The worlds governments must dramatically cut emissions of heat-trapping gases or learn to live with more droughts, heat waves, powerful rainstorms and rising sea levels that will affect millions, leading climate scientists said after the release of a landmark global warming report today.
The United Nations-backed Intergovernmental Panel On Climate Change issued its most definitive, detailed and sobering report in Paris, saying they are more than 90 percent certain that humans are the leading cause of the worlds temperature rise in the last 50 years.
The panel projected that by the end of the century, global temperatures will rise 3.2 to 7.2 degrees Fahrenheit and sea level will rise 7 to 23 inches and possibly more, depending on how much carbon dioxide and other greenhouse gases are emitted from power plants, factories and automobilies. For the first time, the panel also linked global warming to more powerful hurricanes that hit the Americas and projected they would become even more intense.
Temperatures and sea levels will continue to rise for centuries because gases persist in the atmosphere so long -- no matter what steps governments, businesses and individuals take. But continued emissions at or above current levels would likely lead to accelerated warming and other climate changes, the report warned.
Northern latitudes can expect less snow, earlier frosts and more heavy rain storms -- phenomena that parts of the Northeast are already experiencing. Winds will become stronger. There will be more frequent hot days and fewer cold days.
Warming of the climate system is unequivocal, the report said, noting that 11 of the last 12 years ranked among the 12 warmest since accurate record-keeping began in 1850.
Response to the study was swift from world leaders, with United Nations Climate Secretary Yvo de Boer proposing an emergency climate summit of the worlds leaders to push for emission reductions. French President Jacques Chirac called for an economic and political revolution. Italian Environment Minister Alfonso Pecoraro Scanio called for a global tax on emissions of carbon dioxide, the main heat-trapping gas.
In Washington, where the Bush administration has long questioned the science behind climate change and opposes mandatory limits on greenhouse gas emissions, the White House released a statement noting that the administration "continues to support and embrace" the science behind the IPCC report and that the United States is playing a leading role in studying climate change and funding research.
Energy Secretary Samuel W. Bodman said that the U.S. was a small contributor to global warming compared with other countries. Yet, scientists say the US emits about 25 percent of the worlds greenhouse gas emissions, the largest single source in the world.
Environmentalists reacted angrily to what they said was Bushs attempts to appear progressive on climate change, while several members of Congress took Bodman to task for his remarks.
"What the scientific community is telling us is that we face catastrophic problems if we do not move boldly," said US Senator Bernie Sanders, a Vermont independent. "For the administration to suggest the United States is not a major emitter of greenhouse gases is preposterous.''
Members of both parties are supporting various measures to restrict emissions, but Senate Majority Leader Harry Reid, a Nevada Democrat, said in a statement: "The Congress and the American people are waiting to see how this President responds. ... We cannot leave future generations with a disaster by failing to act today."
Scientists involved in the report and other researchers who have read it said worldwide emissions of carbon dioxide need to be sliced 70-80 percent as soon as possible to slow the world's path toward a dramatically warmer existence and the swallowing of coastal regions. While they acknowledged it was a mammoth undertaking, many appeared somewhat hopeful, saying they have noticed a shift in public awareness of global warming and expected political change to follow.
"We are in reality therapy," said Jerry Mahlman, a leading climate scientist at the National Center for Atmospheric Research in Colorado. "Now it is how does the world get energized? We are up against it."
The climate change panel is made up of hundreds of scientists from more than 130 countries that reports on the state of climate science about every five years. Its last report, released in 2001, said emissions of greenhouse gases were "likely" the major contributor to the warming earth, but the scientists were less certain then. Scientists' higher confidence in today's conclusion is based on a thorough review of voluminous research done in the last six years, they said.
The IPCC will issue three more reports this year -- two this spring on more specific effects and how to solve or offset the problem of global warming, and a third pulling together all their work.
Today's report revealed a world in enormous flux: Deeper sections of the oceans are warming because they are absorbing most of the climate's added heat, causing the water to expand and sea levels to rise. The panel was able to draw strong links between human-induced warming and changing wind patterns, ocean warming, and temperature extremes.
Scientists were also able to simulate expected changes in smaller geographical areas of the world for the first time, to project, for example, that the Northeast and southern Canada will continue to see increasing precipitation and a decrease of snow depth. The southwestern US will see less rain and increases in maximum summer temperatures.
The IPCC is approaching the scale where people actually live, said Linda Mearns of the National Center for Atmospheric Research and a lead author of the report's regional climate predictions.
For its projections of the climate at the century's end, the IPCC assumed a range of six scenarios of emissions, from one of rapid economic and population growth that would result in as much as three times current emissions, to one where emissions would be less than current levels.
Under all scenarios, sea ice is projected to shrink in the Arctic and Antarctic.
There is some good news, however: None of the models projects the shutting down of a worldwide ocean circulation system that brings warm air to northern regions. Some scientists have suggested that the system, often called the ocean conveyor belt, could shut down in a warming world, plunging northern regions into a mini-ice age.
One of the most debated scientific issues is over sea level rise, which has the potential to flood millions of homes, especially in low-lying developing countries. The IPCC estimated sea levels would rise 7 to 23 inches by the end of this century, but new evidence shows that sea levels are rising faster than the IPCC predicted in its 2001 report.
Scientists involved in the study today said their report only could calculate sea level rise from the expansion of water as it is heated and did not take into account the accelerated melting of Arctic and Antarctic ice sheets. If the ice sheet melting continued to accelerate, the report notes, sea level could rise 4 to 8 additional inches by 2100 than the new report projects.
Overall, Patrick Gonzales, a climate change scientist at The Nature Conservancy who was a reviewer of the IPCC report, said the study strengthens the evidence of the need to take action right now to avert the most serious effects of climate change.
John Donnelly of the Globe staff and Globe wire services contributed to this report. Beth Daley can be reached at firstname.lastname@example.org.