President Trump assured the American public that the onset of warmer weather could halt the spread of the coronavirus. But experts caution there’s no evidence to support that idea.

His assertion raises new questions about the role temperatures have on infectious diseases as Earth gets warmer. The impacts of climate change on the coronavirus are unknown, but research related to other illnesses suggest that the risk of pandemics is growing as rising temperatures ignite animal migrations and other changes.

The COVID-19 virus continues to spread even as the first hints of spring begin to appear across the Northern Hemisphere.

It’s true that in temperate parts of the world, like the United States, Europe and much of Asia, flu season tends to spike in the winter and drop off in the spring. And some other types of coronaviruses, which have been around longer and been better studied than COVID-19, have also exhibited seasonal patterns.

But COVID-19, being a novel disease, still holds more questions than answers. Scientists aren’t sure what kinds of patterns to expect as it spreads or how it might be affected by weather and climate.

Confirmed reports of the coronavirus have now topped 100,000 cases worldwide, with no signs of slowing down. More than 3,000 people around the globe have already died, the majority in China.

Even if it does turn out to have some seasonal components in the future, that effect will likely be small this year, experts say. Since it’s a new disease with very little immunity built up in the human population, it will likely continue to spread quickly.

Answering these kinds of questions about the coronavirus will take time. But in general, links between climate and infectious disease are a growing subject of interest among scientists.

As the Earth continues to warm, many scientists expect to see changes in the timing, geography and intensity of disease outbreaks around the world. And some experts believe climate change, along with other environmental disturbances, could help facilitate the rise of more brand-new diseases, like COVID-19.

Challenge for research

Figuring out what those changes will look like is difficult — especially for directly transmitted diseases like COVID-19, which spreads easily from one person to another.

There’s a great deal of research about climate and vector-borne diseases — these are illnesses that are transmitted to humans by other animals, such as mosquitoes or ticks. But it’s much harder to research climate impacts on human-to-human disease transmission.

"We can put mosquitoes in a lab," said Rachel Baker, an expert on climate and infectious diseases at the Princeton Environmental Institute. "Put mosquitoes in labs, looking at everything from life length and egg-laying properties and all these different physiological life cycle characteristics and relating those back to climate drivers."

Studies suggest that vectors like mosquitoes and ticks may shift their ranges as the climate warms. This means that certain vector-borne diseases, such as malaria, dengue fever or Lyme disease, may move into new territories in the future.

But with directly transmitted diseases, like influenza or COVID-19, it’s much harder to run experiments. Some viruses — flu, for example — can be tested in animals like guinea pigs. But that’s not true for every viral illness. And animals don’t provide a perfect analogy for the way diseases spread in human societies.

Much of what we know about climate and directly transmitted diseases comes from large-scale observations of the way these diseases behave in the world. In this way, scientists are slowly starting to gain insight into how climate affects some of the most common viral diseases.

But there are more questions than answers. Take influenza, for instance.

In temperate parts of the world, flu exhibits strong seasonal patterns and tends to peak in the winter. Experts believe the virus survives better in colder, drier conditions. Human behavior may have something to do with it, as well — people tend to stay indoors more in the winter, meaning they’re more likely to be in close quarters with one another and may infect others more easily.

In the warmer tropics, on the other hand, flu season tends to spread out throughout the year, with some spikes during the rainy season. As a result, some experts suggest that climate change may cause flu outbreaks in temperate regions to become less intense but more evenly distributed across the seasons, Baker noted.

Researchers have observed similar patterns in RSV, another common, directly transmitted respiratory virus.

But limited studies have suggested climate change could have other effects, as well.

A paper in 2013 found that unusually warm winters tend to be followed by earlier, more severe flu seasons the next year. The researchers suggest this is because fewer people come down with the flu during warmer winters, leaving their immune systems more vulnerable the following year.

Another paper, published earlier this year, suggested that rapid swings in the weather may also make flu epidemics worse.

Flu certainly isn’t representative of all directly transmitted diseases. But the research on flu, one of the most common and well-studied viruses in the world, helps demonstrate the challenges of parsing out the influence of climate change.

Much of the research on common diseases, like the flu, is still focused on how climate and weather affect the disease today — which is the first step to understanding how changes in the climate might affect the disease in the future. The same foundation will be necessary for scientists to make predictions about the future of emerging diseases, like COVID-19.

"We really need to have that understanding before we can think about climate change," Baker said. "There are still a lot of open questions in terms of how is climate important."

Novel diseases

The rapid spread of the coronavirus is sparking challenging conversations about how to prepare for epidemics, especially new or little-known diseases. Climate change may make these conversations even more important.

For one thing, climate change may cause diseases that are common in some places to shift into new geographic locations. That’s a particular risk with vector-borne diseases, as mosquitoes and ticks expand their ranges.

In that scenario, the disease itself isn’t unknown to the world — but it may be new to many of the places it affects in the future.

"It seems like what we expect from mosquito-borne diseases with climate change is that they’re going to change in their distribution and affect new populations that are not used to being under that threat," said Christine Johnson, director of the EpiCenter for Disease Dynamics at the University of California, Davis’ School of Veterinary Medicine. "And in some cases, very vulnerable populations that don’t have a lot at the ready in terms of mosquito control."

Scientists are working on ways to improve their projections of where these types of diseases may crop up in the future, so communities can prepare to deal with them.

It’s also possible that climate change may affect the emergence of entirely novel diseases, like COVID-19.

Exactly how is highly uncertain. But it’s worth keeping in mind that most novel diseases originate in wildlife before they spread to humans, said Johnson. The COVID-19 virus, for instance, is thought to have originated in bats.

As the climate changes, many animal species are likely to change their behavior or migrate to new areas. It’s possible that in some cases, this could increase their likelihood of coming into contact with humans.

Climate change isn’t the only environmental disturbance to keep an eye on. Other human activities may also increase the likelihood of human-wildlife contact and the risk of emerging diseases.

Deforestation is one major potential factor. Wildlife markets are another, Johnson added.

That said, the effects of environmental disturbances on novel diseases remain highly uncertain.

"I think we can say that things are going to change, and that we expect the risk to increase," Johnson said. "But we can’t say with any certainty which diseases, in which locations and at which time."

For now, some of the greatest lessons the world is learning from coronavirus may simply be the value of preparing for the unexpected. And that’s a lesson the world is learning from climate change, as well.

"Something I hear a lot in this field is, we can’t predict next year’s flu season, so how could we possibly make predictions out to 2100 or 2050 on what the flu season is going to look like with climate change?" Baker said. "I think there’s a great analogy here with climate science itself. People make the same case: How do we know what climate change is going to look like in 50 or 100 years when we don’t know what next week’s snowfall is going to be?"

The key thing to remember in both cases, she said, is that short-term fluctuations may be hard to predict — but observing long-term patterns over many years can give scientists great confidence in their predictions about what the future might hold. Keeping up these efforts in both climate science and infectious disease research is critical.

"I think the analogy there is an important one," Baker said.