The New York Times reported on Wednesday that the changes that were made play up doubts on whether so-called "greenhouse gases" from human activity really cause climate change.
But Press Secretary Scott McClellan says the resulting reports were still "scientifically sound."
The changes were made by Philip Cooney, who led the American Petroleum Institute's fight against greenhouse gas limits before he joined President Bush's Council on Environmental Quality in 2001.
The Times documents several handwritten notes by Cooney on drafts of reports issued in 2002 and 2003. Cooney removed or adjusted descriptions of climate research that government scientists and their supervisors, including some senior Bush administration officials, had already approved.
While sometimes as subtle as the insertion of the phrase "significant and fundamental" before the word "uncertainties," tend to produce an air of doubt about findings that most climate experts say are robust, reports the Times.
In one instance detailed by the Times, an October 2002 draft of a summary of government climate research, Cooney adding the word "extremely" to this sentence: "The attribution of the causes of biological and ecological changes to climate change or variability is extremely difficult."
In another cited example, Cooney crossed out a paragraph describing the projected reduction of mountain glaciers and snowpack. His note in the margins explained that this was "straying from research strategy into speculative findings/musings."
The Times said Cooney has no scientific training. But McClellan calls him a "policy person" whose editing is "part of the interagency review process."
Environmentalists have accused Mr. Bush and top aides of claiming doubt about mankind's role in global warming -- when the vast majority of scientific evidence supports a link.