Part 8 of a multi-part article on the causes and implications of global capitalism’s descent into an era when infectious diseases are ever more common. My views are subject to continuing debate and testing in practice. I look forward to your comments, criticisms, and corrections.
- Part 1: An Existential Threat
- Part 2: Relentless Viral Evolution
- Part 3: Systematically Unprepared
- Part 4: Deforestation and Spillover
- Part 5: The Pandemic Machines
- Part 6: China’s Livestock Revolution
- Part 7: Wildife Farms and Wet Markets
- Part 8: Deadly Heat
by Ian Angus
Previous articles in this series have focused on two global trends that are fueling the emergence of new viral diseases in our time. Deforestation and urban growth have reduced or eliminated the natural barriers that prevented most ‘spillover’ of viruses from wildlife to farmed animals and humans, and the concentration of livestock in factory farms has created ideal environments for such viruses to evolve into more contagious and deadly forms.
A full account of capitalism’s new plagues must also include the impact of the global climate crisis. The usually-cautious Intergovernmental Panel on Climate Change concludes, with very high confidence, that “climate hazards are increasingly contributing to a growing number of adverse health outcomes.”
“Climate variability and change (including temperature, relative humidity and rainfall) and population mobility are significantly and positively associated with observed increases in dengue globally; chikungunya virus in Asia, Latin America, North America and Europe (high confidence); Lyme disease vector Ixodes scapularis in North America (high confidence); and Lyme disease and tick-borne encephalitis vector Ixodes ricinus in Europe (medium confidence). Higher temperatures (very high confidence), heavy rainfall events (high confidence) and flooding (medium confidence) are associated with an increase of diarrhoeal diseases in affected regions, including cholera (very high confidence), other gastrointestinal infections (high confidence) and food-borne diseases due to Salmonella and Campylobacter (medium confidence).”[1]
Indeed, as Colin Carlson of Georgetown University’s Center for Global Health Science and Security points out, “human-caused climate change has already caused mass death on a pandemic-like scale.”
“Excluding COVID-19 … climate change has exceeded the combined death toll of every World Health Organization (WHO)-recognized public health emergency of international concern. Every year, climate change kills 14 times as many people as the 2014 Ebola epidemic in west Africa.”[2]
Deadly results of climate change include floods, forest fires and droughts, but our focus in this series is on corporeal diseases. In that respect, global heating’s major threats to human health involve life-threatening heat waves, expanded vector ranges, and disruption of the global virome.
Heat Waves
Unless decisive action slashes greenhouse gas emissions, climate change will in time make large parts of the Earth uninhabitable, characterized for most or all of the year by temperatures that the human metabolism cannot survive. But the path to Hothouse Earth is not linear. Short of a generalized catastrophe, we are already seeing ever more heat waves — intervals of extreme temperatures that can cause heat exhaustion, heat cramps, and heatstroke, often leading to premature death. Between 1990 and 2019, heat waves that lasted 2 days or more caused over 153,000 additional deaths a year. Nearly half of the deaths occurred in Asia, and about one-third in Europe.[3] Just one European heatwave, in 2022, killed 62,000 people.
Source: Lancet 2023 Countdown on Health and Climate Change. Click for larger image
Because heat waves are increasing in frequency, duration, and intensity, they affect more people every year. The Lancet Countdown on Health and Climate Change, the most comprehensive assessment of the subject, tells us:
“Adults older than 65 years and infants younger than 1 year, for whom extreme heat can be particularly life-threatening, are now exposed to twice as many heatwave days as they would have experienced in 1986–2005…. Over 60% of the days that reached health-threatening high temperatures in 2020 were made more than twice as likely to occur due to anthropogenic climate change and heat related deaths of people older than 65 years increased by 85% compared with 1990–2000.”[4]
The Lancet report projects that even if the global temperature increase is kept to just under 2ºC, there will still be a 1120% increase in heatwave exposure for people over 65 by 2041-2060, and a 2510% increase by 2080-2100. “Under a scenario of no further mitigation, the projected increases are even higher, rising to 1670% by mid-century, and 6311% by 2080–2100.”[5]
Without major mitigation efforts, a global temperature increase of just under 2ºC is projected to cause a 370% increase in annual heat-related deaths by 2050.[6]
Vector Range
About 17% of all infectious diseases, and over 30% of newly emerging infectious diseases, are spread by vectors — insects, ticks and other organisms that carry parasites, bacteria or viruses from infected humans or animals to uninfected humans. The best-known and most deadly example is malaria: transmitted by mosquitoes, it kills over 400,000 people, mostly children under five, every year. Other mosquito-borne illnesses include dengue, West Nile virus, chikungunya, yellow fever, encephalitis, Zika and Rift Valley fever.
