Arctic climate: Unique vulnerability and complex response to aerosols

Transcription

1 Arctic climate: Unique vulnerability and complex response to aerosols Mark Flanner November 2, 2011 Santa Fe Conference on Global and Regional Climate Change 1 / 18

2 Arctic: Unique vulnerability to positive RF Pervasiveness of reflective surfaces increases the likelihood that atmospheric aerosols exert a positive radiative forcing (e.g., Cess) Deposition of absorbing species to snow and ice increases the radiative lifetime of these species Arctic is a net sink for black carbon (deposition > emission) Prevalence of thin clouds facilitates a significant longwave aerosol indirect effect in the Arctic (Lubin and Vogelmann, 2006; Garrett and Zhao, 2006), offsetting indirect cooling effects 2 / 18

3 Complexities and Challenges Unique cloud properties (mixed-phase, thin) Huge seasonality in shortwave forcing and spectrum of dominant forcing Stably stratified atmosphere during much of the year Reduced coupling between atmospheric energy forcings and surface temperature Increased coupling between surface energy forcings and surface temperature A large component of the Arctic energy budget is meridional energy transport ( 100 W m 2 ). Small perturbations to this term can overwhelm local forcings. To what extent do local forcings matter? 3 / 18

4 High reflectance, positive aerosol RF, negative feedback Smoke over the Canadian Arctic (NASA) 4 / 18

5 Atmospheric aerosol forcing over pure snow Top-of-atmosphere: Mixtures with SSA< (λ = 500 nm) produce warming Small cooling from sulfate Warming from organic matter Flanner et al (2009), ACP 5 / 18

6 Atmospheric aerosol forcing over pure snow Top-of-atmosphere: Surface: Mixtures with SSA< (λ = 500 nm) produce warming Small cooling from sulfate Warming from organic matter Large dimming from absorbing aerosols, but only a slight cooling effect because of snow s high reflectance Flanner et al (2009), ACP 6 / 18

7 Snow darkening from black carbon Part-per-billion levels of BC significantly reduce snow albedo because: Black carbon visible absorptivity is 10 5 greater than ice Snow scatters visible radiation efficiently via refraction A reflected blue photon typically undergoes 1000 scattering events before emerging from the top of snowpack BC persists longer in near-surface snow than atmosphere 7 / 18

8 Albedo perturbation from impurities 8 / 18

9 Atmospheric aerosol forcing over dirty snow α: Snow/atmosphere column burden ratio. Controlled by: Deposition efficiency Meltwater removal Mean estimate is 0.07 Snow darkening: Increases TOA forcing Reverses the sign of surface forcing (darkening > dimming) Over global snow: darkening 6 > dimming 9 / 18

10 Spatial/temporal characteristics of BC/snow forcing Latitude ( N) Latitude ( N) 1998 Central BC Snow Forcing (W m 2 ) J F M A M J J A S O N D 2001 Central BC Month Snow Forcing (W m 2 ) Strong Fire Year Weak Fire Year 0 J F M A M J J A S O N D Month Forcing operates mostly in local spring, when and where there is large snow cover exposed to intense insolation, coincident with peak snowmelt Global forcing dominated by FF+BF sources of BC, but strong biomass burning events can dominate Arctic forcing Global-mean forcing (including snow and sea-ice): W m 2 (Flanner et al, Koch et al, Rypdal et al, Hansen et al, Jacobson), with adjustments from Arctic measurements (Doherty et al) 10 / 18

11 Sources of Arctic BC+OC radiative forcing AMAP Report (2011). Left: Atmosphere, Right: Snow/Ice ROW sources dominate Arctic atmosphere burden Results sensitive to simulation of polar dome and deposition 11 / 18

12 Normalized Arctic BC+OC radiative forcing AMAP Report (2011) Unit emissions from Nordic countries exert largest Arctic forcing 12 / 18

13 Arctic climate response to Arctic BC Arctic is vulnerable to positive local forcing, but does it matter? Complex response 1: Arctic atmospheric aerosol forcing Figure: Shindell and Faluvegi (2009) Arctic cooling in response to Arctic atmospheric BC, caused by reduced meridional energy transport 13 / 18

14 BC snow forcing: High efficacy Complex response 2: Snow darkening Strong high-latitude warming BC/snow forcing efficacy: 3 ± 1 (derived from equilibrium climate experiments) Reasons: 1 All of the forcing energy is deposited directly in the cryosphere 2 Energy is deposited to surface 3 Snow metamorphism feedbacks 14 / 18

15 Spring/summer vulnerability F (r)s(r) dr (1) NH F : surface insolation S: Snow or ice fraction N. Hemisphere Surface Insolation (Watts) x Incident on Land Snowpack Incident on Sea Ice Incident on Snow+Sea Ice J F M A M J J A S O N D Month 15 / 18

16 Causality? Vulnerability Complex Response Sharma et al (2006) Sept. sea-ice extent Argument that BC is driving Arctic warming is challenged by these observations. Considerations: 1 BC forcing operates in combination with all other forcings 2 Arctic amplification in response to uniform forcings is robust. Regional response to non-uniform forcings is uncertain 3 Equilibrium Arctic climate is warmer when global BC emissions influence atmosphere and snow 4 Non-linear efficacy? A unit forcing may drive greater response in a system with large spatial/temporal coverage of ice near T melt 16 / 18

17 Motivation for exploring alternative forcing mechanisms N. Hemisphere TOA Forcing (W m 2 ) Change in Cryosphere Forcing from 1979 to Land Snow 1.8 b Sea Ice 1.6 Snow+Ice J F M A M J J A S O N D J Figure: Flanner et al (2011), Nature Geosci. 30-year change in land CrRF: ( ) W m 2 30-year change in sea-ice CrRF: ( ) W m boreal cryosphere albedo feedback during ( F cryo / T s ): 0.62 ( ) W m 2 K 1 CMIP N. Hemisphere albedo feedback (18 models): 0.25 ± 0.17 W m 2 K 1 17 / 18

18 Conclusions Vulnerability Complex Response Arctic is uniquely prone to positive radiative forcing by aerosols Virtually any aerosol mixture exerts positive TOA forcing over snow Amplified climate response within the Arctic is a robust feature of both GCM simulations and paleo-sst data Forcing mechanisms which deposit energy directly within the cryosphere have high efficacy Observed cryosphere albedo feedback over the last 30 years is stronger than that simulated by CMIP3 models, suggesting a dynamical or radiative amplifier on cryospheric evolution that is not currently represented Recent trends in Arctic BC and climate change do not support a dominant role for Arctic BC in driving Arctic climate change, but the presence of BC may amplify Arctic response The partitioning of regional change to local and global forcings remains uncertain 18 / 18