Researchers may have solved the mystery of the African Humid period, which was a period of intense rainfall following a long-dry spell almost 10,000 years ago.
A recent study looked at increased rainfall in two regions of Africa that took place right after the end of the last ice age. They found the wet periods were linked to an increase in greenhouse gas concentrations, Oregon State University reported.
"This study is important not only because it explains a long-standing puzzle, but it helps to validate model predictions of how rising greenhouse gas concentrations might change rainfall patterns in a highly populated and vulnerable part of the world," said Peter Clark, an Oregon State University paleoclimatologist and co-author on the study.
To make their findings the researchers used computer simulations of the past climate, focusing on the era following the last ice age. During this time ice sheets in North America and northern Europe were retreating and there was a long dry spell in central Africa that lasted until about 14,700 years ago. After that time rainfall dramatically increased, turning the region into a lush grassland.
Past studies have suggested these puzzling changes occurred as a result of changes in the Earth's orbit, but this did not explain why rainfall increased in two regions of Africa: one north of the equator and one south.
When the Earth came out of the ice age, atmospheric carbon levels increased to almost pre-industrial levels, which caused the planet to continue warming as ice sheets melted and the Atlantic Meridional Overturning Circulation (which brings warm water up from the tropics) weakened.
These effects moved precipitation towards southern Africa, suppressing rainfall in the eastern and northern region. When the ice sheets stopped melting, circulations strengthened and move the precipitation up north. The changes combined with the warming climate were believed to triggered the period, dubbed the African Humid Period.
"This study provides yet another demonstration of the sensitivity of the Earth's climate to small changes in atmospheric greenhouse gases," Clark said.
The study was led by the National Center for Atmospheric Research (NCAR) and funded by the National Science Foundation and the U.S. Department of Energy. The findings were published in a recent edition of the journal Science.
