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 Study Will Help Find the Best Locations for Thermal Power Stations in Iceland
19.01.2017 | University of Gothenburg
Water - as the underlying driver of the Earth’s carbon cycle
17.01.2017 | Max-Planck-Institut für Biogeochemie
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
20.01.2017 | Awards Funding
20.01.2017 | Materials Sciences
20.01.2017 | Life Sciences