[asa] tipping point?

On Monday, Jim Hansen spoke at the National Press Club. The charts he used for the presentation are at: (beware--5MB file so don't try it with dialup) http://www.columbia.edu/~jeh1/dots_feb2007.ppt
 
In his own words, two of his main points are:
 
"A. Climate Sensitivity: What can be called the "Charney" or "fast feedback" climate sensitivity is now well-established: it is approximately 3C for doubled CO2 (forcing of 4 W/m2). However, some of the "boundary forcings" in this "Charney" definition are themselves variables or slow feedbacks. Indeed, the boundary forcings (surface albedo and long-lived GHGs) that are the principal mechanisms for climate change on the 100 ky time scales are in fact feedbacks on those time scales. The problem for humanity is that some of these "slow" feedbacks are not so slow - they may experience significant change this century and increase the climate response beyond that which is calculated by the typical model used in IPCC simulations. [Vegetation feedback - forests and shrubs are moving poleward, into regions that were tundra. Ice sheets are getting darker (wet) and before the century is out they could get smaller. Long-lived GHGs may provide a positive feedback, as in paleo experience, e.g., from methane hydrates.] So the effective climate sensitivity on the century and longer time scales is greater than the Charney sensitivity. We will begin to notice these additional feedbacks now, as we have entered the period of significant almost-monotonic global warming with isotherms moving poleward.
 
B. Dangerous Level. We do not know what long-term level of CO2 constitutes "dangerous human-made interference" (the level can be raised somewhat if we reduce other GHGs such as CH4), but it has become clear that it is not greater than about 450 ppm, and may be considerably lower. Given the fact that at least ~ one-quarter of fossil fuel CO2 emissions remains in the air "forever" (more than 500 years), and given the magnitude of the oil, gas, coal and unconventional fossil fuel reservoirs, it is follows that readily available oil and gas are going to take atmospheric CO2 to at least ~450 ppm. Thus even with responsible efforts to slow emissions, it is likely that atmospheric CO2 amounts will exceed the dangerous level, and we will need to find ways counteract the warming effect of these GHGs."

Randy
 

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Received on Thu Mar 1 21:16:23 2007