1. Tom Breider, Loretta Mickley, Daniel Jacob, Cui Ge, Jun Wang, Melissa Payer, Betty Croft, David Ridley, Sangeeta Sharma, Kostas Eleftheriadis, Joe McConnell, Henrik Skov, Lee Husain Arctic Climate Response to Decadal Changes in Radiative Forcing from Aerosols Source : NASA

2. Arctic – “A region in transition” 60-90N Source: Shindell et al., 2009 1880 1900 1920 1940 1960 1980 2000 0.50 0.00 1.00 1.50 2.50 2.00 SAT anomaly (°C) Arctic surface air temperatures are increasing (2x global average) What processes are driving the observed trends? 1980 1990 2000 2010 Mass Conc, ug S m -3 0.6 0.9 0.3 0.0 At high Arctic sites sulfate and BC concentrations decreased by 2-3% per year between 1980 and 2010 Spring sulfate mass Ny-Alesund

3. 15 SO 2 BC 0.6 0 0 Tg S yr -1 Tg C yr -1 NORTH AMERICA 15 SO 2 BC 0.6 0 0 1980 2010 Tg S yr -1 Tg C yr -1 EUROPE 25 SO 2 BC 0.8 0 0 1980 2010 Tg S yr -1 Tg C yr -1 EAST ASIA 20 SO 2 BC 3.0 0 0 1980 2010 Tg S yr -1 Tg C yr -1 RUSSIA 15 SO 2 BC 0.8 0 0 1980 2010 Emissions Source: MACCcity Inventory (Granier et al., 2011) 1980–2010: Dynamic Anthropogenic Emission Trends Emissions have decreased in N.America & Europe and increased in East Asia Image Source : NASA Earth Observatory

4. GEOS-Chem v9.01.03 driven by MERRA meteorology at 4x5 degrees for the period 1980-2010 1980 --> 2000 ACCMIP (Lamarque et al., 2010) 2005 -> 2010 RCPs / regional inventories Anthropogenic Emissions = MACCcity (Granier et al., 2011) 1980 --> 1996 Duncan et al., 2002 1997 -> 2010 GFED3 (van der Werf et al., 2010) + GFAS corrections (Kaiser et al., 2012) Update precipitable water in low-level precipitating Arctic clouds in summer to 1x10-7 g m -3 Scale annual SO x ems to match Smith et al., 2011 and BC ems to match Cohan and Wang, 2014 Retain spatial and seasonal emissions Updates

5. Observed trends in Arctic surface sulfate and BC are reproduced to within 30% in all seasons Spring sulfate Ny-Alesund Steep drop after 1988 = collapse of the former Soviet Union Arctic AERONET AOD observations are underestimated by 20% 9% Spring; 28% Summer; 34% Fall Season R 2 Spring0.37 Summer0.29 Fall 0.0 Annual0.38 1980 1990 2000 2010 0.6 0.9 0.3 0.0 Mass Conc, ug S m -3

6. 0.14 0.10 0.08 0.06 0.04 0.12 0.02 0.00 -90 -60-30 0 Latitude 3060 90 GEOS-Chem Arctic sulfate AOD in 2006 is towards the higher end of AEROCOM phase II simulations Source: Myhre et al., 2013 Sulfate 550nm AOD AEROCOM models Optical Depth, dimensionless This simulation

7. *Forcing does not include BC on snow Net all-sky surface aerosol RF, W m -2 DJFMAMSONJJA Early 1980s = 1980-1982 Mid 2000s = 2004-2007 Arctic surface aerosol radiative forcing decreased between the early 1980s and mid 2000s Early 1980s Mid 2000s Net Forcing

8. Net Arctic surface aerosol radiative forcing is largest in spring and summer and is mainly sulfate (75%) *Net forcing does not include BC on snow Net all-sky surface aerosol RF, W m -2 Sulfate OC BC DJF MAM SONJJA Net Arctic aerosol forcing of +0.1 W m -2 between early 1980s and mid 2000s

9. Summary Surface trends in spring Arctic sulfate and BC are captured to within 30%. We find a smaller total surface aerosol forcing compared to Quinn et al., 2008 in spring (-0.48 W m -2 vs -0.72 W m -2 ) and summer (-0.26 W m -2 vs -0.93 W m -2 ) Net Arctic surface aerosol RF between the early 1980s and mid 2000s is +0.1 W m -2, largest in summer and spring, and is mainly sulfate (75%) Net Arctic surface forcing driven by emissions decreases in Russia (+0.04 W m -2 ), FSU+EBLOC (+0.04 W m -2 ), Europe (+0.03 W m -2 ), North America (+0.01 W m -2 )

10. Net all-sky surface RF, W m -2 Russia Europe FSU+EBLOC North America Net surface forcing mainly from Russia, FSU and Europe

11. Net all-sky surface RF, W m -2 The sum of the regional net Arctic surface forcing is larger than the net surface forcing Net Forcing Russia Europe FSU+EBLOC North America

12. 15 SO 2 BC 0.6 0 0 Tg S yr -1 Tg C yr -1 NORTH AMERICA 15 SO 2 BC 0.6 0 0 1980 2010 Tg S yr -1 Tg C yr -1 EUROPE 25 SO 2 BC 0.8 0 0 1980 2010 Tg S yr -1 Tg C yr -1 EAST ASIA 20 SO 2 BC 3.0 0 0 1980 2010 Tg S yr -1 Tg C yr -1 RUSSIA 15 SO 2 BC 0.8 0 0 1980 2010 Emissions Source: MACCcity Inventory (Granier et al., 2011) 1980–2010: Dynamic Anthropogenic Emission Trends Image Source : NASA Earth Observatory

13. Net forcing from emissions increases in China and other regions reduces the net forcing from emissions decrease in spring by 36% Net all-sky surface RF, W m -2 Russia FSU+EBLOC Europe North America China Other

14. Summary Surface trends in spring Arctic sulfate and BC are captured to within 30%. We find a smaller total surface aerosol forcing compared to Quinn et al., 2008 in spring (-0.48 W m -2 vs -0.72 W m -2 ) and summer (-0.26 W m -2 vs -0.93 W m -2 ) Net Arctic surface aerosol RF between the early 1980s and mid 2000s is +0.1 W m -2, largest in summer and spring, and is mainly sulfate (75%) Net Arctic surface forcing driven by emissions decreases in Russia (+0.04 W m -2 ), FSU+EBLOC (+0.04 W m -2 ), Europe (+0.03 W m -2 ), North America (+0.01 W m -2 )