As global temperatures increase, the geographic areas in which disease-carrying mosquitoes and ticks can survive and reproduce are expanding, exposing ever-larger numbers of people to infection. West Nile virus, once limited to parts of central Africa, is now found in North America and Europe. Cases of dengue fever have doubled every decade since 1990 — The Lancet estimates that “almost half the world’s population is now at risk of this life-threatening disease.”[7]
By mid-century, a global temperature increase of just 2ºC will cause a 23% expansion of the areas of the world in which malaria mosquitoes can prosper,[8] and at least 500 million previously out-of-area people will be exposed to mosquitoes that carry dengue, chikunguyna, Zika and other pathogens.[9]
Virome Disruption
As we’ve seen, the majority of newly emerging diseases are zoonotic — they originated in wild animals and jumped, often passing through intermediate species, to humans.
About 263 viruses are known to infect humans.[10] Although they have caused massive harm, they are a small fraction of the viral threat. “At least 10,000 virus species have the ability to infect humans, but, at present, the vast majority are circulating silently in wild mammals.”[11] For millennia, each group of viruses has circulated only among a few species of mammals, simply because there is little overlap between most species’ ranges.
Now, however, climate change is forcing animals to expand or leave their traditional territories, taking their viruses with them.
“Even in a best case scenario, the geographic ranges of many species are projected to shift a hundred kilometers or more in the next century. In the process, many animals will bring their parasites and pathogens into new environments. This poses a measurable threat to global health.”[12]
In an important study published In Nature in 2021, Colin Carlson, Greg Alpery and their associates mapped the probable geographical range shifts of 3,129 mammal species through 2070.
They found that even under moderate heating, hundreds of thousands of animals that have never interacted before will meet, leading to at least “15,000 cross-species transmission events of at least one new virus (but potentially many more) between a pair of naïve host species.“[13] The long-term shrinking of forests and wilderness areas means that the new areas of mammalian viral spillover and evolution are likely to be near human population centers and farms. That in turn will increase the probability that new zoonotic diseases will infect humans.
“The effects of climate change on mammalian viral sharing patterns are likely to cascade in the future emergence of zoonotic viruses. Among the thousands of expected viral sharing events, some of the highest-risk zoonoses or potential zoonoses are likely to find new hosts. This may eventually pose a threat to human health: the same general rules for cross-species transmission explain spillover patterns for emerging zoonoses, and the viral species that make successful jumps across wildlife species have the highest propensity for zoonotic emergence. …
“Climate change could easily become the dominant anthropogenic force in viral cross-species transmission, which will undoubtedly have a downstream effect on human health and pandemic risk.”[14]
Of particular concern, the study found that although sizeable migrations will continue through the coming century, “the majority of first encounters occur by the 2011-2040 period.”[15]
In short, climate change is already forcing a global redistribution of wildlife, and in the process bringing thousands of potentially pathogenic viruses into closer contact with humans. In coming years, the massively disrupted global virome will be more dangerous than ever.
As Alpery told The Guardian, “This work provides more incontrovertible evidence that the coming decades will not only be hotter, but sicker.”[16]
Footnotes
[1] Intergovernmental Panel On Climate Change (IPCC), Climate Change 2022 – Impacts, Adaptation and Vulnerability: Working Group II Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, (Cambridge University Press, 2023), 1045.
[2] Colin J. Carlson, “After Millions of Preventable Deaths, Climate Change Must Be Treated like a Health Emergency,” Nature Medicine 30, no. 3 (March 2024): 622–623, .
[3] Qi Zhao et al., “Global, Regional, and National Burden of Mortality Associated with Non-Optimal Ambient Temperatures from 2000 to 2019: A Three-Stage Modelling Study,” The Lancet Planetary Health 5, no. 7 (July 2021): e415–25.
[4] “The 2023 Report of the Lancet Countdown on Health and Climate Change: The Imperative for a Health-Centred Response in a World Facing Irreversible Harms,” The Lancet 402, no. 10419 (December 2023): 1.
[5] Ibid., 13.
[6] Ibid., 2.
[7] Ibid., 17.
[8] Ibid., 17.
[9] Sadie J. Ryan et al., “Global Expansion and Redistribution of Aedes-Borne Virus Transmission Risk with Climate Change,” PLOS Neglected Tropical Diseases 13, no. 3 (March 28, 2019): e0007213.
[10] Dennis Carroll et al., “The Global Virome Project,” Science 359, no. 6378 (February 23, 2018): 872–74.
[11] Colin J. Carlson et al., “Climate Change Increases Cross-Species Viral Transmission Risk,” Nature 607, no. 7919 (July 21, 2022): 555–62.
[12] Ibid., 555.
[13] Ibid., 558.
[14] Ibid., 559, 561.
[15] Ibid., 560.
[16] Oliver Milman, “‘Potentially Devastating’: Climate Crisis May Fuel Future Pandemics,” The Guardian, April 28, 2022.