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.