The world's leading climate scientists have for the first time established a limit on the amount of greenhouse gases that can be released before the Earth reaches a tipping point and predicted that it will be surpassed within decades unless swift action is taken to curb the current pace of emissions.
The warning was issued Friday by a panel of U.N.-appointed climate change experts meeting in Stockholm.
The Intergovernmental Panel on Climate Change estimated that once a total of 1 trillion tons of carbon dioxide are emitted into atmosphere, the planet will exceed 3.6 degrees of warming, the internationally agreed-upon threshold to the worst effects of climate change.
"We've burned through half that amount" since preindustrial times, Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University who reviewed the report and is a co-author of the panel's upcoming report on the effect of climate change, said in an interview. "Because the rates of emissions are growing, it looks like we could burn through the other half in the next 25 years" under one of the more dire scenarios outlined in the report.
Other scenarios show that the threshold will be reached later this century.
The finding constitutes a warning to governments to cut emissions of carbon dioxide, the main greenhouse gas, which is generated by the burning of fossil fuels, industrial activity and deforestation.
Calling climate change "the greatest challenge of our time," panel co-chair Thomas Stocker said humankind's fate in the next 100 years "depends crucially on how much carbon dioxide will be emitted in the future."
In the report, the panel said it is 95% certain that human activity is the dominant cause of the global warming observed since the 1950s. That is up from 90% six years ago.
"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," the report said.
The report is the panel's fifth major assessment since 1990. It reaffirms many of the conclusions of past reports, but with greater confidence.
"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 panel wrote in a 36-page summary of its findings, released Friday. "Each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850."
The panel's full 2,500-page report will be published Monday.
The report also addressed the so-called hiatus, a slowdown in the rise of surface temperature that has been observed over the last 15 years. That slowing of the increase in temperatures has been seized on by skeptics to cast doubt on the science of climate change.
The report touches the subject only briefly, saying that temperatures fluctuate naturally in the short term and "do not in general reflect long-term climate trends."
Brenda Ekwurzel, a climate scientist for the Union of Concerned Scientists, said the slowdown is more like a speed bump, a result of heat being trapped and circulated through the ocean and atmosphere in different ways rather than a fundamental change in the climate. She said surface temperature is just one of many expressions of climate change, including sea level rise, melting ice and ocean acidification.
"The global average temperature is one kind of a thermometer, but an even bigger thermometer is the ocean, which is absorbing most of the excess heat that climate change is creating," she said.
The report updates predictions of how temperature and sea level are expected to rise over the century.
The panel now expects sea level to rise globally by 10 inches to 32 inches by century's end, up from the rise of 7 inches to 23 inches it projected in 2007.
Those figures now include the contribution of massive ice sheets in Antarctica and Greenland that are creeping toward the ocean as they melt. The panel failed to account for that variable in its previous report, prompting criticism from the scientific community that its previous sea level rise projections were too low.
The panel also lowered the bottom of the range of temperature increase expected over the long term if carbon dioxide concentrations in the atmosphere double. The planet would warm by at least 2.7 degrees even if aggressive action is taken to cut emissions, but temperatures could rise as much as 8.1 degrees in other scenarios.
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