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The IPCC's journey from "probable" to "unequivocal" on human-caused warming

Out of the more than 3,000 pages in Monday's landmark climate report, one word stood out: "unequivocal."

What they're saying: "It is unequivocal that human influence has warmed the atmosphere, ocean and land," the report stated.

  • This is how solid the tie is between a warming planet and human emissions of greenhouse gases, scientists and government representatives agreed.

Why it matters: It's the strongest description the U.N. IPCC has used to attribute climate change to human activities, but it's far from the first time the word has been used in its reports.

  • Looking over time at the panel's past assessments sheds light on just how much of a departure the latest one is.

Context: The use of the word in Monday's report is part of a pattern of IPCC language used in its "Summary for Policymakers" sections that date to its first assessment report, issued in 1990.

  • Along with a series of remarkably prescient climate projections through 2030, that 1990 report stated: "The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more."
  • Jump ahead to 1995, when the IPCC came out with what was then a bombshell finding, stating: "The balance of evidence suggests a discernible human influence on global climate."

Be smart: That demonstrated the high stakes associated with individual word choices in these summaries.

Between the lines: Next, fast-forward to the IPCC's fourth assessment in 2007. By then, the science had advanced sufficiently — as had global warming's effects, for the panel to state: "Warming of the climate system is unequivocal." However, it didn't attribute that warming to human emissions of greenhouse gases in such strong terms.

  • Before Monday's report, the most recent full IPCC assessment of climate change was in 2013. That report expressed climate attribution in more confident language than prior assessments, but still not with the same impact:
  • "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century," the report stated.

The big picture: The IPCC's evolution on these statements is slow and methodical, in part because consensus-based science is inherently somewhat conservative.

  • Also, the IPCC is unique in that scientists and governments together approve the summaries word by word, since they get the most attention from the media and world leaders.

The bottom line: There is actually an even stronger attribution statement in Monday's report, hidden in the technical summary but noticed by the AP's Seth Borenstein.

  • It states that the "Human influence on the climate system is now an established fact." It also describes the human influence on extreme weather and climate events as an "established fact."
  • Case closed.

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