"It's a 95 percent probability that more than half of the climate change is from human actions," Field said. "The 95 percent leaves only a sliver of doubt. In most of science when we say were convinced, we say less than 1 in 20 chance. So that's one of the reasons the 95 percent standard is appropriate here."
The 5th IPCC report has a higher confidence that the 4th report, which stated the same cause and effect at 90 percent. That's because measurements of the Earth's climate are more accurate, more widespread and cumulatively larger than the last report, Field said.
Still, the Earth's atmosphere is not a test tube. There are other factors that come into play, such as the natural variability of the climate over time, as well as effects of volcanic gasses.
In fact, the 95 percent figure cited in the summary of the four-part IPCC assessment applies to the overall report, not individual statements on the human impacts of climate change, according to Shang-Ping Xie, another report author and climatologist at the Scripps Institution of Oceanography in La Jolla, Calif.