Can we still avoid dangerous human-made climate change?

Abstract

THE EARTH'S TEMPERATURE, WITH RAPID GLOBAL WARMING 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 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 CO2 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 wanning 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 (80 feet). Contrary to lethargic ice sheet models, real world data suggest substantial ice sheet and sea level change in centuries, not millennia. The century time scale offers little consolation to coastal dwellers, because they will be faced with irregular incursions associated with storms and with repeatedly rebuilding above a transient water level. This grim "business-as usual" climate change can be avoided through an "alternative" scenario in which growth of greenhouse gas emissions is slowed in the first quarter of this century, primarily with concerted improvements in energy efficiency, thus reducing the growth rate of atmospheric CO2, and a parallel reduction of human-made climate forcings that drive global warming, especially the air pollutants methane, carbon monoxide, and black soot. Before midcentury, advanced energy technologies will be needed to reduce CO2 emissions faster. The required actions make practical sense and have other benefits, but they will not happen without strong policy leadership and international cooperation. Action must be prompt, otherwise CO2-producing infrastructure that may be built within a decade will make it impractical to keep further global warming under 1°C.