Global Warming: Why Only 95 Percent Certainty?

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Climate scientists meeting Friday in Stockholm declared that there’s a 95 percent likelihood that climate change is the result of human activity, primarily the burning of fossil fuels which release carbon dioxide and other heat-trapping gases into the atmosphere.

Scientists agree climate change is one of the most serious issues facing humanity, but still, debate over the issue continues to rage on.
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But why isn’t that figure 99, or even 100 percent?

It turns out that the massive report by the Intergovernmental Panel on Climate Change can say some things with absolutely certainty, or 100 percent probability. That includes findings that the planet is warming, the seas are rising and glaciers in Antarctica, Greenland and other mountain regions are melting. It also includes the fact that human activity is pushing up levels of atmospheric carbon dioxide that are the highest in 800,000 years.

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But when it comes to the links between human activity and the climate, scientists have to rule out all other kinds of natural variability, according to Christopher Field, a climate scientist at the Carnegie Institution for Science who worked on the IPCC report.

“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.

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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.

“If A, B and C each have 90 percent chance of occurrence, their occurrence together indicates 99.9 percent certainty,” Xie said in an email to Discovery News. “We have more than three lines of evidence.”

Xie pointed to an important statement in the IPCC report to remove any lingering doubts: "Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system."

"Since it is factual," Xie said. "It is 100 percent true."