President Bush's top science advisors issued a comprehensive report Thursday that for the first time endorses what most scientific experts have long asserted: that greenhouse gases from fossil fuel combustion "are very likely the single largest cause" of Earth's warming.
The 271-page report could undercut opposition to the more aggressive provisions of climate legislation, which is to be debated in the Senate next week.
The Bush administration had long resisted a congressional mandate, the 1990 Global Change Research Act, requiring the White House to report every four years on the science and impact of global warming and other environmental forces.
A U.S. District Court in August ordered Bush to comply with a 2004 deadline for an updated report, after the Tucson-based Center for Biological Diversity and other environmental groups filed suit.
Sharon Hays, deputy director of the White House Office of Science and Technology Policy, said the report did not represent a changed assessment but "a rolling up of a whole bunch of reports on the science, showing that climate change is primarily caused by human activity of the last 50 years."
The administration had earlier issued reports on the effect of climate change on transportation, agriculture and human health.
But environmentalists celebrated what they saw as a long-overdue admission from an administration that has been reluctant to join global efforts to curb greenhouse gases, such as the 1997 Kyoto Protocol.
"This report represents a stark shift in what the administration has been saying since 2001," said Philip Clapp, deputy managing director of the Pew Environment Group.
"For the first time, it has had to admit that global warming is already having clear impacts in the United States, and the impacts are going to get worse even with the most aggressive action to cut emissions," he said.
The report by the National Science and Technology Council and the U.S. Climate Change Science Program asserts that natural causes alone cannot explain recent extremes of heat and cold, warming seas and an increase in the frequency and intensity of hurricanes.
It also shows that regions of North America could warm faster over the next few decades than the global average. In Los Angeles, heat-related deaths, which averaged 165 a year in the 1990s, could jump to between 319 and 1,182 by the 2080s, the report says.
The warming climate also will accelerate the spread of diseases carried by water, food and insects. Among the most vulnerable people are the young, elderly, frail and poor, the administration's scientists concluded.
The few positive effects of climate shifts are outweighed by negatives. For example, warming and higher levels of carbon dioxide are expected to speed up growth of forests and certain crops, but will also increase insect outbreaks and lead to more wildfires, which are likely to take a larger toll on crops, forests and property, the report predicts.
Warmer, less-snowy winters will decrease winter road maintenance costs, but increased coastal and river-related flooding and landslides will cause more serious problems. Heat spells, the report says, "could cause railroad tracks to buckle or kink and could affect roads through softening and traffic-related rutting."
The cost of heating is likely to fall, but the increased demand for air conditioning "would require the building of additional electricity production facilities (and probably transmission facilities) at an estimated cost of many billions of dollars."
Industry representatives greeted the report with a shrug.
"It's well known that autos represent about 20% of the total in the U.S. of man-made [greenhouse gas] emissions," said Charles Territo, spokesman for the Alliance of Automobile Manufacturers.
Jim Owen, spokesman for the Edison Electric Institute, said the power industry "abandoned the science debate years ago. It's universally recognized in our industry that climate change is very real."
Nonetheless, said Kassie Siegel, climate program director for the Center for Biological Diversity, the report underscores "the reality and urgency of the climate crisis."
"Administration officials have spent nearly eight years trying to deny and downplay the science," she said. "They just cannot do it anymore. They are boxed in by court order."