The Role of Soil Carbon Sequestration in Climate Mitigation over the Next Century

The future trajectory of carbon emissions over the next century depends upon many factors. The IPCC recently developed a range of standard reference emission scenarios (SRES) to provide estimates of possible emissions under a range of different possible futures (IPCC, 2000b). These possible futures depend upon the degree to which society or policy becomes global and whether environmental or economic concerns take precedence in the next century.

Among the A1 family of scenarios (global - free market), a number of possible emission trajectories exist depending upon whether the energy sector remains fossil fuel-intensive (A1FI), the rapid introduction of new energy technologies allows a move away from carbon-intensive energy sources (A1T) or a balanced mix of fossil fuel and alternative energy sources (A1B) is achieved.

In all of these scenarios, the global population will grow, become wealthier and per capita energy demand will increase over the next century (IPCC, 2000b). The extent to which these changes will occur varies between different scenarios, with some showing larger increases than others, but in all of them, these trends are observed. For each of the scenarios carbon emission trajectories have been determined (IPCC, 2000b). Annual carbon emissions (billion tonnes C/year) by 2100 would be A1FI = ~30, A1B = ~17, A1T = ~7, A2 = ~28, B1 = ~6, and B2 = ~18.

Emission trajectories can also be calculated for a range of atmospheric CO2 stabilization targets (e.g. 450, 550, 650 and 750 ppm). For each stabilization target, the allowed carbon emission trajectories, which cannot be exceeded if the target is to be reached, can be calculated. The difference between the allowed emission trajectory for stabilization at a given target concentration and the emissions associated with the estimated global energy demand is the carbon emission gap. For each of the IPCC scenarios, the carbon emission gaps by 2100 (Pg C/year) would be A1FI = 25, A1B = 12, A1T = 2, A2 = 22, B1 = 1 and B2 = 13 (IPCC, 2001).

The current annual emissions of CO2 to the atmosphere are 6.3 ± 1.3 Pg C/year. Carbon emission gaps by 2100 could be as high as 25 Pg C/year, which means that the carbon emissions problem could be up to four times greater than at present. The maximum annual global carbon sequestration potential is 0.9 ± 0.3 Pg C/year, which means that even if these rates could be maintained until 2100, soil carbon sequestration would contribute a maximum of 25% towards reducing the carbon emissions gap under the highest emission scenarios. When we consider the limited duration of carbon sequestration options in removing carbon from the atmosphere, we see that carbon sequestration can play only a minor role in closing the emissions gap by 2100. It is clear from these figures that if we wish to stabilize atmospheric CO2 concentrations by 2100, the increased global population and its increased energy demand can be supported only if there is a large-scale switch to non-carbon-emitting technologies for producing energy.

Given that soil carbon sequestration can play only a minor role in closing the carbon emissions gap by 2100, is there any role for carbon sequestration in climate mitigation in the future? The answer is 'yes'. If atmospheric CO2 levels are to be stabilized at reasonable concentrations by 2100 (e.g. 450-650 ppm), drastic reductions in emissions are required over the next 20-30 years (IPCC, 2000b). During this critical period, all measures to reduce net carbon emissions to the atmosphere would play an important role - there will be no single solution (IPCC, 2000b). Given that carbon sequestration is likely to be most effective in its first 20 years of implementation, it should form a central role in any portfolio of measures to reduce atmospheric CO2 concentrations over the next 20-30 years whilst new energy technologies are developed and implemented.

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