A 67-page report on North American climate effects, which is part of an international report on climate-change impact, identified “water at large” as the central global-warming problem for the U.S. The report was released at a press conference featuring eight American scientists who were lead authors of the Intergovernmental Panel on Climate Change study.

The report said reduced snow melt supplying water for the Sacramento Valley in California means that by 2020 there won‘t be enough water “to meet the needs of the community.” On the East Coast, rising sea levels will make storm surge “the No. 1 vulnerability for the metropolitan East Coast.”

Even the air will become more deadly. More heat means about a 4 to 5 percent increase in smog-related deaths.

The scientists held out hope, however, that dramatic cuts in fossil fuel emissions could prevent much of the harm they are predicting. But the U.S. government — and the rest of the world — has to act now.