On the trail of African carbon
Africa is not only the forgotten continent in terms of development. Its climatology is also poorly understood, despite the major role it plays in the Earth's atmospheric processes. Hence the importance of projects such as CarboAfrica, which is attempting to analyse the inner workings of the carbon cycle in Africa.
It looks like one of those complex structures made entirely of matchsticks that street artists used to build in times past to squeeze a few coins from tourists, except that in this case the matchsticks are steel tubes and the tower rises some 65 metres above the ground. Tourists are thin on the ground here, where the towering edifice stands in the heart of Ghana's Ankasa forest. About 30 metres or so below the solar panels at its summit stretches an impenetrable equatorial forest canopy as far as the eye can see.
The Ankasa tower is the flagship of the CarboAfrica project, which received €2.8 million in funding from the 6th Framework Programme.
It is a flux tower, a sophisticated device comprising two coupled sensors: an ultrasound anemometer which provides the wind speed and direction in three dimensions, and an infrared analyser for continuously measuring air temperature as well as water vapour and carbon dioxide concentrations in the air.
Jonas Ärdo, a specialist in African ecosystems from Lund University (SE) provides a simple explanation for the uninitiated: "By combining these two data sets with a little mathematics and physics, we obtain the local flux of CO2 - in other words the quantity of gas that the forest releases or, conversely, the quantity it absorbs. Although the device performs 20 measurements per second, the data are automatically consolidated to provide 30-minute period averages to facilitate the use of the data." They are used to monitor the behaviour of CO2 from day to day, from season to season and from year to year.
Mysteries of the carbon cycle
Why bother to monitor the respiration of this lost African forest to such a precise degree?
The answer is that it could tell us more about one of the major unknowns of our climate's future: the carbon cycle. We already know that each year humans are responsible for releasing more than 27 billion tonnes of carbon dioxide into the atmosphere, mainly from burning fossil fuels. Although this is an enormous amount (around 30 times the mass of the largest lake in Western Europe, Lake Geneva), it is a figure that pales in comparison with natural processes. For instance, the respiration of all living things releases 10 times more carbon dioxide than humans do, but fortunately it is more than reabsorbed by photosynthesis. The ocean, too, dissolves an equal amount of CO2 into its waters, mainly in cold zones, whilst temperate zones release only a little. And if we look at the worldwide reservoirs of carbon, and not simply exchanges, the figures are even more impressive: the atmosphere contains 750 billion tonnes, vegetation three times that amount, and the ocean a staggering 40 000 billion tonnes.
The CO2 that humans release circulates between these different reservoirs. It reaches the atmosphere and is then absorbed by the oceans, plants, soil and rivers, before being released again. How much CO2 remains in the air? What proportion is soaked up? What quantity is released? From which reservoirs and at what rate? While all these data elements are key to modelling the future of our climate, collecting them is no easy matter.
The carbon cycle has been a focus of concern for climatologists for a number of years now. As a result, towers like the one at Ankasa, which continuously spy on the comings and goings of CO2, have sprung up everywhere in recent years. There are currently more than 400 such towers throughout the world. One problem is that, while the climate knows no frontiers, the same cannot be said for human constructions. Africa is seriously lacking in instrumentation compared with the rest of the world. Its network of meteorological stations falls well below the minimum recommendations of the World Meteorological Organization (WMO), with only 15 flux towers to cover the entire continent's 30 million square kilometres (equivalent to six times the surface area of the European Union). The same problem affects regional modelling, which is neglected compared with the rest of the world.
Africa - carbon sink or source?
Christopher Williams, a specialist from the University of Colorado (USA), recently wrote in the journal Carbon Balance and Management: "Our knowledge of Africa's current role in the global carbon cycle is still remarkably limited. At the moment, we have no idea whether Africa is a net sink or source of atmospheric carbon, and have only vague information about the continent's temporal and spatial patterns of carbon exchange." This is a situation that CarboAfrica hopes to remedy. Project coordinator Professor Riccardo Valentini from the University of Tuscia (IT) explains that to achieve its ambitious goal of "Quantification, understanding and prediction of carbon cycle and other greenhouse gases (GHG) in sub-Saharan Africa," the project will first consolidate and expand the network for monitoring terrestrial carbon and other GHG fluxes of sub-Saharan Africa. Although the network is still very stretched, it has the advantage of covering very different ecosystems.
The tropical forest is probably the ecosystem with the most active ‘carbon metabolism' and will be scanned by the Ankasa tower.
Another tower is being erected in the wetlands of Botswana's Okavango delta. The El Obeid station in Sudan, which was installed by Jonas Ärdo, will provide data on the dry savannas.
