A study led by an atmospheric scientist from UC Riverside predicts that uncontrolled carbon emissions will force tropical rains to shift northward in the coming decades, significantly impacting agriculture and economies near the equator.

The northward shift in rainfall will cause complex changes in the atmosphere driven by carbon emissions affecting the formation of intertropical convergence zones. These zones are essentially atmospheric engines that drive about a third of the world's precipitation, as Liu and his co-authors noted in a paper published on Friday, June 28, in the journal "Nature Climate Change".

Tropical regions on both sides of the equator, such as central African countries, northern South America, and Pacific island nations, among others, would be most affected. Major crops grown in the tropics include coffee, cocoa, palm oil, bananas, sugarcane, tea, mango, and pineapple.

However, the northward shift will last only about 20 years before stronger forces resulting from the warming of the southern oceans pull the convergence zones back southward and keep them there for another thousand years, said Wei Liu, associate professor of climate change and sustainability at UCR's College of Natural and Agricultural Sciences.

Intertropical convergence zones are areas near or along the equator where trade winds from the northern and southern hemispheres meet and move upward into cooler altitudes, drawing in large amounts of moisture from the oceans. When this moist air mass cools at higher altitudes, storm clouds form, allowing for abundant rainstorms. Tropical rainforests can receive up to 4.2 meters of rain annually.

“The change in precipitation is very important,” Liu said. “This is an area with very large amounts of precipitation. So, a small shift will cause significant changes in the agriculture and economy of societies. It will affect many regions.”

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

“This climate model includes many components of the atmosphere, oceans, sea ice, and land. All these components interact with each other,” he said. “Basically, we are 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 considered how carbon emissions affect the amount of radiation at the top of the atmosphere. It also considered changes in sea ice, water vapor, and cloud formation. These and other factors resulted in conditions that push the rain convergence zones northward by up to 0.2 degrees on average.

The title of the paper is “Contrasting fast and slow intertropical convergence zone migrations linked to delayed Southern Ocean warming.” The co-authors are Shouwei Li from UCR; Chao Li from the Max Planck Institute for Meteorology, Hamburg, Germany; Maria Rugenstein from Colorado State University, Fort Collins; and Antony P. Thomas from UCR.

Source: University of California