(adds green groups, industry reaction; edits throughout)
By Tom Doggett and Chris Baltimore
WASHINGTON, June 3 (Reuters) - The Bush administration acknowledged for the first time that U.S. greenhouse gas emissions will increase significantly over the next two decades due mostly to human activities, but again rejected an international treaty to slow global warming.
A report quietly released last week by the Environmental Protection Agency gave a surprising endorsement to what many scientists have long argued -- that oil refining, power plants and auto emissions are important causes of global warming.
Gradually increasing temperatures are likely to threaten coastal barrier islands and mountain meadows, the report said.
The White House previously said there was not enough evidence to link industrial emissions to global warming.
"Greenhouse gases are accumulating in the Earth's atmosphere as a result of human activities, causing global mean surface air temperatures and subsurface ocean temperatures to rise," the report said.
That position put the administration at odds with its supporters in the U.S. auto, oil and electricity industries, who contend that more research is needed to determine if the changes are naturally occurring or caused by industry.
In the inter-agency report sent Friday to the United Nations, the administration forecast that total U.S. greenhouse gas emissions will increase 43 percent between 2000 and 2020.
On the same day, all 15 European Union nations ratified the Kyoto pact, a U.N.-backed plan to cut emissions of greenhouse gases such as carbon dioxide. EU officials also criticized Washington for rejecting the treaty.
The United States is the world's largest emitter of greenhouse gases, mostly from utilities and factories.
Last year, U.S. President George W. Bush triggered international outrage when he said the United States would not participate in the Kyoto Treaty. He said the pact's goal of cutting U.S. emissions by about 7 percent from 1990 levels during 2008-2012 would be too costly to the American economy.
The administration repeated in the new report that voluntary measures by U.S. polluters, rather than Kyoto's mandatory approach, are the best way to slow the growth of carbon emissions.
A voluntary approach is "expected to achieve emission reductions comparable to the average reductions prescribed by the Kyoto agreement, but without the threats to economic growth that rigid national emission limits would bring," it said.
The White House aims to cut the amount of emissions per unit of U.S. gross domestic product by 18 percent over the next decade through a combination of voluntary, incentive-based and mandatory measures.
The U.S. approach to global warming is expected to face heavy criticism at an August global summit on climate change in Johannesburg, South Africa. Some 60,000 delegates and 100 heads of state are expected to attend.
MAJOR CHANGE SEEN
But environmental groups said the new report marked a major change for the administration. Air, water and land pollution issues are expected to play an important role in many congressional elections in November.
"The administration has finally acknowledged the science," said Jon Coifman at the Natural Resources Defense Council. "It's unfortunate that they're still not taking action on the solutions."
Philip Clapp, president of the National Environmental Trust, said the report "undercuts everything the president has said about global warming since he took office."
A White House spokesman said many questions remained about the impacts of global warming.
"The report notes there is considerable uncertainty on the science of climate change," said Scott McClellan. "We've always said it's a serious issue that needs to be addressed."
The report warned that global warming will have a greater impact on certain regions of the United States.
Average temperatures in the contiguous United States will rise an estimated 5 degrees to 9 degrees Fahrenheit (3-5 degrees Celsius) during this century.
Sensitive ecosystems, such as Rocky Mountain meadows and coastal barrier islands, will likely disappear, it said. Southeastern forests may see major changes in growth patterns.
The sea level will rise an average 19 inches (48 centimetres) and threaten homes, roads, and power lines in coastal areas, the report said.
Although not mentioned, the impacts spell significant dangers for coastal cities like New York, according to green groups. With sea level rises referenced in the report, the lower tip of Manhattan would be submerged, Clapp said.
Energy industry officials downplayed the report. They contend there is no scientific proof of links between global warming and specific regional effects.
"That's our concern, that regional results played such a prominent part in the report," said Russell Jones, an economist with the American Petroleum Institute. "We just don't think the science can do that yet."