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African Rainfall Rises with Greenhouse Gases

Dec 04, 2014 05:05 PM EST
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African rainfall may rise with greenhouse gas levels, according to a new study.

A long-standing mystery known as the African Humid Period began at the end of the last Ice Age, during which intense cumulative rainfall increased abruptly and lasted for nearly 10,000 years. And considering that greenhouse gas levels reached a record high in 2013, researchers believe their findings could foreshadow what Africa can expect as a result of climate change in the future.

"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," co-author Peter Clark, an Oregon State University paleoclimatologist, said in a statement.

Some 21,000 years ago during the last days of the Ice Age, glaciers in North America and northern Europe began to melt away. But it was out with the cold and in with the dry, with central Africa consequently experiencing a dry spell that didn't end until about 14,700 years ago, when intense rain suddenly fell. This drastic climatic shift turned African deserts into grasslands, and caused scientists to scratch their heads.

It wasn't until this latest study, published in the journal Science, that researchers found a plausible answer.

Rainfall actually increased in two separate regions of Africa - one north of the equator, the other south. Previous studies suggested that the switch from dry to wet could be contributed to changes in the Earth's orbit, however, lead author Bette Otto-Bliesner says orbital patterns could not possibly explain such a radical rise in rainfall in both regions.

According to the new study, as the end of the Ice Age atmospheric levels of carbon dioxide and methane soared - almost to pre-industrial levels. This increasingly warming world thereby caused ice sheets to melt and freshwater from North America and northern Europe to weaken what's called the Atlantic Meridional Overturning Circulation, which brings warm water up from the tropics and gives Europe its temperate climate.

The weakening of the Atlantic ocean current helped move precipitation southward toward the tip of Africa, creating a dry spell in the east and northern parts of the continent. It was when the ice sheets stopped melting and circulation regained its strength that rainfall in the north came abruptly.

It was a combination of this change, Earth's orbital shift and atmospheric and ocean warming due to greenhouse gases that led to the mysterious African Humid Period, researchers concluded.
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