Today’s level of belief that global warming is already apparent is similar to what Gallup found in 1997 and from 2001 through 2005. During those periods, however, somewhat fewer than today’s 15% said its effects would never happen.

The view that global warming is already causing a rise in sea levels and affecting weather and rainfall around the world is central to climate-change researchers’ and others’ concerns about the issue.

While barely half of Americans agree that the effects of global warming are already manifest, an additional 29% say the effects will start to happen within a few years (4%), sometime in their lifetime (10%), or sometime further into the future (15%). Fifteen percent of Americans say the effects will never happen.