Speaking to Al Roker of NBC News, in an interview scheduled to be shown Wednesday morning on the “Today” show, Mr. Obama said “This is not some distant problem of the future. This is a problem that is affecting Americans right now. Whether it means increased flooding, greater vulnerability to drought, more severe wildfires — all these things are having an impact on Americans as we speak.”

In the Northeast, the report found a big increase in torrential rains and risks from a rising sea that could lead to a repeat of the kind of flooding seen in Hurricane Sandy. In the Southwest, the water shortages seen to date are likely just a foretaste of the changes to come, the report found. In that region, the report warned, “severe and sustained drought will stress water sources, already overutilized in many areas, forcing increasing competition among farmers, energy producers, urban dwellers and plant and animal life for the region’s most precious resource.”

The report did find some benefits from climate change in the short run, particularly for the Midwest, such as a longer growing season for crops and a longer shipping season on the Great Lakes. But it warned that these were likely to be countered in the long run by escalating damages, particularly to agriculture.

“Yes, climate change is already here,” said Richard B. Alley, a climate scientist at Pennsylvania State University who was not involved in writing the report. “But the costs so far are still on the low side compared to what will be coming under business as usual by late in this century.”

The report was supervised and approved by a large committee representing a cross section of American society, including representatives of two oil companies. It is the third national report in 14 years, and by far the most urgent in tone, leaving little doubt that the scientists consider climate change an incipient crisis. It is also the most slickly produced, with an elaborate package of interactive graphics on the Internet.