Africa's carbon budget
Dr Bob Scholes
Is Africa breathing in or out? The global carbon budget is a hot potato.
Dr Bob Scholes of the CSIR in South Africa reports.
The build-up of CO2 in the atmosphere is one of the main causes of
global warming. The international treaties that seek to keep climate change
below "dangerous levels" mainly do so by defining how much CO2 given
nations are allowed to emit. As yet, South Africa and other developing countries
do not have an upper limit on their emissions, but within the next decade or so,
they may need to join the international efforts to control this growing problem.
One of the complications in the global carbon budget is that the total
emissions of CO2 resulting from human activities - mostly the burning
of coal, oil and natural gas, but also deforestation, the manufacture of cement,
and a variety of other minor sources - is about twice the amount that actually
appears as a rise in the atmospheric concentration.
The rest is taken up by the oceans and by ecosystems on land. This
"helping hand" from nature is not limitless, and has its own
disadvantages. For instance, the oceanic capacity to dissolve excess CO2
was once thought to be much more than we could use up. In recent years, it has
been discovered that one consequence is that the acidity of the ocean has been
increasing. This hinders the metabolism of microscopic marine organisms that
build their shells out of calcium carbonate - the very process that helps the
ocean to keep sucking up CO2 .
It has been suggested for about a decade that the capacity of land-based
ecosystems to absorb excess CO2 is likely to be used up sometime this
century. Meanwhile there is a rapidly growing market for "carbon
credits": carbon removed from the atmosphere, which can be offset against
emissions in countries that have emission limits.
Placing a value on carbon, and allowing it to be traded between those who
reduce emissions more than they need to, and those who exceed their targets, is
an economically efficient way of solving the global problem and one of the few
ways that developing countries can actually get some benefit out of climate
change.
For most African countries that do not have large industrial CO2
sources that they can reduce, thereby generating credits for sale, storing
carbon in forests and soils is a relatively cheap option. The problem is that
those same soils and forests are mopping up CO2 already, all by
themselves - so how do the customers know that they are getting what they paid
for? We need to know what the size of these natural sinks are, what causes them
and where they are located.
The global distribution of these socalled "carbon sinks" can be
calculated in broad latitudinal bands, by a clever technique known as inverse
modelling. Very precise measurements of the northern hemisphere to southern
hemisphere gradient of atmospheric CO2 measurements are compared to
computer simulations of what that gradient should look like for a known
distribution of sources of CO2 (mainly in the northern hemisphere)
and a speculated distribution of sinks.
One of the sources of these measurements is the Global Atmospheric Watch
station at Cape Point, set up by the CSIR over 30 years ago, and now run by the
South African Weather Service. These methods tell us that Africa as a whole is
probably a small sink for CO2 . But the ability of these techniques
to narrow down the sources and sinks to smaller regions, and to decrease the
uncertainty associated with the estimates is limited by the absence of
high-quality measurements over land, especially over Africa.
This is where two large CSIR projects come in, involving the Ecosystem
Processes and Dynamics Research Group of CSIR Natural Resources and the
Environment. The three-year African Carbon Experiment, funded by the National
Aeronautics and Space Administration (NASA) and the National Oceanics and
Atmospheric Administration (NOAA), and now in its second year, will feed into a
European Union-funded project called CarboAfrica.
Very precise CO2 measurements over land are not easily achieved
because the land surface everywhere is very patchy (unlike the ocean), and a
large nearby source or sink can completely distort the measurements. One
solution is to take measurements from very tall towers - a kilometre or more
high - but these are uncommon in Africa, and very expensive to build. The CSIR
and its collaborators from the USA are working on an innovative technique to
combine "flux" measurements (up-and-down exchanges between the land
and atmosphere) and concentration measurements, both taken close to the land
surface, to estimate the measurements that could come from tall towers. The CSIR-operated
flux tower near Skukuza is crucial to this experiment.
A second approach to the problem is "bottom-up" rather than the
"top-down" inverse modelling studies. If we could predict, using a
combination of models and satellite observations, what each bit of the African
land surface was doing with respect to carbon, we could add them up to get the
national or all-Africa estimates, which could then be verified against the
atmospheric measurements. To do this, we need to understand the processes of
photosynthesis and respiration from African ecosystems much better than we
currently do. This is the objective of the CarboAfrica project. It aims to
increase the number of flux measurement sites in Africa from the current one or
two, to about 20.
Their data will fine-tune the models used for predicting carbon exchange from
land ecosystems to allow them to work much more accurately in Africa. Apart from
narrowing the knowledge gap on the global carbon cycle, there are spinoff
benefits in being able to predict forest growth and rangeland productivity for
purposes of sustainable development.
More information:
 Article
courtesy of CSIR ScienceScope
Dr Bob Scholes
Tel: +27 12 841-2045
bscholes@csir.co.za
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