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
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).
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.
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
