If we can hit the most aggressive targets set by the Paris Agreement, we could stave off the worst health impacts for people living in one of the most climate-vulnerable areas on Earth. A study published Monday in Nature Geoscience projects that the tropics will stay habitable to humans if we can keep warming below the 1.5-degree-Celsius (2.7-degree-Fahrenheit) threshold.
The world’s tropical regions are some of the most worrisome areas of the globe when we think about the next few decades and climate change. The area includes all of Central America, much of South America, a wide swath of Africa, and many countries in South and Southeast Asia. It’s home to 3.3 billion people—including 85% of the world’s poorest population—and the population is growing. The tropics are also a ticking time bomb when it comes to climate change, particularly when it comes to extreme heat.
With almost half the world’s population living in such a crucial area, it’s important to figure out how increasing temperatures will impact human health, which is what this study focused on. Using various climate models and observations, the study’s authors mapped out the correlation between overall temperature changes and an important metric for human health known as wet-bulb temperature.
While we often talk in terms of how hot it is outside, wet-bulb temperature, which measures humidity as well as heat, can matter for our health even more than what the outdoor thermometer tells us. Research has shown that our bodies hit their limits at around a wet-bulb temperature of 95 degrees Fahrenheit (35 degrees Celsius). A 2015 study estimated that anything beyond six hours spent in these conditions “would probably be intolerable even for the fittest of humans.”
This study drilled down at the specifics of how wet-bulb temperatures actually function in the tropics. Humidity and heat are not always totally in sync. The study notes that some regions in the world have seen humidity decrease on hotter days, so determining how humidity changes in tandem with rising temperatures is important for thinking about the future of humid areas like the tropics. Nevertheless, the researchers found that wet-bulb temperatures in the tropics seem to be governed by the same types of atmospheric dynamics as dry-bulb temperatures, which means that we can roughly predict that as general temperatures in this region rise, so too will humidity.
“One degree Celsius [1.8 degrees Fahrenheit] of tropical mean warming corresponds to about 1 degree Celsius to annual-maximum wet-bulb temperature increase,” Yi Zhang, a researcher at Princeton and lead author of the study, wrote in an email. “A unique element in this paper is that we have a theory for the wet-bulb temperature changes, and it works very well for climate model projections. It is also an important finding that the observations do confirm our theory.”
And this means we’ve got a clearer picture of how much the world can warm before the tropics hit that uninhabitable 95 degree Fahrenheit wet-bulb mark: We can keep the region inhabitable if we keep warming below 1.5 degrees Celsius, the more aggressive target detailed in the Paris Agreement.
What this study doesn’t take into account is the health impacts associated with even smaller rises in temperature and the changes we’re already seeing with warming in the tropics right now, let alone what the next few decades could hold. After all, “at what humidity can human beings no longer survive” is a pretty low bar to set—and one that doesn’t encapsulate all the health complications that come when our bodies work extra hard to overcome heightened humidity. The tropics have already seen oppressive heat waves with wet-bulb temperatures pushing the edge of what humans can survive with just the roughly 1.8 degrees Fahrenheit of warming of the past century alone.
“To communicate the health implications of our results, we will have to know more about wet-bulb temperatures than just a survival limit,” Zhang said. “Thorough knowledge on the health impact of intensity, frequency, and duration of high wet-bulb temperatures is needed.”
Nor, of course, does this study consider the overwhelming challenges of keeping warming below 1.5 degrees Celsius. But it seems pretty clear that if we want to keep one of the Earth’s most populated regions inhabitable, we better get to work.