Americans are increasingly less worried about global warming and less convinced its effects are already happening, according to an annual Gallup survey released Thursday.

Nearly half, 48%, now believe its seriousness is exaggerated, up from 41% in 2009 and 31% in 1997, when Gallup first asked the question, according to the Gallup Social Series Environment poll of 1.014 adults taken March 4-7.

The poll also finds a sharp turnaround in views about what's causing the rise in global temperatures. Half, or 50%, now agree with most climate scientists that human activities are the cause, down from 61% in 2003.

"The survey results show that the reversal in Americans' concerns about global warming that began last year has continued in 2010 -- in some cases reverting to the levels recorded when Gallup began tracking global warming measures more than a decade ago," Gallup says.

The poll is one of several that have shown such a shift in the last two years.

In a national poll of 1,000 likely voters released last month by Rasmussen Reports, just 35% of respondents said they believed human activity was primarily responsible for global warming, down from 47% in April 2008, according to a story today by USA TODAY colleague Brian Winter.

Another survey, released earlier this month, indicates young U.S. adults -- those under age 35 -- are less concened about global warming than older generations, according to data collected by the Yale Project on Climate Change and the George Mason University Center for Climate Change Communication.

The Gallup survey discusses why attitudes are changing:

Some of the shifts in Americans' views may reflect real-world events, including the publicity surrounding allegations of scientific fraud relating to global warming evidence, and -- perhaps in some parts of the country -- a reflection of the record-breaking snow and cold temperatures of this past winter. Additionally, evidence from last year showed that the issue of global warming was becoming heavily partisan in nature, and it may be that the continuing doubts about global warming put forth by conservatives and others are having an effect.

It finds that a slight majority of Americans still agree that global warming is real, as 53% say the effects of the problem have already begun or will do so in a few years.

Yet that percentage is dwindling. The survey says the average American is now less convinced than at any time since 1997 that global warming's effects have already begun or will begin shortly.

Until 2008, most Gallup measures had shown increasing concern over global warming. One factor, its newest poll says, was the work of former vice president Al Gore, including his best-selling book, An Inconvenient Truth, an Academy Award-winning documentary on the topic and his receipt of a Nobel Peace Prize in 2007.

Readers: Are you worried about Gallup's findings?