflective . . .

Increasing Aerosol Loading• Lower surface solar radiation• Cool surface

Increasing GHGs• Warm surface

Observations show 0.15°C warming per decade over the last 50 years

Warmer world, decreased evaporation??

800

900

1000

1100

1200

1300

1400

1950 1965 1980 2000

Evaporation in mm/season 800

850

900

950

1000

1050

1950 1965 1980 2000

Evaporation in mm/season

Expected Observed

Pan evaporation paradox

Observed reductions in evaporation

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Decreased pan evaporation by97 mm per warm season

between 1950-1995

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INDOEX: Indian Ocean Experiment

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Ramanathan et al. 2001

INDOEX: Indian Ocean Experiment

Ramanathan et al. 2001

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Continental Air (Polluted)

Marine Air (Unpolluted)

Mixed Air

Surface Cooling

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-20 Watts/m2

average

Ramanathan et al. 2001

Compare 20 watts to Total ocean heat flux (40 Watts/m2)

So the big conclusion is . . .

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Strong present-day aerosol cooling

implies a hot future!Andreae et al., 2005

Why?

Because of:The lifetime effect

• CO2 lives about 100 years in our atmosphere.

• Aerosols last days to weeks.

And . . .

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Future emissions of cooling aerosols from the IPCC-SRES A2.

Andreae et al., 2005

Projected Emissions

Climate Sensitivity-Response to a Perturbation

€

ΔT2×CO2= 3.7 ΔT

ΔQ− c d (ΔT )dt

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We need to better understand present day forcing to better predict future warming.

Andreae et al., 2005

Too low

Too low Too high

“Deceitful” observed surface temp

ΔT = 1 K to 1.6 K ΔT = 0.6 K

What does the future hold?

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Andreae et al., 2005

Projected temperature risefrom IPCC-TAR

Scenario A2

Conclusions• Aerosols may be responsible for the

“missing” warming signal.• Climate sensitivity may currently under-

predict warming due to doubling of CO2

• Aerosol pollution levels are changing, and are predicted to decrease world-wide.

References

References• http://www.scienceagogo.com/news/20020204005723data_trunc_sys.s

html

• http://www.realclimate.org/index.php?p=110• Peterson et al., 1995, Evaporation losing its strength,

Nature (377)

Thank you!

Acknowledgements to Dr. Ramanathan for his

assistance and guidance and to Google Images for the pictures.

Global dimming or local dimming?

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Alpert et al., 2005

< 0.1 million

> 0.1 million

-0.41 Wm-2yr-1

-0.16 Wm-2yr-1

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Wild et al., 2005

The future looks “bright”!

Satellites show+0.16 Wm-2yr-1

Pinker et al., 2005Post-1990Increase = 26Decrease = 6

Pre-1990Increase = 8Decrease = 24

1. More effective clean air regulations2. Political transition in Eastern Europe3. Recently, recovery from Pinatubo

Deriving climate sensitivity

€

c d(ΔT)dt

= ΔQ− λΔT

€

λ = ΔQ− c d(ΔT)dt

⎛ ⎝ ⎜

⎞ ⎠ ⎟ 1ΔT

€

ΔT2×CO2=

ΔQ2×CO2

λAndreae et al., 2005

3.7 Wm-2

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The warmest decade since 1861

IPCC-TAR

Shallow Cloud Cooling

Kaufman et al., 2005

Aerosol reflected flux at TOA due to direct (1/3) and Twomey (2/3) effects

Increase in Atlantic shallow clouds by 20% to 40%

4% global increase = 2-3K of cooling

How does it work?

atmosphereS/4*(1-albedo)*(absorbed)

S/4*(1-albedo)

Surface

S/4*(1-albedo)*(1-absorbed) Ts4

Ts4

(1-) Ts4 Ta

4

Ta4

+

+

2 Ta4 -

=

=

=

TOA

What is solar dimming?• Direct: Scatter and absorb solar

radiation.• Indirect: Longer lasting and more

reflective clouds (Twomey Effect)• Net reduction in surface solar

radiation