The world’s hottest year on record was 2014. On the whole, global average temperature has increased by at least 0.8oC since 1880. And human influence on average temperature increases since 1950 is unequivocal.
Hang on. Only 0.8oC?! That doesn’t seem like much at all. Surely that can’t actually mean anything!
This is the reaction I consistently get when attempting to explain how climate change impacts heatwaves. How can a small change on the global scale impact events over my country/state/region?
The answer: well, quite a lot actually.
You see, the relationship between averages and extreme temperatures is disproportionate. By definition, the average is what’s expected, whereas extremes are rare. This figure, part of the Intergovernmental Panel on Climate Change Special Report on extremes graphs this relationship pretty nicely. By shifting the average just a little to the right (ie. it becomes warmer), there’s a disproportionate increase in the number of extreme, or ‘rare’ events. Also, more ‘records’ are seen – these are extreme temperatures that would not have occurred had the average temperature not increased.
THE COMPLEXITY OF HEATWAVES
Heatwaves are just one type of temperature extreme. In fact, they’re quite complex, since their occurrence depends also on weather systems, the time of year (depending how you define them), and even how much rain has recently fallen. Yet increases in their frequency, intensity and duration have been measured globally since at least 1950. This is where most of the observed global warming has occurred.
Exactly what aspects of heatwaves have changed the most and where these changes have occurred does have some variation. In Australia, the city of Canberra has seen incidence double since 1950, however Melbourne has seen the hottest heatwave day increase by over 4oC (see this report for more detail). Globally, increases in heatwave frequency over Europe and parts of Asia are larger than most other regions.
HUMAN ACTIVITY AND HEATWAVES
As I mentioned above, heatwaves are complex, and are caused by multiple other mechanisms other than changes in average temperature. Depending on the region, some mechanisms are more dominant than others. For example, a lack of winter rainfall over Europe greatly influences the intensity and duration of the most extreme heatwaves for this region. Such conditions were key ingredients for severe European heatwaves in 1976, 1994, 2003 and 2005.
How can we be sure that observed changes in heatwaves are due to humans?
In a similar way that doctors can work out how lifestyle factors (eg. smoking) increases the risk of certain diseases (eg. cancer), climate scientists can work out how greenhouse gas emissions from human activity alter the risk of certain extreme events, such as heatwaves. We do this with climate models, as we can switch things on and off in them that we can’t do with the real climate. We can simulate a ‘natural world’, where greenhouse gases are kept at pre-industrial levels, and we can also simulate the ‘current world’ over and over again, with greenhouse gases are prescribed by real-world observations. Then we compare how often the heatwave occurs in the natural and current worlds.
The result? Not great.
While the 2003 European heatwave was, at least in part, due to low rainfall during the previous winter, a 2004 study found that the frequency of such an event had at least doubled due to human influence on the global climate. However, a more recent study in 2014 found that the likelihood of a similar event occurring has increased even further, just over the last decade.
Over Australia, the occurrence of severe heatwave seasons such as 2013 has at least doubled due to human activity. That same year, a severe heatwave season occurred over South Korea, which now occurs 10 time more often than before the industrial revolution (both studies are in this report).
These are just a few examples. There is a whole swathe of literature concluding that human activity has influenced observed changes in heatwaves. While there are other processes that trigger these events, and there is regional variation on how much humans have altered the occurrence of these high-impact events, the common conclusion is crystal clear – the impact of global climate change on these high-impact events is already detectable.
And this is all from a global average temperature change of 0.8oC.
What about the future? Well, if we keep emitting greenhouse gases at the current rate, by 2100, global average temperature could be anywhere between 1.5-5oC. I don’t think I need to spell out the disastrous impact this will have.