This piece is based on my own work and is apart of a blog series profiling climate scientists, economists, social scientists and civil society members who are presenting and discussing innovative climate science at Our Common Future Under Climate Change Conference in Paris, July 2015. For more follow @ClimatParis2015 and #CFCC15 on Twitter.

It’s boiling outside. You can’t remember the last time it was this hot. It feels like the sun is sucking every morsel of water out of your body. Hopes of a short respite in a delicious ice cream fade fast when you realize how quickly it will melt. 

You’re in the middle of a heatwave. 

Heatwaves, measured as prolonged periods of excessive heat, are a complex type of extreme temperature event. These events occur naturally (albeit rarely) as part of our climate, and are driven by a delicate balance of the right weather patterns, local soil moisture conditions, and larger-scale climate variability patterns.

Unfortunately, they’ve increased in their intensity, frequency and duration over many regions of the globe since at least the middle of the 20th Century.

But is that because of human influence on the global climate?

In order to answer this question, I’ve used a special set of simulations from a global climate model. One of these experiments simulates what the global climate would have been like without human activity. It represents an alternate climate, had the industrial revolution never occurred. The other experiment includes observed anthropogenic emissions of greenhouse gases, thus simulating the historical climate. 

From these simulations I calculated trends in heatwaves across the globe and compared them to observed trends. I did this for trends since the 1950s and also since 1998 (when the “hiatus” begun, apparently). And I found some very interesting and sobering results. Though for many, they are probably not surprising.

If we compare long-term trends of heatwaves in a world where human influence is included to a world without humans, it is obvious that we are largely responsible for the rate at which they are increasing. This is not quite the same thing as overall or absolute changes in heatwaves – sure, we are now seeing more of them than we used to. And projections from climate models indicate that even more will occur in the future as our influence of the climate increases. But what is interesting from my findings is that the speed at which they are increasing could not have occurred naturally – heatwaves fluctuated from year to year, but not on timescales in the order of decades.

Had the industrial revolution never occurred, almost everywhere in the world would have seen no, or at most, a very little (and insignificant) changes in the frequency of heatwaves since 1950.  Yet the observations tell us that what actually happened were significant increases across almost every region where there is sufficient data. This pattern is consistent only under a climate that is altered by us, as indicated by the climate model simulations.

As for the “hiatus” period, increasing or decreasing trends in heatwaves differ from region to region. Yet this is expected over shorter timescales, since climate variability processes (think El Niño/Southern Oscillation) dominate. Besides, regional and global trends of heatwaves are not robust on these timescales. We simply don’t have enough information over 10-15 years to have a clear picture of what is really happening in the climate system.  This also stands for other climate variables, such as global surface temperature – changes over short time periods (particularly those that start from extremely warm years) simply cannot tell us the whole story.

What my results for the “hiatus” period also tell us is that while regional cooling trends in heatwaves can still occur under (the current amount of) human influence, warming trends are still far more likely. This means that shorter time periods in the near future are likely to have sharp increases in heatwaves. 

Not only is this bad news for you and your ice cream (more of them will melt more quickly in the future), but it’s also terrible news for the many other people and systems adversely affected by heatwaves. In Australia for example, fruit bats literally fall out of trees when the extreme heat is on. An estimated 70,000 people were killed in Europe during the 2003 heatwave, and although it’s too early to know the full impact, over 2,300 people have already died in the recent Indian heatwave.

With regional trends in heatwaves increasing more quickly than ever before, there is little doubt that the adverse impacts will sharply rise too.

Although written by me, this piece is a part of a blog series profiling climate scientists, economists, social scientists and civil society members who are presenting and discussing innovative climate science at Our Common Future Under Climate Change conference in Paris, July, 2015. For more follow @ClimatParis2015 and #CFCC15 on Twitter.


