By 2020, the US will emit almost one-fifth more gases that lead to global warming than it did in 2000, increasing the risks of drought and scarce water supplies.
That projection comes from an internal draft report from the administration of President George W. Bush that is more than a year overdue at the UN.
The US is responsible for one-quarter of the world's greenhouse gases that scientists say cause global warming.
The draft report, which is still being completed, projects that the current administration's climate policy would result in the emission of 8.3 billion tonnes of greenhouse gases in 2020, a 19 percent increase from 6.9 billion tonnes in 2000.
Doing more than slowing the growth rate of greenhouse gas emissions, which remains the administration's stated goal, will be decided "as the science justifies," the draft report said.
But an authoritative UN report last month from hundreds of scientists and government officials said global warming is "very likely" caused by mankind and that climate change will continue for centuries even if heat-trapping gases are reduced.
The report was approved by 113 nations including the US.
The administration's draft covers inventories of carbon dioxide and other greenhouse gases, projected environmental consequences and policies to limit emissions.
The White House Council on Environmental Quality has been coordinating the draft report.
A spokeswoman, Kristen Hellmer, said the draft "will show that the president's portfolio of actions and his financial commitment to addressing climate change are working. And the president is always looking at ways to address our energy security and environmental needs."
Hellmer blamed the delay in completing the fourth US Climate Action Report on the "extensive interagency review process."
The report, which was due no later than Jan. 1 last year, is required under the UN Framework Convention on Climate Change.
Among the consequences of a warming world anticipated in the report is "a distinct reduction in spring snowpack in the northwestern United States," which supplies much of the water in that region, the report says.
Warmer temperatures expected from more greenhouse gases would only "exacerbate present drought risks in the United States by increasing the rate of evaporation," it says.
Rick Piltz, director of Climate Science Watch, a nonprofit watchdog program, said Saturday he expects the final report will evade a full discussion of how global warming might affect the nation.
"I think it is very likely that the main reason the report has been held up for more than a year beyond the deadline is because the administration is reluctant to make an honest statement about likely climate change impacts on this country," said Piltz, a former senior associate with the US Climate Change Science Program.