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.