Concluding Summary

We have examined the challenge of achieving CO2 stabilization in the context of a sustainable Earth system, by outlining a systems framework and then identifying the wider implications of carbon management options in economic, environmental, and socio-cultural terms.

The systems analysis begins from the familiar global atmospheric carbon budget, and the stabilization requirement that direct-human-induced CO2 emissions are low enough to allow land-air and ocean-air carbon fluxes to buffer atmospheric CO2 at a constant future level. A range of strategies is available to keep net emissions within this constraint: (1) conservation of energy at end-use points, (2) use of non-fossil-fuel energy sources, (3) more carbon-efficient use of fossil fuels; (4) reduction of carbon emissions from land disturbance; (5) sequestration of carbon in terrestrial or oceanic biological sinks; and (6) engineered disposal of CO2 in geological or oceanic repositories. Each strategy also has a range of impacts (benefits and costs), broadly in four classes: climatic, economic, environmental, and sociocultural.

The success of a suite of carbon mitigation strategies will be determined not only by the technical potential of each strategy (the amount of carbon emission that can be avoided, based on biophysical considerations alone), but also by the uptake rates of the various strategies. These rates are determined by the overall benefit-cost outcomes of the entire suite of strategies, judged not only against carbon-mitigation, but also against economic, environmental, and sociocultural criteria. Thus, the uptake rates (and the tra jectories of all parts of the carbon-climate-human system) are emergent properties of the system, resulting from interactions among system components, rather than being imposable properties of isolated components. This situation has several important consequences: First, it is important to maintain a broad suite of mitigation options. Next, successful carbon mitigation depends on exploiting beneficial synergies between mitigation and other (economic, environmental, and sociocultural) goals. Third, societal choices play a major role in determining the relative weightings between these goals and thus the overall outcome.

An order-of-magnitude comparison of likely achievable mitigation potentials (Figure 6.1) shows that major mitigation through sequestration and disposal cannot achieve stabilization unless there is also major mitigation in the energy sector, which has a larger overall impact.

We have assessed a wide range of carbon management options for their economic, environmental, and sociocultural impacts, both positive and negative. The following sweeping (therefore imperfect) generalizations summarize our findings: strategies based on energy conservation and efficiency have broadly beneficial impacts and offer major achievable mitigation, as do strategies involving non-fossil-fuel energy (though with significant environmental and sociocultural negative implications in certain cases). Land-based options offer significant mitigation but with some significant negative impacts mainly by competing with other land uses such as food production. Ocean biological sequestration has major collateral concerns. CO2 disposal in ocean and geological repositories has significant mitigation potential, but its side effects are still poorly known.

A more detailed analysis of a specific case, land-based mitigation through bioenergy and sequestration, has explored the implications for the pressure on land for food and other essentials under six SRES scenarios. These options offer a sustainably achievable mitigation potential only under SRES scenarios A1T and B1.

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