Greenhouse gas linked to major African rainfall thousands of years ago

New research by the National Center for Atmospheric Research and Florida State University has found that an increase in greenhouse gas concentrations thousands of years ago was a key factor in higher amounts of rainfall in two major regions of Africa.

That information could be critical, researchers say, as scientists try to predict how the Earth may react to continued heating due to climate change.

“Climatologists have long assumed that most climate variations on the scale of thousands to hundreds of thousand of years are caused by what we call ‘orbital forcing,’” said Sharon E. Nicholson, professor of meteorology at Florida State and a co-author on the study. “That refers to variations in the spatial relationships between the Earth and sun, like the inclination of the Earth’s axis as it revolves around the sun. This article shows that a major climate change about 5,000 years ago cannot be explained solely on the basis of orbital forcing alone. Greenhouse gases must be included.”

The study, published in the journal Science, used computer simulations and analysis of fossil pollen, evidence of former lake levels and other geological records. Nicholson’s role was to evaluate how realistically the model represented East African climate and to help interpret the results.

The team of scientists, led by NCAR climate scientist Bette Otto-Bliesner, focused on the era following the last ice age.

As ice sheets took over large parts of North America and Europe, Africa’s climate responded differently with an abrupt increase in rainfall. The rainfall was so intense that it turned the Sahara desert into grassland. It’s known in science as the African Humid Period.

Even more puzzling, the rain phenomenon occurred simultaneously in two regions of the continent that were far apart, one north of the equator and one to the south.

Researchers determined that the orbital patterns alone could not cause that much rainfall.

As the ice began to melt, atmospheric levels of greenhouse gases increased significantly — almost to pre-industrial levels. As the Earth continued to warm and ice melted, the influx of fresh water from North America and northern Europe began to weaken a critical circulation system in the Atlantic Ocean that brings warm water up from the tropics.

The weakened current moved precipitation to the southernmost part of Africa, while also suppressing rainfall in east Africa and northern equatorial Africa.

“Africa is expected to be the continent most affected by climate change,” Nicholson said. “Thus, it is important to know what an addition of greenhouse gases can do to its climate.”

In addition to NCAR and Florida State, other institutions participating in the study are Brown University, Oregon State, Columbia University, University of Arizona, University of Colorado, University of Wisconsin-Madison and Peking University.