In the conservative world of science, conclusions are couched in caveats and statements are chosen carefully to not seem overwrought. And in the world of climate science that’s no different. For example, in their 2007 report, the Intergovernmental Panel on Climate Change (IPCC) issued a series of defined levels of certainty about their conclusions that looked like this:
• “Virtually certain” (considered more than 99% likely to be correct)
• “Very likely” (more than 90%)
• “Likely” (more than 66%)
• “More likely than not” (more than 50%)
• “Unlikely” (less than 33%)
• “Very unlikely” (less than 10%)
• “Exceptionally unlikely” (less than 5%)
So in the IPCC’s final report they made statements like, “Global climate change is “very likely” to have a human cause.”
According to a recent article in New Scientist this use of language may be one of the reasons the public is not sharing the same level of urgency to reduce greenhouse gas emissions that scientists have:
David Budescu of the psychology department at Fordham University in New York and colleagues asked 223 volunteers to read sentences from the IPCC reports that used these expressions. For example: “It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent.”
They then asked participants to estimate on a scale of 0 to 1 the probability conveyed by each sentence.
Participants tended to underestimate the certainty of the sentences. Three quarters of respondents thought “very likely” meant less than 90% certain, and nearly half thought “very likely” meant less than 66% certain. Public understanding of climate change was slightly better if the readers were given a legend to refer to.
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