1U.S. Environmental Protection Agency – Office of Air and Radiation

Short-Lived Climate Forcers

Laura McKelvey

Office of Air Quality Planning and Standards

November 18, 2009

2U.S. Environmental Protection Agency – Office of Air and Radiation

What is a “Short-Lived Climate Forcer”?

• A “climate forcer” is any gas or particle that alters the Earth’s energy balance by absorbing or reflecting radiation:– Greenhouse gases (e.g. carbon dioxide, methane, ozone) warm

the climate by trapping outgoing radiation from earth’s surface– Aerosols (i.e., particles such as black carbon and sulfates) can be

either warming or cooling, depending on composition• Sulfates and nitrates scatter and reflect incoming solar radiation,

producing a cooling effect• Black carbon warms the atmosphere by absorbing incoming sunlight

and by darkening snow and ice, reducing “albedo” (reflectivity)

• Some climate forcers—like ozone and black carbon– are considered “short-lived” because they only stay in the atmosphere for a few days or weeks– By contrast, “long-lived” climate pollutants like CO2 and HFCs can

stay in the atmosphere for hundreds of years or longer

3U.S. Environmental Protection Agency – Office of Air and Radiation

Estimated Radiative Forcing Associated with Short-Lived Climate Forcers as Compared to CO2

Best estimates of climate forcing

(Adapted from IPCC Synthesis Report, 2007)

-1

-0.5

0

0.5

1

1.5

2

CarbonDioxide

Methane BlackCarbon

Ozone Nitrate OrganicCarbon

Sulfates CloudIndirectEffect

Glo

bal

Mea

n R

adia

tive

Fo

rcin

g (

W/m

2 )

Warming Effect

Cooling Effect

4U.S. Environmental Protection Agency – Office of Air and Radiation

Benefits of SLCF Reductions

• Reducing “short-lived” climate forcers (SLCF) can lead to immediate climate benefits– The Earth’s climate system responds quickly to reductions in these

pollutants– This may help us slow the overall rate of warming and avoid

climate “tipping points”, such as melting of ice sheets– Also, reducing SLCFs may be particularly important for protecting

sensitive regions such as the Arctic and the Himalayan glaciers

• Reductions in SLCF’s– esp. ozone and black carbon– can also provide significant public health benefits– The Clean Air Act already provides us authority to address these

conventional air pollutants

• Controls on SLCF will not eliminate need for rapid action on GHGs: controls on both long-lived and short-lived climate forcers are necessary

5U.S. Environmental Protection Agency – Office of Air and Radiation

• Strong congressional interest in BC H.R. 2454 Waxman-Markey Climate & Energy Bill requires Report

to Congress and domestic and international Mitigation Recommendations

Kerry-Boxer includes similar requirements Signed Appropriations language mandates Black Carbon Report

To Congress due May 2011 International Assessments

United Nations Environment Program Black Carbon and Ozone Assessment

Arctic Council Task Force on Short Lived Climate Forcers Convention on the Long Range Transport of Air Pollution

considering further study Many academic studies and reports underway

Mounting Interest in Assessment and Mitigation of Black Carbon

Total: 7900

gigagrams

Pie Chart from Kirk Smith, UC Berkeley

Sources of Black Carbon: Global Emissions by Sector

Mobile

Sourc

e Die

sel

Presc

ribed

and A

g Burn

s

Wild

fires

Natura

l Gas

Com

busion

Resid

entia

l Wood C

ombust

ion

Mobile

Sourc

e Gas

oline

Coal C

ombst

ion

Statio

nary

Diese

l Eng

Comm

erci

al C

ooking

Distil

late

Oil

Comb

0100002000030000400005000060000700008000090000

100000

Ton

s

Domestic EC Emissions, 2005Over 200,000

Expected levels of Mobile Diesel Emissions in 2020

• The biggest categories for elemental carbon (EC) emissions are 1) mobile source diesel (onroad + nonroad); 2) biomass burning; and 3) fossil fuel combustion categories• While not shown here, organic carbon emissions (which are co-emitted with EC from all these sources) are also important.

• Biomass burning has significant amounts of OC emissions, while diesel sources and fossil fuel combustion have much less OC emissions. • Generally, the smaller the amount of OC from a source category, the better mitigation option it is for climate.

8U.S. Environmental Protection Agency – Office of Air and Radiation

Important Considerations for Designing Control Programs for SLCF

• Location of reductions matters because these pollutants are more local/regional in nature than long-lived GHGs

• BC’s warming effect is offset somewhat by cooling from reflective pollutants emitted at the same source, especially organic carbon (OC)

– Diesel engine exhaust is mostly BC– Residential wood smoke is mostly OC

• Some ozone precursors lead to warming (CH4, CO, and nmVOC), but NOx emissions lead to cooling, so the net climate effect of controls on ozone precursors varies

• Significant uncertainties remain: additional research needed on emission inventories and assessing net climate impacts of reductions from particular source categories

9U.S. Environmental Protection Agency – Office of Air and Radiation

Climate Impacts of Black CarbonN

AS

A G

od

dard

Sp

ace

Flig

ht

Cen

ter/

Jeff

Sch

malt

z

Imag

e:

Gla

ciers

On

line

Jurg

Ale

an

NA

SA

God

dard

Sp

ace

Flig

ht

Cen

ter

Changing Precipitation Patterns Shrinking Glaciers

Decreasing Summer Sea IceClean Ice Reflects Ice with BC Absorbs

10U.S. Environmental Protection Agency – Office of Air and Radiation

• Ozone transported from the mid-latitudes is partially responsible for Arctic warming

• Black Carbon has disproportionately large impact– Absorbs more heat over Arctic

reflective surfaces

– Once deposited, BC darkens snow and ice leading to greater melting

SLCF Impacts on the Arctic

(Adapted from Reiersen and Wilson, 2009)

Sn

ow

Atmosphere