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Scientists expect to see changes in the timing, location, and severity of disease outbreaks as global temperatures rise.
President Trump assured the American public that warmer weather could stop the spread of the coronavirus. But experts caution that there is no evidence to support that idea.
His statement raises new questions about the role that temperatures play in infectious diseases as the Earth warms. The impacts of climate change on the coronavirus are unknown, but research related to other diseases suggests that the risk of pandemics is increasing as rising temperatures trigger animal migrations and other changes.
The COVID-19 virus continues to spread even as the first signs of spring begin to appear throughout the Northern Hemisphere.
It is true that in temperate areas of the world, such as the United States, Europe and much of Asia, the flu season tends to increase in the winter and disappear in the spring. And some other types of coronaviruses, which have been around longer and better studied than COVID-19, have also exhibited seasonal patterns.
But COVID-19, being a new disease, still contains more questions than answers. Scientists aren't sure what kind of patterns to expect as it spreads or how it might be affected by the weather.
Confirmed reports of the coronavirus have surpassed 260,000 cases worldwide, with no signs of slowing down. More than 11,000 people have already died around the world, the majority in China.
Even if it turns out to have some seasonal components in the future, that effect will likely be small this year, experts say. Since it is a new disease with very little accumulated immunity in the human population, it is likely that it will continue to spread rapidly.
Answering these kinds of questions about the coronavirus will take time. But in general, the links between climate and infectious diseases are a topic of growing 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 that climate change, along with other environmental shocks, could help facilitate the emergence of more new diseases, such as COVID-19.
CHALLENGE FOR INVESTIGATION
It's difficult to determine what those changes will look like, especially for direct-transmitted diseases like COVID-19, which spreads easily from one person to another.
There is a great deal of research on climate and vector-borne diseases: these are diseases that are transmitted to humans by other animals, such as mosquitoes or ticks. But it is much more difficult to investigate climate impacts on person-to-person disease transmission.
"We can put mosquitoes in a lab," said Rachel Baker, an expert on climate and infectious diseases at the Princeton Environmental Institute. "He places mosquitoes in the labs, looking at everything from life span and egg-laying properties and all these different life cycle physiological characteristics and relating them to climatic factors."
Studies suggest that vectors such as mosquitoes and ticks can change their ranges as the weather warms. This means that certain vector-borne diseases, such as malaria, dengue, or Lyme disease, may move to new territories in the future.
But with directly transmitted diseases, like the flu or COVID-19, it is much more difficult to conduct experiments. Some viruses, like the flu, for example, can be tested in animals like guinea pigs. But that doesn't work for all viral illnesses. 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 behavior of these diseases around the world. In this way, scientists are slowly beginning to understand how climate affects some of the most common viral diseases.
But there are more questions than answers. Take the flu, for example.
In temperate parts of the world, influenza exhibits strong seasonal patterns and tends to peak in winter. Experts believe that the virus survives better in colder and drier conditions. Human behavior may also have something to do with it: people tend to stay indoors longer in winter, which means they are more likely to be around and can infect others more easily.
In the warmer tropics, on the other hand, the flu season tends to extend throughout the year, with some peaks during the rainy season. As a result, some experts suggest that climate change may make flu outbreaks in temperate regions less intense but more evenly spread across the seasons, Baker noted.
Researchers have observed similar patterns in RSV, another common directly transmitted respiratory virus. But limited studies have suggested that climate change could have other effects as well.
A paper published in 2013 found that unusually warm winters tend to be followed by earlier and more severe flu seasons the next year. Researchers suggest this is because fewer people get the flu during warmer winters, making your immune system more vulnerable the following year.
Another article, published earlier this year, suggested that rapid changes in the climate can also make flu epidemics worse.
The flu is certainly not representative of all directly transmitted diseases. But research on the flu, one of the most common and best-studied viruses in the world, helps demonstrate the challenges of analyzing the influence of climate change.
Much of the research on common illnesses, such as the flu, still focuses on how climate and climate affect illness today, which is the first step in understanding how changes in the weather might affect illness in the future. The same foundation will be needed for scientists to make predictions about the future of emerging diseases, such as COVID-19.
"We really need to have that understanding before we can think about climate change," Baker said. "There are still many open questions in terms of how important the climate is."
The rapid spread of the coronavirus is sparking challenging conversations about how to prepare for epidemics, especially new or unknown diseases. Climate change can make these conversations even more important.