New analysis concludes the hots are still getting hotter.
The so-called hiatus or pause in global warming over the past 15 years or so has been much on the minds of climate scientists (see here, here, here, here and here), policy makers (see here, here, here and here) and the media (see here, here, here, here and here).
While a number of theories have been advanced to explain the hiatus (see here, here and here for example), a comprehensive explanation has yet to be established, and what the hiatus portends for future climate change remains unclear. What is clear, however, is that, contrary to what some climate skeptics might claim, the hiatus does not signal the end to concerns about climate change and the need to address the issue.
First of all the hiatus we are currently experiencing has happened previously. Each time, the upward climb in global temperatures has paused for a period of years and then resumed its ascent.
The rise in global temperatures has stalled for about a decade now, but not in places experiencing extreme heat. (From NOAA)
While we don't yet fully understand the cause of these pauses, there's no reason to believe that the current hiatus is any different.
Secondly, while average global temperatures have been relatively flat for the past 15 years, it would not be correct to say that global warming is on a hiatus. Indeed, the climate system continues to warm up -- over the past 15 years much of the excess heat has gone into the ocean and very little into the atmosphere. Again, we're not exactly sure why that is happening, but it does tell us that global warming continues apace.
Now a new analysis by Sonia Seneviratne of the Swiss Federal Institute of Technology in Switzerland and co-authors reported in the journal Nature Climate Change last week adds a new and interesting wrinkle to our understanding of the nature of this rising temperature hiatus.
Extreme Temperatures Becoming More Extreme
Seneviratne et al. used two independent global temperature records to track the temporal trend in extreme hot temperatures over land spanning the period from 1979-2010.* The authors found that the spatial extent of extremely hot days around the globe increased significantly over the study’s 30-year period, for example expanding to cover much of Europe and Greenland. Moreover, they found that the average magnitude of extreme hot temperatures around the globe also increased significantly, with more of the hottest extreme days occurring later in the time series. (See chart.) And, interestingly, the upward trend in the extreme land temperature was significantly larger than that of average global temperature. In other words, it may not be getting hotter on average, but when and where it's hot, it's generally getting hotter.
Why are extreme warm temperatures increasing while the average global temperature is not? The authors advance a few hypotheses:
- That the apparent hiatus in rising global temperatures reflects a slowdown or cooling of the surface ocean and not the land, which was the focus of this study;
- That the hiatus reflects a slowdown or cooling during the winter rather than the summer when extreme hot temperatures occur;
- That the processes that drive extreme hot days are "somewhat" independent of those driving mean temperatures. For example, drought conditions can establish a positive feedback between aridity and hot conditions.
Regardless of the explanation, the Seneviratne et al. analysis suggests that while there may be a hiatus in the global temperature trend, there is no such hiatus in extreme temperatures. Indeed, when it comes to hot days, it would appear that the warming trend continues to march on unabated.
In discussing their results, the authors make an interesting observation. While we often refer to the climate phenomena we are witnessing as “global warming,” it is more aptly called “climate change.” The trend in global average temperature may be a useful metric for talking about climate change, but in the final analysis things like extreme hot temperatures are far more impactful and therefore relevant than the global average temperature. And in that regard the "news you can use" from the Seneviratne et al. analysis may be inconvenient for the skeptics but it's also not welcome for the rest of us. When you're sweating through one of those torrid heat waves this summer I suspect you will derive small comfort from the fact that globally averaged temperatures are on a hiatus.
* Extreme temperatures are defined by the authors as any temperature that falls within the top 10 or top 5 percentile of temperatures for a locale relative to the 1979-2010 average for that locale. If there was no trend in extreme temperatures due to climate change, one would expect extreme hot temperatures to be randomly distributed over the time period. Under this scenario there would not be a temporal trend in the spatial extent of magnitude of extreme temperatures. In fact the authors found significant trends.↩