Site icon Conservation news

Global warming could shift tropical rainfall

Ongoing burning of fossil fuels could flip which portion of the tropics receive more rainfall: the southern hemisphere or the northern. Currently, the northern hemisphere tropics is the wetter of the two, but why this is has long baffled scientists. Now, new research in Nature Geoscience has discovered that rainfall in the tropics is in part driven by massive ocean currents that travel back-and-forth between the Arctic and Antarctic, a process known as ocean overturning circulation.

“Ocean overturning circulation brings a large amount of heat northward across the equator, which makes the Northern Hemisphere warmer,” lead author Dargan Frierson with the University of Washington told “Some of the heat spreads into the Northern Hemisphere tropics, and since warmer ocean waters are where the heaviest rains occur, precipitation peaks in the Northern Hemisphere.”

Frierson and his team employed data from NASA’s Clouds and Earth’s Radiant Energy System (CERES) and computer modeling to determine that this large ocean current likely produces a wetter tropics in the Northern Hemisphere. However, that could change in the next century: many scientists believe that global climate change will slowdown the ocean circulation, potentially changing where rain falls in the tropics.

“Just a few hundred kilometers separates the Sahara Desert from the African rainforests, so even a small shift of the tropical rain belt can have a devastating effect,” notes Frierson.

NASA animation showing precipitation concentrated in the tropics in the form of  average daily rainfall rates during the month of January from 1998-2007.

NASA animation showing precipitation concentrated in the tropics in the form of average daily rainfall rates during the month of January from 1998-2007.

The theory that global warming is set to slowdown—or even stop—this massive marine current was the driving force behind the Hollywood disaster movie, The Day After Tomorrow. Although scientists dismissed how the filmmakers employed climate science (in the film the world enters an implausible new Ice Age), a slowdown in the current is certainly a theoretical possibility in the future.

Still, Frierson cautions that rainfall patterns in the tropics remain complex and therefore difficult to predict.

“Many aspects of climate change affect tropical rain. We aren’t sure which effect will be most important in the future, so tropical rainfall forecasts remain uncertain for now.”

John Fasullo, a researcher at the National Center for Atmospheric Research who was unaffiliated with the research, says that the discovery that ocean circulation likely drives rainfall in the tropics is an “important step” in predicting shifting rainfall on a hotter world. However, he also agrees that much more research will be needed to untangle the complexities driving tropical rainfall.

“It is important to recognize that this work does not explain rainfall’s regional features, such as for example the contrast between the western Pacific Ocean and the eastern Pacific, nor does it say anything about whether more rainfall will fall over land or over ocean,” Fasullo told

He also says that to date scientists have not observed any slowdown in ocean circulation, however it is “extremely important” for researchers to continue studying this possibility.

Rainforest in Borneo. If tropical rainfall patterns shift, ecosystems like rainforests could be hugely impacted. Photo by: Rhett A. Butler.


Related articles

Featured video: How climate change is messing with the jetstream

(05/08/2013) Weather patterns around the globe are getting weirder and weirder: heat waves and record snow storms in Spring, blasts of Arctic air followed by sudden summer, record deluges and then drought.

Controversial research outlines physics behind how forests may bring rain

(01/30/2013) It took over two-and-a-half-years for the journal Atmospheric Chemistry and Physics to finally accept a paper outlining a new meteorological hypothesis in which condensation, not temperature, drives winds. If proven correct, the hypothesis could have massive ramifications on global policy—not to mention meteorology—as essentially the hypothesis means that the world’s forest play a major role in driving precipitation from the coast into a continent’s interior. The theory, known as the biotic pump, was first developed in 2006 by two Russian scientists, Victor Gorshkov and Anastassia Makarieva of the St. Petersburg Nuclear Physics, but the two have faced major pushback and delays in their attempt to put the theory before the greater scientific community.

Gaining from rain: precipitation is an indicator of tropical forest biodiversity

(11/12/2012) Policymakers seeking to conserve forests in southern India should focus on those receiving the highest levels of rainfall, according to new research. Scientists from the Center for International Forestry Research (CIFOR) found rainfall to be the most important environmental determinant of species richness in the Anamalai region of the southern Western Ghats.

Amazon deforestation could trigger drop in rainfall across South America

(09/06/2012) Deforestation could cause rainfall across the Amazon rainforest to drop precipitously, warns a new study published in the journal Nature. Using a computer model that accounts for forest cover and rainfall patterns, Dominick Spracklen of the University of Leeds and colleagues estimate that large-scale deforestation in the Amazon could reduce basin-wide rainfall 12 percent during the wet season and 21 percent in the dry season by 2050. Localized swings would be greater.

Rainforest fungi, plants fuel rainfall

(09/04/2012) Salt compounds released by fungi and plants in the Amazon rainforest have an important role in the formation of rain clouds, reports research published in the journal Science.

The vanishing Niger River imperils tourism and livelihoods in the desert

(06/04/2012) Severely affected by recent turmoil across its northern frontiers, Nigerien tourism pins hope on river valley attractions to play a major role in rebuilding its tourism industry in the upcoming years. Even though the river itself is threatened. Located in the heart of the Sahel Region, the vast desert lands of Niger have captivated European tourists seeking a taste of its immensely varied natural landscapes.

“Strong evidence” linking extreme heatwaves, floods, and droughts to climate change

(03/28/2012) As North America recovers from what noted meteorologist Jeff Masters has called “the most incredible spring heatwave in U.S. and Canadian recorded history,” a new paper argues that climate change is playing an important role in a world that appears increasingly pummeled by extreme weather. Published in Nature Climate Change, the paper surveys recent studies of climate change and extreme weather and finds “strong evidence” of a link between a warming world and the frequency and intensity of droughts, floods, and heatwaves—such as the one that turned winter into summer in the U.S.

New meteorological theory argues that the world’s forests are rainmakers

(02/01/2012) New, radical theories in science often take time to be accepted, especially those that directly challenge longstanding ideas, contemporary policy or cultural norms. The fact that the Earth revolves around the sun, and not vice-versa, took centuries to gain widespread scientific and public acceptance. While Darwin’s theory of evolution was quickly grasped by biologists, portions of the public today, especially in places like the U.S., still disbelieve. Currently, the near total consensus by climatologists that human activities are warming the Earth continues to be challenged by outsiders. Whether or not the biotic pump theory will one day fall into this grouping remains to be seen. First published in 2007 by two Russian physicists, Victor Gorshkov and Anastassia Makarieva, the still little-known biotic pump theory postulates that forests are the driving force behind precipitation over land masses.