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, scientists have discovered.
The finding provides new evidence that today’s increase in greenhouse gases will have an important effect on Africa’s future climate.
Results of the study, led by scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., are published today in a paper in the journal Science.
“The future effect of greenhouse gases on rainfall in Africa is a critical socioeconomic issue,” said NCAR climate scientist Bette Otto-Bliesner, the paper’s lead author. “Africa’s climate seems destined to change, with far-reaching implications for water resources and agriculture.”
The research drew on advanced computer simulations and analyses of sediments and other records of past climate. It was funded by the National Science Foundation (NSF), NCAR’s sponsor, and the U.S. Department of Energy’s Office of Science.
Mysterious period of rain
Otto-Bliesner and colleagues set out to understand the reasons behind the dramatic climate shifts that took place in Africa thousands of years ago.
As ice sheets that had covered large parts of North America and northern Europe retreated from their maximum extent around 21,000 years ago, Africa’s climate responded in a way that had puzzled scientists.
Following a long dry spell during the glacial maximum, the amount of rainfall in Africa abruptly increased, starting about 14,700 years ago and continuing until around 5,000 years ago.
So intense was the rainfall–turning desert into grassland and savanna–that scientists named the span the African Humid Period (AHP).
The puzzling part was why the same precipitation phenomenon occurred simultaneously in two well-separated regions, one north of the equator and one to the south.
Previous studies had suggested that, in northern Africa, the AHP was triggered by changes in Earth’s orbit that resulted in more summertime heating. (Today the northern hemisphere is closest to the Sun in winter, due to a 20,000-year cycle of wobble in Earth’s axis.)
But Otto-Bliesner said the orbital pattern alone would not explain the simultaneous onset of the AHP in southeastern equatorial Africa. Instead, the study revealed the role of greenhouse gases in the atmosphere, along with changes in circulation patterns in the Atlantic Ocean.
As Earth emerged from the last Ice Age, greenhouse gases, especially carbon dioxide and methane, increased significantly–reaching almost pre-industrial levels by 11,000 years ago–for reasons that are not yet fully understood.
Most recent natural global warming and increased greenhouse gases
It was the most recent time during which natural global warming was associated with increases in greenhouse gas concentrations.
The influx of fresh water from melting ice sheets in North America and Scandinavia about 17,000 years ago began weakening a critical circulation pattern that transports heat and salinity in the Atlantic Ocean like a conveyer belt.
The weakened circulation had the effect of moving precipitation to southernmost Africa, suppressing rainfall in northern, equatorial and East Africa.
When the ice sheets stopped melting, the circulation became stronger again, bringing precipitation back north of the equator and to Southeast equatorial Africa.
That change, coupled with the orbital shift and the warming of the atmosphere and oceans by the increasing greenhouse gases, is what triggered the AHP, the scientists believe.
“This study is a step toward solving the puzzle of what triggered abrupt changes in rainfall over southeastern equatorial and northern Africa during early deglaciation,” said Anjuli Bamzai, program director in NSF’s Division of Atmospheric and Geospace Sciences, which funded the research.
“Through an analysis of proxy records and climate model simulations, the team demonstrated that the recovery of what’s calledthe Atlantic Meridional Overturning Circulation, or AMOC, played a role as an initial trigger to wetter conditions.”
Putting together a puzzle
To piece together the puzzle, the researchers drew on fossil pollen, evidence of former lake levels and other proxy records indicating past moisture conditions.
They focused their work on northern Africa, which includes the present-day Sahel region encompassing Niger, Chad and northern Nigeria. They also focused on the largely forested area of today’s eastern Democratic Republic of Congo, Rwanda, Burundi and much of Tanzania and Kenya in southeastern equatorial Africa.
In addition to the proxy records, they simulated past climate with the NCAR-based Community Climate System Model, a powerful global climate model funded by NSF and the U.S. Department of Energy that uses supercomputers at the Oak Ridge National Laboratory.
By comparing the proxy records with the computer simulations, the scientists demonstrated that the climate model had the AHP right.
This helps validate its role in predicting how rising greenhouse gas concentrations might change rainfall patterns in a highly populated and vulnerable part of the world.
“Normally climate simulations cover perhaps a century, or take a snapshot of past conditions,” Otto-Bliesner said. “A study like this, dissecting why climate evolved as it did over this 10,000-year period, was more than I thought I would see in my career.”
