How does climate change affect extreme weather events?

Extreme fire danger

Extreme fire danger: image credit Parkaboy

John Connor, CEO of the Climate Institute, made a speech today talking about bushfires. I’ve been pondering one of his key points for the last two weeks, ever since the bushfires.

Climate change is not just about warmer weather, it is about wilder weather.

One of the lessons I learned early on as an actuary is that the extremes of any distribution do not behave the same way as the middle. So if the middle of a distribution of temperature rises by 1 degree, the effects on the extremes are not intuitive.

Something that was a 1 in 100 chance of occuring in the old distribution is not just slightly more likely. Moving the distribution up the curve can significantly increase the chance of that 1 in 100 event. For an extreme event that is three deviations away from the mean of a distribution, moving the mean up 20% of a standard deviation doubles the chance of that extreme event. So put it in temperature terms – say the summer mean maximum temperature is 25 degrees, with a 1 in 400 chance of a 40 degree day. Increase the mean maximum temperature to 26 degrees, and there is a 1 in 200 chance of a 40 degree day. And that is assuming that the climate follows a nice stable normal distribution model (or bell curve – the one that statisticians like, because it’s easy to work with mathematically).

You don’t need complex models of extreme weather events following climate change to realise that extreme events become much more frequent if the average temperature moves up a little bit. But complex models of extreme weather events suggest that the extremities move even further than a very simple normal distribution would suggest.

The Climate Institute put out a very detailed  report from in September 2007.  They first of all looked at the increase in extreme fire danger in the two global warming scenarios – low global warming (0.4 degree by 2020 and 0.7 degrees by 2050) and high global warming (1 degree by 2020 and 2.7 degrees by 2050)  – based on projections from the Intergovernmental Panel on Climate Change (IPCC). The number of extreme fire danger days in 2020 in the low global warming scenario increased by 5-25%. And the number in 2050 using the high global warming scenario increased by 100-300%.

They developed two new categories beyond the Australian standard ‘extreme’ fire danger days – ‘very extreme’ and ‘catastrophic’. Catastrophic fire danger days, which at the time of the study had occurred 12 times in the more than 30 years of the temperature records studied at the sites studied, were projected to occur once every three years at 7 of the 26 sites studied, and once every 8 years in 12 more.

Given that the flash point of eucalyptus oil is 49 degrees, bushfire risk does not just increase in a linear way with temperature. The closer the air temperature gets to 49 degrees, the more likely that just some random heat point will set something off. The maximum temperature in much of Victoria on Saturday 7 February was around 46 degrees. And there had been little or no rain for the last month, with baking temperatures. Although that weather pattern is quite theoretically possible without global warming, global warming makes it much more likely.

John Connor is right. Time to add Very Extreme and Catastrophic to all those bushfire warning signs you see everywhere in the Australian bush.

  9 comments for “How does climate change affect extreme weather events?

  1. February 21, 2009 at 2:58 am

    I can not tell you how much I love your perspective… it is just fascinating to me…whenever I read one of your posts like this, I *almost* believe that I should find the year twelve teacher who told me I would regret switching from maths to biology and apologise that I spent the whole year walking around the schoolyard carrying the Web of Life and saying ‘he said I’d regret it!’.

  2. February 21, 2009 at 6:26 am

    Add to that the way forests have been mis-managed (at least on the American West Coast) for the last 100 years!

    We have signs like that around here. They sometimes move the arrow to “moderate” but I’ve never seen it at “low.”

  3. February 21, 2009 at 8:07 am

    That’s quite terrifying. People in the UK are all too complacent about the idea of wild weather, somehow managing to avoid noticing all the recent floods, hurricane force storms and blizzards in February that we’ve started getting.

  4. February 21, 2009 at 9:00 am

    Great post. And agreed, we need a scale of warning systems. Though I keep thinking about how I reacted to John Brumby’s warning when I read it in the Age, seeing it as unnecessarily alarmist and unconvincing…not the messenger but the vehicle. The Boy who Cried Wolf seems to apply – the Age and all newspapers have made such a business of inflating bad news stories, that they lack all credibility, even when the news really is bad.

  5. Wizofaus
    February 21, 2009 at 9:50 am

    Good post but the issue with the flash point of eucalyptus oil raises a question – 46 was the shade temperature on Feb 7 – the temperature under the sun must have been well above 49 already, probably as high as 60 in some areas – so wouldn’t the possibility of spontaneous combustion of such oils already exist?

  6. February 21, 2009 at 10:32 am

    A fantastic post Jennifer. You should offer it to a newspaper as a column. I can’t find any way to email you from this blog, but if you want to email me back, I’d be more than happy to send it to several opinion editors suggesting that they publish it.

  7. February 21, 2009 at 8:24 pm

    Thanks everyone.

    Wizofaus – the flash point just means the point at which a spark will start a fire. So you’re right, in the sun, eucalyptus oil on a hot day will probably be hotter than 49 degrees, and if there is enough of it in one place, a tiny flame would start a huge fire.

    That day (in Sydney, only 42 degrees) I was at a birthday party outside on a large lawn, and I remember wondering whether we should have lit the candles. The answer is that we probably shouldn’t have – total fire bans mean TOTAL fire bans.

    Penni, not being in Victoria, I didn’t have to react to the warnings, but after the fact, I’m not sure what anyone could have done – the whole state was at risk, after all – where was it safe to evacuate to? From here it almost feels as if the fires that happened could have happened pretty much anywhere in the state – it was just terrible bad luck for the towns they got to.

  8. February 24, 2009 at 8:23 am

    I think it’s part of the whole living in the bush conundrum. I mean after all it’s not just where we keep our stuff. We LIVE out here. Our kids go to school. This is where we read and watch tellie and cook our dinners and visit our friends and shower and sleep and hang out and do nothing and, for many of us, work. For example yesterday was a day of extreme risk, but it was also my husband’s first day back at uni, a school day for one daughter, a creche day for another and a work day for me…it’s hard to evacuate and disrupt everything on the chance of a fire. In the aftermath it seems obvious enough to say leave early, why even take the smallest of risks, but in practice, evacuating on the off-chance and disrupting all familiar routines is impractical, especially when the fire season extends into the school year for at least 3 months (Feb, Mar, Dec). Again I’m not sure about the solution to this, but we may require one if the weather is going to continue to worsen. Yesterday was a tense and horrible day for me, and we’ve got another one on Friday. Roll on winter.

  9. freebornjohn
    February 28, 2009 at 3:09 pm

    Warming can even be about colder weather too:

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