Climate change is shifting tropical rain north, computer modeling suggests

A study led by a UC Riverside atmospheric scientist predicts that unchecked carbon emissions will force tropical rains to move northward in coming decades, which would profoundly affect agriculture and economies near the Earth’s equator.

The northward shift of rain would be caused by complex changes in the atmosphere induced by carbon emissions that influence the formation of intertropical convergence zones. These zones are essentially atmospheric engines that drive about a third of the world’s precipitation, Liu and his coauthors report in a paper published June 28 in the journal. Nature Climate change. The title of the paper is “Contrasting Fast and Slow Migration of the Intertropical Convergence Zone Associated with Delayed Southern Ocean Warming.”

Tropical regions on both sides of the equator, such as Central African states, northern South America, and Pacific island states, would be most affected. The main crops grown in the tropics include coffee, cocoa, palm oil, bananas, sugarcane, tea, mangoes and pineapples.

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

The Intertropical Convergence Zones are areas along or near the equator where the trade winds from the northern and southern hemispheres meet and shoot upward to cooler altitudes, drawing large volumes of moisture from the oceans. As this moist air cools at higher altitudes, storm clouds form, allowing for heavy thunderstorms. Tropical rainforests can receive up to 14 feet of rain per year.

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

Liu and his colleagues used sophisticated 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,” 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 radiative energy in the upper atmosphere. She also considered changes in sea ice, water vapor and cloud formation. These and other factors have led to conditions that push rain-producing convergence zones northward by up to 0.2 degrees on average.