The world’s largest society of earth scientists, the American Geophysical Union, is warning that human beings have knocked the climate out of balance and that nations will need to sharply cut carbon-dioxide emissions to avoid widespread problems like melting the Greenland ice cap.

In a position statement issued today, the society laid out the evidence that recent changes in climate are attributable to human activity. The organization, which has a membership of 50,000, stated that the world would need to cut emissions of greenhouse gases by more than 50 percent to avoid warming the planet by 2 degrees Celsius, a level at which substantial disruptions would take place, it said.

Much of the statement dovetails with conclusions drawn by the Intergovernmental Panel on Climate Change, which released its multivolume report last year and later shared the Nobel Peace Prize with Al Gore. In fact, the society notes that many of its members helped write the panel’s report.

The new statement by the American Geophysical Union is much stronger and more explicit than the version released four years ago.

One of the most dramatic changes recently has been the decline in Arctic sea ice over the past few decades. The National Snow and Ice Data Center has created an animation of data documenting how the amount of Arctic sea ice has varied each year, reaching a record low in September 2007 (shown in the image above, in which the average September ice extent is marked by the magenta line). —Richard Monastersky