How climate change is raising the risks of another pandemic

As humans degrade earth’s environment, we have created a world in which diseases may be increasingly apt to fester and multiply.

Infection-spreading creatures such as mosquitoes and ticks are thriving on a planet warmed by a blanket of fossil fuel emissions. When pollution, hunting or development push rare organisms to extinction, parasites proliferate because they have evolved to target the most abundant species.

And then there are the harms caused when humans introduce nonnative plants and animals or chemicals such as herbicides and fungicides to fragile ecosystems. That exacerbates losses in biodiversity that leave surviving populations more vulnerable to illness, according to research published Wednesday in the journal Nature.

Researchers said the study is the first to look at the ways such a variety of environmental problems can compound disease risks. It combined hundreds of studies and thousands of observations of all kinds of creatures - humans and other mammals, fish, reptiles, amphibians, worms and arthropods - and all kinds of pathogens, such as viruses, bacteria and fungi.

The analysis reinforced the findings of many of those inquiries: that a hotter world of ravaged ecosystems is one that is more hospitable to many parasites, and less so to humans and other life.

The connection appeared with all types of infections and their hosts, suggesting that as the planet continues to warm and humans continue to disrupt nature, increases in disease spread “will be consistent and widespread,” said Jason Rohr, a professor of biological sciences at the University of Notre Dame and one of the study’s authors. The link was just as clear with humans as it was with wildlife and plants, he added.

“That is despite all the efforts we’re making to control and prevent diseases,” Rohr said.

And if diseases become more rampant in the animal world, that could mean the likelihood of “spillover” events exposing humans to new pathogens - the likely origin of covid-19, and a feared outcome of the ongoing spread of H5N1 bird flu - also will increase, the study suggests.

“It could mean that by modifying the environment, we increase the risks of future pandemics,” Rohr said.

As grim as the findings appear, they underscore that actions to protect the planet can also serve to improve health, researchers said.

“This adds to a very long list of reasons we should be rapidly moving away from fossil fuels and trying to mitigate the impacts of climate change,” said Felicia Keesing, a professor at Bard College who was not involved in the study but whose research focuses on biodiversity and disease risks.

The study used observations of disease outcomes involving a wide variety of parasites infecting a spectrum of hosts around the world. The observations also included information about a range of human influences on the environment: biodiversity changes, chemical pollution, climate change, habitat loss or change and introduction of nonnative species.

Biodiversity has a natural gradient across the planet, with the greatest numbers of species found closest to the equator and at moderate elevations. The researchers gauged the effect of human-caused biodiversity loss on diseases by comparing the observations of infections around the world to average disease prevalence at varying levels of biodiversity across that natural gradient. In nature, reductions in biodiversity are associated with reductions in disease.

But when humans cause losses in biodiversity, diseases increase. The researchers found that levels of disease and mortality in environments affected by human-caused biodiversity losses were nearly nine times worse than disease outcomes expected under earth’s natural biodiversity gradient. Rohr said that is probably because the loss of rarer creatures means pathogens have an easier time finding the more abundant species they have evolved to use as hosts.

The researchers also found that climate change and the introduction of nonnative species have significant links to worsened disease spread, though not as strong as the effect of biodiversity losses.

The analysis found one variable of human influence that actually decreased disease risks: habitat losses. Rohr said the researchers believe that is largely a function of urbanization: Cities tend to have better sanitation and health infrastructure, and are simply home to fewer natural organisms, he said.

Skylar Hopkins, an assistant professor at North Carolina State University who was not involved in the research, cautioned against applying the findings too broadly. Analyses such as this one are composed of a selection of completed studies but cannot represent a truly random sample of pathogens and infections, she said. Not all parasites are “bad,” she added, and one also cannot assume that repairing lost biodiversity will undo an increase in disease.

The research published Wednesday builds on past findings that link disease spread with specific global changes.

For example, it is known that extreme heat and precipitation tied to human-caused climate change have allowed malaria cases to rise, and could drive them to surge even more dramatically in the decades ahead. Biodiversity losses are known to contribute to spillover of diseases such as covid-19, HIV/AIDS, Ebola and severe acute respiratory syndrome, or SARS.

But the researchers wrote that more needs to be learned about how humans’ many influences on the environment might be building upon each other.

“For example, climate change and chemical pollution can cause habitat loss and change, which in turn can cause biodiversity loss and facilitate species introductions,” the researchers wrote. New studies will need to examine whether those factors, in combination, serve to add, subtract or even multiply risks of disease spread.