Natural and Anthropogenic Drivers of Arctic Climate Change Gavin Schmidt NASA GISS and Columbia University Jim Hansen, Drew Shindell, David Rind, Ron Miller and Larissa Nazerenko SEARCH: October 2003

Climate Changes.... Global temperature anomaly SAT anomaly: 0.57°C

Climate forcings

Climate forcings: The past 50 years GHG Volcanic Solar Aerosols

Simulated vs Obs. Climate Change MSU4 SAT MSU2 SICE Hansen et al (2003)

Large changes in amount of black carbon ("Soot") in last century Derived from incomplete combustion (biomass burning, fossil fuel use) Produces significant direct radiative forcing (~0.4 W/m 2 ) Indirect impact on visible albedo of snow Bigger effect in NH (more emissions) Arctic snow: ppbv Alpine snow: ~100 ppbv Greenland: 2-6 ppbv Antarctica: ppbv Black Carbon impact on albedo? Hansen and Nazerenko (submitted)

Theoretically, pure snow ~ 98% albedo in visible (Warren and Wiscombe, 1981) Measurements in Arctic ~ 90-97% Consistent with important effect of BC aerosols: 2-3% decrease in fresh snow albedo (Bohren, 1986) Factor of 2 uncertainty in effects due to mixing assumptions, voids in BC particles, shape etc. Main impact in visible, near IR unaffected. Bigger effect for 'old' snow (up to 9%) Test impact by allowing changes in NH land (5%) and sea ice snow albedo (2.5%), consistent with BC concentrations... Black Carbon impact on albedo? Hansen and Nazerenko (submitted)

Impact of BC albedo change Radiative forcing (different scenarios for anthropogenic effects) Temperature change "Efficiency" of forcing is 2x that of CO 2 due to enhanced ice-albedo feedbacks 0.16 W/m °C

Model results for sea ice show thermodynamic response Good evidence that sea ice thinning is dynamic instead Can we consider atmospheric dynamics as external to Arctic system? Yes, if circulation is determined by non-local factors... Dynamic forcing of Arctic Climate? Winter (JFM) AO Index

(Thompson and Wallace, 1998) Similar to NAO in Atlantic sector EOF Analysis of SLP patterns Largely barotropic structure, becoming more zonally symmetric in the stratosphere Arctic Oscillation pattern

+ve AO index Todd Mitchell, U. Washington

+ve AO index Todd Mitchell, U. Washington

Multiple evidence that AO/NAO can be forced by external factors: Volcanic aerosols "Winter Warming" (Stenchikov et al 2003, Collins 2003, Shindell et al (in press)) Ozone depletion (SH) (Thompson and Solomon, 2003) Long term solar forcing (Shindell et al, 2001) Mechanism involves stratosphere/planetary wave interaction affecting surface winds/pressure Higher AO phase consistent with stronger polar jets Greenhouse gas trends...? (Shindell et al, 1999; Paeth (in press)) Is there a forced component?

Many models show eventual rise in AO under GHG forcing Response is model specific, magnitude not well constrained Dependent on stratospheric representation and...? Multi-model AO changes... Paeth et al, (in press)

Known radiative forcings can explain large part of climate change over last 50 years (GHGs, Solar, Aerosols, Ozone, etc.) Ice-albedo feedbacks imply greater sensitivity in Arctic "polar amplification", particularly for BC albedo effect Dynamic "forcing" by AO/NAO in Arctic is possibly affected by anthropogenic forcings Still uncertainties though… Spatial/Temporal variations in tropospheric aerosols Indirect aerosol effects on cloud formation Modelling of Arctic clouds? Conclusions

(Shindell et al, submitted) "Winter warming" response to volcanic forcing Pinatubo (winter)

Summary of effects: