New climate report: Interpreting 95 percent odds of human-generated global warming

    A major new scientific report on global warming was released today, and the news is still bad. Still bad doesn’t make for flashy headlines. So the new emphasis is on the odds that humans are primarily responsible for warming so far.

    Here’s the New York Times version:

    The 2007 report found “unequivocal” evidence of warming, but hedged a little on responsibility, saying the chances were at least 90 percent that human activities were the cause. The language in the new draft is stronger, saying the odds are at least 95 percent that humans are the principal cause.

    The report came from the Intergovernmental Panel on Climate Change, a scientific group first convened by the United Nations in 1998.

    What does this mean? Scientists’ 95 percent confidence could lead some to believe there’s still a 5 percent chance that global warming is a hoax or that there’s nothing to worry about. But that’s not the case at all. There’s much greater than 99 percent certainty that human activity has changed the composition of our atmosphere by increasing the concentration of carbon dioxide and other greenhouse gases. And there’s well-established experimental and observational evidence that atmospheric carbon dioxide traps heat – that it’s a so-called greenhouse gas.

    The 95 percent simply applies to the confidence that the overall warming trend of the last 50 years is predominantly caused by this known human-generated change to the atmosphere. It’s a little like a doctor saying multiple tests show you have lung cancer and he’s 95 percent sure your well-established case of lung cancer is the dominant cause of your recent cough. That means there’s a 5 percent chance you’ve been coughing for some other reason, but not a 5 percent chance you don’t have cancer.

    Of course there’s never 100 percent certainty in science – it deals in probabilities rather than proofs. An interesting story from the Associated Press made that point, but goofed in making the case for ignoring climate change look much more rational than it really is:

    Some climate-change deniers have looked at 95 percent and scoffed. After all, most people wouldn’t get on a plane that had only a 95 percent certainty of landing safely, risk experts say.

    I noted in a comment on the Knight Science Journalism Tracker that the analogy is backwards. The scientific panel is saying there’s a 95 percent chance we’ve caused the current problem, not a 95 percent chance everything is fine. A better analogy would be a plane that had a 95 percent chance of some mechanical malfunction. The “deniers” as the AP story describes them, would be analogous to people who would board anyway because they don’t want to be inconvenienced over a risk that hasn’t been proven.

    I’m not sure there’s much merit in labeling people as deniers. Let’s just say that some people continue to argue that humans have no role in climate change. Those people would be wrong – or nearly certainly so.

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