The percentages that worry a big deal are:
- pollution of rivers, lakes etc: 46% now (max: 72% in 1989)
- air pollution: 38% (max: 63% in 1989)
- toxins in soil and water: 44% (max: 69% in 1989)
- loss of rain forests: 33% (max: 51% in 2000)
- global warming: 28% (max: 41% in 2007)
While I have always cared about such things, it's just true that I have never encountered an environmental problem (air, water, soil pollution) in the U.S. that would be worth a discussion. The environment has improved in the last 20 years - so the contamination simply ceased to be a serious problem. In this sense, I think that the people react rationally. It's sensible to care about problems that actually exist.
The previous paragraph referred to the issues that people can see in their neighborhood. The AGW and tropical forests don't belong to this category. But it's still true that people have learned that one of these problems has been vastly exaggerated while the other one was de facto completely fabricated.
The polls just see the people's learning - which often took place despite the media's persistent efforts to distort the truth.
Most of these concerns may be classified as "fads" and the number of people who worry about them is changing pretty rapidly. Needless to say, global warming is the most ephemeral one among these fads. A drop from 41% to 28% (to 2/3 of the original value) in just two years is unmatched by any of the other graphs.
However, you should notice that global warming has been the "least worrisome" among the five concerns pretty much since the beginning of the poll in 1989 so we're not experiencing a qualitatively unprecedented situation now.
By the way, most employees of the Science Times section of the New York Times doubt that global warming is a serious issue, too. We can learn it from John "End of Science" Horgan, an übercrackpot who is of course unhappy about this heresy of the Gray Lady. Horgan just returned from a symposium of similar dopes who were trying to find the best ways to spread the AGW propaganda.
Some attendees proposed "neuroframing" which is "spinning plus brain scans" and it was a too strong a cup of tea even for Horgan himself - and believe me, he is a real nutcase.