In a report sent to Congress this week, the administration noted a recent government-sponsored study supported the view of many scientists that human action from driving automobiles to running power plants helped cause global warming.
"North American temperature changes from 1950 to 1999 were unlikely to be due only to natural climate variations," the report said. Warmer temperatures that occurred from 1900 to 1949 were "likely due" to natural causes, the report added.
The brief passage in the report was surprising because President Bush and other senior administration officials have long insisted there was no clear scientific proof to link human activities to global warming.
White House spokesman Trent Duffy said this one study does not change the administration's position and more research on the causes of global warming is still needed. "The president's policy is the same.... we need to fill in the knowledge and the scientific gaps," he said.
Bush withdrew the United States from participating in the Kyoto treaty that sought to reduce global warming emissions produced mostly by industrialized nations. He said the accord's tough requirements would be too costly to the U.S. economy.
The White House has promoted a voluntary program for U.S. power plant and oil refinery owners and other industries to cut their so-called greenhouse gas emissions.
The specific study on temperature changes from 1950 to 1999 was included in a list of research projects on climate change sponsored by various government agencies that were recently completed or are under way.
In its report to Congress on the research, the administration said the studies did not "make any findings of fact that could serve as predicates for regulatory action."
However, one environmental group said the report put pressure on Bush to address the global warming issue when the president lays out his plans for a second term at the Republican convention next week in New York.
"It will be interesting to see whether he plans to do something about global warming or just continue ignoring it for political reasons," said Philip Clapp, president of the National Environmental Trust.
Other recent government-sponsored studies listed in the administration's report found:
+ 5.6 million hectares (13.8 million acres) of U.S. farmland set aside from production across a 13-state region soak up 5.1 million metric tons of carbon dioxide from the atmosphere each year.
+ Emissions from the oil and natural gas industries in the Southwestern United States raised quantities of ethane, propane, and butane in the near-surface atmosphere of Kansas, Oklahoma, and Texas in autumn and spring that were comparable to urban smog.
+ The portion of the Arctic Ocean covered by perennial sea ice has declined by about 9 percent per decade since 1978.