The factors leading to the current Zika outbreak won't be clear for some time, but environmental health experts say there's a good chance such infectious diseases will become more common as the global climate warms.
Mosquitoes, the blood-sucking insects responsible for transmitting Zika virus in Brazil and more than 20 other countries and territories in the Americas, are responsive to changing weather conditions, and experts warn they may increase in numbers as temperatures rise and as changes in precipitation levels create more standing pools of water -- mosquitoes' favorite breeding ground. More of the insects that host the virus could mean a higher chance of being infected.
It's too soon to say whether the Zika outbreak, which is causing babies to be born with smaller heads and brains that aren't fully developed, is driven by a changing climate. But it's certainly the kind of health consequence scientists have anticipated seeing more of as global temperatures rise.
"If I were a gambler and I wanted to place a wager on the healthiest possible future in terms of these mosquito-transmitted diseases, would that future look more like what we are looking at with climate change -- with warmer temperatures and more intense precipitation -- or a future that looks more like the climate of 1900?" Dr. Aaron Bernstein, the associate director of Harvard's Center for Health and the Global Environment, told The Huffington Post. "I would probably bet on the latter."
There are countless factors that contribute to the rise of an infectious disease like Zika, added Kristie Ebi, an expert on health risks and responses to climate change at the University of Washington. There's a good possibility that a warming planet is one of them.
"This is the kind of thing the field has been saying is likely to occur for the last 20-plus years," she said. It's "very difficult to tease out" whether climate change is driving the disease, she added. "We’ll only figure that out in retrospect, but given the known interactions, is this the kind of thing we’d expect from climate change? Yes."
Part of the uncertainty is a result of mosquitoes having different reactions to different levels of heat. While scientists know the insect thrives in a warmer climate and that prolonged warm seasons will increase its breeding season, temperatures that are too high can kill them off.
"The story is not so simple as 'warmer temperatures are better' or 'more rain is better' or 'more drought is better,'" Bernstein said.
For instance, researchers have observed rates of malaria, another mosquito-transmitted disease, slowing with higher temperatures. Malaria, Bernstein explained, is not a virus like Zika but a parasite that must first infect a mosquito before it can be transmitted to humans. Higher temperatures, he said, have been shown to drive mosquitoes to reproduce at a faster rate than they drive the parasite to reproduce, therefore decreasing the rate of malaria.
"Whether that plays for Zika, I have no idea, but it just points to the sort of difficulty in really understanding what’s likely," Bernstein explained.
More standing water may be the bigger concern. With climate change comes more intense rain storms, which leave behind pools of water. It also brings more severe drought, which transforms rushing rivers into slow-moving streams and puddles -- all perfect breeding grounds for mosquitoes.
"We’re seeing more droughts and more heavy downpour, and so arguably that’s more conducive [to mosquito reproduction]," Bernstein said.
Climate change isn't the only human activity creating pools of water and interfering with mosquito behavior.
"We know one of the drivers of these outbreaks is urbanization and urbanization patterns," Ebi said. "We're basically shipping the mosquito around the world."
One contributing example she cited was the global used tire trade. Researchers have observed mosquitoes breeding in pools of water that form in tires' hollows while they're sitting outside. When those tires are shipped to another country, they bring along mosquitoes carrying diseases to places where foreign populations may have less immunity to them.
"As you can see, there are a lot of complex interactions," Ebi said.
If anything, adding climate change into that mix destabilizes what is already a global health challenge.
"The last thing we need to do is throw in some uncertainty into that mix," Bernstein said. "It makes it much more difficult to understand where we might direct our prevention measures, and that’s one of the many reasons addressing climate change is so important."