A tower is also being constructed in the famous Kruger National Park in South Africa, a savannah more heavily populated with trees. In the Congo, measurements are taken in a eucalyptus plantation in order to gauge the carbon balance of cultivated environments. In Benin, too, millet crops are measured for emissions. There are plans to operate a total of 18 stations, two of them brand new. This entire infrastructure will be backed by measurements from two sites covering all atmospheric strata, and by data gathered from overflights by experimental aircraft. "The idea behind all this is to calibrate and validate models reflecting the carbon circulation in different ecosystems," explains Jonas Ärdo. "Knowing the distribution of ecosystems on the continent, we can use these models to start building a global model of the carbon cycle in Africa." These 18 stations will not be excessive in number given the great diversity of ecosystems on the continent, and with the carbon cycle varying significantly from one location to the other. For example, there is little activity in the Sahel region during the eight or nine months of the dry season, but when the rains arrive they trigger photosynthesis on a vast scale, accompanied by equally largescale respiration. Conversely, in forest zones the carbon cycle is very active and remains relatively stable throughout the year. Researchers are particularly interested in interannual variability because Africa is a continent that displays impressive changes of mood, especially in the Sahel region. It is not unusual to see dry years followed by one or more years of above average rainfall. These variations can be three times greater than in India, another part of the world that is affected by monsoons.
"Most of the basic models we use were developed in Europe or America," explains Jonas Ärdo. "So it is particularly important to adapt them to reflect closely the conditions in Africa, which this project should enable us to do." However, this will not solve all the problems: the models work more effectively the better the quality of the meteorological and pedological data that are fed into them. The land maps researchers are working with are full of errors because the Food and Agriculture Organization of the United Nations (FAO) drew them up two or three decades ago. Also, on the African continent it is not uncommon to find oneself more than 300 kilometres from the nearest weather station. These shortcomings could be partially offset by using satellite data.
Sensors like Modis provide information on weather and ground conditions with less than one kilometre resolution, but with a passage frequency of around once a week. "By combining satellite data with the data from weather stations and the flux towers already in operation, we are nevertheless able to make approximations with acceptable accuracy - or at least that is our aim," says Jonas Ärdo.
The challenge facing CarboAfrica is not simply to reflect reality more accurately. It is also to determine how to reduce Africa's impact on global warming. While there is no doubt that the continent's contribution to fossil emissions of CO2 is paltry (about 3 % of the world total, according to Christopher Williams), some 14 % of the Earth's population lives in Africa. Nevertheless, the emissions caused by deforestation, which is responsible for around a quarter of the greenhouse gases released by human activity, are far from negligible. The tropical forests fix huge quantities of carbon, both within the mass of the wood that they contain and in the carbon-rich soil of the forest floor. When tropical forests are cleared, usually by fire, the CO2 generated by the burning wood is released into the atmosphere immediately, and when the cleared ground is ploughed and cultivated, this in turn releases part of the carbon currently locked up in the forests. Africa is responsible for nearly 30 % of the world's tropical deforestation.
Major financial issues
The carbon cycle is also determined by hard economic issues. The Kyoto Protocol explicitly defines financial mechanisms under the Clean Development Mechanism (CDM), which offer enterprises in developed countries the option of offsetting their surplus greenhouse gas emissions by funding emission-reduction programmes in developing countries. Although Africa has made scant use of these North-South transfers so far, investing in tree planting and reforestation can, in principle, be used to obtain credits. Forest preservation financing methods, known under the acronym REDD (Reducing Emissions from Deforestation and Degradation of forests), are currently being negotiated.
There remains the problem of estimating the quantities of carbon stored in the forests and in the ground accurately and incontrovertibly in order to calculate the payments. "This is a particularly complicated procedure," says Laurent Saint André, a specialist in forest ecosystems at the French agricultural research centre for international development (Centre de coopération Internationale en Recherche Agronomique pour le Développement - CIRAD). "We must first find zones representative of the forest in question, divide them into relatively uniform biomes, and then use models to evaluate the carbon absorbed by each variety. Next there is the soil and the forest floor - the dead wood. Decisions have to be taken, especially regarding the depth of the analysis. One of the project goals is to define optimum evaluation procedures and methodologies." Not only must these evaluations be made more rigorous, costs must be reduced too, because cost can become an insurmountable obstacle for the people concerned, including foresters and local authorities.
Promoting carbon storage
If we were able to remunerate tree-planting or forest preservation, even at low levels, this would potentially have a huge impact on carbon storage in Africa, not only in wet forest zones. In the Sahel, for example, where the environment has been severely damaged by over-grazing and tree clearance, there are a number of assisted natural regeneration (ANR) techniques that encourage a return to woodland. An added advantage is that they enhance the soil's organic content and water retention properties, thereby increasing its fertility. Although ANR techniques (building low walls, ditches, micro-dams and other structures) are inexpensive, they still exceed the meagre investment capacity of local communities.
If they were financed via the carbon markets, not only would this have a positive impact on the climate, there would also be useful socioeconomic gains.
Another example: Laurent Saint-André and his team have calculated that a eucalyptus tree plantation on the Congo savannas would store almost 30 tonnes of carbon per hectare each year during its growth period. With CO2 trading at around €20 per tonne, this offers the prospect of income that could finance many development projects for local communities prepared to give over some of their land to growing trees.
There are doubtless other avenues to explore in Africa for reconciling local development and reducing global warming. While projects like CarboAfrica can certainly help to discover and identify those with the best potential, their implementation will depend largely on political decisions, and hence, to some extent, on all of us.