Human-caused climate change "may have played a key role" in the coronavirus pandemic. That's the conclusion of a new study which examined how changes in climate have transformed the forests of Southeast Asia, resulting in an explosion of bat species in the region.
The researchers found that, due to changes in vegetation over the past 100 years, an additional 40 species of bat have moved into the region, carrying with them 100 more types of bat-borne coronaviruses. Bats are known carriers of coronaviruses, with various species carrying thousands of different types. Many scientists believe the virus that started the worldwide COVID-19 pandemic originated in bats in southern China's Yunnan province or neighboring areas before it crossed paths with humans.
These findings have scientists concerned about the probability that climate change will make future pandemics more likely.
"If bats carrying around 100 coronaviruses expanded into a new area due to climate change, then it would seem likely that this increases, rather than decreases, the chance that a coronavirus harmful to humans is present, transmitted, or evolves in this area," explains Dr. Robert Beyer, lead author of the study and a researcher at the University of Cambridge.
The researchers used climate records to create a map of the world's vegetation as it was a century ago. Using knowledge of the type of vegetation required by different bat species, they determined the global distribution of each species in the early 1900s.
They then compared this to current bat populations. Their results reveal that bat species richness — the number of different bat species found in a given area —has flourished in this pocket of Southeast Asia more than any other place on Earth.
The image below, from the study, shows how the forests of southern China, Myanmar and Laos have changed over the past century, enhancing the habitat favored by bats and allowing more species to proliferate. This distinct bullseye over the region shows the increase in bat species richness. (The study does not consider overall population sizes, just the diversity of bat species in the area.)
According to the authors, climate changes such as increases in temperature, sunlight and carbon dioxide, which affect the growth of plants and trees, have shifted the makeup of vegetation in southern China, turning tropical shrubland into tropical savannah and deciduous woodland. This type of forest, the authors contend, is more suitable to bat species.
The study calls this area in Southeast Asia "a global hotspot" for bat species and points to genetic data suggesting that SARS-CoV-2, the virus that causes COVID-19, originated in this region.
This, the authors say, provides the first evidence of a way that climate change could have played a direct role in the emergence of the virus.
"We estimate that, over the past century, climate change caused a significant increase in the number of bat species in the location where SARS-CoV-2 likely originated," said Beyer. "This increase suggests a possible mechanism for how climate change could have played a role in the origin of the pandemic."
A team of researchers from the World Health Organization was finally allowed into Wuhan, China, in January to investigate the source of the outbreak, which was first reported in that city a little over one year ago. A leading theory among scientists is that the virus originated in bats before making the jump to humans, potentially through an animal host like pangolins. Some of the first cases were linked to a wildlife market in Wuhan. But as of now, this is only a theory, and researchers are just beginning to formally investigate the origins of the pandemic.
Dr. Rick Ostfeld, a disease ecology expert from the Cary Institute of Ecosystem Studies in Millbrook, New York, finds the research compelling, even though he doesn't agree with all of its conclusions. He says it is not surprising that climate change was found to have transformed forests and bat communities. He also agrees with the study authors that the movement of animals can help spread viruses.
"Moving animal communities around a region can have strong impacts on disease transmission by exposing animal hosts to new pathogens," he said.
But he is cautious about drawing conclusions beyond that.
"The link to emergence of coronaviruses is highly speculative and seems unlikely," said Ostfeld.
"What the study apparently gets wrong is the assumption that the increased diversity of bats (which they postulate) leads to an increased risk of a bat-borne virus jumping to humans. This is simply not the case," he said. "The vast majority of bats are harmless to humans — they don't harbor viruses that can make us sick. So adding more of those species doesn't increase risk."
Kate Jones, a professor of ecology and biodiversity at University College London, is also somewhat cautious. She said, "Climate change certainly has a role to play in changing species distributions to increase ecological hazard. However, spillover risk is a complex interplay of not only ecological hazard but human exposure and vulnerability."
Beyer does agree that "caution is well-justified" when it comes to connecting climate change directly to the pandemic because, as he explains, assessing the degree to which climate change contributed to any stage between a bat carrying the virus and a human getting infected will take a lot more work. In particular, he says, this involves the use of epidemiological models that analyze the interactions of different species and viruses across space and time.
While it's widely understood that exponential growth of the human population, and our rampant exploitation of the natural world, like destroying forests and expanding the animal trade, is increasing the risk that contagious pathogens can more easily make the jump from animals to humans, it has been less clear the extent to which climate change factors in.
However, over the past century, because of human-caused climate change, many ecosystems have have warmed — sometimes by several degrees — and precipitation patterns have shifted, with some areas getting less and others getting more. These ecosystem changes are shifting the habitat of many species, putting more species in contact with one another, potentially allowing viruses to spread more easily.
When asked about the climate connections to the spread of disease, most experts agree there is an impact, but some say direct human actions like deforestation, development, or industrial-scale animal agriculture, are a bigger concern.
"It may turn out that increases in human populations, human movement and degrading natural environments through agricultural expansion have a more important role to play in understanding the spillover process of SARS-CoV-2," explained Jones.
Ostfeld observed, "We can predict which wildlife species are most likely to carry pathogens that can make people ill. These are generally the ones that thrive when we replace natural habitat (like forests and savannas) with agriculture, residential developments, and strip malls."
Beyer does not take issue with those assessments. "We absolutely agree that the expansion of urban areas, farmland, and hunting grounds into natural habitats is a key driver of zoonotic disease transmissions — they are what puts many pathogen-carrying animals and humans into contact in the first place," he said.
But given the findings of his research on how climate reshaped the region, Beyer feels climate change can be a significant driver.
"Climate change can drive where these animals occur; in other words, climate change can move pathogens closer to humans. It can also move a species that carries a virus into the habitat of another species that the virus can then jump to — a step that might not have occurred without climate change, and that might have major long-term consequences for where the virus can go next."
Beyer also sees climate connections beyond just the increase in bat species. "In some cases, higher temperatures can increase the viral load in species, which can make it more likely that the virus is transmitted," he said. "And: Increased temperature can increase the tolerance of viruses to heat, which in turn can increase infection rates since one of our primary defense systems to infectious diseases is to raise our body temperature (fever)."
While there is some caution in the scientific community about the specific impact of climate change on the current coronavirus pandemic, there is widespread agreement that, in the future, climate change will be a growing driver of emerging infectious disease and pandemics.
"Climate change will shift the geographic distributions of pathogen-carrying species in such a way that they overlap with species that they did not overlap with before," said Beyer. "These new interactions will provide dangerous opportunities for viruses to spread and evolve."
"Climate change definitely is an important driver in disease emergence and spread. It can increase transmission in a number of ways," said Ostfeld. "So, yes, climate change definitely concerns me as a driver of future pandemics."
