Now they have to figure out just what to do about it.
"It is critical for us to get a much better understanding of the impact of climate change in some parts of the world," Rajendra Pachauri, chairman of the U.N.'s Intergovernmental Panel on Climate Change, told The Associated Press in an interview Tuesday.
Scientific warnings of potential catastrophe have been the backdrop for talks among more than 10,000 delegates and environmentalists negotiating a treaty to control the emission of greenhouse gases, which have grown by 70 percent since 1970. The treaty, due to be completed in one year, would replace the Kyoto Protocol, which expires in 2012.
Pachauri said he was concerned that negotiators were sparring and probing — and leaving key decisions for the last moment.
"My concern is that if we leave everything to the end, we might end up with a weak agreement that doesn't really address the problem," he said.
Last year, Pachauri's IPCC, which collected the work of more than 2,000 scientists, said climate change is "unequivocal, is already happening, and is caused by human activity."
It listed likely effects of global warming: arid regions will grow dryer, rising seas will flood coastal areas, melting glaciers will flood communities downstream and then dry up the source of future water supplies, and up to 30 percent of all plant and animal species may become extinct.
Since then, new evidence has emerged showing that ice caps in the Arctic and Antarctic are melting, which threatens to dramatically raise the level of the oceans and flood coastal cities and low-lying islands.
"Small island states are living in a state of fear," he said.
But Pachauri said there was no conclusive evidence the world is in imminent danger.
"I don't think we should jump to conclusions if we get material that is based on the last one or two years," the Indian scientist said. The IPCC issues its reports every five or six years.
The 2007 report cited a scientific consensus that global warming should be limited to 2 degrees Centigrade (3.6 degrees Fahrenheit) to avoid the worst scenarios. To contain global warming to that target, carbon emissions must peak by 2015, then begin a rapid decline.
Pachauri now says governments should reconsider whether even that goal goes far enough, since it would still raise sea levels from between 15 inches (40 centimeters) to 4.6 feet (1.4 meters).
Dozens of scientists were among the delegations or nongovernment groups attending the Poznan conference, exhibiting some of the latest technologies and scientific studies.
"The skeptics are doing a good job because they are making us present ironclad proof," said Lawrence E. Buja, a climate change researcher for the U.S. National Center for Atmospheric Research in Boulder, Colorado.
But since that battle is over, he said scientists need to move on and look at the detailed impact of climate change.
"That's a much harder question," he said.
Buja, who contributed to the IPCC report, said scientists are looking at futuristic solutions to halt global warming, such as imitating the cooling effects of a massive volcanic eruption by spreading sulfur in the atmosphere, or scattering billions tiny refractors high in the air to dim the sun and lower the temperature.
But he said such radical solutions involve risks.
"How are you going to go up and find all those little refractors and pull them down if something bad starts to happen?" he asked.