Americans are less concerned about environmental issues than they've been in 20 years or more, a new poll from Gallup shows. 

The shift in public attitude could mean people think environmental conditions are improving or simply that economic concerns vastly outweigh nervousness about issues like global warming and water pollution. Or both. 

The poll asked people whether they worry a "great deal" about eight distinct environmental issues -- the percentage that answered yes was lower than last year in every category. For all but two categories, global warming and fresh water supply maintenance, the number was the lowest since Gallup started measuring 20 years ago. 

The survey data could help explain why the urgency behind enacting comprehensive climate change legislation appears to have waned. A bill passed the House last year but the Senate has not approved anything similar -- inaction by the U.S. Senate was considered an obstacle to seeking a global climate treaty in Copenhagen in December. 

According to the Gallup poll, 28 percent of people view global warming as a major concern. That's down from 35 percent in 1989 and 33 percent just last year. The low point came in 1997, when 24 percent of people listed the issue as a major worry. 

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On other issues, public concern has plummeted. Seventy-two percent of Americans said they were very concerned, for instance, about water pollution in lakes and rivers in 1989. In 2010, that number is 46 percent. 

The survey of 1,014 adults was taken March 4-7. It had a margin of error of 4 percentage points.