New research demonstrates for the first time that an increase in greenhouse gas concentrations thousands of years ago was a key factor in causing substantially more rainfall in two major regions of Africa. The finding provides new evidence that the current increase in greenhouse gases will have an important impact on Africa’s future climate.
The study, led by the National Center for Atmospheric Research (NCAR), is being published this week in Science.
“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.”
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, which is NCAR’s sponsor, and the Department of Energy Office of Science.
A mysterious period of rain
Otto-Bliesner and her co-authors in the United States and China set out to understand the reasons behind dramatic climate shifts that took place in Africa thousands of years ago.
As the ice sheets that had covered large parts of North America and northern Europe started retreating from their maximum extent around 21,000 years ago, Africa’s climate responded in a way that has puzzled scientists. Following a long dry spell during the glacial maximum, the amount of rainfall in Africa abruptly increased, starting around 14,700 years ago and continuing until around 5,000 years ago. So intense was the cumulative rainfall, turning desert into grasslands and savannas, 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 a ~20,000-year cyclic wobble in Earth’s orbit that resulted in increased summertime heating north of the equator. (In contrast, the northern hemisphere today is closest to the Sun in winter rather than summer.) That summertime heating would have warmed the land in such a way as to strengthen the monsoon winds from the ocean and enhance rainfall.
But Otto-Bliesner said the orbital pattern alone would not explain the simultaneous onset of the AHP in southeastern equatorial Africa, south of the equator, since the wobble in Earth's orbit led to less summertime heating there rather than more. Instead, the study revealed the role of two other factors: a change in Atlantic Ocean circulation that rapidly boosted rainfall in the region, and a rise in greenhouse gas concentrations that helped enhance rainfall across a wide swath of Africa.
Tracing multiple causes of a wetter Africa
As Earth emerged from the last Ice Age, greenhouse gases, especially carbon dioxide and methane, increased significantly—reaching almost to pre-industrial levels by 11,000 years ago—for reasons that are not yet fully understood. It was, the authors note, the most recent time during which natural global warming was associated with increases in greenhouse gas concentrations. (Because of feedbacks between the two, greenhouse gas concentrations and global temperature often rise and fall together across climate history.)
The end of the last Ice Age also triggered an influx of fresh water into the ocean from melting ice sheets in North America and Scandinavia about 17,000 years ago. The fresh water interfered with a critical circulation pattern that transports heat and salinity northward through the Atlantic Ocean, much like a conveyer belt. The weakened circulation led to African precipitation shifting toward southernmost Africa, with rainfall suppressed in northern, equatorial, and east Africa.
When the ice sheets stopped melting, the circulation became stronger again, bringing precipitation back into southeastern equatorial and northern Africa. This change, coupled with the orbital shift and the warming by the increasing greenhouse gases, is what triggered the AHP.
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 (the present day Sahel region encompassing Niger, Chad, and also northern Nigeria) and southeastern equatorial Africa (the largely forested area of today’s eastern Democratic Republic of Congo, Rwanda, Burundi, and much of Tanzania and Kenya).
In addition to the proxy records, they simulated past climate with the NCAR-based Community Climate System Model, a powerful global climate model developed by a broad community of researchers and funded by the National Science Foundation and Department of Energy, and using supercomputers at the Oak Ridge National Laboratory.
By comparing the proxy records with the computer simulations, the study demonstrated that the climate model got the AHP right. This helps to 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 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.”
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
About the paper
Title: Coherent changes of southeastern equatorial and northern African rainfall during the last deglaciation
Authors: Bette L. Otto-Bliesner, James M. Russell, Peter U. Clark, Zhengyu Liu, Jonathan T. Overpeck, Bronwen Konecky, Peter deMenocal, Sharon E. Nicholson, Feng He, Zhengyao Lu
On the Web
For news releases, images, and more:
David Hosansky, NCAR/UCAR Media Relations
Bob Henson, NCAR/UCAR Media Relations
David Hosansky | newswise
New technologies and computing power to help strengthen population data
22.03.2018 | University of Southampton
New interactive map shows climate change everywhere in world
22.03.2018 | University of Cincinnati
Satellites in near-Earth orbit are at risk due to the steady increase in space debris. But their mission in the areas of telecommunications, navigation or weather forecasts is essential for society. Fraunhofer FHR therefore develops radar-based systems which allow the detection, tracking and cataloging of even the smallest particles of debris. Satellite operators who have access to our data are in a better position to plan evasive maneuvers and prevent destructive collisions. From April, 25-29 2018, Fraunhofer FHR and its partners will exhibit the complementary radar systems TIRA and GESTRA as well as the latest radar techniques for space observation across three stands at the ILA Berlin.
The "traffic situation" in space is very tense: the Earth is currently being orbited not only by countless satellites but also by a large volume of space...
An international team of researchers has discovered a new anti-cancer protein. The protein, called LHPP, prevents the uncontrolled proliferation of cancer cells in the liver. The researchers led by Prof. Michael N. Hall from the Biozentrum, University of Basel, report in “Nature” that LHPP can also serve as a biomarker for the diagnosis and prognosis of liver cancer.
The incidence of liver cancer, also known as hepatocellular carcinoma, is steadily increasing. In the last twenty years, the number of cases has almost doubled...
In just a few weeks from now, the Chinese space station Tiangong-1 will re-enter the Earth's atmosphere where it will to a large extent burn up. It is possible that some debris will reach the Earth's surface. Tiangong-1 is orbiting the Earth uncontrolled at a speed of approx. 29,000 km/h.Currently the prognosis relating to the time of impact currently lies within a window of several days. The scientists at Fraunhofer FHR have already been monitoring Tiangong-1 for a number of weeks with their TIRA system, one of the most powerful space observation radars in the world, with a view to supporting the German Space Situational Awareness Center and the ESA with their re-entry forecasts.
Following the loss of radio contact with Tiangong-1 in 2016 and due to the low orbital height, it is now inevitable that the Chinese space station will...
Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, provider of research and development services for OLED lighting solutions, announces the founding of the “OLED Licht Forum” and presents latest OLED design and lighting solutions during light+building, from March 18th – 23rd, 2018 in Frankfurt a.M./Germany, at booth no. F91 in Hall 4.0.
They are united in their passion for OLED (organic light emitting diodes) lighting with all of its unique facets and application possibilities. Thus experts in...
A new scenario seeking to explain how Mars' putative oceans came and went over the last 4 billion years implies that the oceans formed several hundred million...
23.03.2018 | Event News
19.03.2018 | Event News
16.03.2018 | Event News
23.03.2018 | Life Sciences
23.03.2018 | Materials Sciences
23.03.2018 | Process Engineering