Climate Scientists 95 Percent Certain that Global Warming Is Man-Made

Reuters has gotten hold of a copy of the new report by the United Nations' Intergovernment Panel on Climate Change that aims to summarize the best scientific data with regard to the causes, effects, and trends of climate change. From Reuters:

Drafts seen by Reuters of the study by the U.N. panel of experts, due to be published next month, say it is at least 95 percent likely that human activities - chiefly the burning of fossil fuels - are the main cause of warming since the 1950s.

That is up from at least 90 percent in the last report in 2007, 66 percent in 2001, and just over 50 in 1995, steadily squeezing out the arguments by a small minority of scientists that natural variations in the climate might be to blame.

That shifts the debate onto the extent of temperature rises and the likely impacts, from manageable to catastrophic….

The report will flag a high risk that global temperatures will increase this century by more than that level, and will say that evidence of rising sea levels is now "unequivocal".

Projected sea level rise by 2100 has been increased from 18 to 59 centimeters in the last report to 29 t0 82 centimeters (11.4 inches to 32.3 inches). The report also apparently backs off on the claim that warming has already made hurricanes worse. The new IPCC report will evidently try to explain why global average temperatures have been essentially flat for the past decade:

An IPCC draft says there is "medium confidence" that the slowing of the rise is "due in roughly equal measure" to natural variations in the weather and to other factors affecting energy reaching the Earth's surface.

Scientists believe causes could include: greater-than-expected quantities of ash from volcanoes, which dims sunlight; a decline in heat from the sun during a current 11-year solar cycle; more heat being absorbed by the deep oceans; or the possibility that the climate may be less sensitive than expected to a build-up of carbon dioxide. (emphasis added) 

It will be very interesting to see how the new report handles the recent studies that suggest that climate sensitivity - conventionally defined as to how much warming can be expected from a doubling of carbon dioxide concentrations in the atmosphere - may be significantly less than had been earlier maintained. If those new studies are correct, that means that catastrophic warming is less likely and that humanity would have more time to address whatever problems future warming might cause.

The new IPCC Working Group 1 report on the physical sciences will be officially released on September 27.