A recent post at WUWT discusses projections for increase in the number of very hot days in northern Australia, based on expected temperature change estimated by computer models. It’s actually a follow-up to a previous post on that topic. The theme seems to be doubt about the projections based on general incredulity, combined with devising some “rules” designed to make people mistrust computer models in general.
I expect that as the world gets hotter (which it will), so will Australia, and as a consequence the number of very hot days will increase. The original projections of how much they will increase are based on taking recent temperature data and increasing it by the forecast warming. This amounts to assuming that as Australia warms, the mean of the distribution will increase but the shape of the distribution will remain the same.
That’s not a bad approach as a first approximation. But I’m interested in whether or not we might also see a change in the shape of the temperature distribution, and how that might affect the number of very hot days.
To investigate the issue, I’ll look at the daily high temperature data from Darwin, Australia according to the Bureau of Meteorology (BOM). The daily high temperature since 1910 looks like this:
The horizontal red line marks 35°C (95°F); the adopted definition of “very hot” is a daily high temperature above that. We can count the number of such days each year, giving this:
Clearly the number of very hot days has been on the rise. But what about the distribution of daily high temperatures?
I divided the time range into 20-year intervals, starting with 1910-1930, ending with 2010-present (which is only a little less than 7 years). Then I estimated the probability distribution of daily high temperature in each time interval. That gives this:
The steady increase in average temperature is clear. But so too is the steady decrease in the width of the distribution. As a result, the probability of “cool” days has decreased faster than the probability of “hot” days has increased.
We can also look at the survival function at each temperature (the probability of being that hot or hotter):
The probability of a “very hot” day is now more than three times what it used to be. It’ll rise even more in the future.
But it may not rise quite as fast as some expect, because the shape of the distribution is narrowing. Still, it is rising at a disturbing rate. Also, the conclusion that the shape of the distribution is narrowing (that the probability of “cool” days has decreased faster than the probability of “hot” days has increased) is based on observed data, which makes it more real than a computer model simulation, but also means that we don’t really have a theoretical basis for expecting that trend to continue, other than to extrapolate based on assuming the trend will continue.
By the way, counting the number of very hot (T > 35°C) days per calendar year has a drawback, in that a very hot summer will be split between two calendar years because in the southern hemisphere, the summer season straddles the new year. A better idea, in my opinion, is to count the number of very hot days reckoning the “year” as July-through-June rather than January-through-December. On that basis, the counts look like this:
Note that when the yearly count includes the entire hot season like this, the hot summers of 2002-2003 and 2012-2013 really stand out, with 29 “very hot” days each. Note also that 2015-2016 already has seen 19 such days, in spite of the fact that the data for calendar year 2016 isn’t yet listed so that counts only half of the July-through-June year.
Deniers like the regular crew at WUWT will continue to try to persuade you not to expect any bother from the increase in hot weather, be it northern Australia or anywhere else. Meanwhile, the people of Darwin will swelter.
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