Sunday, February 8, 2009

Australian wildfires and climate statistics

The Aussies are fighting for their lives and homes again. Thirty five people have died already.

The increased frequency and intensity of Australian brush fire seasons is another of those extreme weather phenomena that are linked to climate change statistically, but that a climate scientist is not allowed to say "is caused by" climate change.

Sooner or later, when we're the ones whose homes and lives are threatened, we might begin to give up on that scientific nicety or point of etiquette, so that we can begin to communicate with ordinary people in the way that they understand.

You see, a scientist who knows that there was any probability at all that there might be bush fires in Australia (or in California, or hurricanes in the Gulf, or high summer temperatures and tornadoes in Maine, or any of a dozen other extreme weather phenomena) knows that she cannot say scientifically that this bush fire was caused by climate change.

What she might instead say is, ceteris paribus, if we put more GHGs into the atmosphere, the likelihood of bush fires in any given year goes up, as well as the likelihood that there are more and larger brush fires.

Even then, you may still get a year without brush fires.

Likewise, in Maine, all other factors being what they currently are, we expect increased GHGs will lead to warmer and shorter winters. But we also expect that they will lead to more precipitation, ie: snow.

And we can't rule out the possibility that the random winter will be quite cold.

This is because climate change means that the frequency of warm winters will likely increase, and the average temperature of winters will likely increase, but the variability of winters may also increase.

This could be called the "mean-variability paradox," and it's one reason why Rush Limbaugh is able to get away with telling people that this year's cold in Iowa and Maine means that climate change is definitely not happening, when 99% of climate scientists will tell you it is, despite the cold.

To understand, you need to learn a statistic called the frequency distribution.

If you take the frequencies of occurrence of mean winter season temperature in Maine, and plot their distribution, you will get a bell or normal curve like the one above.

If you increase the mean temperature, but not the variability, you move the curve to the right, but it keeps its overall shape.

If, however, you increase the mean temperature, but also the variability, you may move the peak of the curve to the right, but flatten the curve out at the same time.

That's when you notice that it remains possible, although less likely, to have winter as cold as some winters used to be.

The mean-variability paradox, in political terms, is a sucker-punch. It sucks you in. It makes "big fat liars" like Limbaugh seem more believable on climate change than thousands of climate scientists. It allows a large, even politically crucial percentage of people to go on believing that climate change is not happening, even when it is, and, when combined with the bit of scientific etiquette that prevents scientists from making plain-spoken statements related to causation, adds up to decades of lost time.

Which leads directly to dead bodies like the ones in Australia last week. The Aussies, it seems, either have already or will soon accept the new reality and make their dispositions accordingly: more firefighters better ecological planning rules, firebreaks, possibly abandoning some areas for settlement as just too dangerous. More taxes to pay for this. Reduce fossil fuel consumption.

They will need our help with this last one, or they'll be piddling into a very hot wind for decades of hot summers to come.

I keep saying this: we're all going to have to learn to think more complex thoughts if we're to get through these next few decades with civilization intact.

Postscript: Real Climate Blog has an excellent piece by an Aussie scientist giving the actual details of the calculation I summarized above.

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