Guest Essay by Kip Hansen – 30 June 2022
The esteemed journal Science carries this story from Latin America in Climate section:
“An increase of 1°C could mean thousands of additional deaths on very hot days, according to a new study” by Rodrigo Pérez Ortega
It leads with:
“In mid-January, the southern tip of South America suffered its worst heat wave in years. In Argentina, temperatures in more than 50 cities rose above 40°C [ 104°F ], more than 10°C [ 18°F ] warmer than the typical average temperature in cities such as Buenos Aires. The scorching heat sparked wildfires, worsened a drought, hurt agriculture, and temporarily collapsed Buenos Aires’s electrical power supply. It also killed at least 3 people, although experts estimate the true number might be much higher.
With climate change, heat waves and cold fronts are worsening and taking lives worldwide: about 5 million in the past 20 years, according to at least one study. In a new study published today in Nature Medicine, an international team of researchers estimates that almost 900,000 deaths in the years between 2002 and 2015 could be attributable to extreme temperatures alone in major Latin American cities. This is the most detailed estimate in Latin America, and the first ever for some cities.”
There is a study! A real study published in nature medicine authored by Josiah L. Kephart and eleven others. “City-level impact of extreme temperatures and mortality in Latin America” [ .pdf here ].
Let’s start with the abstract and compare it to the lede in Science.
“Climate change and urbanization are rapidly increasing human exposure to extreme ambient temperatures, yet few studies have examined temperature and mortality in Latin America. We conducted a nonlinear, distributed-lag, longitudinal analysis of daily ambient temperatures and mortality among 326 Latin American cities between 2002 and 2015. We observed 15,431,532 deaths among ≈2.9 billion person-years of risk. The excess death fraction of total deaths was 0.67% (95% confidence interval (CI) 0.58–0.74%) for heat-related deaths and 5.09% (95% CI 4.64–5.47%) for cold-related deaths. The relative risk of death was 1.057 (95% CI 1.046–1.067%) per 1 °C higher temperature during extreme heat and 1.034 (95% CI 1.028–1.040%) per 1 °C lower temperature during extreme cold. In Latin American cities, a substantial proportion of deaths is attributable to nonoptimal ambient temperatures. Marginal increases in observed hot temperatures are associated with steep increases in mortality risk. These risks were strongest among older adults and for cardiovascular and respiratory deaths.”
One of the amusing things we see right off is the use of Large Numbers: 15,431,532 deaths, 2.9 billion person-years of risk. Well, they have a huge population over a very large area (1.5 continents) over 13 years during which 15.4 million people died. But what of the results?
The excess death fraction of total deaths was 0.67% (95% confidence interval (CI) 0.58–0.74%) for heat-related deaths
The excess death fraction of total deaths was 5.09% (95% CI 4.64–5.47%) for cold-related deaths
Bottom Line: Excess Death Fraction for cold-related deaths is 7.5 times higher than for heat-related death.
And for Relative Risk (RR) change per 1°C change in highest/lowest temperature?
The relative risk of death was 1.057 (95% CI 1.046–1.067%) per 1 °C higher temperature during extreme heat and 1.034 (95% CI 1.028–1.040%) per 1 °C lower temperature during extreme cold.
Bottom Line: While the study makes a big deal about the difference in these two RRs, with a difference of only 0.023 – they are in terms of medical science, often considered identical.
There is no reason, however, to believe that this small difference is not real. It may just show that human bodies have a different limit responses to small changes at highest and lowest temperatures when averaged across a large enough population.
At every turn in this paper, the authors make the attempt to make heat the villain despite the far greater risk of dying from cold:
“Overall, a substantially higher proportion of deaths is attributable to ambient cold than to ambient heat, which corroborates findings from similar analyses in other settings. A 2021 analysis by Zhao et al. estimated temperature–mortality associations in 750 locations from 43 countries (including 66 locations in Latin America and the Caribbean), and extrapolated these estimates glob ally at 0.5° × 0.5° grid size (approximately 55 × 55 km2 at the equator) using meta-predictors. The Zhao et al. study reported global EDFs of 8.52% for cold and 0.91% for heat for all-age, all-cause mortality. This global EDF for cold (8.52%) is almost twice our estimated EDF for cold within Latin American cities (4.71%).”
“A 2017 study, which included 32 locations in Mexico, Brazil and Chile, projected that, under multiple climate-change scenarios, midcentury decreases in cold-related mortality would approximately counterbalance increases in heat-related mortality, yet by the end of the twenty-first century overwhelming heat-related mortality would cause a substantial net increase in temperature-related excess mortality.”
[ Yes, that 2017 study finding uses RCP8.5. – kh ]
The Pérez Ortega study we are looking at today summarizes its findings in this table:
I have written about Cause of Death and its uses in studies more than once: Cause of Death: A Primer and Cause of Death: Follow-up. This study is not about heat deaths or cold deaths. It is about All Cause Deaths with details about the major causes: Cardiovascular Deaths, Respiratory Deaths, and Respiratory Infection Death with breakouts for All Ages and Ages 65+.
This study does not even consider categories of deaths caused by extremes of temperature, hot or cold. There are cause of death codes for excessive natural heat “2022 ICD-10-CM Diagnosis Code X30 Exposure to excessive natural heat” and cold “2022 ICD-10-CM Diagnosis Code X31 Exposure to excessive natural cold”. Quite simply, they did not count people killed by heat or people killed by cold, at all, not one.
The question the study asks and tries to answer is “Do more people than normal die in Latin America when it is unusually hot or when it is unusually cold?”
What they fail to ask and fail to analyze are the most likely culprits in the issue itself: what are the poverty and development levels in the cities studied? Surely poverty and lack of development — lack of electricity, lack of clean water, lack of appropriate housing and lack of even basic healthcare and social support have far more impact on the extreme numbers of “temperature related deaths” than the temperatures themselves.
BOTTOM LINE: While it comes as no surprise, this study confirms that more people die when ambient temperatures are at extreme levels (much higher or much lower than usual) for the locality. This study confirms that far more die when it is unusually extremely cold than when it is unusually extremely hot. This is a fact affecting older people (65+) more than younger people and these excess deaths are a result of heart (cardiovascular) and breathing (respiratory) problems – but do not from directly from the heat or cold itself.
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Readers here already know that cold kills far more than heat. This study finds this to be true once again. The authors have made feeble attempts – not based on their own study but on speculative RCP8.5 studies – to claim that this will cause more, not less, future deaths if general climates continue to warm.
As with all studies that use All Cause mortality, there is no “cause” found, only various vaguely related correlations. All Cause Mortality is one of the absolutely worst indicators to be used in such studies and is used, quite frankly, because it is easy. Cause of Death is hard, complicated, complex, and records of ICD-10 codes are unreliable (doctors are in a hurry or doctors lie…). It is hard to determine the real causes of individual deaths but easy to determine and count dead bodies.
We already knew that more people, particularly (us) old folks die when it is very hot or very cold. We already knew that far more die when it is very cold than when very hot. I am not convinced that this study found anything that makes mankind more knowledgeable or anything that will help policy makers in nations or localities set better policy to make a better world. In that sense, this study is “useless”.
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