By 2100, the latest state-of-the-art global climate models project a global average temperature rise of 2.6-4.8 degrees C under a high emissions scenario. While initially this may not sound like much, the impacts of such an increase will be disastrous to many systems, both human and natural, if they cannot adapt. However, what is likely more important for the adaptation of these systems is the rate of change towards the projected increases, over that of the absolute increases. 

Understanding this rate of change has been central to Yann Chavaillaz’s work, a Ph.D student studying at the Laboratoire des Sciences du Climat et de l'Environnement in France. Looking at an ensemble of climate models that project the above absolute warming, he’s developed indicators directly linked with this rate. 

These indicators regionally compare how much change has occurred over a 20-year period, compared to the prior 20 years. This method has the benefit of determining how quickly the climate is changing, rather than the overall change from an arbitrary baseline state.

What he’s found is that the rate of change is not uniform throughout the 21st century.  In fact, the influence of this rate on temperature distributions peaks for many regions around or just after the middle of the century.

Take for example West Africa. Currently, about 15% of the region has a one in two chance to experience an extremely warm year, compared to the last 20 years. By about 2060 - and compared to 20 years immediately before – the indicator peaks at 75%. That’s 5 times the current figure, and, frightfully, accounts for future warming that hasn’t even occurred yet. 

This means the rate of warming will become increasingly rapid over the next few decades, despite accounting for any sort of adaption to recent changes.

“The definition of an extremely warm year will be outdated within an increasingly short timescale,” says Yann. 

“The acceleration in the temperature rise will induce drying and moistening trends that will persist in some critical regions”. 

And this result is pretty robust over the models he’s analysed. Taking into account how the models represent climate variability and other differences in their representations of the earth system, qualitative conclusions remain near identical from one model to another. 

The above results, along with those Yann will present at the Our Common Future Under Climate Change conference, mainly focus on the RCP8.5 scenario. This is the scenario with the greatest change and most potential damage over the remainder of the century, and is unfortunately the scenario we are currently tracking.

However, should substantial mitigation practices be put in place and lower emission scenarios followed, indicators linked to the rate of change are not so disturbing. 

Under RCP2.6, which involves immediate and extensive mitigation, all regional indicators return to historical values by 2050. Under RCP4.5, a more middle-of-the-road scenario, indicators remain fairly consistent until 2040, and afterwards return to near-historical values. 

So while some absolute changes to the climate do occur under these emission scenarios, adaptation to changes over the prior 20 years might be much more possible, since the rate of change in the climate is slower than under RCP8.5.

Yann’s current research has applied this approach of seasonal or yearly changes in average temperature and rainfall. However its versatility means that it can be applied to basically any climate variable to understand when its rate of change may peak. He’s even working on extending the method to see if start of spring and the duration of summer are influenced by such rates of change.  

“I really hope that my results might me helpful for adaptation planning,” Yann says,  “as climate continues to change, natural and human systems will need to continuously adapt to a moving target”.

By quantifying this moving target, Yann has given insight on how much of a challenge this may be.

Right now I’m on a working holiday (though far more on the work side!) in Europe. Also right now there is a rather large heatwave that has hit Spain, is currently over France, the southern U.K., Switzerland and Italy and is starting to move towards Germany, Austria and the Czech Republic (currently where I am). The other day, the hottest ever temperature in London during July was recorded, and many locations in Spain and France have tipped over 40 (somewhat ironically the next leg of my journey is a climate conference in Paris).

I’ve also seen a bit of crap floating around the internet saying that Europe isn’t really in a heatwave and, frankly, that everyone should suck it up and enjoy the warm rays of sunshine.

So I thought this would be as good a time as ever to talk about how heatwaves are measured.

The U.K. Met Office uses this definition: at least 5 consecutive days where the temperature is at least 5oC above the daily maximum (get that?). {Edit - I originally reported this is what the World Meteorological Organisation uses, but I have been corrected. It is, however the definition in Wikipedia}. The threshold used is specific to each day of the year, so thresholds in spring are cooler than summer thresholds, and you can feasibly have a heatwave in winter too. It’s also relative to the location, so thresholds in London, say, will be cooler than Melbourne.

