Global Warming Causing Heat Fatalities

One of the fallback positions of climate denial after the assertions that "It's not happening" and "It's not us" fail is "It's not bad."  The latest incarnation of this myth courtesy of Pat Michaels' serial data deletion colleague Chip Knappenberger argues that those who seek to mitigate global warming are actually endangering public health because, and believe it or not this is a direct quote:

"“longer, more intense and more frequent heat waves” may actually improve the public health and welfare"

Here is the specific argument Knappenberger makes in attempting to defend this seemingly absurd thesis:

"more frequent exposure to heat waves will lead the population to adapt to them, better preparing them for their occurrence, and ultimately reducing the rate of mortality and morbidity."

By this logic gang violence is great because it makes people more adept at dodging bullets. 

Knappenberger cites a paper he co-authored, Davis et al. (2003), which found that heat-related deaths are less common in hotter cities.  This makes sense, as hotter cities have the infrastructure (i.e. air conditioning units) to cope with hotter temperatures.  They do not have to adapt - they are already adapted to the heat, whereas most heat-related deaths come in regions which experience uncommon heat events (but are now experiencing them more and more frequently due to global warming).  Thus this point does not support Knappenberger's argument that more heat waves would be beneficial.

Knappenberger describes the second point in Davis et al. as follows.

"over time, the rates of heat related mortality across virtually all the cities that we studied were declining even in the face of rising summer temperatures"

This point, however, is contradicted by more recent research.

Zanobetti et al. on Short-Term Heat Impacts

Zanobetti et al. (2012) take an interesting approach in investigating the relationship between hot weather events and mortalities.  Since the age group most at risk for heat deaths are the elderly (those over 65 years of age) with predisposed illnesses, Zanobetti compared Medicare data from 1985 to 2006 from 135 U.S. cities to summer temperatures.  The authors explain the reasoning behind their approach:

"By restricting the analysis to within a city, we avoid all confounding by factors that can vary across a city or region. By looking only at year-to-year variations around the city-specific trend in exposure, we eliminate potential confounding by trends in other exposures, such as smoking, and focus on whether essentially random meteorological events are related to health."

Zanobetti et al. find that larger summer temperature variability leads to more deaths among the elderly.  Each 1°C increase in summer temperature variability increased the death rate for elderly with chronic conditions between 2.8% and 4.0%, depending on the condition (emphasis added):

"A 1°C increase in temperature SD [standard deviation] is a plausible increase in some regions. Based on our findings, this increase in temperature SD would increase all-cause mortality in our MI [myocardial infarction] cohort by 5%, for example. Based on an average of 270,000 deaths per year across all four cohorts, a 5% increase in mortality would correspond to 14,000 additional deaths per year due to an increase in temperature variability in the United States."

Sherwood and Huber on Long-Term Heat Impacts

A 2009 paper by Sherwood and Huber examines a worst case scenario in which the average global surface temperature warms in the ballpark of 10°C a few centuries in the future.  They note that a wet-bulb temperature (Tw) exceedence of 35°C for extended periods should induce hyperthermia in humans and other mammals, as they become unable to sufficiently dissipate heat.  In short, if Tw(max) of a particular region were to exceed 35°C for long periods of time, that region would effectively become uninhabitable to mammals.

"A 4°C increase in Tw would then subject over half the world’s population annually to unprecedented values and cut the “safety buffer” that now exists between the highest Tw(max) and 35°C to roughly a quarter. A shift of 5°C would allow Tw(max) to exceed 35°C in some locations, and a shift of 8.5°C would bring the most-common value to 35°C."

Based on their climate model simulations, Sherwood and Huber found that Tw increases somewhat more slowly than the average global surface temperature, such that a 1°C average global warming corresponds to a 0.75 to 1°C Tw increase.  Therefore, an 8.5°C Tw increase would require approximately 11°C global warming.

"We conclude that a global-mean warming of roughly 7°C would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11–12°C would expand these zones to encompass most of today’s human population."

"A global-mean warming of only 3–4°C would in some locations halve the margin of safety (difference between Tw(max) and 35°C) that now leaves room for additional burdens or limitations to cooling."

"If warmings of 10°C were really to occur in next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level."

In short, Sherwood and Huber find that there is a limit to what humans and other mammals can adapt to in terms of rising temperatures.  It will likely take a few centuries for global temperatures to reach that limit, but eventually large regions of the planet could become effectively uninhabitable, beyond what mammals can adapt to.

McInerney & Wing (2011) also examined the Paleocene-Eocene Thermal Maximum (PETM); a period about 56 million years ago during which global temperatures increased 5 to 8°C over a period of about 200,000 years.  They found that most species were able to avoid extinction by adapting to the increasing temperatures, for example by becoming smaller (increasing their surface area to volume ratio and thereby being better able to shed bodyheat).  Secord et al. (2012) similarly concluded that many species became smaller during the PETM and grew larger after the PETM (Figure 1).

Secord Fig 1

Figure 1: Summary of percent mean body size change in genera that exhibit change from the latest Paleocene to the PETM (left), and from the PETM to the post-PETM (right). No genus exhibits a size increase in the PETM or a decrease after the PETM. Compiled from published sources, except for Sifrhippus from this study. Asterisks indicate genera that first appear in the PETM (Secord et al. 2012).

Our problem is that current climate change is occurring much faster, over just centuries rather than the millennia of the PETM, and thus species will not have sufficient time to evolve in this manner.

Warmer is Not Better

The Knappenberger argument that higher temperatures will decrease heat-related deaths and thus benefit humanity thus suffers from two major flaws.  The first is that while fewer heat-related deaths occur when humans are adapted to high local temperatures, heat-related deaths will nevertheless rise in unprepared regions until they become adapted to those rising temperatures (i.e. by installing the necessary cooling infrastructure).  Zanobetti et al. illustrate that increasing heat-related deaths is already a reality.

The argument also neglects the long-term limit - there is a point at which temperatures can become too hot for humans and other mammals to survive.  If we continue on a business-as-usual path as Knappenberger promotes, we will likely reach that point within a few centuries, and the costs of losing the habitability of large regions of the planet are incalculable.

Posted by dana1981 on Tuesday, 24 April, 2012

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