Climate change doesn’t just shift weather patterns. It can force the migration of plants, people, animals, bugs — and disease.

By the end of this century almost all of the world’s population could be exposed to mosquito-borne diseases once limited to the tropics, according to a new study from PLOS Neglected Tropical Diseases.

Aedes aegypti and Aedes albopictus are two species of mosquitoes known to carry and transmit potentially deadly viruses like dengue, chikungunya, Zika and yellow fever. These bugs can thrive in urban environments, often traveling alongside humans as we ship goods and travel around the world.

And, according to the study, these tropical mosquitoes are poised to expand their ranges, exposing a billion additional people to the diseases they carry in the next 50 years.

Given estimates for future temperatures around the globe, the researchers created a model to determine which areas may become suitable for mosquitoes to transmit viruses.

By using these models to predict temperature, a critical factor in mosquito and virus survival, the team could also predict places where dengue, yellow fever, chikungunya and Zika might be able to make the leap from person to person, causing an outbreak of disease.

These models of our future world could help governments and health organizations prepare for and prevent the spread of disease by controlling standing water, providing mosquito-blocking screens or spraying pesticides.

There’s not going to be a ‘somebody else’s problem’ 20 to 30 years from now.

According to the researchers, developed countries have often regarded tropical diseases as out of sight and out of mind — somebody else’s problem. This kind of prediction can draw attention (and funding).

“Part of the punchline with climate change is that there’s no way to keep diseases in one place,” said co-lead author Colin Carlson, a global change biologist from Georgetown University. “There’s not going to be a ‘somebody else’s problem’ 20 to 30 years from now.”

What the researchers did

The researchers used four commonly used models for future climate to look at the temperature predictions for four scenarios, called representative concentration pathways or RCPs. Each scenario represented some course of action on the part of humanity — the most optimistic, known as RCP 2.6, shows a future in which ambitious action is taken to curtail greenhouse gases, while the most drastic, RCP 8.5, stands for a future where no changes are made to fossil fuel consumption or climate policy.

That’s because temperature has a huge impact on the transmission of mosquito-borne diseases, Carlson said.

Inside an Aedes mosquito, viruses take time to develop, requiring what’s known as a “Goldilocks” range of temperatures — not too cold, and not too hot. Outside of that temperature range, a presumably infected mosquito could bite a person and no virus would be transmitted.

The team looked month-by-month at the predicted temperatures generated by each climate model. They found more than 6 billion people already live in tropical and temperate areas that are suitable for disease transmission for at least one month out of the year.

In a worse-case scenario, there may actually be large areas that become too hot for the mosquitoes to transmit viruses. That’s not really good news, though.

But by 2050, the areas considered “exposed” to Goldilocks temperatures will creep northwards and southwards, away from the equator. For at least some months of the year, both species of mosquitoes would be in the “Goldilocks zone” throughout Eurasia and North America, where a billion more people could be at risk.

“A billion people exposed for the first time is a lot more terrible than a billion people living where they’ve built up immunity,” Carlson said. First-time exposures to these diseases in heavily populated areas have the potential to become explosive outbreaks, he said.

By 2080, in the best-case scenario where global fossil fuel emissions drastically decrease, the risky zone will still spread to affect a little fewer than 455 million. In the worse-case scenario, where business as usual continues and temperatures rise unchecked, there may actually be large areas that become too hot for the mosquitoes to transmit viruses.

That’s not really good news, though. Areas that heat up too much for dengue or Zika transmission, Carlson said, will leave their inhabitants at consistent and overwhelming risk of heat stroke and dehydration.

“I don’t think there’s any way to slice climate change that looks good for health,” Carlson added.

Hope in the face of global pandemics

“Not every region in the world is prepared for climate change conditions,” said Stephanie Thomas, a geoecologist from the University of Bayreuth in Germany who was not involved in this study. “We [in Germany] are not prepared for this situation.”

This kind of research, she said, is valuable forewarning for public health organizations that can get a jump-start on mosquito surveillance and pest control.

It’s important to note that this study is on a global scale, she said, and mosquito management efforts will need nation-by-nation or even city-by-city information while trying to combat the viruses.

In addition, Carlson and Thomas both argued that the study predicts areas where the diseases could possibly be transmitted by mosquitoes, not necessarily where the mosquitoes and diseases will actually find purchase in human populations. Wind patterns and humidity could have effects on where the disease and mosquitoes can pop up, too, Thomas added.

But Carlson is optimistic. “I think we don’t talk about [hope] enough. We’re not staring down a certain apocalypse, because I don’t think there’s a future where people take no steps to combat the effects of climate change.”