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This story was updated at 9:05 a.m. EDT.
The Intergovernmental Panel on Climate Change's latest report on the science of global warming squarely blames humans as the primary cause of climate change, saying it is "extremely likely" that human activities have caused most of the warming of the planet's surface that has occurred since the 1950s.
The assessment, released today (Sept. 27), is the first major report from the Intergovernmental Panel on Climate Change (IPCC) since 2007, and presents the strongest case yet for human-caused global warming since the IPCC was established in the late 1980s.
"There's much stronger evidence connecting human activity to changes in temperature, melting glaciers and ocean warming," said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research and one of the authors of the new report. "There's a lot more evidence that connects human activity to changes in the climate system."
In the new summary, climate scientists say they are at least 95 percent certain that people are responsible for the warming oceans, rapidly melting ice and rising sea levels that have been observed since the 1950s. The 2007 report linked human activities to climate change with 90 percent certainty, which was a considerable leap from the 66 percent probability stated in the organization's 2001 report. [8 Ways Global Warming is Already Changing the World]
"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 new report says. "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
Dangerously high emissions
The scientists also warned that emissions of carbon dioxide and other greenhouse gases will need to be cut in order to keep the increase in average global temperature to less than 3.6 degrees Fahrenheit (2 degrees Celsius), and avoid the most devastating effects of global warming. The benchmark of 3.6 degrees Fahrenheit was set by climate negotiators in Copenhagen in 2009.
Given current patterns of emissions, approximately 1 trillion tons of carbon could be burned and emitted into the atmosphere before the increase in the planet's average surface temperature creeps above 3.6 degrees Fahrenheit, the panelists said. However, more than half of that amount has already been burned, Thomas Stocker, co-chair of the so-called Working Group I report, said at a news briefing this morning in Stockholm, Sweden.
"We cannot emit more than 1,000 billion [1 trillion] tons of carbon, of which already 54 percent has been emitted," Stockersaid. The report does not specify when and how emissions should be reduced, he said, "but we give very relevant guidance for the total amount of carbon that cannot be exceeded, in terms of emissions, in order to stay below 2 degrees Celsius [3.6 degrees Fahrenheit]."
Yet, some estimates indicate that even if measures are introduced today to reduce greenhouse gas emissions, the 3.6-degree limit will likely be surpassed by the end of the century.
The effects of climate change
The new assessment also contains updated projections for various climate scenarios, including sea level rise, melting glaciers and rising global average temperatures. [FAQ: IPCC's Upcoming Climate Change Report Explained]
If greenhouse gas emissions are not curbed, sea levels could rise as much as 3 feet (0.9 meters) by the year 2100, the scientists said. This is an increase from the estimated 0.9 to 2.7 feet (0.3 to 0.8 meters) of sea-level rise that was predicted in the 2007 IPCC report.
"The top estimate is about 3 feet, and that's quite a lot to deal with," Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University in Princeton, N.J., and a contributor to previous IPCC reports, told LiveScience. "Sea level rise is a real problem because it poses very high risks around coasts."
Global temperatures are also likely to rise by between 0.5 and 8.6 degrees Fahrenheit (0.3 degrees and 4.8 degrees Celsius) this century, depending on global levels of carbon emissions.
"Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system," Stocker said. "Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions."
The report found that atmospheric concentrations of carbon dioxide, methane and nitrous oxide are now at levels that are "unprecedented" in at least the last 800,000 years.
Panel members said they are hopeful the report will shape climate negotiations between nations, particularly in the lead up to the UN Conference on Climate Change scheduled to take place in Paris in 2015.
"Thanks to the dedicated work of [this] global community of scientists, we know the nature of the problem, and the options for addressing it," UN secretary general Ban Ki-Moon said in a video address to the audience in Sweden this morning. "The heat is on. Now we must act."
Today's assessment is part of the IPCC's latest summary on climate change, called the Fifth Assessment Report of AR5. The IPCC reports consist of four sections: the Working Group I report on the science of climate change; the Working Group II report on the vulnerabilities and socioeconomic impacts; the Working Group III report on possible ways to mitigate climate change; and the Synthesis Report, which reviews the findings from all the working groups and integrates relevant information for policymakers. The other sections of the IPCC's Fifth Assessment Report will be released in 2014.
The IPCC was established by the United Nations in 1988 to comb through the most recent published and peer-reviewed research on global warming, and put together comprehensive reports on the risks and impacts of climate change.
The assessments undergo an extensive review process that involves thousands of scientists and government officials, and the final products represent consensus within the scientific community. As a result, the IPCC reports are considered the authority on the risks and impacts of global warming.
LiveScience Staff Writer Becky Oskin contributed reporting.