SAN FRANCISCO—A mantra that has driven global negotiations on carbon dioxide emissions for years has been that policy-makers must prevent warming of more than two degrees Celsius to prevent apocalyptic climate outcomes. And, two degrees has been a point of no return, a limit directly or indirectly agreed to by negotiators at international climate talks.
New, extensive study of the paleoclimate record going back 50 million years by Hansen and others now shows that the two-degree target for global temperature rise "is a prescription for disaster," Hansen said here at a news conference during the American Geophysical Union meeting.
Hansen came to that conclusion after reviewing average and extreme perturbations in the paleoclimate record that have been more thoroughly documented in the past few years. The record shows that 50 million years ago, Earth was free of ice, and sea level was 70 meters higher on average than it is today. Both phenomena resulted from natural variations in mean temperatures due to slight changes in the sun’s output and Earth’s orbit over geological time scales. Rising temperatures today, over far shorter time scales in which neither the sun nor the orbit are factors, are caused primarily by higher levels of CO2 and other greenhouse gases in the atmosphere.
Atmospheric carbon dioxide levels before the industrial revolution were about 280 parts per million on average. They have been rising ever since, and today are about 397 ppm. A level of 450 ppm has generally been associated with an average global temperature rise of two degrees C. However, the latest analysis shows that a level of 450 ppm is enough to melt a significant portion of the world’s ice, because feedback mechanisms kick in; melting ice hastens the melting of even more ice, for example, and thawing permafrost emits methane that accelerates warming, prompting permafrost to thaw even more.
If the number reaches 560 ppm, a doubling of preindustrial values, sea level globally could rise 25 meters, according to Eelco Rohling, professor of ocean and climate change at University of Southampton in the U.K., who presented data at the AGU meeting with Hansen. Many large cities worldwide lie at that elevation or lower. The two scientists agreed that if nations continue to emit CO2 at current rates, the world could reach 560 ppm by 2100.
The paleoclimate record also shows that 560 ppm would be enough to melt all the ice in the Arctic, and later the Antarctic. Rohling said that once the Antarctic melts, sea levels would rise by 60 to 70 meters. "If governments keep going the way they are going," Hansen added, "the planet will reach an ice-free state."
Hansen concluded with a message to negotiators at the current climate talks in Durban, South Africa. If the world begins reducing CO2 emissions by 6 percent a year starting in 2012, Hansen said, atmospheric levels can return to the "safe" level of 350 ppm that he and others have long called for. "If the world waits until 2020 to begin," he noted, "it will need to reduce CO2 by 15 percent a year to reach 350 ppm. We are out of time."
Photo of James Hansen courtesy World Development Movement