Major science organisations on climate change

he scientists of the American Geophysical Union (AGU) warn that greenhouse gas (GHG) emissions must be slashed in half to keep temperatures from rising 3.6 degrees Fahrenheit (2 degrees Celsius)—or else. "Warming greater than 2 degrees Celsius above 19th-century levels is projected to be disruptive, reducing global agricultural productivity, causing widespread loss of biodiversity and—if sustained over centuries—melting much of the Greenland ice sheet with ensuing rise in sea levels of several meters," the AGU declares in its first statement in four years on "Human Impacts on Climate."

The statement, released today, is the latest—and strongest—statement from the Washington, D.C.–based scientific organization on human-induced climate change. "The record of the Earth's climate since the invention of the thermometer is much better understood now," says physicist Tim Killeen, AGU president and director of the National Center for Atmospheric Research (NCAR) in Boulder, Colo. "This detailed understanding of the climate of the 20th century gives confidence in the ability to project into the future. It is now agreed that we can't explain the detailed temperature record of the 20th century without bringing to bear human effects and GHG emissions. That, in a way, is the smoking gun, the fingerprint."