A%20Slippery%20Slope

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A Slippery Slope

• How much Global Warming Constitutes Dangerous
  Anthropogenic Interference?

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Why is this issue important?
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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

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Submerged regions from 100m increase in seal
level
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• 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

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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

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Hansens arguments
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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

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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

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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

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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.

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Important Factors

• Global energy imbalance
• 1 W/m2 more energy absorbed from sun than
  emitted out to space
• Due mainly to rapid growth of GHGs
  (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

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Positive Feedbacks

• Higher Ts 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.

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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?

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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

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Key Issues

• Ice Sheet growth is dry process, take millennia
• Limited by snowfall
• Disintegration is wet process.
• Can occur more rapidly
• If T3 T1T2, once ice sheet change passes
  critical point, will be impossible to prevent
  serious disintegration

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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

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IPCCs Stance
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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.

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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.

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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.

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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.)

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Model Predictions
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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
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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

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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.