The Deadly Combination of Heat and Humidity (a technical note)
Jon Buzan, Matt Huber and I authored an article in the June 7, 2015, New York Times that looks at the recent heat wave in India – one of the most deadly in the planet’s history – and considers the role of humidity and projected increases in heat and humidity under climate change. I think that, with some excellent guidance from the editorial staff at the Times and from Bob Tanner at Climate Nexus, the article came out quite clearly; but as is always true in such venues, there are a few technical points that couldn’t be captured but may be of interest to some readers. We annotate these here.
The death toll is still being tallied, and many heat-related deaths will be recognized only after the fact. Yet it’s already the deadliest heat wave to hit India since at least 1998 and, by some accounts, the fourth- or fifth-deadliest worldwide since 1900.
As of June 2, Reuters was reporting about 2,500 deaths during the heat wave. The EM-DAT database tabulates
- Europe, July-August 2003: 72,210 direct deaths
- Russia, June-August 2010: 55,736 direct deaths
- Europe, June-July 2006: 3,418 direct deaths
- India, May 1998: 2,541 direct deaths
(These numbers are based on our parsing of the database – the numbers are slightly different than those listed in Jeff Masters’ nice post on the Indian heat wave.)
These numbers differ somewhat from those we quote in the text for Europe 2003 and Russia 2010, as we drew upon two studies that (1) estimated excess mortality, not just deaths directly identified as being tied to the heat wave, and (2) drew somewhat narrower time windows to focus around the main body of the heat waves.
The July 1995 heat wave in the Midwest caused over 700 deaths in Chicago. The August 2003 heat wave in western Europe led to about 45,000 deaths. The July-August 2010 heat wave in western Russia killed about 54,000 people.
The Chicago number comes from Whitman et al. (1997). The western Europe number comes from Robine et al. (2008); the August total in that paper is 44,878, although a total of 71,449 excess deaths are estimated for the summer as a whole. The Russia numbers are from Revich (2011).
In the Indian state of Andhra Pradesh, the highest wet-bulb temperatures of the latest heat wave have peaked around 86 degrees — levels approaching the worst of the 1995 Midwest heat wave, which set records in the United States for humid heat.
These numbers are based on Jon’s own calculations, using meteorological observations from Weather Underground and his HumanIndexMod code. HumanIndexMod calculates wet-bulb temperature using an algorithm with inputs of atmospheric moisture, surface pressure, and air temperature.
The plots below show the daily maximum and minimum wet-bulb temperatures for Kakinada, Andhra Pradesh, this past month. It’s worth noting that there are days when wet-bulb temperature didn’t fall below 82°F – a truly miserable situation, affording no opportunity to cool off.
In work one of us (Robert Kopp) led for the Risky Business Project, we found that over the period from 1981 to 2010, the average American experienced about four dangerously humid days, with wet-bulb temperatures exceeding 80 degrees. By 2030, that level is expected to more than double, to about 10 days per summer. Manhattanites are expected to experience nearly seven uncomfortably muggy weeks in a typical summer, with wet-bulb temperatures exceeding 74 degrees, about as many as residents of Washington have experienced recently.
These results come from the forthcoming book Economic Risks of Climate Change: An American Prospectus, much of the text of which is available as a report at ClimateProspectus.org. The wet-bulb temperature projections are detailed in the section on ‘Humidity’ in chapter 4; the numerical projections are detailed further in the data tables available at the same site. Results for the ‘average American’ refer to a population-weighted average of county-level values across the U.S. Results for New York City are taken as the average of the values provided in the data tables for Connecticut and New Jersey, as the New York state values are biased by conditions upstate. The ‘uncomfortable’ and ‘dangerous’ conditions refer to Categories I and II on the American Climate Prospectus (ACP) Humid Heat Stroke Index; the conditions in Kakinada were skirting close to Category III (‘extremely dangerous’).
The American Climate Prospectus analysis is probabilistic: it comes up with probability distributions over space and time for the different physical and economic variables analyzed. The results presented in the column are ‘expected’ in the statistical sense; that is to say, they represent the probability-weighted average across possible future states of the world.
