Tropical Storms, Global Consequences

Rising levels of carbon dioxide (CO₂) are driving recent changes to the global climate, and these changes are sometimes referred to as global warming because of the warming effect of that greenhouse gas.

But temperature isn’t the only part of the climate that will change; society can also expect changes in rainfall, and according to Shang-Ping Xie, a climate scientist and Roger Revelle Chair in Environmental Science at the Scripps Institution of Oceanography, UC San Diego, this could be even more important. Xie is a co-author of a paper recently published in the journal Nature Geosciences that evaluated the effect of climate change on rainfall in the tropics.

“In a sense, the most crucial variable of global warming isn’t really temperature, it’s rainfall,” Xie said,” But it is also more challenging, because with temperature you will see increase everywhere, although the degree will be variable. Rainfall is more challenging because in some places it will increase and in others it will decrease.”

Climate scientists have been divided into two camps over the mechanisms used to predict changes in rainfall, known as “wet gets wetter” and “warmer gets wetter” scenarios. This has added to the complexity of predicting regional rainfall. Xie’s paper evaluated results from 18 different climate models and data on current rainfall in the tropics to test both theories.

The “wet gets wetter” mechanism is based on the idea that as global average temperatures increase, the atmosphere can hold more water vapor, and so tropical areas that are already wet will experience even more rainfall.

Xie proposed the “warmer gets wetter” theory in a paper in the Journal of Climate in 2010. The results of several climate models predicted that the regions with the biggest rise in sea surface temperature would experience the greatest rainfall increase because the moisture increase is larger than elsewhere, inhibiting the vertical movement of atmospheric properties known as convection.  This theory gains support from scientists’ current understanding of El Niño, the wet weather system caused by warm ocean temperatures in the tropics.  According to this mechanism, rainfall will depend on changes in sea surface temperature. This makes accurate predictions of regional rainfall tricky because spatial variation in sea surface warming will depend on ocean currents, winds, and other effects.

“So both theories sound reasonable…but this paper shows that the truth is like [a] coffee mug,” Xie said. “If you look from the top, it’s circular. From the side, it appears square, and if you look from the right angle, you can see it’s three-dimensional. But it’s still the same mug!”

Xie said that the climate model comparison in the Nature Geosciences paper showed that both predictions hold true, but it depends on which rainfall measurement one looks at—the seasonal rainfall mean or the annual mean. Ocean warming will likely be greatest near the equator, and this warming will cause increased rainfall according to the “warmer-gets-wetter” pattern. But this warming will follow the suns path, going back and forth across the equator with the seasons, so only the seasonal rainfall will follow the “wet-gets-wetter” prediction. Overall, the “wet-gets-wetter” hypothesis will contribute more to the seasonal rainfall changes, but annual changes will follow the “warmer-gets-wetter” scheme.

These results are good news for countries that depend on seasonal rainfall in the form of monsoons. Many of these countries—such as China and India—are already suffering water shortages due to burgeoning populations and rising standards of living. Beyond this direct effect on countries in tropical regions, rainfall changes in the tropics will certainly impact the climate of the rest of the globe. The ocean around the equator is the warmest, so this is where the most heat is released from the ocean into the atmosphere. This energy drives the patterns of the entire atmospheric system. The dramatic effects of El Niño in far-away places like California evidence the far-reaching effects of tropical weather. Changes in tropical rainfall will cause “re-organization” of the climate systems everywhere, including North America, according to Xie.

“People want to know what will happen to San Diego rainfall in 50 years,” he added, “But unfortunately the science is not yet there. Our work is a step forward to figure out what the patterns in tropical rainfall will be…the tropics are remote geographically, but its [impacts] are very close.”

Study lead author, Ping Huang, and two of the co-authors, Kaiming Hu and Ronghui Huang, are from the Center for Monsoon System Research at the Chinese Academy of Sciences. The fourth author, Gang Huang, is at the Key Laboratory of Regional Climate-Environment Research for Temperate East Asia and is also part of the Chinese Academy of Sciences.

– Mallory Pickett is a masters student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography, UC San Diego