1. Impact of Short-Lived Pollutants on Arctic Climate (SPAC) Background and Progress Since January 2007 GISS Meeting SPAC Workshop, November 5 – 7, 2007 Oslo, Norway Sponsored by NILU, CATF, IGAC, CPC

2. From NSIDC Summer 2007 NW passage open for the first time in human memory NE passage closed by a narrow band of sea ice. 1979 - 2000

3. September sea ice extent from 1979 – 2007 The September rate of sea ice decline is now  10% per decade. Spring melt is occurring earlier and autumn freezing is starting later. The earlier onset of spring melt is of particular concern as this is the season of maximum snow-albedo feedback. Snow-Albedo Feedback As the area of ice and snow cover decreases, there is less reflection of solar radiation and more absorption by the darker remaining ocean and land surfaces. This increase in total absorbed radiation contributes to continued and accelerated warming.

4. Impact of Short-Lived Pollutants on Arctic Climate Reductions in CO 2 are the backbone of any climate forcing mitigation strategy But reductions in CO 2 most likely will not be swift enough to delay Arctic melting Targeting short-lived climate forcing agents may be the best strategy for delaying the onset of spring melt and constraining the length of the melt season Particularly important are the species that impose a radiative forcing that leads to a surface temperature increase and that trigger regional scale climate feedbacks pertaining to sea ice melting Methane Tropospheric Ozone Tropospheric Aerosols

5. Seasonality of Pollutant Transport to the Arctic Quinn et al., Tellus, 2006. Monthly averaged values Arctic Haze Arrives late winter/ early spring Includes tropospheric aerosols, ozone, and ozone precursors

6. Barrow ”normal” value [Stohl et al. (2006): JGR, 111, D22214, doi:10.1029/2006JD007216] Summertime transport of smoke from boreal forest fires Barrow, Alaska Summit, Greenland Zeppelin, Svalbard Springtime transport of smoke from agricultural fires in Eastern Europe Iceland [Stohl et al. (2006): ACP, 7, 511 - 534.] Transport from warm, lower latitudes possible because of abnormally warm Arctic temperatures Erosion of the Polar Dome

7. Seasonality of Pollutant Transport to the Arctic Quinn et al., Tellus, 2006. Monthly averaged values For the short-lived species with lifetimes of days to weeks, the timing of transport to the Arctic can be a factor in the seasonality of the forcing and the corresponding temperature response. Methane is an exception due to its longer lifetime of  10 years. It is globally well-mixed so that transport is not seasonal. The next few slides indicate the seasonality of the different forcing mechanisms and emphasize the season of maximum surface temperature response for each forcing agent.

8. Forcing Mechanism and Season of Maximum Temperature Response Winter – Enhanced Cloud Longwave Emissivity (ΔT > 0) Thin, clean cloud Poor insulator Heat escapes Thin, polluted cloud. Better insulator. Heat is trapped and re-emitted. [e.g., Garrett and Zhao, Nature, 2006] O 3 CH 4 GHG Warming ΔT > 0

9. Forcing Mechanism and Season of Maximum Temperature Response Spring – Black carbon deposition / snow albedo reduction (ΔT > 0)

10. Forcing Mechanism and Season of Maximum Temperature Response Summer – Shortwave Aerosol Direct & Indirect Effects (ΔT < 0) Haze layer: Direct forcing Aerosol Modified Cloud: Indirect forcing

11. Enhanced Cloud Longwave Emissivity ΔT > 0 SUMMER O 3 CH 4 WINTER Black carbon aerosol / Snow albedo ΔT > 0 SPRING Aerosol Direct & Indirect Effects ΔT < 0 GHG Warming ΔT > 0

12. How does each forcing agent impact Arctic Climate?  sign and magnitude of forcing at the surface  surface temperature response  feedbacks triggered (function of seasonality of the forcing and response) What sources should be targeted to lessen impact in the Arctic?  most bang for the buck – which species have the largest impact?  local or extra-polar sources? Successful Mitigation Strategy Requires Knowing:

13. Starting Point (i.e., quick and dirty approach where you use what you already have) After the Jan 2007 GISS meeting, output from global climate models was used to obtain: Seasonally averaged values of instantaneous forcing and surface temperature response Averaged over 60° to 90°N Calculated forcing was based on present-day fossil fuel + bio-fuel + biomass burning emissions relative to present-day biomass burning emissions. The Arctic response was forced globally with changing composition. Exception was the cloud LW emissivity. F s was based on measurements of the sensitivity of low-level cloud emissivity to pollution at Barrow. Not a seasonal average as it only includes times when pollution aerosol and clouds were coincident.

14. Seasonally averaged values of F s and ΔT s for 60° to 90°N FsFs ΔTsΔTs Quinn et al., ACPD, submitted, 2007.

15. Seasonally averaged values of F s and ΔT s for 60° to 90°N ΔT s > 0 FsFs ΔTsΔTs

16. Seasonally averaged values of F s and ΔT s for 60° to 90°N ΔT s > 0 O 3 and CH 4 ΔT s maximum is in the winter when forcing is at a minimum BC / Snow albedo F s and ΔT s are both at a maximum in the spring ? Not a seasonal average FsFs ΔTsΔTs

17. Seasonally averaged values of F s and ΔT s for 60° to 90°N ΔT s > 0 O 3 and CH 4 ΔT s maximum in winter when forcing is at a minimum BC / Snow albedo F s at a maximum in spring along with is ΔT s ? BC – Added atmospheric heating will ultimately increase LW radiation and warm the surface Indirect Effect - Largest F s is in the spring but this is when ΔT s is at a minimum Not a seasonal average

18. FsFs ΔTsΔTs Comparison of Short-Lived Pollutants and Well-Mixed Greenhouse Gases Includes cloud LW emissivity so is an overestimate of the seasonal average Need an apples to apples comparison!

19. Framework for Discussion of Climate Impacts of Short-lived Pollutants What is the best way to calculate forcing and the resulting temperature response in order to assess impacts on Arctic melting and to find “smoking gun(s)”? Seasonal averages do not capture the response due to episodic intense events. What is the impact of a short, intense forcing vs. a continuous moderate forcing on a non-linear system? Transient simulations do not allow for equilibration with atmospheric temperatures. How do we implement an apples-to-apples comparison in order to design an effective mitigation strategy? What are the largest uncertainties in current models and measurements? How can we reduce those uncertainties?

20. State of Understanding forcing & response calculations uncertainties required measurements local vs. remote forcing potential feedbacks Recommendations for Policy Makers What is the climate impact of each pollutant? What are the sources of that pollutant? What are mitigation strategies for each pollutant? Day 1 Day 3 Day 2 Emerging Issues boreal forest fires shipping activity source of BC deposited to snow pack BC and O 3 mitigation strategies effects of a warming Arctic