African Humid Period
December 8, 2014

Mysterious Period Of African Rainfall Explained

Eric Hopton for redOrbit.com - Your Universe Online

Scientists discover link between greenhouse gases and African rainfall

A new study is shedding new light on a sudden intense increase in rainfall that occurred in parts of Africa around 15,000 years ago. Until now science has struggled to explain the dramatic climate change, known as the African Humid Period, in which deserts and savannahs were rapidly turned into wet grasslands. The African Humid Period followed a prolonged dry period at the end of the last Ice Age and lasted for almost 10,000 years.

This latest research proposes a link between the increase in rainfall in two regions of Africa and an increase in greenhouse gas concentrations.

The findings are important, not just in an historical context, but also because they demonstrate that increasing levels of carbon dioxide and other greenhouse gases could significantly change Africa’s climate in the future.

The study was carried out by the National Center for Atmospheric Research (NCAR) using computer simulations and analysis of data from the geologic record of historic African climate. Examination of pollens, fossils, former lake levels and other geologic data were used to simulate past climate with NCAR’s own powerful global climate model. The study was based on information from the Sahel region of Africa to the north, including Niger, Chad and northern Nigeria and the south eastern equatorial region of Africa, including the Democratic Republic of Congo, Rwanda, Burundi, Tanzania and Kenya. The sophisticated computer climate model accurately reflected the fossil record, demonstrating its potential to predict how rising greenhouse gas concentrations might change rainfall patterns.

According to Peter Clark, an Oregon State University paleoclimatologist and co-author on the study, “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.”

Following the last Ice Age, an extensive and prolonged dry spell covered much of central Africa and lasted until about 14,700 years ago. Then came the heavy rainfall of the African Humid Period, affecting two different areas which lay north and south of the equator.

There have been many attempts to explain the phenomenon, including one suggestion that changes in the Earth's orbit may have triggered the event. Bette Otto-Bliesner, lead author of the study, believes that orbital patterns alone could not have caused the level of increased rainfall seen in both regions.

Levels of atmospheric carbon dioxide and methane increased significantly after the Ice Age. As the Earth continued to warm and ice sheets melted, increasing amounts of fresh water flowed out from North America and northern Europe. This caused big changes in oceanic current systems, in particular, a weakening of what scientists term the “Atlantic Meridional Overturning Circulation” that carries warmer water up from the tropics and helps to maintain a temperate climate in Europe.

At the same time as the Atlantic Meridional Overturning Circulation weakened, it brought with it higher precipitation to the southernmost part of Africa but also had the effect of suppressing rainfall in east Africa and northern equatorial Africa during the long dry spell, according to the study.

Eventually, when the ice sheets stopped melting, the Circulation grew stronger. It then carried higher precipitation back to the northern regions. The NCAR team suggest that this change, as well as the orbital shift and warming of the atmosphere and oceans by greenhouse gases, brought about the African Humid Period.

If the proposals are correct, they demonstrate clearly that the world’s climate is highly sensitive, even to relatively small shifts in greenhouse gas levels.

“The future impact of greenhouse gases on rainfall in Africa is a critical socioeconomic issue,” said Otto-Bliesner. “Africa's climate seems destined to change, with far-reaching implications for water resources and agriculture in ways that may generate new conflicts.”

This is the first time that scientific research has shown that an increase in greenhouse gas concentrations thousands of years ago could be a key factor in the major rainfall increase in the two African regions.

“The future impact of greenhouse gases on rainfall in Africa is a critical socioeconomic issue,” said NCAR scientist Bette Otto-Bliesner, the lead author. “Africa’s climate seems destined to change, with far-reaching implications for water resources and agriculture. Normally climate simulations cover perhaps a century or take a snapshot of past conditions. A study like this one, dissecting why the climate evolved as it did over this intriguing 10,000-year period, was more than I thought I would ever see in my career.”

Results of the study are published in the journal Science.

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