Abstract

The earth's temperature, with rapid global warming over the past 30 years, is now passing through a period of relatively stable climate that has existed for more than 10,000 years. Further warming of more than 1°C will make the earth warmer than it has been in a million years. "Business-as-usual" scenarios, with fossil fuel carbon dioxide emissions continuing to increase approximately 2 percent annually for several more decades, yield additional warming of 2° to 3°C this century and imply changes that constitute practically a different planet. Multiple lines of evidence indicate that the earth's climate is nearing, but has not passed, a tipping point, beyond which it will be impossible to avoid climate change with far-ranging undesirable consequences. The changes include not only loss of the Arctic as we know it, with all that implies for wildlife and indigenous peoples, but losses on a much 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 West Antarctic ice is softened and lubricated by melt-water and as buttressing ice shelves disappear because of a warming ocean, the balance will tip to rapid ice loss, bringing multiple positive feedbacks into play and causing cataclysmic ice sheet disintegration. The earth's history suggests that with warming 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 meters.