Climate change and droughts and flooding rains

The crazy weather of the 2012-13 Australian summer – which apparently isn’t over yet – prompts the question of whether climate change means this is what we should expect from now on. The answer is yes, that’s the trend, but the details are a bit more complicated, and definitely worth knowing.

First though, the simple answer is that, as the Australian Government’s Climate Commission pointed out in their report, The Angry Summer:

Extreme heatwaves and catastrophic bushfire conditions during the Angry Summer were made worse by climate change… All extreme weather events are now occurring in a climate system that is warmer and moister than it was 50 years ago. This influences the nature, impact and intensity of extreme weather events.

Having said that, it doesn’t mean that it’s always-always going to be like this. People tend to think that any weird weather is completely unprecedented and at the same time a permanent change, when in fact variation is a normal part of the system.

To take an example: Melbourne weather is famously variable, so you’d think Melburnians would be used that. But every year when the first hot days arrive in September or October everyone says “summer’s come early! Time to put away the warm clothes.” Then it gets cold again, and everyone says “where did that come from? Oh well, guess there’s not going to be much of a summer this year.” And so on. Every year. Even this past summer.

However, under climate change, the odds are skewed towards hotter weather, even as this variation continues. If there was no global warming, you’d still expect occasional records broken for extreme hot and cold weather. But with the 0.8°C warming we’ve seen under the past 100 years, that same variation around the average will produce slightly more hot records than cold ones.

Graph showing how a shift in average temperature changes the proportion of extreme events (click to embiggen)
Image from the Climate Commission’s Angry Summer report, showing the connection between a shifting average and the proportion of extreme events (adapted from IPCC report, 2007)

It’s important to remember this, so you don’t get fooled into thinking that global warming has stopped when it gets cold again in a few months time. After all, 0.8°C isn’t a very noticeable change when considered on a daily basis.

So that’s the big picture, but to find out exactly what it means for weather in your area you have to turn to the climate scientists and their computer models. You can look them up yourself: for an overview, see www.climatechange.gov.au. For more technical and detailed report from the CSIRO and the Bureau of Meteorology, see www.climatechangeinaustralia.com.au.

When you go to these sites, you’ll see patterns that are largely a continuation of what we’ve experienced so far. Temperatures will rise on average, with the greatest warming in the middle of Australia and the north-west.

But there will also be more hot days and warm nights, including an increase in the number of days per year over 35°C. For instance, Melbourne currently has 9 days above 35°C per year (that’s average: in 2013 we’ve already had 14), but by 2070 it could be as high as 26. Brisbane will go from an average of 1 to up to 21. Darwin could be up to 308 days above 35°C every year.

Rainfall is a little more complicated. On average, warmer weather puts more moisture into the atmosphere, but changing temperatures also change wind patterns. The top end is not going to have much of a drop in average rainfall, but the rest of the continent will – especially the southwest. Perth is already experiencing much drier conditions.

Rainfall trend map showing the changes in annual totals across Australia from 1910 to 2012, with the biggest drops in the southwest and the biggest increases in the northwest (click to embiggen)
Rainfall trend map showing the changes in annual totals across Australia from 1910 to 2012 (Image from the Bureau of Meteorology)

But not only will rainfall reduce, it will also vary much more. We’re likely to have more very wet days and more very dry days. So yes, more droughts: again, the south-west will be worst hit, with an 80% increase in drought by 2070.

What’s that you say, Mr Andrew Bolt? Doesn’t the recent flooding in Queensland and NSW prove these predictions wrong? No, because I said there’d be more variation.

Some climate models predict that storms will get more intense, particularly tropical cyclones like Oswald that caused most of the trouble (Knutson TR, McBride JL, Chan J, Emanuel K, Holland G, Landsea C, Held I, Kossin JP, Srivastava AK & Sugi M 2010, “Tropical cyclones and climate change”, Nature Geoscience, vol. 3, no. 3, pp. 157-163, doi:10.1038/ngeo779 [PDF 641 KB]).

At the same time though, there’s an effect called Hadley circulation, which causes dry air in the subtropics. What happens is that air travels from the subtropics to the equator, rises to about 10-15 km, then travels back and descends again. Most of the world’s deserts can be found in the latitudes where the Hadley circulation brings dry air down.

With climate change, that circulation is predicted to expand, and the areas of drier climate will also expand. And for Australia, that means reduced rainfall on average, with longer periods of drought punctuated by these more intense storms.

But don’t just take my word for it: let’s look at the weather records again.

For this purpose, I’ve chosen to look at the weather station at Fairymead Sugar Mill. It’s just outside Bundaberg, which experienced such devastating floods in late January and early February 2013.

Fairymead also happens to be in the Bureau of Meteorology’s high quality climate site network, meaning it has good quality data stretching back many years – in this case, back to 1881. And it’s very easy for anyone to go to the bureau’s website and generate graphs of mean annual rainfall for one of these sites, complete with a trendline.

Graph of total annual rainfall records for Fairymead Sugar Mill weather station, with a trendline showing a decline of 12.69 mm per decade from 1881 to 2012 (click to see more)
Total annual rainfall records for Fairymead Sugar Mill weather station, showing a decline of 12.69 mm per decade from 1881 to 2012 (Image from Bureau of Meteorology)

When you do that, you see a drop in rainfall from an average of close to 1200 mm per year, down to 1000 mm per year. That’s a 16% decrease over 130 years.

The bureau also issues monthly drought statements, showing the total rainfall levels for a season. The January 2013 drought statement – that’s prior to the floods – showed the Bundaberg area was in “serious deficiency” for the 5 months from August to December 2012, i.e. it was in the bottom 10 per cent of rainfall. The record breaking rainfall (and tornadoes) experienced from 22-29 January was enough to change that, but it’s entirely consistent with the predictions of drier averages with occasional bursts of heavy rains and floods.

The fact that it’s consistent with predictions means that yes, we can expect more weather like this in future. Just hopefully not every year.

(This story first aired on 7 February 2013 – you can listen to the podcast.)

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