Since we can’t avoid it now, we must make our communities more resilient to heat and humidity extremes. One step is to expand access to air-conditioning for those who can’t afford it. We must also improve cooling in stiflingly hot factories and warehouses, strengthen public health systems, improve public warnings when heat and humidity are dangerously high, and be willing to shift outdoor work schedules.
There are some additional options we didn’t have space to mention here. These include technologies for passively cooling buildings and urban areas, such as cool roofs and pavements, as well as the broader set of energy efficiency measures to reduce the need for active cooling.
If we choose not to reduce emissions of heat-trapping gases and instead continue to rely upon fossil fuels, the average American could expect to see about 17 dangerously humid days in a typical summer in 2050 and about 35 in 2090.
The results here are for continued fossil-fuel-intensive growth, represented by Representative Concentration Pathway (RCP) 8.5. (See discussion of RCPs below.)
Some summers would have days so stiflingly muggy that a healthy individual would suffer heat stroke in less than an hour of moderate, shaded activity outside.
These stiflingly muggy days are Category IV (‘extraordinarily dangerous’) on the ACP Humid Heat Stroke Index, with wet-bulb temperatures exceeding 92°F. Such days have no precedent in U.S. history. In the ACP analysis, they are expected with 6%/year probability in Illinois and 1%/year probability in New Jersey under RCP 8.5 in 2040-2059. By 2080-2099, 4 such days are expected per summer in Illinois and 1 such day in New Jersey. The average American is expected to experience such a day with 1%/year probability in 2040-2059 and 80%/year probability in 2080-2099.
About an hour of shaded, 150-watt activity at a wet-bulb temperature of 92°F leads to skin temperatures of 100°F and core body temperatures of 104°F (the threshold for heat stroke), based on a Danish study conducted on twelve trained, male endurance athletes, ages 23–34. 150 watts corresponds to ‘moderate effort’ on a stationary bike – about 40% of maximal aerobic capacity for these individuals. The athletes, dressed only in swim trunks and shoes, were asked to pedal to exhaustion on a stationary bike in a room with an air temperature of 95°F and relative humidity of 87%, corresponding to a wet-bulb temperature of about 91°F. After 45 ± 3 minutes of exercise without acclimation, or 52 ± 2 minutes with acclimation, these individuals reached exhaustion and a core temperature of 103.8 ± 0.2°F.
And carrying on this way through the 22nd century locks in a trajectory where summer outdoor conditions could become physiologically intolerable for humans and livestock in the eastern United States — and in regions currently home to more than half the planet’s population.
This remark is based upon a 2010 paper Matt co-authored with Steven Sherwood. This study found that conditions physiologically intolerable for humans (conservatively defined there as areas with peak wet-bulb temperatures exceeding 95°F during the peak of the summer, well into ACP Category IV, and well beyond the current planetary experience) cover regions home to more than half the planet’s population with about 11°C (20°F) of global warming. The regions affected include much of the eastern U.S., China, India, Brazil, and north Africa. Based on simulations with the MAGICC simple climate model, as run for the ACP, such conditions have about a 20% chance of being realized by 2200 under RCP 8.5.
But this fate is not yet locked in. Moderate reductions in emissions of heat-trapping gases — sufficient to stop global emissions growth by 2040 and bring emissions down to half their current levels by the 2070s — can avoid those paralyzing extremes and limit the expected late-century experience of the average American to about 18 dangerously humid days a year. And strong reductions — bringing global emissions to zero by the 2080s — can cap the growth of humidity extremes by the midcentury.
The two alternative pathways referred to here are RCP 4.5 and RCP 2.6, shown alongside RCP 8.5 and historical emissions below. The RCPs are the current standard pathways of emissions used by the climate modeling community. For reasons a little too complicated to get into here, they don’t directly correspond to socio-economic scenarios, but RCP 4.5 is somewhat below what is possible to achieve in the absence of climate policy and RCP 2.6 requires quite severe reductions in greenhouse gas emissions, with global emissions begin to decline in the 2020s. RCP 2.6 is the pathway most consistent with the 2°C temperature target to which the nations of the world have notionally agreed in the 2009 Copenhagen Accord and the 2010 Cancun Agreements.
