Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Greenhouse Gases Linked to Past African Rainfall

08.12.2014

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.


Wikimedia Commons photo by Sfivat.

Lakes and other water features, such as the Ubari Oasis in southern Libya, were more prevalent across now-dry parts of Africa during times when precipitation was more plentiful.

“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

Journal: Science

On the Web

For news releases, images, and more:
www.ucar.edu/atmosnews

Contact Information
David Hosansky, NCAR/UCAR Media Relations
303-497-8611
hosansky@ucar.edu

Bob Henson, NCAR/UCAR Media Relations
303-497-8605
bhenson@ucar.edu

David Hosansky | newswise
Further information:
http://www.ucar.edu

More articles from Earth Sciences:

nachricht Live from the ocean research vessel Atlantis
13.12.2018 | National Science Foundation

nachricht NSF-supported scientists present new research results on Earth's critical zone
13.12.2018 | National Science Foundation

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Data use draining your battery? Tiny device to speed up memory while also saving power

The more objects we make "smart," from watches to entire buildings, the greater the need for these devices to store and retrieve massive amounts of data quickly without consuming too much power.

Millions of new memory cells could be part of a computer chip and provide that speed and energy savings, thanks to the discovery of a previously unobserved...

Im Focus: An energy-efficient way to stay warm: Sew high-tech heating patches to your clothes

Personal patches could reduce energy waste in buildings, Rutgers-led study says

What if, instead of turning up the thermostat, you could warm up with high-tech, flexible patches sewn into your clothes - while significantly reducing your...

Im Focus: Lethal combination: Drug cocktail turns off the juice to cancer cells

A widely used diabetes medication combined with an antihypertensive drug specifically inhibits tumor growth – this was discovered by researchers from the University of Basel’s Biozentrum two years ago. In a follow-up study, recently published in “Cell Reports”, the scientists report that this drug cocktail induces cancer cell death by switching off their energy supply.

The widely used anti-diabetes drug metformin not only reduces blood sugar but also has an anti-cancer effect. However, the metformin dose commonly used in the...

Im Focus: New Foldable Drone Flies through Narrow Holes in Rescue Missions

A research team from the University of Zurich has developed a new drone that can retract its propeller arms in flight and make itself small to fit through narrow gaps and holes. This is particularly useful when searching for victims of natural disasters.

Inspecting a damaged building after an earthquake or during a fire is exactly the kind of job that human rescuers would like drones to do for them. A flying...

Im Focus: Topological material switched off and on for the first time

Key advance for future topological transistors

Over the last decade, there has been much excitement about the discovery, recognised by the Nobel Prize in Physics only two years ago, that there are two types...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

ICTM Conference 2019: Digitization emerges as an engineering trend for turbomachinery construction

12.12.2018 | Event News

New Plastics Economy Investor Forum - Meeting Point for Innovations

10.12.2018 | Event News

EGU 2019 meeting: Media registration now open

06.12.2018 | Event News

 
Latest News

Data use draining your battery? Tiny device to speed up memory while also saving power

14.12.2018 | Power and Electrical Engineering

Tangled magnetic fields power cosmic particle accelerators

14.12.2018 | Physics and Astronomy

In search of missing worlds, Hubble finds a fast evaporating exoplanet

14.12.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>