When it comes to environmental issues, Americans are more concerned about immediate problems like contaminated soil, polluted drinking water, and even the loss of the rainforests than they are about global warming. From Gallup:

Americans' Worry About Environmental Problems, March 2012

It would seem Americans’ environmental concerns follow a rather clear hierarchy based on both physical and temporal proximity to us. The top environmental issues are things that could at this very moment hurt individuals or their families, such as polluted drinking water.

Americans’ second biggest environmental concerns are things that we can see happening right now but don’t appear to put us in immediate danger, such as the loss of rainforests and animal extinction.

Global warming ranks the lowest most likely because we see it as something that will be a problem in the future.

This is human nature. Evolutionarily it makes most sense to be most concerned about problems that are immediate and nearby as opposed to problems that are far away in time or space. This is a dynamic that people in the environmental movement need to deal with when trying to win public support.