Sunday, January 13, 2013

The heat is on

If you are in Australia and are reading this, you'll know it's been hot lately, really hot, unprecedented in fact.

 So how hot has it been? Well, last Monday the 7th of January was the hottest day in Australia since records began (check out the statement from the Bureau of Meteorology (BOM) here). Not only that, but the first 8 days of 2013 were all amongst the top 20 hottest days since records began. So even by last Tuesday the length of a heatwave of this intensity was unprecedented. Currently the BOM data was last updated on the 9th, but the heat has continued, so it will be interesting to see how many more records have been broken.

So this really is an unprecedented heatwave, not only has it been bloody hot, but the area covered by the heatwave has been huge. It is always interesting when a single region breaks its all-time temperature record, as Hobart did on the 4th of January. But think about the amount of heat needed to break a record in a single place, and then think about the amount of heat required to break the heat record for a country the size of Australia and you can see which is more significant.

It is also worth noting that not only was the Australia-wide maximum temperature record set on the 7th, but so was the record for the highest minimum temperature, until it was broken again the next day, that is.

Less appreciated, but also interesting is how long this heatwave (in the broader sense) has actually been going. According to the BOM, the last four months of 2012 were the hottest on record, so in some respects this current heatwave actually started in September.

Combine this unprecedented heatwave event with record heat over the last 4 months and very dry conditions since mid-2012 and it is no surprise that bushfires have followed.

Unprecedented heatwaves such as what we are currently experiencing raise the question of if climate change is involved. The answer, quite simply, is yes. I had been trawling through Australian climate data on the BOM website to help show this, but thankfully several climate scientists have written a short report to do my homework for me (it's only a couple of pages so I recommend you download it if you want to know more).

Over the last ~50 years average temperatures in Australia have risen by more than half a degree, a trend that has also played out worldwide. Looking at the data you can see the whole country has been getting warmer. So we can see that our climate has already changed, it is hotter than it used to be and this means more heatwaves, hotter heat waves and/or longer heatwaves.

Or, think of it this way. The maximum temperature each day can be described using the "bell curve" in the picture below. Average temperatures are those that occur most often and they represent the tallest part of the curve. Temperature extremes (both low and high) are rare and so form the left and right hand edges of the curve.

What global warming is doing is pushing the curve to the right (called "new climate"). This means the temperatures that occur most often (the "mean") are now warmer. But perhaps more importantly, what was previously hot weather (ie: a heatwave) that occurred rarely is now more common and new record hot temperatures now become possible.

We are seeing this in Australia. Warming has lead to increases in the number in hot days (over 35 degrees) and hot nights. While record hot days are now outstripping cold records 3:1, with the number of record hot days doubling since 1960 (ref). So while it wouldn't be accurate to say that global warming has caused this current heatwave, we do know it is making heatwaves more common, more severe and more likely to last longer. Global warming is worsening this heatwave and (although this will need to be tested by climate scientists) the unprecedented nature of the heatwave makes it quite possible that a heatwave of this magnitude simply would not have occurred without global warming.

And what of the future? Well as climate scientist Professor David Karoly points out, over the next 50 years we are expecting:
 "two or three times the warming we've seen already leading to much greater increases in heatwaves and extreme fire danger days".

In other words, the unprecedented nature of this heatwave won't stay unprecedented for long.

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