A landmark report released last week in Paris makes it very clear that climate change is under way, that it is a result in part of human activity and that it is likely reversible to a degree.

The three-year study, by the Intergovernmental Panel on Climate Change, leaves little or no room for squirming away from its conclusions. The panel falls under the oversight of the United Nations Environment Program and the World Meteorological Organization and involves hundreds of scientists and analysts from some 112 countries in its work. The new assessment is the fourth offered by the panel, after previous studies in 1990, 1995 and 2001. Each has been firmer in the conclusions it offered.

The study offers evidence that documents a buildup of heat-trapping gases -- so-called greenhouse gases -- in the atmosphere, largely a result of burning oil and coal. It notes that 11 of the 12 warmest years on record have occurred since 1995, although evidence is lacking that the current increase in temperature is permanent rather than cyclical. The report notes that the world's sea level rose by 6 to 9 inches in the 20th century, and it projects a rise of 7 inches to 2 feet in the 21st century. Arctic sea ice has declined by 2.7 percent since 1972, and Greenland is melting.

The panel judges that results of the buildup of greenhouse gases will include warmer temperatures, rising sea levels and changes in weather patterns. The United States, with 5 percent of the world's population, produces about 25 percent of the heat-trapping gases. China is second.

Opposition to doing something about this phenomenon falls roughly into three categories: nonbelievers in the science; people who believe but are too frozen to act, like deer in the headlights; and businesses that don't want to adopt more expensive energy sources, backed up by their political thralls. The panel's report will heap more science on the matter to address the reservations of the first group. The second group, which accepts that the trouble is coming but believes that what can be done will be too little to make a difference, is hard to deal with. This is the same group that says all of this will happen after we are long gone.

The third group of opposition to trying to reverse or reduce global warming is found embedded in the Bush administration, sometimes trying to modify scientific reports emanating from the U.S. government or from American academic figures, and is prompted for the most part by American business interests, specifically the oil and coal industries.

The panel's summary report and the more detailed analysis to follow make it more difficult for the U.S. government to do nothing. Maybe it will take years and years of catastrophic storms or rising seas that wash away their houses at the beach to catch the attention of America's moguls. By that time, of course, it will be, if not too late, much later in the game and the problem will be even harder to deal with than it is today.

The panel's report makes it very clear that this particular bell tolls for us and calls for serious action now.