Global Warming as a Threat to Global Health - Review in Nature

Nature has a review this week on the Impact of regional climate change on human health(1) that is an interesting read.

Contrary to the previous article we discussed which suggested what I think is a non-existent link between climate change and chronic disease, this article discusses the very real likelihood of increased acute mortality from respiratory and cardiovascular disease with extreme weather.

Exposure to both extreme hot and cold weather is associated with increased morbidity and mortality, compared to an intermediate 'comfortable' temperature range¹⁵. Heat mortality follows a J-shaped function with a steeper slope at higher temperatures¹⁶. The comfortable or safest temperature range is closely related to mean temperature, with an upper bound from as low as 16.5 °C for the Netherlands and 19 °C for London¹⁷, to as high as 29 °C in Taiwan¹⁸. Hot days occurring earlier in the summer season have a larger effect than those occurring later¹⁷. It should be noted that although the majority of temperature-mortality studies have taken place in developed countries and in regions with temperate climates, the same pattern of temperature-mortality relationship found in European and North American cities occurs in São Paulo, Brazil, a developing city with subtropical conditions¹⁹.

The temperature/mortality curves look like this:

This figure from this paper (2) shows the J-shaped function for eleven American cities over 20 years (referred to in the Nature paper), and that both cold and hot weather can negatively impact mortality. Further, Curreiro et al. showed that mortality was more severe when colder climates experienced warm temperatures or vice-versa (the dotted lines are southern cities, and experience more severe responses to cold, the solid are northern and mortality sharply increases with heat), as well as a socio-economic trend - people with heat and air-conditioning of course do better during extremes of temperature. One can see how if climate change brings more severe or variable climate to different regions, hot or cold, we will need to adjust to make sure excess death does not result from lack of preparation for extreme or unexpected weather in both warm and cold climates.

They also note the problem of increased infectious disease with expanding regions of warm climate.

Climatic variations and extreme weather events have profound impacts on infectious disease. Infectious agents (such as protozoa, bacteria and viruses) and their associated vector organisms (such as mosquitoes, ticks and sandflies) are devoid of thermostatic mechanisms, and reproduction and survival rates are thus strongly affected by fluctuations in temperature^{4, 22}. Temperature dependencies are seen in correlations between disease rates and weather variations over weeks, months or years²³ and in close geographic associations between key climate variables and the distributions of important vector-borne diseases^{24, 25}.

While they conclude that malaria has not strongly correlated with climate predictions so far - likely due to confounding variables complicating the analysis - other vector-borne diseases, such as Dengue fever, the Ross River virus, plague, and salmonellosis have correlated tightly with warming of local climates.

And, as usual, the impacts of climate will fall heaviest on the poorer regions of the world.

This figure, from the review, is pretty self-explanatory. Based on projections from the "business-as-usual" models of climate from the IPCC, projects the mortality from "cardiovascular diseases, diarrhoea, malaria, inland and coastal flooding, and malnutrition, for the years 2000 to 2030" The projected mortality per million will likely be most severe in countries along the equator, with limited resources and infrastructure.

However, that doesn't leave the rich countries off the hook. While we are likely to be safe from vector-borne illnesses, and have the infrastructure to deal with many of these problems, we still will likely face excess mortality from heatwaves:

To consider changes in future heatwave probabilities, GCM projections of future climate for conditions contributing to heatwaves are now capable of estimating the occurrence of stagnant, warm air masses that can determine the severity of a heatwave, including variables such as consecutive nights (three or more) with high minimum night-time temperatures⁵⁹. A recent analysis of the 1995 Chicago and 2003 Europe heatwaves predicted intensified magnitude and duration of heatwaves over portions of Europe and the United States, suggesting that heatwaves in Chicago and Paris will be 25% and 31% more frequent, respectively, by 2090 and that the average length of a heatwave in Paris will have increased from 8-13 days to 11-17 days. Large increases in heatwaves were also projected for the western and southern USA and the Mediterranean region⁵⁹.

These will also likely be accompanied by increases in pollutants and irritants like ozone, which are known to increase the probability of respiratory and cardiovascular hospital visits. Although, these risks are more uncertain, the authors project a potential doubling in heat-related mortality in some areas of the country by the end of the century.

The conclusions of their review? Heat related mortality will likely manifest as an increase in acute deaths from events like heat waves, and likely increases in infectious disease from the spread of habitat of disease vectors.

What should we do? Well, early warning systems that can help identify when extreme weather or specific weather patterns like the El Nino/Southern Oscillation, will likely lead to increases in infectious disease or heat related mortality should be invested in. Very simple things can reduce acute heat-related mortality as well, such as areas of refuge, especially for the elderly, during extreme heat, basic education and preparation, as well as making sure homes have suitable climate control technology for unexpected weather events. In other words, Chicagoans may want to invest in some air conditioning in the near future. Although, the most effective strategy, especially in the places that will be affected the most, will likely be increasing economic equality, and investment in public health infrastructure. Sadly, things we have consistently failed to do for Africa over the last century.

Also in Nature this week the effects of climate change on water availability will likely be significant for populations that rely on ice melt for water supplies.

1. Patz,J.A., Campbell-Lendrum,D., Holloway,T. & Foley,J.A. Impact of regional climate change on human health. Nature 438, 310-317 (2005).
2. Curriero,F.C. et al. Temperature and Mortality in 11 Cities of the Eastern United States. Am. J. Epidemiol. 155, 80-87 (2002).