For decades, scientists have been making predictions about how climate change will hurt health around the world.
But actually showing a link? That's been pretty tough.
Take for example, mosquito-borne diseases. It's easy to blame rising temperatures for the global spread of Zika or the explosion of dengue fever. Mosquitoes thrive in higher temperatures, right?
Yes and no. As we reported earlier this year, warmer weather doesn't necessarily mean mosquitoes are more likely to spread viruses like dengue, yellow fever and Zika. Higher temperatures can actually reduce transmission of viruses because the insect's lifespan can decrease in warmer weather. So the mosquito may die before the virus has time to mature and become infectious inside of it.
In other words, climate's connection to health is extraordinarily complicated.
Now international team of scientists has taken a step toward untangling this problem on a global scale.
"All of the work we present is pretty tricky," says Dr. Nick Watts, at University College London, who led the study. "I don't think any of us would ever say that this has been easy."
Around the world, people have experienced an average increase in temperature about 1.5 degrees Fahrenheit, and the study — published Monday in The Lancet journal — finds several signs that even this small amount of warming threatens the health of hundreds of millions of people each year.
First, the number of vulnerable people exposed to heat waves has surged worldwide, the study finds. In the past few years, more about 125 million people over age 65 experienced heat waves each year, compared to about 19 million people each year in the 1990s.
"That's a pretty stunning number," says Kim Knowlton, an environmental researcher at Columbia University, who studies climate change and health at but wasn't involved in this study. "Heat waves aren't just an inconvenience. Heat kills." And it also exacerbates existing problems, such heart disease and kidney problems.
This recent surge in heat waves is consistent with previous studies looking at health and climate change, including meta-analyses published by the Intergovernmental Panel on Climate Change in 2014. "Rising temperatures have [likely] increased the risk of heat-related death and illness," the IPCC wrote.
The second major consequence of warming temperatures is an increase in weather-related disasters. The frequency of floods, droughts and wildfires, collectively, has increased by 46 percent since the 1980s, rising from about 200 events each year to 300 events per year. And some of that increase is due to climate change, the Lancet study finds.
Families around the world — including those in the U.S. — are already experiencing these events firsthand, Knowlton says.
"Communities are hurting. People are reeling globally," she says. "And I think this experience might mark a turning point in the public's perception of climate change because people are connecting the dots to their health, here and now."
Surprisingly, the study finds that deaths from weather-related disasters has not increased during the same time period.
"There's no discernible upward or downward trend in the lethality of these extreme weather events," Watts says. "That may simply be because the data is not over a long enough period of time to isolate that trend."
And then there's the question of mosquito-borne diseases. Since 1990, annual cases of dengue worldwide have doubled each decade. Much of this rise is likely due to rapid urbanization and global travel, the World Health Organization says.
But Watt and his collaborators do find that climate change has contributed to dengue's explosion — at least a little bit.
"We're not trying to say that all cases of dengue fever are the result of climate change," Watts says. "But we've identified a very strong signal in the climate trends that are increasing the capacity of the Aedes mosquitoes to spread dengue." Although they don't yet know why.
Specifically, the team estimates that climate change has increased dengue transmission by Aedes aegypti and Aedes albopictus by 3 percent and 6 percent, respectively, since 1990.
Finally, the Lancet study also analyzes what countries are doing to slow down climate change. "That's probably the part of this report that really surprised me," Watts says. "There are glimmers of hope in that data."
For the past 25 year, he says, countries have been basically doing very little to reduce carbon emissions. "Progress has been woefully inadequate."
But now there are signs the tide may be turning by a small degree.
"In the past five years, we have started to see an acceleration in the response to climate change," Watts says.
In particular, the use of coal around the world has slowed down, the study finds and even possibly even peaked in 2013. Some countries are relying more on natural gas and some are starting to swap in renewable energy sources — like geothermal, hydropower, ocean energy, solar energy and wind energy — which not only reduce carbon emissions but also make the air healthier for people to breath.
"That's really exciting," Watt says. "Because we could all use a little bit of hope at the moment."
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