ALN logo; link to Arid Lands Newsletter home page No. 49, May/June 2001
Linkages between Climate Change and Desertification

The potential benefits of carbon sinks in dryland agricultural soils

by Lennart Olsson, Andrew Warren, and Jonas Ardö

"We believe that the parties in the reconvened climate negotiations, whatever form these ultimately take, should carefully consider the inclusion of soils in degraded dryland agroecosystems as an accountable carbon sink."

Carbon in the form of CO2 is accumulating in the atmosphere at a rate of about 3.5 petagrams per year (Pg/yr).(1) Additions are thought to come from burning of fossil fuel and land use conversion (Siegenthaler & Sarmiento 1993). The scientific consensus now supports the UN Framework Convention on Climate Change (UNFCC) in its opinion that this is unprecedented and that it will alter global climate (IPCC 2001). If not consensus, there is also considerable political backing for this opinion in many parts of the globe. Furthermore, there is scientific consensus that land degradation continues to be associated with poverty among millions of people in the drylands of the developing world (Levia 1999), an issue covered by the UN Convention to Combat Desertification (UNCCD), which has been reinvigorated by its ratification in the U.S. Congress in November 2000. Land degradation also poses a threat to biodiversity, the focus of yet another UN Convention, namely on biodiversity (UNCBD). It is at the intersection or "crosscutting" of these three major UN conventions that we are likely to find the most effective interventions.

The Kyoto Protocol aimed to reduce global emissions of greenhouse gases (GHG) by 5.2 % below 1990 levels (Grubb et al. 1999). But controversy over the so-called "flexible mechanisms," by which emissions reductions might be achieved, stalled the negotiations in the COP-6 meeting on the Protocol in the Hague in November 2000 and may have been a factor in the repudiation of the Protocol by U.S. President George Bush in March 2001.

The most controversial of these mechanisms is the Clean Development Mechanism (CDM, UNFCCC Article 12). This allows industrial countries to invest in projects to promote sustainable development in developing countries (those countries that have no commitment under the Kyoto Protocol). Reductions in emissions, if certified, must play a major role in fulfilling these aims, but terrestrial biospheric sinks of carbon could be another part of the CDM. At present the European Union (EU) wishes to minimize the use of sinks. The attitude of the U.S. government is now unclear, but during the negotiations at the COP-6 meeting Hague, the U.S. was keen on them.

The use of these sinks can indeed be criticized on several grounds, although the criticisms can themselves be countered. It is claimed that sinks could not wholly solve the problem, because the amount of carbon they would sequester is not enough to reduce the rate of increase of atmospheric CO2 to acceptable levels, and because offsets might delay emission reductions. Yet sinks do have potential to play an immediate role in reducing the rate of increase in atmospheric CO2. A further criticism is that carbon sequestered in forests or soils can easily be released again, and that plantations intended specifically for this purpose might compromise biodiversity. Yet it is not difficult to suggest monitoring and financial controls that could control these threats.

These problems, if they exist, are much less acute in the case of sequestration in agricultural soils than in forests (the latter being the main focus of debate until recently). Indeed, there are strong reasons for encouraging sequestration schemes in degraded agroecosystems. One is that land degradation, particularly in the tropics, is as urgent an environmental issue as climate change. The sequestration of carbon in these soils, if properly managed, has the potential to counter degradation, and, by increasing water-holding capacity, cation exchange capacity and resistance to erosion, even to increase productivity, resilience and sustainability of these agroecosystems. This would also increase food security and reduce poverty. A second reason is that the low-input agroecosystems of most of the developing world, which would benefit most from such a program, may have a higher potential for net carbon accumulation than do intensive forms of agriculture, where the inputs already have already a high carbon cost (Schlesinger 1999). A third reason is that low-input agriculture is less damaging to biodiversity than intensive forms of agriculture, even when these latter systems are "no-tillage" (many no-tillage systems use herbicides to clear weeds).

Projects funded by developed-world utilities or governments to sequester carbon in developing-world smallholder systems would, in other words, fulfil the "crosscutting" role of our introduction. They would address all three of the major international environmental conventions that stemmed from the 1992 UN Conference on Environment and Development (UNCED, also known as the Rio Convention): UNFCC, UNCCD and UNCBD. This process could allow large areas of land that would otherwise have been cultivated to revert to semi-natural vegetation, at least in temporary, sometimes long-term fallow. Such schemes would also enable developing countries to become active participants in the fight against climate change: something that is as close to a win-win (even win-win-win) situation as one can get.

