Climate change could dump even more rain on the tropics
The world’s tropical regions might see far more rain than expected as the climate warms.
Most global climate models may be underestimating just how much rain will fall in the tropics as ocean and surface temperatures continue to rise because of human activities, NASA scientists said in a new study.
If that’s the case, that could mean more dangerous deluges and damaging floods are on the way for countries and islands near the equator. Other parts of the world, meanwhile, may become drier, harming crops and straining water supplies.
Hui Su, a scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, led the research for the study, which was published last week in the journal Nature Communications.
The reason for a wetter tropical future has to do with the clouds.
Recent NASA observations suggest that tall, high-altitude clouds in the tropics will continue to shrink in coming decades in response to warming at the planet’s surface. Although it seems intuitively that fewer clouds should equal less precipitation, that’s not necessarily the case.
These high clouds trap heat in the atmosphere, which helps to balance the sun’s incoming energy with Earth’s outgoing heat energy. With fewer heat-trapping clouds around, the air above the tropics is expected to cool, and cooler air means more unstable air, and hence, more rain.
Another counterintuitive force is at play here. The increased rainfall in the tropics is expected to warm the atmosphere, not cool it down. That’s because in the cold upper atmosphere, when water vapor condenses into ice particles, it releases its heat energy and warms the atmosphere. This is known as the release of latent heat.
At a broader scale, the large-scale flow of air around the tropics is known as the “General Atmospheric Circulation,” and includes a wide zone of rising air centered close to the equator. Such an area of rising air, coupled with sinking air offset from this region, is known as a Hadley Cell.
NASA observations from the past few decades show that such zones are narrowing as the climate warms, and that in turn has reduced the number of high clouds. This has major implications for the future climate in many vulnerable, relatively poor tropical countries, and even for what may happen to drought-prone areas that lie in the descending regions of the Hadley Cell, where sinking air snuffs out precipitation.
As fewer clouds result in more rainfall, the zone of rising air at the center of the Hadley Cell is expected to narrow even more, Su and his colleagues found.
For the study, the researchers compared climate data from the past 30 to 40 years — including satellite measurements and ground-level observations — with 23 climate model simulations from the same period.
They found that most climate models underestimated the rate of increase in precipitation for every degree of surface warming that’s occurred in recent decades. The models that came closest to matching cloud observations from today’s climate showed a greater increase in precipitation in the future.
Su said that updating the models’ assumptions about tropical high clouds and air flow could improve the climate models that scientists, policy leaders, and citizens all depend on to prepare for the changing climate.
“This study provides a pathway for improving predictions of future precipitation change,” Su said in a statement.