Some of the effects being seen today and cited in the report are familiar, like more powerful tropical storms and erosion of ocean coastlines caused by melting Arctic ice. The study also cites an increase in drought in the Southwest and more intense heat waves in the Northeast as a result of growing concentrations of carbon dioxide and other climate-altering gases in the atmosphere.

Reduced mountain snowpack means earlier melt-offs and reduced stream volumes across the West and Northwest, affecting residential and agricultural water supplies, habitats for spawning fish and reduced hydroelectric power generation, the study found.

But the speed and severity of these effects in the future are expressed with less certainty in the report and will depend to some extent on how quickly the United States and other nations move to reduce emissions.

“What we would want to have people take away is that climate change is happening now, and it’s actually beginning to affect our lives,” said Thomas R. Karl, director of the National Climatic Data Center at the National Oceanic and Atmospheric Administration and a principal author of the report. “It’s not just happening in the Arctic regions, but it’s beginning to show up in our own backyards.”

Dr. Karl said that unless the country acted soon to reduce emissions and to adapt to inevitable effects of a changing climate, the costs would be severe.