September 2, 2020

Macro Series

Climate change and disease spread: from nature to humans

BY John Llewellyn, Llewellyn Consulting

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Rising temperatures and destruction of ecosystems are set to increase health risks

Covid-19 shows how an animal-borne virus can wreak havoc on humankind. It is vital that its precise origins and transmission process become better understood, so as to avoid similar, or even more infectious and fatal, pandemics in the future.

The ecosystem of which humans are a part is complex. Matters hang in a delicate balance; and small disturbances, such as those arising from rising temperatures, changing land-use, and habitat and biodiversity loss can have major, and at times seemingly disproportionate, consequences.

Increasingly, public health issues are one such area. They fall broadly into three categories.

Zoonotic diseases

Around 60% of human infections are of animal origin, and the figure is even higher (around 75%) for new and emerging human infectious diseases. 1

Hence increasing the frequency and intensity of contact between humans and animals, whether by higher density living or by pushing wildlife out of natural habitats – deforestation, farming, urbanisation – is proving to be a risky combination. 2

Even before Covid-19, some 2 million people annually died from zoonotic diseases, mostly in poorer countries. 3 Recent cases include Ebola, SARS, MERS, HIV/AIDS, and Nipah viruses. However, there may be as many as 1.7m harmful viruses, present in mammal and bird populations that have not yet transmitted to humans. 4 Any of these could potentially be the next ‘Disease X’.

Furthermore, the likelihood of a zoonotic disease turning to a pandemic has been heightened in recent decades by increased global connectivity, particularly through air travel. 5

Vector-borne diseases

Warmer, more humid conditions foster vector-borne disease transmission (e.g. Zika virus, malaria, dengue fever) by the increasing vector population (e.g. sand flies, mosquitos, and ticks); by lengthening their seasonal viability; and by expanding their geographic ranges. 6 These insects are ectothermic (i.e. cannot regulate their body temperature), and so the warmer the environment, the warmer the insect, and the faster the virus replicates. 7

One study has concluded that, with continued global warming, mosquito-borne diseases alone could reach an additional one billion people by 2080, mostly in higher latitudes. 8

Diseases stored in the permafrost

Scientists are suggesting that the rapidly-warming climate in the Arctic risks exposing long-dormant viruses, which may be tens, or even hundreds, of thousands of years old. 9 The anthrax outbreak in northern Siberia in 2016, for example, is believed to have originated from the frozen (infected) carcass of a reindeer becoming unearthed by the thawing of the (no longer perma) frost.

With the Arctic now warming at least twice as fast as the rest of the planet, and temperatures this northern-hemisphere summer breaking records, further outbreaks of ‘long-forgotten’ or even unknown diseases seem increasingly likely.


The emphasis to date has been largely on vector-borne diseases. Research into animal-borne diseases is still embryonic, but Covid-19 is shifting that emphasis, not least because zoonotic diseases are now seen to be an issue not only for poorer, economically less developed countries, but also for the highest per capita income countries.

Covid-19 is already impelling massive funding and research around the world; and the basic research in particular may well increase the chances of earlier detection and/or a cure. Given that a single zoonotic outbreak can result in trillions of dollars in costs, prevention is much more cost-effective than any after-the-event response.

It is, however, more appropriate to treat the fundamental underlying cause, rather than just the symptoms. The longer that humans keep exploiting wildlife and destroying biodiversity, the greater the likelihood of transmission of pathogens from animals to humans.

The likely effects of climate change have often seemed distant and slow-moving. But it is now widely recognised that a pandemic can have a devastating effect across the globe, and very quickly. As crucial links get better understood, climate change will increasingly concentrate minds.

  1. UN Environment Programme, 2020. Preventing the next pandemic – Zoonotic diseases and how to break the chain of transmission [online]. Available at: [Accessed: 25 July 2020]
  2. Climate Home News, 2020. Next UN climate science report to consider lessons from coronavirus. [online]. Available at [Accessed: 23 July 2020]
  3. Carrington, D., 2020. Coronavirus: world treating symptoms, not cause of pandemics, says UN. The Guardian, 6 July [online]. Available at: [Accessed: 27 July 2020]
  4. Dunne, D., 2020. Q&A: Could climate change and biodiversity loss raise the risk of pandemics? Carbon Brief [online]. Available at: [Accessed: 27 July 2020]
  5. Ibid.
  6. Evans, L., 2020. Climate change is fuelling the spread of deadly tropical diseases. Wired, [online] Available at: [Accessed: 27 July 2020]
  7. As insects cannot control their body temperature, they are more directly affected by warming of their environment, and thus, become more hospitable hosts for viruses. In one study, for example, groups of mosquitoes were given Zika virus and exposed to temperatures between 17 and 31°C. The researchers then studied Zika virus infectivity at different temperatures and found that the virus could be detected in the insects’ salivary glands – and that therefore they were infectious – at temperatures of 19°C and above. For more, see Lu, D., 2020. Climate change may push Zika virus into southern and eastern Europe. New Scientist, [online]. Available at: [Accessed: 20 July 2020]
  8. Dunne, D., 2020.
  9. Cockburn, H., 2020. Climate crisis: Thawing Arctic permafrost could release deadly waves of ancient diseases, scientists suggest. Independent [online]. Available at: [Accessed: 23 July 2020]

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