There are pros and cons to this definition. Let’s go with the pros first.

The definition is relative to location and time of year.  It’s worth keeping in mind that we’re all adapted to the climates we live in. The temperature in Český Krumlov today was around 28-30, to me this is lovely summer weather that I really enjoyed exploring this beautiful town in, but then I’m Australian (o.k., so today was more holiday than work…..). Many residents the Czech Republic did not seem to be coping so well. Smack on a few consecutive days under these conditions and it will wear them down. The same goes for any other living things in this climate – trees, pets, wild animals, crops.

The relativeness to the time of year is also a nice feature. We generally associate heatwaves as summer events since this is when the most disastrous impacts occur. Indeed, some abnormally warm weather in winter can be pretty enjoyable. But we’ve got to remember that such conditions are not normal to the time of year, and can disrupt the reproductive cycle of staple crops, and, as what’s currently happening back home, preventing some much anticipated snowfalls.

Now let’s go with the cons.

Firstly, the definition stated above is antiquated. This group, who came up with the index about 15 years ago don’t actually use it anymore. They realized (as did I when I wrote this paper) that it doesn’t work everywhere (particularly the tropics), which is a bit redundant for an index that’s meant to be relative.

Secondly, 5 consecutive days is too long to be the minimum length of time a heatwave should last for. Over many locations in Australia, we rarely get heatwaves of over 5 days - due to the synoptic patterns that govern our weather -  yet we still definitely get heatwaves. The Australian Open heatwave in 2014, which received worldwide media coverage, is a perfect example of this.  Also, the impacts of heatwaves on human health, infrastructure, and our native ecosystems can kick in from 2 consecutive days. Particularly in the case for human health, this is a major issue if nighttime temperatures aren’t cool enough.

So, what’s the right way to measure heatwaves then?

Truth be told, there is no ONE way. In fact, there are literally hundreds of heatwave definitions. Sure, a lot of them are closely related, but how they are calculated is different. Some incorporate relative humidity. Some are a mixture of daily maximum and minimum temperatures. Some only measure summer events. Some used fixed thresholds (e.g. days above 300C). Believe me, the list goes on and on and on…..

Jee, I don’t even stick to a single definition! In a lot of my research activities, I use the Excess Heat Factor, the official definition of the Australian Bureau of Meteorology. This index is kinda neat, since it includes a measure of acclimatisation, as well as identifying how hot it is against the background climate. However on my website scorcher, heatwaves are measured based on at least 3 consecutive days exceeding the daily 90th percentile (each day is in the hottest 10% for that day of the year), since this was easier to calculate and show on graphs. All in all, the events measured by these definitions are pretty much the same, though they aren’t 100% identical.

Given the vast range of impacts heatwaves have, I don’t think we’ll ever fully agree on one definition. Sure we might (and should!) be able to narrow it down, but I think a complete one-size-fits-all approach is a little out of reach.

But I certainly do think it’s time that the guidance on a formal definition is changed. 5 days is certainly too long for a minimum length. And a threshold that does not work everywhere, particularly in regions where impacts can potentially be very high, needs quick attention. Based on the current state of research, it really is time for more formal guidance, perhaps by something like the World Meteorological Organisation, ton an overarching definition. That way, weather and climate services the world over can adopt their own heatwave definitions that are more in line with current scientific research.

Putting all of the above aside, I do hope everyone who is in the thick of the hot European weather is taking care. If it is 1, 3, 5, or even more hot days in a row, if the temperature is 30C, 35C or 40C, I hope you’re taking the appropriate measures to keep cool, regardless if a heatwave has been officially declared.  Remember to keep yourselves hydrated, avoid any strenuous exercise or activities, and stay out of the sun. Don't forget to look after those around you too.

I originally wrote this piece as a guest blogger for International Innovation. You can check it out here, published on the 24th April 2015

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.

WARMING WORRIES

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.