The top authority for assessing scientific research on climate change has concluded that global warming is "unequivocal," that humans are "extremely likely" to be the dominant cause, and that there's stronger evidence now for placing the blame with us.
The Intergovernmental Panel on Climate Change, first convened by the United Nations in 1988, periodically releases reports to assess global warming research findings, and on Friday, its 259 authors from 39 countries announced the IPCC's fifth report -- a major update since the last one in 2007. The new report won't surprise anyone who's followed scientists' conclusions about how greenhouse gases including carbon dioxide trap heat and thus warm the atmosphere and oceans.
"Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes," said a summary of the IPCC report (PDF) released Friday. "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century," the report said, and the evidence for human influence is stronger now than when the group wrote the 2007 report.
Several facets of global warming have been observed, the report said, and the evidence leaves no doubt:
Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.
The evidence of human involvement is stronger, too. In the previous report, the IPCC researchers concluded it was likely that humans played a role in cold days getting warmer and in hot days getting hotter. Now human influence is rated as "very likely" for both of those phenomena.
The full report is scheduled for release on Monday.
In the report, "likely" means scientists are 66 percent to 100 percent sure of a particular conclusion, and "very likely" means 90 percent to 100 percent.
"Observations of changes in the climate system are based on multiple lines of independent evidence," said Qin Dahe, a co-chair of IPCC Working Group I that wrote the report.
The ocean is absorbing most of the heat, he added, and some global warming effects will accelerate. "As the ocean warms, and glaciers and ice sheets reduce, global mean sea level will continue to rise, but at a faster rate than we have experienced over the past 40 years," he said.
The changes will lead to more climate extremes at the local level, said Thomas Stricker, also a co-chair. "As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions," he said.