Americans who think global warming is caused by human activity, including vehicle and industrial emissions, are now a minority for the first time in nearly two years, a CNN/Opinion Research Corp. poll said Monday.

Only 45 percent of the 1,041 adults surveyed on December 2-3 believed global warming is a proven fact and mostly caused by human activity, down from 56 percent in October 2007, the survey found.

By comparison, Americans who believe global warming is caused by natural changes unrelated to man have increased to 22 percent from 20 percent two years ago, and those who believe global warming is a yet unproven theory grew to 31 percent from 23 percent.

Regarding how the United States should tackle global warming, 58 percent of those surveyed said it should cut carbon dioxide emissions unilaterally, down from 66 percent in October 2007.

Those who thought the United States should cut CO2 emissions only if other countries do so as well remained basically unchanged at 17 percent, from 16 percent two years ago.

But those who said carbon emissions should not be cut regardless of what the rest of the world does jumped to 24 percent, from 15 percent two years ago.

With a three percent margin of error, the poll coincided with the start Monday of a landmark, 12-day conference on tackling climate change opened in Copenhagen attended by more than 110 heads of state or government.

The White House on Monday dismissed as "silly" the notion that global warming science had been compromised by emails exposing a row between top climate scientists.

The emails have been pounced on by climate skeptics, particularly in the United States to suggest the scientific community is hyping the threat from carbon dioxide emissions ahead of the Copenhagen climate conference.