PARIS – Climate change is real, it is already here and its consequences may be worse than anticipated, say early drafts of an upcoming report from an international group of climate scientists. The Intergovernmental Panel on Climate Change (IPCC) is set to release on Friday a summary of the report—its fourth on the state of global warming since the group was formed by the World Meteorological Organization (WMO) and U.N. Environment Program (UNEP) in 1988—and the news is bleak.

The body of several thousand atmospheric scientists, climatologists, glaciologists, oceanographers and other scientists, hailing from 154 countries, are more certain than ever that humanity is to blame for global warming, which may be linked to odd events like trees blossoming in the Luxembourg Garden here in the middle of winter. The consensus stems from new evidence (among other things, proxies that extend the climate record back in time and six more years that are among the hottest ever recorded) brought forward since the last assessment in 2001. And it is unanimous, including the U.S. and other previously skeptical governments as well.

Negotiators continue to haggle behind closed doors of the United Nations Education, Scientific and Cultural Organization (UNESCO) as they work to iron out the wording of the final document. U.S. representatives, for instance, in response to early drafts of the report argued that "modifying solar radiance may be an important strategy if mitigation of emissions fails for one reason or another" and asked for its inclusion in the forthcoming summary for policymakers. Such geo-engineering, in the form of space-based sunshades or seeding the atmosphere with sulfate particles similar to those thrown up in a volcanic eruption, has been proposed as a last-ditch response to runaway global warming.

This painstaking process of reaching a consensus means that the IPCC assessment will be extremely conservative, confining temperature sensitivity and sea level rise to narrow bands. For example, early drafts fail to take into account factors such as the dramatically increasing melt rate of Greenland's glaciers—now up to six times the average flow of the Colorado River. "Greenland is probably going to contribute more and faster to sea level rise than predicted by current models," said Eric Rignot, a glaciologist at NASA's Jet Propulsion Laboratory who studied the glacial flow in a paper in Science last year. If all the ice in Greenland were to melt in coming decades (an unlikely scenario), it would raise sea levels by seven meters (more than 20 feet)—enough to swamp New Orleans, Florida's coast, Bangladesh and the Netherlands, among other low-lying lands.

Global warming skeptics are already gearing up to deconstruct the IPCC report, whatever its conclusions. The Fraser Institute—a Canadian think tank devoted to denying climate change—plans to release its own independent summary on February 5 and conservative Sen. James Inhofe (R-Okla.) has decried the politicization of climate change science. IPCC head Rajendra Pachauri's comment that he hoped the report "will shock people" into action has led some, including political scientist Roger Pielke, Jr., of the University of Colorado—proponent of a middle of the road plan neither denying the existence of climate change nor succumbing to extreme solutions—to question the organization's credibility.

But the report may well shock people: Early drafts noted that the frequency of extreme weather events will increase, Arctic Sea ice may completely disappear and rising sea levels will inundate existing coastlines—without taking into account variables like Greenland's increased rate of melting. It remains to be seen whether the final summary will contain these stern warnings, but UNEP is already pressuring new U.N. Secretary-General Ban Ki-moon to hold an emergency climate summit, according to Reuters. Even then, it will be too late to preserve the early blossoms of Parisian and other northern trees. "We'll lose some of those flowers for this season," says Nina Bassuk, director of the Urban Horticulture Institute, about prematurely flowering trees in the U.S. northeast. "But the long-term health of most plants probably won't be affected." If, that is, the climate does not change permanently.