Are the tornadoes in the USA, or the floods in Queensland and Victoria, or the record drought in southwest Australia, or the Russian heatwave of last year or western Europe in 2003, or Black Saturday, anything to do with global warming?
It’s a big question, with a complicated answer. So let’s start with heatwaves.
The last decade has seen record high temperatures in many parts of the world, including Europe in 2003, southeast Australia in early 2009, and Russia in 2010. Some of the recent temperatures have absolutely smashed previous records, not just broken them.
For instance, the new record daily maximum temperature established in Melbourne, Australia on Black Saturday, 7 February 2009, was more than 3˚C hotter than the previous record Melbourne February temperature (and nearly a degree hotter than the previous all-time record high temperature).
Later that year, at Windorah in western Queensland, the record temperature for August was broken six times in a single month.
Before 2009, the temperature in August at this station had never reached 35˚C. This temperature was exceeded seven times in August 2009 and a new record August maximum temperature of 38˚C was set, a full 3.1˚C higher than the previous record.
And there are many more examples like Windorah. In November 2009, 41% of NSW set record high November temperatures, in the one month.
Heatwaves are also setting new duration records. The week before Black Saturday, Melbourne saw three days in a row with temperatures above 43˚C. Previously the city had never recorded three consecutive days exceeding 42˚C.
The previous year, in March 2008, Adelaide had 15 days in a row hotter than 35˚C. Before 2008 the Adelaide record run of hot days was only eight days.
Record runs of hot days were again experienced in Adelaide in January/February 2009 and again in November that year.
In Sydney the first week of February 2011 saw seven consecutive days above 30˚C, a heatwave two days longer than had ever been seen in the 152 years of data. A new Sydney minimum temperature record was set, fully one degree above the previous all-time record.
Almost everywhere where the Bureau of Meteorology has good data, we are seeing more high temperatures and longer warm spells.
Of course, we still set new cold records, even in a period of global warming. But research at the Bureau of Meteorology has shown that over the last decade hot records are being set more than twice as frequently as cold records are being broken.
And it is not just Australia.
In Russia last year, and in western Europe in 2003, “mega-heatwaves” meant that their summer was most likely hotter than any summer for at least 500 years.
More than half of Europe broke the 500-year record twice in the one decade. Both these heatwaves caused tens of thousands of deaths. The last decade has truly been the decade of unprecedented heatwaves.
None of these heatwaves was caused by global warming. They were the result of naturally occurring weather events.
In the case of Melbourne such events lead to strong, sustained northerly winds from the interior of the continent. Similar weather systems were causing high temperatures centuries before we started to affect the composition of the atmosphere by increasing emissions of greenhouse gases such as carbon dioxide.
But the warming caused by our carbon dioxide emissions is pushing temperatures in these weather events to new extremes.
Other extremes are more difficult to link to global warming. The Queensland floods, for instance, were the result of a record-breaking La Niña event, a natural phenomenon that has been causing heavy rains and widespread floods in Australia for at least thousands of years.
Has global warming exacerbated the La Niña, or its impact on Australia? Perhaps, but the evidence is pretty thin, as yet.
Similarly, with the USA tornadoes – we just don’t know if there is a link. And we don’t even know if this is because the data are poor, or our analyses are insufficient, or if there really isn’t much of an influence of global warming on these sorts of extreme events.
We don’t even know if more and better research will answer this question.
But the evidence is clear for hot temperatures and heatwaves – humans are making these worse. And without political agreement to restrain greenhouse gas emissions we can look forward to worse heatwaves in the future.
We don’t cope well with heatwaves, even now. Mortality increases substantially on hot days, as do ambulance callouts and infrastructure damage.
So governments around the world are increasing their efforts to deal with the ever-increasing problems caused by heatwaves, by introducing and improving systems for alerting the vulnerable to an impending heatwave, and providing support to ensure we improve our ability to cope.
The alert systems won’t save everyone, but they do make a difference. And if by some fluke the world doesn’t continue to warm, if the fairies at the bottom of the garden manage to offset the increases in greenhouse gases, these new heatwave alert systems will still save lives. That’s not such a bad thing.