Global Warming "very likely" caused by humans, scientists say

In a bleak and powerful assessment of the future of the planet, the leading international network of climate change scientists has concluded for the first time that global warming is "unequivocal" and that human activity is the main driver, "very likely" causing most of the rise in temperatures since 1950.

They said the world is already committed to centuries of warming, shifting weather patterns and rising seas, resulting from the buildup of gases in the atmosphere that trap heat. But the warming can be substantially blunted by prompt action, the panel of scientists said in a report.

The report summarized the fourth assessment since 1990 by the group, the Intergovernmental Panel on Climate Change of the United Nations, sizing up the causes and consequences of climate change. But it is the first in which the group asserts with near certainty — more than 90 percent confidence — that carbon dioxide and other heat-trapping greenhouse gases from human activities have been the main causes of warming since 1950.

In its last report, in 2001, the panel, consisting of hundreds of scientists and reviewers, put the confidence level at between 66 and 90 percent. Both reports are online at http://www.ipcc.ch.

“Policy makers paid us to do good science, and now we have high very scientific confidence in this work — this is real, this is real, this is real,” said Richard B. Alley, one of the lead authors and a professor at Penn State University. “So now act, the ball’s back in your court.”

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