The 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."

That fingerprint is now clearly visible on Earth's climate, according to physicist Michael Prather of the University of California, Irvine, who chaired the AGU committee that wrote the statement. Humanity's touch has tipped the scales in favor of more of the sun's heat being trapped by the sky and sea. "Earth's energy system is out [of] balance, excess heat is being pumped into the ocean," Prather says. "That means we're on the move to someplace different."

"Someplace different" will be a lot warmer; 11 of the past 12 years were warmer than any since 1850, and 2007 tied 1998 as the second-warmest since instrument-based records began, according to NASA. Global average sea level has risen by roughly 0.11 inch (3 millimeters) per year since 1993 due to a combination of water expanding as it warms and melting ice sheets. "The scales of change we are seeing is something that modern society has never experienced," Prather notes.

But cutting global (GHG) emissions by 50 percent by 2050 is a major challenge that would require curbs on the smokestacks of power generators and the tailpipes of vehicles as well as a halt to deforestation, among other efforts. Although California, the European Union (E.U.) and others have pledged to meet that goal, global emissions continue to climb rapidly—as do concentrations of GHGs in the atmosphere. "We're on a trajectory upwards for the next five or 10 years no matter what we do. We need to turn that over and bring it down," Prather says. "What we're really looking for is much more substantial cuts by the end of the century."

Regional emission efforts—such as the E.U.'s new plan to reduce emissions by 20 percent by 2030—are already underway. Most rely on so-called cap-and-trade mechanisms: an overall cap on emissions is set, allowances to meet that cap are distributed to polluters, and those who emit more or less than their quota can buy or sell these allowances. Killeen notes that those programs will rely on unprecedented precision in measuring and monitoring greenhouse gas emissions, a role AGU scientists might fill.

But the AGU believes that a broader solution is needed, which is why the statement calls on members to become more involved not only in researching the problem but also spreading the word about the urgency of controlling climate change, something many scientists have been loathe to do in the past, Killeen admits. "Each scientist can do it in their own way," says NCAR senior scientist Bette Otto-Bliesner, also a member of the AGU committee that wrote the statement. "From one-on-one conversations, to giving talks to schools or communities, to going up to Congress."