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Thursday, April 21, 2011

Hansen et al. concludes “the rate of sea level rise is likely to accelerate during the next several years.” Humanity's "Faustian aerosol bargain"

Humanity's "Faustian aerosol bargain"

by Joseph Romm, Climate Progress, April 20, 2011

James Hansen has posted a new 52-page draft paper entitled “Earth’s Energy Imbalance and Implications.”

While this paper is a tougher read than most, the good news is that Hansen gave an hour-long talk on this topic last week, which is posted to YouTube:

The video makes it clearer than the article that Hansen is kidding about his grandchildren, Sophie and Connor — though not about the implications of his analysis about the “net forcing”:
Global warming has been limited, as aerosol cooling partially offsets GHG warming. But aerosols remain airborne only several days, so they must be pumped into the air faster and faster to keep pace with increasing long-lived GHGs. However, concern about health effects of particulate air pollution is likely to lead to eventual reduction of human-made aerosols. Thereupon the Faustian payment will come due.
If Sophie’s +2 W/m2 is close to the truth (aerosol forcing -1 W/m2), even a major effort to clean up aerosols, say reduction by half, increases the net human-made forcing only 25%. But Connor’s aerosol forcing (-2 W/m2) means that reduction of aerosols by half would double the net climate forcing. Given global climate effects already being observed (IPCC, 2007), a doubling of the climate forcing suggests that humanity may face a grievous Faustian payment.
Most climate models in IPCC studies use aerosol forcing about -1 W/m2. We will argue that this understates the true aerosol effect.
Later he notes, “If the negative aerosol forcing is understated by as much as 0.7 W/m2, it means that aerosols have been counteracting half or more of the GHG forcing. In that event, humanity has made itself a Faustian bargain more dangerous than commonly supposed.”  

Hansen has said this is a “Faustian aerosol bargain” because it can be maintained only through “continued exponential growth of the atmospheric aerosol load.”

Here is what the paper says on the “Implications for sea level”:
Based on our inferred planetary energy imbalance, we conclude that the rate of sea level rise is likely to accelerate during the next several years.    Reasons for that conclusion are as follows. 
First, the contribution of thermal expansion to sea level is likely to increase above recent rates. The nearly constant rate of sea level rise since 1993 masks the fact that thermal expansion must have been less in the Argo era than in the prior decade, when ice melt was less but sea level rose 3 mm/year. Solar minimum and a diminishing Pinatubo rebound effect both contributed to a declining rate of thermal expansion during the past several years. But the Pinatubo effect is now essentially spent and solar irradiance change should now work in the opposite sense. 
Second, the rate of ice melt is likely to continue to accelerate. Planetary energy imbalance now is positive, substantial, and likely to increase as greenhouse gases and solar irradiance increase. Thus, despite year-to-year fluctuations, global temperature will increase this decade and there will be a substantial flux of energy into the ocean. Increasing ocean heat content provides energy for melting sea ice and ice shelves. Sea ice protects the ice sheets from heating and ice shelves mechanically buttress the ice sheets. It has been argued that loss of these protections may be the most important factor causing more rapid discharge from ice sheets to the ocean (Hansen, 2005, 2007).
Indeed, the rate of ice melt is likely to continue to accelerate — see JPL bombshell: Polar ice sheet mass loss is speeding up, on pace for 1 foot sea level rise by 2050.

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