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When heat waves turn deadly: A look at patterns that are worsening worldwide

One-third of the global population is already in danger 20 days per year; by the end of the century, it could be three-fourths.

It’s likely that more than 100,000 deaths caused by heat occurred worldwide between 1980 and 2014.
REUTERS/William Hong

Everybody knows that heat waves can kill people. Most of us also accept that extreme hot weather events are increasing in severity, frequency and geographic extent as the globe continues to warm.

But where, exactly, is the risk of heat-induced fatality rising? And by how much?  What does the future seem to hold?

These are surprisingly difficult questions to answer, but an interesting — and, I have to say, discouraging — effort appeared Monday in the respected journal Nature.

This research is a meta-analysis of nearly 2,000 peer-reviewed studies of heat-related mortality, and the precise conditions that caused it, between 1980 and 2014. From these snapshots the research team, led by biogeographer Camil Mora of the University of Hawaii at Manoa, arrived at some big-picture conclusions:

  • It’s likely that more than 100,000 deaths caused by heat occurred worldwide during that period, in nearly 800 separate events in 164 cities and 36 countries.
  • A lethality index of the temperature/humidity combinations that prove fatal can be calculated from these deaths, and it suggests that 30% of people in the world now experience life-threatening heat at least 20 days a year.
  • If current trends in greenhouse gas emissions continue, the proportion of people at that risk level will rise to 74% by the end of the century; if drastic reductions were made, at a pace that probably is not yet feasible, 48% would still be at risk.
  • It is possible to reduce the risk of death in these conditions — though not the conditions themselves — with “social adaptation,” by which the authors mean, essentially, more air-conditioning. (Of course this option tends to be out of reach in the poorer, hotter places that need it most; also, as long as the AC is powered by fossil fuels, it raises the problematic emissions.)
  • As for biological adaptation, they say, “Given the speed of climatic changes and numerous physiological constraints, it is unlikely that human physiology will evolve the necessary higher heat tolerance, highlighting that outdoor conditions will remain deadly even if social adaptation is broadly implemented.”
  • Oh — and those risk figures can be taken as conservative, because heat-related deaths are often misclassified; also, because faulty recordkeeping in poorer countries understates their toll.

Calculating lethality

So, how hot is fatally hot?

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The researchers’ methods were sophisticated and are described in depth in a section whose beginning I’ll quote for the technically minded:

To identify the climatic conditions related to lethal heat events, we assessed daily climatic data (that is, surface air temperature, relative humidity, solar radiation, wind speed, and several other metrics), for the duration of lethal heat episodes reported in the literature and an equal number of non-lethal episodes (that is, periods of equal duration from the same cities but from randomly selected dates); then we used Support Vector Machines (SVMs) to identify the climatic conditions that best differentiated lethal and non-lethal episodes. SVMs generate a threshold that maximizes the difference in the attributes of two or more groups, allowing for classification of objects in either group based on where their given attributes fall with respect to the threshold.

All this leads to a conclusion that is both simple and commonsensical: A heat wave turns deadly to humans when it matches or exceeds normal body temperature and stays there for a while, halting our ability to shed body heat to the surrounding air. Since sweating is our primary way of doing that, humidity can lower the red line well below 98 degrees Fahrenheit (37 degrees Celsius).

Combining the two factors, the researchers wrote,

We found that in 2000, ~13.2% of the planet’s land area, where ~30.6% of the world’s human population resides, was exposed to 20 or more days when temperature and humidity surpassed the threshold beyond which such conditions become deadly.

(Using a slightly different data set, by the way, the numbers were worse: 16.2 percent of land area, holding 37 percent of population.)

Looking to the future, they assessed the outcomes under three widely accepted scenarios for future emissions and their consequences, called Representative Concentration Pathways or RCPs. Their conclusions, omitting footnotes but including the standard-deviation ranges:

We found that by 2100, even under the most aggressive mitigation scenario (that is, RCP 2.6), ~26.9% (±8.7%) of the world’s land area will be exposed to temperature and humidity conditions exceeding the deadly threshold by more than 20 days per year, exposing ~47.6% (±9.6%) of the world’s human population to deadly climates.

Scenarios with higher emissions will affect an even greater percentage of the global land area and human population. By 2100, ~34.1% (±7.6%) and ~47.1% (±8.9%) of the global land area will be exposed to temperature and humidity conditions that exceed the deadly threshold for more than 20 days per year under RCP 4.5 and RCP 8.5, respectively; this will expose ~53.7% (±8.7%) and ~73.9% (±6.6%) of the world’s human population to deadly climates by the end of the century.

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Of course, where in the world you are matters a great deal in these projections:

At mid-latitudes (for example, New York), temperatures approach the deadly threshold only during the summer, which represents a smaller proportion of the year; compared to tropical locations (for example, Jakarta), which have consistently warm temperatures near the deadly threshold year-round.

Although tropical humid areas will experience less warming than higher latitudes, they will be exposed to the greatest increase in the number of deadly days over time, because higher relative humidity in tropical areas requires lower temperatures to cross the deadly threshold. … Subtropical and mid-latitude areas will have fewer days beyond the deadly threshold, but such deadly days will be much hotter in the future.

Among regions of the U.S., the eastern and especially the southeastern states would show the greatest increase. The south and southwest would remain very hot, with Orlando and Houston being in the danger zone almost all summer.

Prisoners in our homes

The Mora paper has been generating a fair amount of buzz in media since its release, and commentary from other climate scientists has been generally complimentary and admiring. An exception was Daniel Mitchell, an atmospheric scientist at Oxford who told Popular Science he thought the team’s calculation of deadliness was flawed because it didn’t account for non-weather variables.

“There are lots of things that can lead to mortality that have nothing to do with the climate. A good example was Egypt 2015, the heat wave there where a large number of people died, but they all died in places like prisons and psychiatric hospitals,” Mitchell says. “And the reason was because the timing of the heat wave was such that they didn’t have people around to take care of them.”

Similarly, the 2003 European heatwave was so deadly in France because it coincided with the country’s fairly ubiquitous habit of going on vacation in August. Many of the cities emptied out, leaving behind elderly people without caregivers.

I guess this strikes me as an odd basis for discounting a couple of horrible events. The 2003 heat wave killed nearly 5,000 people in Paris (and some 70,000 across Europe);  the death toll in Egypt was only 100, but the event was part of a pattern across the Middle East, India and Pakistan that summer that took more than 4,700 lives.

Still, perhaps it underlines one of Mora’s more pungent if less technical conclusions from his work. Across much of the world and for much of the year, he says, “we will become prisoners in our homes.”

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The full paper, “Global risk of deadly heat,” can be read here but access is not free.