The Mega-Heat Wave Feedback Loop

New science reveals exactly how the warming globe will give rise to unprecedented heat waves.

Apr 21 2014, 7:28pm
Image: Wikimedia

My first memories of Europe are clouded with sweat and dust. It was the summer of 2003, and I was an impertinent teenager in the midst of an unfortunately timed family vacation to France. But forget the sweltering temperatures outside this castle or that chalet, or the melodramatic suffering of linen-draped, red-faced tourists—across France, Portugal, the Netherlands, and Spain, people were dying. Over 70,000 people perished from the heat wave that summer, which brought the highest temperatures recorded in Europe since the 1500s. 

Some 14,000 people died in France alone—one of the richest countries in the world. There were so many dead in Paris that undertakers had to turn a refrigerated warehouse into a makeshift mortuary. Seven years later, Russia would see a similar crisis, when a crushing heat wave swept Moscow and the surrounding region. These catastrophes have been termed 'mega-heat waves' by climatologists, and in the warming world, we're going to see more of them.

"Although events of this magnitude were unprecedented from a historical perspective, they are expected to become common by the end of the century," write the authors of a new study published in Nature Geoscience. Scientists have long suspected that the changing climate fueled these events, and there's ample evidence suggesting as much. But they hadn't yet confirmed the mechanism that brought the unprecedented scorch to bear. 

"The mega-heat waves that parched Europe in 2003 and Russia in 2010 were exacerbated by a vicious feedback loop between soil and atmosphere," Hannah Hoag writes of the findings in Nature. "Drying ground added more heat into air close to Earth's surface, a process that repeated over time to produce record-breaking warmth that shriveled crops, set forests ablaze and claimed tens of thousands of lives." 

We get heat waves when high-pressure systems move in and trap hot air over land. But when they start over land that has low soil moisture to begin with—as was the case with both the French and Russian heat waves—it gets a lot hotter, faster. That heat then further dries out the soil, causing a feedback loop that can blast a blanket of heat into the troposphere two and a half miles high.

"This study spells out for the first time a feedback mechanism that many of us have been worried about for years," the physicist and climate writer Joe Romm wrote me in an email. "Global warming and warming-driven heat waves make drought and soil drying worse, and that drying in turn worsens heat waves."

Romm says that "this fits with other recent research making clear that warming will lead to more droughts and drying even in regions that may not see a drop in total annual precipitation." He points to another study published in Climate Dynamics that had me concerned, too. Those researchers found that a decline in rainfall alone would drape 12 percent of the world's land in drought—but that increased rates of evaporation would spread drought conditions further; to nearly one third of the planet's land surface.

That means there's a lot more room for mega-heat waves to form, many more nations and cities eligible to be subjected to prolonged, record-breaking temperatures.

"The greatest challenge facing humanity this century will be feeding 9 billion people post-2050 in a world that is running out of arable land and potable water."

"What this new study means is that the same 30 percent of land that will be subject to more drought by 2100 will also be subject to more monster heat waves like the European one in 2003 and Russian one in 2010," Romm says.

By that estimation, one third of the planet's land will be prone to mega-heat waves by the end of the century. And like so many phenomena spurred on by climate change—melting ice cover, thawing permafrost—it will create an ominous feedback loop. Heat waves dry out the soil and cause worse heat waves which dry out the soil even more and prime the ground for mega-heat waves, which—and so on. 

Such a cycle will make combating desertification and growing crops in hotter climes even more challenging. We'll have to embrace new water management regimes and adaptation technologies—like this solar farm/cropland hybrid concept, maybe—that help keep the soil moist while hastening a transition away from the fossil fuels that are heating the globe. But these mega-heat waves are all but assured to become a fact of life in the near-future. Some studies have determined that such massive heat events will be up to ten times more likely in coming years.

As Romm says, "this continues to makes clear that the greatest challenge facing humanity this century will be feeding 9 billion people post-2050 in a world that is running out of arable land and potable water."

I think back to my summer in Europe in 2003, and I remember sweat glistening on foreheads, dampening armpits, people driven to the shade in droves, weak in the knees. It's a strange thing to remember sweat. Or a dryness so distinct that events seem painted on a landscape of parched pastel, but I do. I remember the heat: it was heavy; it had its own gravity, a slimy burden bearing down on every pore. 

And I was on vacation. I can't imagine exercising, or working—doing physical labor; building, hauling, tilling land—under those conditions. No one should have to suffer through that. But we're going to, because we're living in the age of the mega-heat wave.