Carbon sequestration can be achieved in these low-input systems by the application of immediately deployable low-input land management practices such as increased use of green fallow periods, conservation tillage (Lal 1997), increased used of rotational crops, return of crop residues to the soil and agroforestry. The highest per-hectare potentials for sequestering carbon may indeed occur in moist areas, where they may be best achieved with additions of nitrogen (N) and phosphorus (P), but by far the largest areas of degraded soils, and thus the greatest total potential for carbon sequestration, occur in low-rainfall zones. Inputs here are likely to remain very low, for in these systems the greatest potential for sequestration might be in the lengthening of fallow periods. If grazed or used for tree-crops, such as gum arabic, fallows can also contribute substantially to income, given markets and some system to compensate for the loss of the security that food crops bring. Batjes (1999) estimated that between 0.6 and 2 Gt C/year could be sequestered by large-scale application of appropriate land management of these lands. This would account for between 18 - 60% of the annual accumulation of CO2 in the atmosphere.

table of historic and projected soil carbon levels
Thumbnail link to table of historic and projected soil carbon levels in study area

We have investigated, by means of soil samples and modeling, the potential for increasing soil carbon content in semi arid agroecosystems in the Sudan, and found that increasing fallow periods would result in increased soil organic matter (SOM) content. Converting marginal agricultural areas to rangeland would restore the carbon levels to 80% of the carbon content of the natural savanna in 100 years. In this region it can also be shown that the economic gain from future carbon sequestration programs has the potential of making a significant contribution to the household economy.

Such sinks have advantages over forests. Whereas forests might compete for land with more socially desirable products, improving the productivity of agricultural land would almost always be socially beneficial. Forests are vulnerable to the temptation of quick economic return by logging. There are fewer temptations to release SOM in cultivated soils, because it contributes to fertility. Furthermore, even should such releases occur, they happen much more slowly than release of SOM from forests after logging . Some forms of SOM have very long residence times (hundreds and even thousand of years [Lal et al. 1998]), compared with carbon stored in above-ground vegetation. This is particularly so in the case of soil carbon stored in fallows, and less so of soil carbon added as manure to more productive systems. Finally, the monitoring and verification of storage agroecosystems is probably easier due to the fact that forests are much more complex ecosystems.

We believe that the parties in the reconvened climate negotiations, whatever form these ultimately take, should carefully consider the inclusion of soils in degraded dryland agroecosystems as an accountable carbon sink.


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(1) One petagram = 1 thousand million metric tons; sometimes also described as one gigaton.
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Batjes, N. H. 1999. Management options for reducing CO2-concentrations in the atmosphere by increasing carbon sequestration in the soil. Wageningen: International Soil Reference and Information Centre.

Grubb, M., C. Vrolijk, and D. Brack. 1999. The Kyoto Protocol - A guide and assessment. London: Earthscan.

IPCC (Intergovernmental Panel on Climate Change). 2001. Summary for policymakers. Working Group I, Third Assessment Report. Geneva: IPCC.

Lal, R., J.M. Kimble, R.F. Follet, and B.A. Stewart, eds. 1997. Soil Processes and the carbon cycle. Boca Raton, Fla.: CRC Press.

Lal, R., J.M. Kimble, R.F. Follet, and B.A. Stewart, eds. 1998. Management of carbon sequestration in soil. Boca Raton, Fla.: CRC Press.

Levia, D. F. 1999. Land degradation: Why is it continuing? Ambio 28(2):200-201.

Schlesinger, W. H. 1999. Carbon sequestration in soils. Science 284:2095.

Siegenthaler, U., and J.L. Sarmiento. 1993. Atmospheric carbon dioxide and the ocean. Nature 365:119-225.

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Author information

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Dr. Lennart Olsson is Director of the Center for Environmental Studies at Lund University, Sweden. You can reach him for comment as follows:
Dr. Lennart Olsson
Centre for Environmental Studies
Lund University
Box 170
S-221 00 Lund
Phone: +46 (46) 2220511
Fax: +46 (46) 2220475

Dr. Andrew Warren is a professor of geomorphology at University College London. You can contact him as follows:
Dr. Andrew Warren
Department of Geography
University College London
26 Bedford Way
London, WC1H 0AP, UK

Jonas Ardö, Ph.D., is a lecturer and researcher at Lund University. You can contact him as follows:
Jonas Ardö
Department of Physical Geography
Lund University
Sölvegatan 13
S-223 62 Lund

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