PARIS -- For the first time, a panel of climate experts has confirmed that global warming is occurring and that it is "very likely"--90 percent certain--man-made. The Intergovernmental Panel on Climate Change (IPCC), a working group of some 3,000 delegates from 113 countries, today issued its final report here on the state of climate change--and the findings were grim. "There can be no question that the increases in these greenhouse gases are dominated by human activity," says Susan Solomon, co-chair of the working group and an atmospheric scientist with the U.S. National Oceanic and Atmospheric Administration (NOAA). "Warming of the climate system is now unequivocal. That is evident in observations of air and ocean temperature as well as rising global mean sea level."

"The 2nd of February in Paris will be remembered as the day that the question mark was removed from the idea that humans had anything to do with climate change," adds Achim Steiner, executive director of the United Nations Environmental Programme (UNEP) "The focus of attention will now shift from whether climate change is linked to human activity and whether the science is sufficient to what on earth are we going to do about it."

A wealth of new data in the years between this report and the last one in 2001 provided improved accuracy and precision. For instance, thanks to a diversity of computer models--as well as several runs of each--the scientists can now provide a best estimate for the temperature change based on a doubling of carbon dioxide (CO 2 ) levels in the atmosphere: three degrees Celsius. This doubling is based on preindustrial levels of the most prevalent greenhouse gas--roughly 280 parts per million (ppm). Concentrations of CO 2 in the atmosphere had already reached 379 ppm in 2005.

"Some of the models show an ice-free Arctic. We see more severe extremes, heat waves. We see a lot of heavier precipitation, drought increases in a lot of regions. Tropical cyclones are projected to become more intense in a lot of areas with ongoing increases in sea surface temperatures," says Gerald Meehl, an atmospheric scientist at the U.S. National Center for Atmospheric Research (NCAR) and a contributing author. "We see what we've already seen but everything becoming a lot more extreme."

This warming would vary from place to place, with some regions experiencing far more. "The last time our polar regions of the earth were significantly warmer than they are today over an extended period occurred 125,000 years ago," Solomon notes. "At that very different time we did see reductions in the polar ice sheets that led to four to six meters (13 to 20 feet) of sea level rise." Adds geoscientist and lead author Jonathan Overpeck of the University of Arizona, some scientists "drilled through an Antarctic ice sheet and it wasn't there then. That's going to be a big focus for the future."

The process of drafting this summary report proved contentious at times. For example, a sentence was ultimately removed that said man-made greenhouse gases outweigh the contribution of the sun by a factor of five. "The difference is really a factor of 10," says atmospheric scientist and lead author Piers Forster of the University of Leeds in England. In fact, this report, despite its gravity, represents a very conservative estimate of what may happen as a result of man-made climate change.

Future reports from the IPCC will focus on impacts and strategies for mitigation, culminating in November in a summation of their findings. Action is already being taken: many countries have committed to reducing greenhouse gases under terms of the Kyoto Treaty; but this IPCC document represents the international consensus on the state of climate science. "It's important that all governments have agreed to the conclusions of this science," notes Yvo de Boer, executive secretary of the U.N. Framework Convention on Climate Change (UNFCCC). "The economic costs of waiting to act will be severe."

The U.S., which emits the most greenhouse gases, has so far not accepted any form of reduction in such emissions, either as part of international efforts or domestically. But it also acceded to this summary, saying it "summarizes the current state of climate change research and will serve as a valuable source of information for policymakers," according to a statement from Sharon Hays, leader of the U.S. delegation and deputy director for science at the White House Office of Science and Technology Policy.

Efforts to combat climate change include proposals to mimic the natural cooling effect of volcanic eruptions or to place an enormous parasol in space to block sunlight. "We could probably offset the effects now by having a big volcano every 10 years or so," Forster says. "You would have to do it now, you'd have to do it for the rest of time or until you find some other alternative. It can only ever be temporary."

But there are other, less intrusive options. "The largest opportunity is energy efficiency," says Halldor Thorgiersson, deputy executive secretary for scientific and technological advice at the UNFCCC. "There are also new technologies such as carbon capture and storage." Already, potential impacts of climate change should be taken into account for long-term planning, such as hydroelectric projects or sewer systems that will last a century or more, says lead author David Wratt, principal scientist for New Zealand's National Climate Center. "Let's not do the minimum, let's put in some tolerances or at least make it so that putting [in] more pipes in 50 years isn't too difficult."

It is now clear that the world will undergo even more rapid changes this century if the levels of greenhouse gas emissions are not slowed. "If we were to have continued emissions at or above the current levels, the changes in the 21st century would very likely be larger than they were in the 20th century," Solomon says, just as the rate of sea level change in the 19th century is dwarfed by the rate of sea level change in the 20th century. "We now know we have warming and it is due to humans, there should be no real debate," Overpeck adds. "We know there have been a variety of associated changes: stronger hurricanes, reduction of snowpack. These are the kind of changes we have had that are detectable with just a fraction of a degree of warming."

"We're going to see these same patterns continue in the future and get more and more severe," he continues. "We have a really clear picture of what is going to happen if we don't do anything or if we make some reduction in emissions."