Public concern about global warming has dropped sharply since the fall of 2008, according to the results of a national survey released by researchers at Yale and George Mason universities.

The survey found:

+ Only 50 percent of Americans now say they are "somewhat" or "very worried" about global warming, a 13-point decrease.

+ The percentage of Americans who think global warming is happening has declined 14 points, to 57 percent.

+ The percentage of Americans who think global warming is caused mostly by human activities dropped 10 points, to 47 percent.

In line with these shifting beliefs, there has been an increase in the number of Americans who think global warming will never harm people in the United States or elsewhere or other species.

"Despite growing scientific evidence that global warming will have serious impacts worldwide, public opinion is moving in the opposite direction," said Anthony Leiserowitz, director of the Yale Project on Climate Change.

"Over the past year the United States has experienced rising unemployment, public frustration with Washington and a divisive health care debate, largely pushing climate change out of the news.

"Meanwhile, a set of emails stolen from climate scientists and used by critics to allege scientific misconduct may have contributed to an erosion of public trust in climate science."

The survey also found lower public trust in a variety of institutions and leaders, including scientists. For example, Americans' trust in the mainstream news media as a reliable source of information about global warming declined by 11 percentage points, television weather reporters by 10 points and scientists by 8 points.

They also distrust leaders on both sides of the political fence. Sixty-five percent distrust Republicans Arnold Schwarzenegger and Sarah Palin as sources of information, while 53 percent distrust former Democratic Vice President Al Gore and 49 percent distrust President Barack Obama.

Finally, Americans who believe that most scientists think global warming is happening decreased 13 points, to 34 percent, while 40 percent of the public now believes there is a lot of disagreement among scientists over whether global warming is happening or not.

"The scientific evidence is clear that climate change is real, human-caused and a serious threat to communities across America," said Edward Maibach, director of the Center for Climate Change Communication at George Mason University.

"The erosion in both public concern and public trust about global warming should be a clarion call for people and organizations trying to educate the public about this important issue."

The results come from a nationally representative survey of 1,001 American adults, age 18 and older. The sample was weighted to correspond with U.S. Census Bureau parameters. The margin of sampling error is plus or minus 3 percent, with 95 percent confidence.

The survey was designed by researchers at Yale and George Mason Universities and conducted from December 23, 2009, to January 3, 2010 by Knowledge Networks using an online research panel of American adults.

A copy of the report can be downloaded here.