Photo: Bev Sykes/Flickr
Over the coming decades extreme heat waves will only increase in frequency, thanks to all the greenhouse gas emissions we've already spewed into the atmosphere. In fact, a new study published in Environmental Research Letters shows, even if we make strong cuts in emissions we won't be able to avoid a four-fold increase in them—but if we do so we'll still be able to keep them from getting even worse in the second half of this century.
I'll take it as a given that you're aware of the exceptional number of extreme heat waves that we've seen in the past several years and that these have been increasing. Statistically, extreme heat waves now cover about 5 percent of the Earth's land surface every summer. In the 1960s such events were largely, but not totally, absent.
Examining different emissions scenarios this study finds that irrespective of what happens with greenhouse gas emissions in the near-term, extreme heat waves become much more common. By 2020, extreme heat waves will cover about 10 percent of land surface each summer, increasing to 20 percent by 2040. What's more, unprecedented heat waves, heat rising to levels currently all but absent in the record, emerge by 2040. These will cover roughly 3 percent of land during summer.
Again, what this study shows is that this is happening no matter what how deeply we cut back on pollution. Where emissions cuts start to matter, significantly, is in the post-2040 period.
Under a low emission scenario, summer heat stabilizes at 2040 levels—meaning that in the tropics what we now consider extreme heat essentially becomes the new normal temperatures for about half of the summer months and what's now unprecedented heat occurs in about 20 percent of summertime. In Europe, extreme heat waves occur about 20 percent of the time in the summer.
If we don't reduce emissions, however, things become much worse. Under a high emissions scenario the amount of land under extreme or unprecedented heat waves increases by 1 percent per year after 2040. That means by 2100, what's now extreme heat covers 85 percent of the land in summer, with unprecedented heat (by now not exactly unprecedented, fair enough) covering 60 percent of land. In some parts of the tropics, unprecedented heat covers the entirety of the land.
As for what this means for adapting to these new temperature regimes, the authors make an important point: "Society and ecosystems are adapted to extremes experienced in the past and much less so to extremes outside the historic range...Expected future increase in extremes beyond thresholds defined by the historic variability is likely to pose serious adaptation challenges."
In financial terms, this will take us into uncharted territory. One heat wave in California cost $5.3 billion in damages; the Russian heat wave of 2010 knocked $15 billion from GDP; in the European heatwave of 2003, which killed 52,000 people, it too caused $15 billion in damages. And these are all events for which there's historical precedent.