Rising sea levels, sweltering temperatures, deeper droughts, and heavier downpours — global warming's serious effects are already here and getting worse, U.S. President Barack Obama's administration warned on Tuesday in the grimmest, most urgent language on climate change ever to come out of the 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 aggressively to reduce world emissions of heat-trapping gases, primarily 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 both 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 adds "urgency to the growing momentum in Congress" for passing a law.

'Thresholds will be crossed,' report warns

"It's not too late to act," said Jane Lubchenco, one of several agency officials at a White House briefing. "Decisions made now will determine whether we get big changes or small ones."

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."

Lubchenco, a marine biologist, heads the National Oceanic and Atmospheric Administration.

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.

The document, a climate status report required periodically by Congress, was a collaboration by about three dozen academic, government and institute scientists. It contains no new research, but it paints a fuller and darker picture of global warming in the United States than previous studies.

Bush issued draft report

Bush was ultimately forced by a lawsuit to issue a draft report last year, and that document was the basis for this one. Obama science adviser John Holdren called the report nonpartisan, started by a Republican administration and finished by a Democratic one.

"The observed climate changes that we report are not opinions to be debated. They are facts to be dealt with," said one of the report's chief authors, Jerry Melillo of Marine Biological Lab in Woods Hole, Mass. "We can act now to avoid the worst impacts."

Among the things Melillo said he would like to avoid are more flooding disasters in New Orleans and an upheaval of the world's food supply.

The scientists softened the report from an earlier draft that said "tipping points have already been reached and have led to large changes." Melillo said that is because some of the changes seen so far are still reversible.

Even so, Tom Karl of the National Climatic Data Center said that at least one tipping point — irreversible sea level rise — has been passed.

A point of emphasis of the report, which is just under 200 pages, is what has already happened in the United States. That includes rapidly retreating glaciers in the American West and Alaska, altered stream flows, trouble with the water supply, health problems, changes in agriculture, and energy and transportation worries.

"There are in some cases already serious consequences," report co-author Anthony Janetos of the University of Maryland told The Associated Press. "This is not a theoretical thing that will happen 50 years from now. Things are happening now."

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

Shorter winters require adjustments:report

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

The "major disruptions" already taking place will only increase as warming continues, the authors wrote. The world's average temperature may rise by as much as 6.3 degrees by the end of the century, the report said. And the U.S. average temperature could go even higher than that, Karl said.

Environmental groups praised the report as a call for action, with the Union of Concerned Scientists calling it what "America needs to effectively respond to climate change."

Scott Segal, a Washington lobbyist for the coal industry, was more cautious: "Fast action without sufficient planning is a route to potential economic catastrophe with little environmental gain."