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.

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.

(Article continues below)

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.