The new UN climate report to be released on Friday in is expected to cast a gloomy picture of the consequences of global warming.
The report will project that one-fifth of all animal and plant species are threatened with extinction if warming continues at the current pace, according to an advance look at the draft document.
The Intergovernmental Panel on Climate Change (IPCC) study, six years in the making and drawing on the work of more than 2,000 experts, also projects that hundreds of millions of people will be threatened with flooding from rising sea levels, fed by ice melt at the north and south poles.
Extreme rainfall will increase in frequency and intensity. In Europe alone, the report says that every year from now until 2080 could see up to an additional 2.5 million people affected by flooding.
The expected climate changes will hit hardest the poorest regions of the world that are least responsible for producing greenhouse gasses like carbon dioxide, and threaten especially the health of their children. Drought areas and deserts will expand. The risk of starvation will grow, the draft report says.
Friday's report is also expected to say it may already be too late to prevent some of global warming's impacts - humans can only adapt to so much, especially when it comes to rising sea levels flooding islands.
The scientists began meeting on Monday in Brussels for the final report.
"Many of us believe we are on the threshold of a massive extinction event," Jeff Price of California State University at Chico, a lead author of the IPCC report's chapter on ecosystems, said.
All of the findings are already known through the reams of scientific research published since the IPCC's last series of reports in 2001. The job of the more than 2,000 scientists was to sift through those findings.
What singles out this report, scientists say, is the strong consensus found within the worldwide literature - and the development of new models since 2001 that have allowed scientists to set specific timeframes for events.
"We have for the first time started putting bounds, temperature limits on when things will start happening," Price said.
Though Price would not discuss specific findings ahead of the April release, he said it was generally agreed that just a 2-degree-Celsius hike in global temperature would lead to a "serious conversion of habitats," while anything above 2 degrees Celsius could result in "major ecosystems' collapses".
The IPCC report's first section released in February, which dealt with the scientific evidence, found an "unequivocal" trend of rising global temperatures and sea levels and placed the blame squarely on man-made emissions.
It predicted the Earth would heat up between 1.8 and 4 degrees Celsius by the end of this century, and up to 6.4 degrees Celsius at the poles, which heat up twice as fast as around the equator. Melting ice has led sea levels to climb 17 centimetres in the 20th century, and at a rate of 3.1 mm per year since 1993.
On Monday, the environmental group Friends of the Earth in Brussels called for "urgent assistance" for developing countries that will be hardest hit.
European Union Environment Commissioner Stavros Dimas criticized the US - which produces 25 percent of greenhouse emissions - for dragging its feet and lashed out at Australia for not supporting the Kyoto protocol under which international industrial nations have committed to cut greenhouses gases.
Belgian Prime Minister Guy Verhofstadt called on the US, China and India "to get on board for more extended measures" in fighting climate change.
The IPCC's third section will be released in Bangkok in May, offering specific advice on how policy makers can reduce global warming.
In a development related to climate change, the US Supreme Court on Monday in Washington declared global warming a serious and urgent problem in its first ever ruling on the subject. The court ruled that the federal government has the authority under a 1990 clean air law to cap carbon dioxide and other emissions blamed for global warming.
It said the government is not obligated to control the emissions but said the Environmental Protection Agency (EPA) would have to come up with scientific reasons for not doing so.