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SEARCH: October 2003
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
Climate Changes....
Global temperature anomaly
1880-2002
2002 SAT anomaly: 0.57°C
Climate forcings 1850-2000
Climate forcings: The past 50 years
GHG
Solar
Aerosols
Volcanic
Simulated vs Obs. Climate Change
MSU4
MSU2
SAT
SICE
Hansen et al (2003)
Black Carbon impact on albedo?
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
Hansen and Nazerenko (submitted)
Black Carbon impact on albedo?
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...
Hansen and Nazerenko (submitted)
Impact of BC albedo change
Radiative forcing
2
0.16
W/m
(different scenarios for anthropogenic effects)
Temperature change
"Efficiency" of forcing is 2x that of CO2 due to
enhanced ice-albedo feedbacks
0.24°C
Dynamic forcing of Arctic Climate?
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...
Winter (JFM)
AO Index 1950-2003
Arctic Oscillation pattern
EOF Analysis of SLP
patterns
Largely barotropic
structure, becoming
more zonally
symmetric in the
stratosphere
Similar to NAO in
Atlantic sector
(Thompson and Wallace, 1998)
+ve AO index
Todd Mitchell, U. Washington
+ve AO index
Todd Mitchell, U. Washington
Is there a forced component?
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))
Multi-model AO changes...
Paeth et al, (in press)
Many models show eventual rise in AO under GHG forcing
Response is model specific, magnitude not well constrained
Dependent on stratospheric representation and...?
Conclusions
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?
"Winter warming" response to
volcanic forcing
Pinatubo (winter)
(Shindell et al, submitted)
Summary of effects: