Scientists looked at emission plans from 192 nations and calculated what would happen to global warming. The projections take into account 80 percent pollution cuts from the U.S. and Europe by 2050, which are not sure things.
The U.S. figure is based on a bill that passed the House of Representatives but is running into resistance in the Senate, where debate has been delayed by health care reform efforts.
Carbon dioxide, mostly from the burning of fossil fuels such as coal and oil, is the main cause of global warming, trapping the sun's energy in the atmosphere. The world's average temperature has already risen 1.4 degrees (0.8 degrees Celsius) since the 19th century.
Much of projected rise in temperature is because of developing nations, which aren't talking much about cutting their emissions, scientists said at a United Nations press conference Thursday. China alone adds nearly 2 degrees (1 degree Celsius) to the projections.
"We are headed toward very serious changes in our planet," said Achim Steiner, head of the U.N.'s environment program, which issued the update on Thursday.
Even if the developed world cuts its emissions by 80 percent and the developing world cuts theirs in half by 2050, as some experts propose, the world is still facing a 3-degree (1.7 degree Celsius) increase by the end of the century, said Robert Corell, a prominent U.S. climate scientist who helped oversee the update.
Corell said the most likely agreement out of the international climate negotiations in Copenhagen in December still translates into a nearly 5 degree (2.7 degree Celsius) increase in world temperature by the end of the century. European leaders and the Obama White House have set a goal to limit warming to just a couple degrees.
The U.N.'s environment program unveiled the update on peer-reviewed climate change science to tell diplomats how hot the planet is getting. The last big report from the Nobel Prize-winning Intergovernmental Panel on Climate Change came out more than two years ago and is based on science that is at least three to four years old, Steiner said.
Global warming is speeding up, especially in the Arctic, and that means that some top-level science projections from 2007 are already out of date and overly optimistic. Corell, who headed an assessment of warming in the Arctic, said global warming "is accelerating in ways that we are not anticipating."
Because Greenland and West Antarctic ice sheets are melting far faster than thought, it looks like the seas will rise twice as fast as projected just three years ago, Corell said. He said seas should rise about a foot every 20 to 25 years.
Other problems that have worsened since the 2007 report include the oceans getting more acidic - a threat to some sea creatures - and projections for regular long-term droughts in the U.S. Southwest.
"As sobering as this report is, it is not the worst case scenario," said U.S. Rep. Edward J. Markey, co-author of the bill that passed the U.S. House. "That would be if the world does nothing and allows heat-trapping pollution to continue to spew unchecked into the atmosphere."
On the Net: U.N. Climate Change Science Compendium: http://www.unep.org/compendium2009/