Short-Lived Climate Forcers

1. Short-Lived Climate Forcers Laura McKelvey Office of Air Quality Planning and Standards November 18, 2009

2. 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

3. Estimated Radiative Forcing Associated with Short-Lived Climate Forcers as Compared to CO2 Best estimates of climate forcing Warming Effect Cooling Effect (Adapted from IPCC Synthesis Report, 2007)

4. 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

5. Mounting Interest in Assessment and Mitigation of Black Carbon • 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

6. Sources of Black Carbon: Global Emissions by Sector Total: 7900 gigagrams Pie Chart from Kirk Smith, UC Berkeley

7. Domestic EC Emissions, 2005 100000 90000 80000 70000 60000 Tons 50000 40000 30000 20000 10000 0 Wildfires Coal Combstion Distillate Oil Comb Mobile Source Diesel Stationary Diesel Eng Commercial Cooking Mobile Source Gasoline Natural Gas Combusion Prescribed and Ag Burns Residential Wood Combustion Over 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.

8. 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

9. Climate Impacts of Black Carbon NASA Goddard Space Flight Center/ Jeff Schmaltz Image: Glaciers OnlineJurg Alean Changing Precipitation Patterns Shrinking Glaciers NASA Goddard Space Flight Center Clean Ice Reflects Ice with BC Absorbs Decreasing Summer Sea Ice

10. SLCF Impacts on the Arctic • 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 Atmosphere Snow (Adapted from Reiersen and Wilson, 2009)