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James E. Hansen Can We Still Avoid Dangerous Human-Made Climate Change? 1. SUMMARY THE EARTH’S TEMPERATURE, W ITH RAPID GLOBAL W ARM ING OVER THE past 30 years, is now passing through the peak level of the Holocene, a period of relatively stable climate that has existed for more than 10,000 years. Further warm ing of m ore than 1°C will make the earth warm er than it has been in a million years. “Business-as-usual” scenarios, with fossil fuel CC>2 em issions continuing to increase approxim ately 2 percent annually for several m ore decades, yield additional warm ing of 2° to 3°C this centuiy and imply changes that constitute practically a different planet. Multiple lines of evidence indicate that the earth’s climate is near­ ing, but has not passed, a tipping point, beyond which it will be impossi­ ble to avoid climate change w ith far ranging undesirable consequences. The changes include not only loss of the Arctic as we know it, w ith all that implies for wildlife and indigenous peoples, but losses on a m uch vaster scale because of worldwide rising seas. Sea level will increase slowly at first, as losses at the fringes of Greenland and Antarctica due to accelerating ice streams are partly balanced by increased snowfall and ice sheet thickening in the ice sheet interiors. But as Greenland and W est Antarctic ice is softened and lubricated by melt-water and as buttressing ice shelves disappear because of a warm ing ocean, the balance will tip to rapid ice loss, bringing m ultiple positive feedbacks social research Vol 73 : No 2 : Summer 2006 949 into play and causing cataclysmic ice sheet disintegration. The earth’s history suggests that w ith warm ing of 2° to 3°C, the new equilibrium sea level will include not only most of the ice from Greenland and West Antarctica, but a portion of East Antarctica, raising sea level of the order of 25 m eters (80 feet). Contrary to lethargic ice sheet models, real w orld data suggest substantial ice sheet and sea level change in centuries, not m illen­ nia. The century tim e scale offers little consolation to coastal dwell­ ers, because they will be faced w ith irregular incursions associated w ith storm s and w ith repeatedly rebuilding above a transient w ater level. This grim “business-as usual” clim ate change can be avoided through an “alternative” scenario in w hich grow th of greenhouse gas emissions is slowed in the first quarter of this century, prim arily w ith concerted im provements in energy efficiency, thus reducing the grow th rate of atm ospheric C 02, and a parallel reduction of humanm ade clim ate forcings th at drive global warm ing, especially the air pollutants m ethane, carbon monoxide, and black soot. Before m id­ century, advanced energy technologies will be needed to reduce C02 emissions faster. The required actions m ake practical sense and have other benefits, but they will not happen w ithout strong policy leader­ ship and international cooperation. Action m ust be prompt, otherwise C02-producing infrastructure that may be built w ithin a decade will make it impractical to keep further global warm ing under 1°C. 2. GLOBAL WARMING I m ust begin w ith two caveats. First, the views I present are my personal scientific opinion. They are based on my 39 years of experience at NASA, but I am not representing the agency or the government. Second, I do not attem pt to define policy, which is up to the people and their elected representatives, and I do not criticize policies. The climate science has policy relevance, but I let the facts speak for themselves about conse­ quences for policymakers. 950 social research I will show that the answ er to the question “Can we still avoid dangerous human-made climate change?” is yes, we can, but we are not on a path to do that. And if we do not begin actions to get on a different path w ithin the next several years we will pass a tipping point, beyond w...


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