The report is the official view of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), the body of several hundred scientists advising governments on global warming since 1990. The IPPC's findings, seen as the most authoritative estimates of changing climate, are designed to be used as the basis of policy by all governments.
The 1,000-page study complements a similar report on the science of global warming last month in Shanghai, which concluded that the world's average temperature will rise by up to 6 degrees Centigrade (34.5 to 42.44 degrees Fahrenheit) by 2100--faster than originally thought--with sea levels rising by an average of up to 88 centimeters (nearly three feet). In high latitudes, the temperature increase is likely to be greater than the average.
This means glaciers and polar ice caps melting; countless species of animals, birds and plant life dying out; farmland turning to desert; fish-supporting coral reefs destroyed; and low-lying island states in the Pacific and Caribbean under water.
Northern hemisphere countries would probably become hotter, bringing a rise in deaths from heatstroke in cities and the onset of once-tropical diseases such as malaria and the West Nile virus, which arrived in the United States in 1999.
Africa, already suffering severe economic and social problems, would be most vulnerable to the effects of warming. Disease levels could shoot up, especially in crowded cities on the coast, which could also face inundation as sea levels rise.
In Asia, mangrove forests that protect river and sea banks could be swamped, especially in Bangladesh. Forest fires could become more frequent, and warmer conditions could increase the risk of infectious disease. In the Middle East, the report suggested, wars could break out over water resources.
The melting of glaciers in the Himalayas, which provide water to about 500 million people, could first cause huge flooding, then massive water shortages. Much of Latin America could see a decline in crop yields, shrinkage of forests, and the arrival of new diseases.
In Europe, the southern countries are more likely to be affected, with an increased risk of water shortage and a deterioration in soil quality that would affect agriculture. In more northerly countries, such as Britain, flooding would be worse and tropical diseases could become endemic.
Roger Higman, senior climate campaigner for the environmental group Friends of the Earth's, said: "This catastrophe was made in the rich countries of the north. Governments in industrial countries must agree radical cuts in our use of coal, oil and gas, and big increases in the use of renewable power. If we don't act now it may be too late."
The U.N. report, released in Geneva Monday, said that projected changes this century "have the potential to lead to large-scale and possibly irreversible changes in earth systems resulting in impacts at continental and global scales."
The report also predicted that:
Equity issues and development constraints could arise if weather-related risks become uninsurable, prices increase, or availability becomes limited.
Harvard University Professor James McCarthy co-chaired a four-day IPCC meeting in Geneva last week that was sponsored by the U.N. Environment Program and the World Meteorological Organization and attended by more than 100 government and scientific experts. The attendees approved by consensus, after heated discussions, both the 1,000-page report and the policymakers' summary.
McCarthy told reporters that while some, such as farmers in temperate zones, might reap gains in farm output if temperatures rise by a few degrees, most people on earth "will be on the losing side."
Dr. Robert Watson, chief scientist at the World Bank and chairman of the IPCC panel, said most global warming in the last 50 years has been due to human activity. He said tens of millions of people are at risk due to storms and sea-level rises in coastal areas.
"No country can afford to ignore the coming transformation of its natural and human environment. The report is a timely reminder that we need to pay more attention to the costs of inaction," said UNEP's executive director, Klaus Toepfer.
Environmental advocacy groups taking part in the IPCC session also called for action Monday by Washington and other key capitals.
The conservation group World Wildlife Fund International said in a statement that it believes governments "must use the G-8 environment ministers meeting in two weeks time in Trieste, Italy, to show they have taken on board the results of the IPCC'S work."
"This week we're seeing what's in the firing line. It's time governments such as the United States get serious about reducing their carbon dioxide emissions," said WWF's climate change campaign director, Jennifer Morgan.
Bill Hare, climate change director for Greenpeace International, in a statement released in Amsterdam, said, "It's time for governments, particularly the new Bush administration, to show that they're taking the reality of climate change seriously."
McCarthy, in an interview with United Press International, dismissed fears held by some key industrialized and developing nations that the new Bush administration might take a hard-line stance, given Bush's close ties to the oil industry.
Asked about the approach of the Bush team on climate change, he said he was not pessimistic, adding that the president's father, when he was in the White House in the early 1990s, succeeded in changing the tone on the issue, and went to Rio in 1992 to sign the U.N. Convention on Climate Change.
"If he [Bush] wants to be re-elected, he will need to address these issues. He can show leadership here," McCarthy said.
Scientific skeptics include Roger Pielke, a Colorado State University atmospheric scientist who believes the computer models used by the U.N. panel to make climate predictions are too simplistic.
Pielke, who is also with the National Center for Atmospheric Research, said he's not suggesting climate change isn't a problem, but that today's science is not capable of reliably predicting that change.
"People are affecting Earth's climate but in unpredictable ways," he told MSNBC. "It's surprising we think we can predict 50 years into the future when we can't predict next year's weather."
Pielke said that while the U.N. panel limits its models to how carbon dioxide and other gases impact the atmosphere, he and many colleagues look at additional factors like how carbon affects plant biology or how land and water are impacted by population, not just global warming.
"We're showing that these other effects are at least as significant" as changes in the atmosphere, Pielke added.
A better planning policy, he contended, would be to look at "vulnerability to all kinds of environmental stresses, which includes changes in long-term weather, and then decide where the threats are most serious."
Anthony Janetos, who co-chaired the group that prepared the U.S. contribution to the U.N. report, acknowledged the limitations of modeling but defended it as the best tool available for looking at plausible scenarios of climate change.
Those limitations "certainly shouldn't be an excuse for people not to do anything," Janetos, a vice president at the World Resources Institute, also told MSNBC.