Climate change is causing tropical rains to shift north, computer modeling suggests

A study led by a UC Riverside atmospheric scientist predicts that uncontrolled carbon emissions will cause tropical rainstorms to move north in the coming decades. This would have a major impact on agriculture and economies in the equatorial areas.

The northward rainfall shift would be caused by complex changes in the atmosphere, fueled by carbon emissions that influence the formation of the intertropical convergence zones. Those zones are essentially atmospheric engines that drive about a third of the world’s precipitation, Liu and his co-authors report in a paper published June 28 in the journal Nature Climate change. The title of the article is “Contrasting fast and slow intertropical convergence zone migrations linked to delayed Southern Ocean warming.”

Tropical areas on either side of the equator, such as Central African countries, northern South America and Pacific island states, among others, would be hardest hit. The major crops grown in the tropics include coffee, cocoa, palm oil, bananas, sugar cane, tea, mangoes and pineapples.

However, the northward shift will only last about 20 years before larger forces from warming southern oceans pull the convergence zones back south and keep them there for another millennium, says Wei Liu, associate professor of climate change and sustainability at the UCR’s College of Natural and Agricultural Sciences.

Intertropical convergence zones are areas along or near the equator where trade winds from the Northern and Southern Hemispheres converge and surge upward to cooler altitudes, drawing large amounts of moisture from the oceans. As this moist air cools at higher altitudes, thunderstorm clouds form, making drenching rain showers possible. Tropical rainforests can receive as much as 4 meters of rain per year.

“The change in rainfall is very important,” Liu said. “It is a region with very high rainfall. So a small shift will cause big changes in agriculture and the economy of the societies. It will affect many regions.”

Liu and his colleagues used advanced computer models to predict the atmospheric impact of carbon dioxide emissions from the continued burning of fossil fuels and other sources, Liu said.

“This climate model included many components of the atmosphere, ocean, sea ice and land. All of these components interact with each other,” he said. “Basically, we’re trying to simulate the real world. In the model, we can increase our carbon dioxide emissions from pre-industrial levels to much higher levels.”

The analysis took into account how carbon emissions affect the amount of radiant energy at the top of the atmosphere. It also took into account changes in sea ice, water vapor, and cloud formation. These and other factors resulted in conditions that pushed the rain-forming convergence zones north by as much as 0.2 degrees on average.