The proportion of Americans who say that the earth is getting warmer has decreased since January 2007, mostly because of a decline among Republicans, according to the latest national survey by the Pew Research Center for the People & the Press.

Overall, 71 percent of Americans say there is solid evidence of higher global temperatures, compared with 77 percent at the beginning of last year.

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Republicans in particular are increasingly skeptical - just 49 percent of Republicans say there is evidence that the earth’s average temperature has been rising, down 13 points since January 2007.

Roughly half of Americans (47%) say the earth is warming because of human activity.

Groups are working hard to raise awareness about global warming. The Alliance for Climate Protection will spend $300 million over the next three years on a global warming marketing campaign.