Transcript A Slippery Slope - Georgia Institute of Technology

A Slippery Slope
How much Global
Warming Constitutes
“Dangerous
Anthropogenic
Interference”?
Why is this issue
important?
Climate Impact of Increasing CO2
• Increases the surface air temperature
• Decrease sea ice which leads to an increase in
sea level
• Possible melting of glaciers halts deep ocean
circulation in North Atlantic (similar to Younger
Dryas Event)
• Decrease of soil moisture leads to drought
• Impacts on El Niño and Hurricanes
Submerged regions from 100m
increase in seal level
•Model Simulates Large Scale
Atmospheric Phenomena
•Increase in temperature changes
the storm track and predominate
wind flow
•Leads to desertification of the
eastern half of North America
•Adversely affects agriculture and
growing seasons
Impacts on El Niño and
Hurricanes?
• Models cannot
resolve impacts on El
Niño and hurricanes
…yet
• Increasing SSTs
could influence
strength of storms
• Changing large scale
patterns could
influence storm paths
Hansen’s arguments
The Slippery Slope Argument
• Based on the contention
that ice sheet
disintegration is a wet,
potentially rapid process
• Thus, sea level rise from
melting glaciers requires
that only low limits on
global warming can be
tolerated without risking
dangerous anthropogenic
interference with climate
Problems with Ice Sheet
Modeling?
• IPCC estimates between 9-88 cm sea
level rise over next 110 years
• Based mainly on thermal expansion of
ocean water, secondarily on melting
glaciers
• Simulations show both Greenland and
Antarctic ice sheets growing at a rate
equivalent to sea level drop of 12 cm per
century
Problems with Ice Sheet
Modeling?
• 2002 study by Zwally,et al. shows that ice
sheet flow accelerates as melt-water is
delivered via moulins to the ice sheet base
• 2004 study by Parizek & Alley found that
the Greenland ice sheet is likely to make a
larger contribution to sea-level rise that
previously believed
– Doubling CO2 results in 0.6-6.6 cm rise in sea
level
Problems with Ice Sheet
Modeling?
• Hansen argues that
even these
calculations are too
conservative
• Models do not
currently incorporate
realistic and important
processes that will
accelerate ice sheet
disintegration.
Important Factors
• Global energy imbalance
• +1 W/m2 more energy absorbed from sun than emitted out to
space
• Due mainly to rapid growth of GHG’s (anthropogenic)
• Based on sea level rise, only 5-10% of energy imbalance
went into melting of ice during 20th century
– Hansen suggests that this percentage will increase with time, as
atmosphere becomes more moist and transports energy more
efficiently to the ice.
– Accelerating ice streams increases transport of ice to the ocean,
cooling the ocean, and maintaining global energy imbalance.
– Increased melt-water contributes to sea level rise, but more
importantly breaks up ice sheets, accelerating movemnt of ice
towards the ocean
Positive Feedbacks
• Higher T’s in low/mid latitudes will increase
rainfall intensity.
– Transferring more energy to ice sheets via increased
atmospheric latent heat transport
• Sea sfc cooling increases planetary energy
imbalance, thus increasing flux of heat into the
system
• Air pollution (soot) accelerates ice melting by
causing snow to “age” into larger, wetter
particles.
Disaster?
• Net effect of
prolonged exposure
to these forcings is
out of control cycle
resulting in demise of
entire south dome of
Greenland ice sheet?
• How long is “long
enough”?
Temporal Factors: The Slippery
Slope
• 3 critical time constants:
– T1 = time required for
climate (specifically ocean
sfc temp) to respond to a
forced change in global
energy imbalance
– T2 = time required for
society to change its
energy systems enough to
reverse GHG growth
– T3 = time required for ice
sheets to respond to a
large, positive planetary
energy imbalance
• Hansen estimates:
– T1 ~ 50-100 yrs
– T2 ~50-100 yrs
– T3 = unknown,
controversial
• IPCC believes millennia
• Hansen thinks centuries
Key Issues
• Ice Sheet growth is dry
process, take millennia
– Limited by snowfall
• Disintegration is wet
process.
– Can occur more rapidly
• If T3 ~ T1+T2, once ice
sheet change passes
critical point, will be
impossible to prevent
serious disintegration
Problem
• What levels of anthropogenic climate
forcings will alter the climate to the
precipice of disaster?
• “Dangerous” anthropogenic interfence
could occur more quickly than IPCC
models project
IPCC’s Stance
Sea Level Changes
• “Global mean sea level has risen 10-25
cm over the last 100 years.”
• “There has been no detectable
acceleration of sea level rise during this
century. However, the average rise during
the present century is significantly higher
than the rate averages over the last
several thousand years.”
Effects of Global Warming
• “Global warming should, on average,
cause the oceans to warm and expand,
thus increasing sea level.”
• “Global warming should, on average,
increase the melt rates of glaciers and ice
caps, causing sea level to rise.
Observational data indicates that, globally,
there has been a general retreat of
glaciers during this century.”
Greenland Ice Sheet
“With respect to the
Greenland ice sheet, a
warmer climate should
increase the melt rates at
the margins. The
increase in melting
should dominate over any
increase in accumulation
rates in the interior,
causing sea level to rise.”
Antarctic Ice Sheet
• “With respect to the
Antarctic ice sheet, a
warming climate should
increase accumulation
rates, causing sea level
to fall.”
• (Even if climate warms,
Antarctica will still be
below freezing, and the
warmer air would hold
more water vapor, which
would increase snowfall
and offset any melting.)
Model Predictions
Outlook
• Scientists unsure if
polar ice sheets are
growing or shrinking.
• Projections of sea
level rise have a high
degree of uncertainty
due to lack of specific
knowledge:
Source of the
sea level rise
Change
Thermal
expansion of
oceans
+ 15 cm
Melting of
glaciers, ice caps
+ 12 cm
Greenland ice
sheet melting
+ 7 cm
Antarctic ice
sheet
- 7 cm
Best total
estimate
+ 27 cm
What We Think
• Hansen raises good points about the
shortcomings of the models – they should
take more feedbacks into account
• His theories seem tenuous – similar
uncertainties in data and effects that
plague current IPCC
• We do need to be cautious about CO2
emissions
Sources
• Geophysical Fluid Dynamics Laboratory:
http://www.gfdl.gov/~tk/climate_dynamics/
• 100m figures from: An End to Global
Warming by Laurence O. Williams
• Hansen, J. 2004. A Slippery Slope: How
much global warming constitutes
“dangerous anthropogenic interference”?,
Clim. Change.