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 on Tuesday in the grimmest, most urgent language on climate change ever to come out of any White House.
The report finds that greenhouse-gas emissions are "primarily" responsible for global warming and that rapid action is needed to avert catastrophic shifts in water, heat and natural life. 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.
Other differences are the report's scope — at 196 pages, the report attempts to present the fullest picture yet of the threats to the United States — and its timing. It comes out as Congress is considering a mammoth bill that would impose the first national cap on emissions and then seek to reduce them sharply over the next 41 years.
That bill, supported by Obama, has spurred some Republicans to say that they are not certain climate change is happening. It has also been criticized, from both sides of the aisle, as a measure that would impose significant new costs on energy use and manufacturing.
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.
"It's not too late to act," said Jane Lubchenco, one of several 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 back yards, 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 warns, "Thresholds will be crossed, leading to large changes in climate and ecosystems." The survival of some species could be affected, it said.
Another effect of warmer seas is stronger hurricanes. "Sea-level rise and the likely increase in hurricane intensity and associated storm surge will be among the most serious consequences of climate change," the report says.
"If sea-level rise increases at an accelerated rate … a large portion of the Southeast coastal zone could be threatened."
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 paints a fuller, darker picture of global warming in the United States than previous studies have.
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.
Among its findings:
• The average U.S. temperature has risen 2 degrees Fahrenheit over the past 50 years and might rise more rapidly, between 4 and 11 degrees, before 2100.
• Precipitation in the United States has increased an average of about 5 percent over past 50 years. In the future, computer models show that northern areas will become wetter, and southern areas will become drier, especially in the West.
• The heaviest rainstorms are even heavier now, with the amount of rainfall in major storms having increased 20 percent nationwide over the past century. The hardest-hit areas have been the Northeast — where heavy storms are now 67 percent heavier — and the Midwest, with a 31 percent increase.
• Extreme heat waves will also become more common. A temperature reached only once in 20 years until now might be reached every other year or so by the end of the century.
• Sea levels have been rising along most of the U.S. coast over the past 50 years, increasing up to 8 inches in some places. That trend is expected to continue as warmer temperatures melt glaciers.
Tom Karl of the National Climatic Data Center said that at least one tipping point — irreversible sea level rise — has been passed.
"There are, in some cases, already serious consequences," report co-author Anthony Janetos of the University of Maryland said. "This is not a theoretical thing that will happen 50 years from now. Things are happening now."
The Associated Press and Washington Post contributed to this report.