WASHINGTON - Rising sea levels, sweltering temperatures, deeper droughts, and heavier downpours - global warming's serious effects are already here and getting worse, the Obama administration warned yesterday in the grimmest, most urgent language on climate change ever to come out of any White House.

But amid the warnings, scientists and government officials seemed to go out of their way to soften the message. It is still not too late to prevent some of the worst consequences, they said, by acting to reduce world emissions of heat-trapping gases, mostly carbon dioxide from the burning of fossil fuels.

The new report differs from a similar draft issued with little fanfare or context by George W. Bush's administration last year. It is paradoxically more dire about what's happening and more optimistic about what can be done.

The Obama administration is backing a bill in Congress that would limit heat-trapping pollution from power plants, refineries, and factories. A key player on a climate bill in the Senate, California Democrat Barbara Boxer, said the report added "urgency to the momentum in Congress" to pass a law.

"It's not too late to act," said Jane Lubchenco, one of several agency officials at a White House briefing. But what has happened already is not good, she said: "It's happening in our own backyards and it affects the kind of things people care about."

For example, winters in parts of the Midwest have warmed by 7 degrees in just 30 years and the frost-free period has grown a week, the report said.

Shorter winters have some benefits, such as longer growing seasons, but those are changes that require adjustments just the same, the authors note.

The world's average temperature may rise by as much as 11.5 degrees by the end of the century, the report said. And the U.S. average temperature could go even higher.

In one of its key findings, the report warned: "Thresholds will be crossed, leading to large changes in climate and ecosystems." The survival of some species could be affected, it said.