The National Academies of Science and Engineering yesterday delivered a mixed verdict on the cumulative effects of oil and gas exploration on Alaska's North Slope, crediting the oil industry and government with doing much to minimize the environmental impact but warning that adverse consequences have not been eliminated and will continue to accumulate in the region.

In a report ordered by Congress to assess the effects of drilling since the discovery of huge oil reserves in Prudhoe Bay in 1968, a committee of the Academies' National Research Council said oil production has transformed life on the North Slope, spurring an economic boom that is viewed by most residents as positive. But it said the benefits have come at the cost of some environmental degradation, disruption of wildlife and habitat and negative social changes. Environmental groups and their congressional allies seized on the unanimous report to buttress arguments against President Bush's call for expanded oil and gas exploration into a part of the Arctic National Wildlife Refuge. But Edward Porter, research manager of the American Petroleum Institute, argued that the report "at least recognizes that progress is being made and that we have had some remarkable achievements in advancing exploration with minimal effects on the environment."

The committee did not address the ANWR issue, but said oil exploration involves unavoidable environmental consequences. In a section of the report titled "The Essential Trade-Off," the committee said that despite "considerable efforts" by the oil industry and government agencies to minimize the adverse impact, expanded drilling is certain "to exacerbate some existing effects and to generate new ones."

"Whether the benefits derived from oil and gas activities justify acceptance of the inevitable accumulated undesirable effects that have accompanied and will accompany them is an issue for society as a whole to debate and judge," the report said.

The report criticized government agencies for "weak and sporadic" communications and for failing to develop a comprehensive plan for oil and gas development on the North Slope. In a telephone news conference yesterday, Gordon H. Orians, professor emeritus of zoology at the University of Washington and chairman of the committee, said that "unless this is improved substantially, undesirable effects are likely to be greater" from future drilling.

The report said that some widely feared consequences of drilling on the North Slope have not materialized. Chief among these are large and frequent oil spills. It said most spills have been small and had only local effects, but warned that a large spill off the coast could have a substantial impact because current cleanup methods can remove only a fraction of spilled oil.

But in other sections, the report said the building of roads and off-road travel have damaged the Alaskan tundra and affected the habits of wildlife. For example, the noise from seismic exploration has driven bowhead whales further offshore, forcing the local people to travel further to hunt them.

The report said continued warming may limit the use of ice roads and other technologies that protect the environment. It also said that virtually no thought has been given to restoring oil drilling areas once production dries up, with the result that "abandoned and unrestored infrastructure are likely to persist for centuries."

Porter said the report recognized that the oil industry has gotten much better at managing the adverse impact of oil and gas drilling. But Philip Clapp, president of the National Environmental Trust, said that even if new technology has lessened the environmental damage, the report makes clear that "it's not the drilling itself, but the other activities such as road building, housing for workers, the infrastructure needed to support them that cause the damage."

Sen. Ted Stevens (R-Alaska) criticizes a study by the National Academies of Sciences on oil exploration in Alaska during a news conference yesterday.