New NASA Study Suggests Tropics Will Get More Rainfall With A Warming Climate

A warming climate may drive more rainfall in tropical areas, thanks to a decrease in high tropical cloud cover, which in turn may further warm the air, according to new research.

Although the term global warming brings to mind hot temperatures and dry conditions, that’s not always going to be the case for all regions and all seasons, and the more accurate term global climate change speaks more to the future of the world’s weather patterns, which are already changing from historic norms. And not to understate the ecological, economic, and health impacts of a changing climate, but it would be one thing if the dry regions got wetter, and the wet ones got a bit drier, and another completely if the wet regions get wetter and the hot regions get warmer, which is kind of what seems to be happening currently.

While there may be quite a lot to learn about our planet’s climate and weather systems, one of the sure things we know is that they are more complex than we know, and that changes in one aspect can bring about counterintuitive changes in other systems. One case in point is a recent study by scientist Hui Su of NASA’s Jet Propulsion Laboratory in Pasadena, California, the results of which seem to indicate that most of our global climate models underestimate the amount of rain that will fall in the tropics as our planet continues to warm. The counterintuitive side effect of more rainfall in the tropics is more heat released into the atmosphere, which seems like it ought to go the other way, but because of the role that tropical cloud cover plays in the bigger picture, which is to trap heat close to the surface, the upper atmosphere may bear the effects of the heat with decreased cloud cover.

The study suggests that the decrease in high tropical cloud cover is “one result of a planet-wide shift in large-scale air flows that is occurring as Earth’s surface temperature warms.” The flows are called the Atmospheric General Circulation, and include a wide region of rising air which is centered around the equator, but several decades of monitoring have shown that this rising air zone is narrowing as the climate warms, which then causes a decrease in high tropical cloud cover.

Figure 1: A schematic of model simulated changes in the Hadley Circulation and tropical clouds along with the OLR and precipitation changes in a warmer climate. (Image: Nature Comms)

Su’s team found that most of the climate models underestimated the rate of increase in precipitation for each degree of surface warming that has occurred in recent decades. The models that came closest to matching observations of clouds in the present-day climate showed a greater precipitation increase for the future than the other models.

Su said that by tracing the underestimation problem back to the models’ deficiencies in representing tropical high clouds and the atmospheric general circulation, “This study provides a pathway for improving predictions of future precipitation change.” – NASA JPL

One of the salient points in the study’s conclusion is that “understanding the complex interactions between circulation and cloud changes would be of utmost importance for accurate climate change predictions,” which is an important tool in resilience planning and design.

The research was published in the journal Nature Communication under the title “Tightening of tropical ascent and high clouds key to precipitation change in a warmer climate.

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