Natural and Anthropogenic Drivers of Arctic Climate Change Gavin Schmidt NASA GISS and Columbia...
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Transcript of Natural and Anthropogenic Drivers of Arctic Climate Change Gavin Schmidt NASA GISS and Columbia...
Natural and Anthropogenic Drivers of Arctic Climate Change
Gavin SchmidtNASA GISS and Columbia University
Jim Hansen, Drew Shindell, David Rind, Ron Miller and Larissa Nazerenko
SEARCH: October 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/m2)
Indirect impact on visible albedo of snow
Bigger effect in NH (more emissions)
Arctic snow: 10-30 ppbv Alpine snow: ~100 ppbv
Greenland: 2-6 ppbv Antarctica: 0.1-0.3 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 CO2 due to
enhanced ice-albedo feedbacks
0.16 W/m2
0.24°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 1950-2003
(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
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