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What ‘likely’ and ‘extremely likely’ really mean in climate-speak

Rajendra Pachauri, the head of the U.N. Intergovernmental Panel on Climate Change (IPCC), left, and Sweden's Environmental ministers Lena Ek, right, comment on the U.N. IPCC climate report, in Stockholm, Friday Sept. 27, 2013. Scientists can now say with extreme confidence that human activity is the dominant cause of the global warming observed since the 1950s, a new report by an international scientific group said Friday. Calling man-made warming "extremely likely," the Intergovernmental Panel on Climate Change used the strongest words yet on the issue as it adopted its assessment on the state of the climate system.

Jessica Gow/AP

Climate scientists announced Friday that they now believe it is "extremely likely" that warming of the Earth's climate is caused by humans, marking a rise in confidence from "very likely" in 2007.

What do these terms actually mean?

The authors of the Intergovernmental Panel on Climate Change (IPCC) report use terms that range from "extremely unlikely" to "virtually certain" as a way of qualifying in words how confident they are in their findings.

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There is a percentage range attached to each of those terms.

Since 2001, the IPCC has continuously raised its level of confidence that climate change is man-made to 95 per cent this year, from 90 per cent in 2007 and 66 per cent in 2001.

The IPCC's likelihood scale:

  • Virtually certain: 99 to 100 per cent probability
  • Extremely likely: Over 95 per cent. According to the IPCC, it is now “extremely likely” that human activity is to blame for climate change
  • Very likely: Above 90 per cent
  • Likely: Above 66 per cent
  • More likely than not: 50 per cent and above
  • About as likely as not: Between 33 and 66 per cent
  • Unlikely: Zero to 33 per cent
  • Very unlikely: Zero to 10 per cent
  • Extremely unlikely: Zero to five per cent
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About the Author
National Food Reporter

Ann Hui is the national food reporter at The Globe and Mail. Previously, she worked as a national reporter and homepage editor for theglobeandmail.com and an online editor in News. More

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