Is increased greenhouse gas a dire situation? Well, not in Africa – 14 700 years ago!

New research has found that, thousands of years ago, an increase in greenhouse gas (GHG) concentrations was a key factor in causing substantially more rainfall in two major African regions. It provides evidence that the current increase in GHG will also have an important impact on our continent’s future climate …

GHG emissions have long been branded a destroyer of the environment. Many research papers have proved that they could, and probably would, end life as we know it …

(In SHEQ MANAGEMENT’s July/August 2014 edition we featured a piece, which proves that, over the next two decades, the world faces a substantially increased risk of a major slowdown in the growth of global crop yields as a result of climate change.)

This is in the “short run”, however. The new study: Coherent changes of south-eastern, equatorial and northern African rainfall during the last deglaciation, led by the United States-based National Centre for Atmospheric Research (NCAR), takes things to a whole new level …

To predict the future impact of GHG on rainfall in Africa, the research drew on advanced computer simulations and analyses of sediments of past climate. “The future impact of greenhouse gases on rainfall in Africa is a critical socio-economic issue,” says NCAR scientist Bette Otto-Bliesner, the lead author. “Africa’s climate seems destined to change, with far-reaching implications for water resources and agriculture.”

In the past, as the ice sheets that covered large parts of North America and northern Europe started to melt (around 21 000 years ago), Africa’s climate responded in an interesting way … Following a long dry spell, during the peak of the last glacial period, the amount of rainfall in Africa suddenly increased – starting around 14 700 years ago and continuing until approximately 5 000 years ago. This is known as the African Humid Period (AHP) as it saw desert areas turn into savannas.

The puzzling part was why these conditions were seen in two different regions at the same time; 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, which 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.) It’s believed that the summertime heating would have warmed the land in such a way that it strengthened the monsoon winds from the ocean and enhanced rainfall.

Otto-Bliesner points out that the orbital pattern, alone, would not explain the simultaneous onset of the AHP in south-eastern equatorial Africa, south of the equator, since the wobble in the Earth’s orbit led to less summertime heating there, rather than more.

Instead, the study reveals the role of two other factors: a change in Atlantic Ocean circulation, which rapidly boosted rainfall in the region, and a rise in GHG concentrations that helped enhance rainfall across a wide strip of Africa.

This came about as planet Earth emerged from the last Ice Age. GHGs, especially carbon dioxide and methane, increased significantly (almost reaching pre-industrial levels) 11 000 years ago – for reasons that are not yet fully understood.

The authors note that this was the most recent time during which natural global warming was associated with an increase in GHG concentrations. (Because of feedbacks between the two, GHG 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, in which heat and salt were transported northward through the Atlantic Ocean. This weakened circulation led to Africa’s rainfall shifting toward its southernmost point, with it being suppressed in northern, equatorial and eastern Africa.

When the ice sheets stopped melting, the circulation became stronger again, bringing rainfall back into south-eastern, equatorial and northern parts of Africa. This change, coupled with the orbital shift and the warming caused by the increasing GHGs, is what triggered the AHP.

To piece this puzzle together, 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 south-eastern 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, which uses supercomputers at the Oak Ridge National Laboratory.

By comparing the proxy records with the computer simulations, the study demonstrates that the climate model got the AHP right. This helps to validate its role in predicting how rising GHG 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 points out. “A study like this one, which disects why the climate evolved as it did over this intriguing 10 000-year period, is more than I thought I would ever see in my career.”