In a report by the Intergovernmental Panel on Climate Change (IPCC) due out next month, researchers say it is at least 95 percent likely that human activities are the main cause of global warming since the 1950s, according to Reuters who acquired drafts of the study ahead of schedule.
This marks an increase from 90 percent from the last report issued in 2007, 66 percent in 2001 and just over 50 percent in 1995.
Among the many contributions made by humans, the report cites the burning of fossil fuels as the number one driver of any anthropogenic climate change.
However, growing certainty regarding the source of climate change is not currently being met in terms of certainty of its regional effects, researchers say.
"We have got quite a bit more certain that climate change ... is largely manmade," Reto Knutti, a professor at the Swiss Federal Institute of Technology in Zurich, told Reuters. "We're less certain than many would hope about the local impacts."
Drew Shindell, a NASA climate scientist, said when asked what the temperature in a region will be in a given area in the next 20 to 30 years, the answer is unclear.
"We can't really tell," he said, citing a lack of research since the release of the 2007 report. "It's a shame."
One of the main challenges of the report is that of explaining why temperatures have risen more slowly over the the last 15 years or so even as greenhouse gas concentrations have repeatedly reached record highs, especially in areas such as China.
In response to this the UN panel of experts report "medium confidence" that this tapering off is "due in roughly equal measure" to natural variations in the Earth's weather as well as other factors, such as higher quantities of ash from volcanoes and a decline in heat from the Sun during an 11-year solar cycle.
In short, Gabriele Hegerl of Edinburgh University said, the last decade or so could simply be a blip or it could be "minor contributions that all add up."
All told, the draft, which uses more complex models than the one from 2007, estimates that temperatures could rise anywhere from less than 1 degree Celsius to 5 degrees Celsius within the century -- a wider range than the one presented in the report six years ago.
In the end, however, the leaked document represents only a draft and not the final product.
"It is guaranteed it will change," IPCC spokesman Jonathan Lynn said, according to the BBC, explaining that the scientists have yet to go through the 15-page summary aimed at policymakers "line by line."
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