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The plains of the Serengeti, Tanzania, Africa
Main image: The plains of the Serengeti, Tanzania, Africa. Credit: mdd/Shutterstock
17 March 2015 14:10

Farming Africa’s wet savannahs would have a high climate cost, study warns

Robert McSweeney


Robert McSweeney

17.03.2015 | 2:10pm
Food and farmingFarming Africa’s wet savannahs would have a high climate cost, study warns

As the global population rises, some scientists have suggested that Africa’s wet savannahs could be ideal for growing the extra crops needed to meet the growing demand for food and bioenergy.

But it isn’t quite the solution it seems, according to new research. The idea that Africa can provide food and biofuels while keeping emissions low “does not add up”, the researchers say.

The wet savannah

‘Wet savannah’ describes warm, tropical areas areas that are wet enough to support crops and aren’t covered with dense forest. Africa is home to around half of the world’s wet savannah. Much of it is found in the Guinea Savannah, which makes up around  a third of sub-Saharan Africa.

Map of the Guinea Savannah

The Guinea Savannah covers all the coloured areas on the map. The dark greens show areas with more dense vegetation, and the reds show areas already used for farming. Source: Searchinger et al. (2015).

Past studies have suggested that turning Africa’s wet savannah into cropland could be a potential solution for meeting global food and energy demand, the researchers say.

For example, a 2009 World Bank and United Nations Food and Agriculture Organisation (FAO) report described the Guinea Savannah as “one of the world’s largest underused agricultural land reserves”, with around 400 million hectares available to convert into farmland.

But the new paper, published in Nature Climate Change, says that these studies underestimate the environmental impact of converting the land for farming.

While the World Bank and FAO acknowledge the effect on biodiversity, they do not consider the amount of carbon that would be released, the researchers say.

‘Lower cost is not low cost’

When land is cleared and cultivated, carbon dioxide is released into the atmosphere from the soil, and the plants, shrubs and trees that grow there. The more densely the land is packed with vegetation, the more carbon is released when it is cleared.

Previous studies assume that Africa’s wet savannahs are ripe for cultivation because they aren’t covered by trees. The new study’s lead author Dr Timothy Searchinger, a research scholar at Princeton University, tells Carbon Brief:

“Prior studies have assumed wetter savannahs are expendable because they are lower cost than forests. But lower cost is not low cost.”

So, the researchers set about working out how much of Africa’s wet savannahs could be converted to cropland without large emissions of carbon dioxide.

Low carbon option

The researchers used a crop simulation model to estimate how much maize and soybean could be potentially be grown on the Guinea Savannah. The model takes into account the soil and weather conditions, risk of failures, and how the crops are managed.

The scientists then worked out the proportion of the potential yield that could be grown in a ‘low carbon’ way.  They define ‘low carbon’ as emitting a third less carbon than the global average emissions for converting land for farming.

The study works out the carbon emitted per ton of food produced, rather than the traditional method of measuring emissions by area of land cleared. This is a more precise way of measuring the food gains against the environmental costs of carbon emissions, the researchers say.

You can see their results on the map below. Just 2% of the Guinea Savannah met the low carbon criteria for growing maize (yellow areas), with around 11% suitable for soybean (light green).

Map of low-carbon potential cropland sites for maize, soybean and both.

Low-carbon potential cropland sites for maize (yellow), soybean (light green) and both (dark green). Source: Searchinger et al. (2015).

Payback time

Previous studies also suggested the Guinea Savannah could be used to grow biofuel crops. A biofuel is any form of energy derived from plants or animal waste. Crops such as long-lasting grasses can be fermented to create ethanol and then blended with petrol for use in cars, thus reducing the amount of fossil fuels used.

The researchers used the same crop model to test growing these grasses rather than a food crop. The researchers calculated how many years of growing biofuels it would take to payback the carbon emitted when the land was converted for farming.

The map below shows that for around half of the Guinea Savannah, it would take over 50 years of growing biofuels to save the same amount of carbon dioxide that was emitted when the land was cleared (see orange and red areas).

Only 0.6% of the land would payback the carbon within 10 years (navy blue areas), which is the standard set by the European Union, the researchers say.

Map showing carbon payback times (in years) for growing grasses for producing ethanol

Carbon payback times (in years) for growing grasses for producing ethanol. Source: Searchinger et al. (2015).

Rob Bailey, research director at Chatham House, tells Carbon Brief that the biofuel finding is particularly important:

“Given the poor economics of biofuels as a mitigation strategy, and the fact that bioenergy production competes with food production in an already food insecure region, it would seem that this would represent a particularly bad use of land.”

Growing demand

Previous studies have overestimated the potential for low carbon cropland in the wet savannahs, the researchers conclude.

But for sub-Saharan Africa to be able to feed its growing population at improved nutrition levels, overall food production will need to quadruple by 2050, the study notes. So what are the alternatives?

Some expansion of cropland is probably necessary, but the focus should be on boosting yields and limiting demand for biofuels and meat worldwide, Searchinger says.

Bailey agrees that reducing meat demand could make a big difference:

“[Meat production] is terribly wasteful way to produce calories and protein. For the two crops considered in the analysis, globally, about 70% of maize production and 85% of soybean goes to animal feed.”

But the question is broader than the competing demands of food, energy and environmental concern, says Bailey. Development is also a key objective, and improvements in farming practice and technology would help Africa meet its food needs without the need to expand farmland unsustainably.

Searchinger, T.D. et al. (2015) High carbon and biodiversity costs from converting Africa's wet savannahs to cropland, Nature Climate Change, doi:10.1038/nclimate2584

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