The world's largest society of Earth and space scientists has released a new statement on climate change that unequivocally names human activity as the cause of global warming.

"Earth's climate is now clearly out of balance and is warming," according to the first paragraph of the statement by the American Geophysical Union.

The statement cites many components of the Earth system that are changing at unnatural rates, including rising global temperatures, ice melt, sea level rise and the distribution of precipitation around the globe.

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"The facts are well-established now that the Earth's climate is warming," said Bette Otto-Bliesner of National Center for Atmospheric Research in Colorado at a press conference today in Washington, D.C., held to release the statement.

These climate changes, the statement says, are "best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century."

The second paragraph of the three-paragraph statement warns of problems global warming could cause, including rising sea levels, declining agricultural productivity and loss of biodiversity.

Specifically, the effects of a warming of 2 degrees Celsius (3.6 Fahrenheit) globally are cited because more research has been done on the effects of that change and how to avoid it than for other temperature changes, said Michael Prather of the University of California, Irvine, at the press conference.

(Some scientists contend that smaller temperature changes could be as disastrous.)

The statement notes that climate projections inherently have uncertainties, but says "none are known that could make the impact of climate change inconsequential."

AGU President Timothy Killeen says the statement is "a fair representation of the basic scientific understanding" of climate change, a collective opinion that has evolved over the past few years and given scientists a firmer understanding of natural climate change and human-caused changes.

Prather noted that the committees that helped draft the AGU statement included scientists outside the field of climate change.

The statement also calls for the need to reduce emissions of carbon dioxide by 50 percent globally by the end of this century to avoid the more catastrophic potential effects of climate change.

The final paragraph of the statement calls for cooperation among scientists, industry and government to develop strategies to mitigate climate change.

The new statement was adopted on Dec. 14, 2007, by the AGU's governing body, the AGU Council, at their annual meeting in San Francisco.

Its release today comes about 11 months after the Intergovernmental Panel on Climate Change first announced its initial and similar findings.

Later in 2007, the IPCC and Vice President Al Gore won the Nobel Peace Prize for their efforts to learn more about man-made climate change and to disseminate that knowledge broadly.