5.4.1.5 Vulnerability of carbon pools

Impacts of climate change on managed systems, due to the large land area covered by forestry, pastures and crops, have the potential to affect the global terrestrial carbon sink and to further perturb atmospheric CO₂ concentrations (IPCC, 2001; Betts et al., 2004; Ciais et al., 2005). Furthermore, vulnerability of organic carbon pools to climate change has important repercussions for land sustainability and climate-mitigation actions. The TAR stressed that future changes in carbon stocks and net fluxes would critically depend on land-use planning (set aside policies, afforestation-reforestation, etc.) and management practices (such as N fertilisation, irrigation and tillage), in addition to plant response to elevated CO₂. Recent research confirms that carbon storage in soil organic matter is often increased under elevated CO₂ in the short-term (e.g., Allard et al., 2004); yet the total soil carbon sink may saturate at elevated CO₂ concentrations, especially when nutrient inputs are low (Gill et al., 2002; van Groenigen et al., 2006).

Uncertainty remains with respect to several key issues such as the impacts of increased frequency of extremes on the stability of carbon and soil organic matter pools; for instance, the recent European heatwave of 2003 led to significant soil carbon losses (Ciais et al., 2005). In addition, the effects of air pollution on plant function may indirectly affect carbon storage; recent research showed that tropospheric ozone results in significantly less enhancement of carbon-sequestration rates under elevated CO₂ (Loya et al., 2003), because of the negative effects of ozone on biomass productivity and changes in litter chemistry (Booker et al., 2005; Liu et al., 2005).

Within the limits of current uncertainties, recent modelling studies have investigated future trends in carbon storage over managed land by considering multiple interactions of climate and management variables. Smith et al. (2005) projected small overall carbon increases in managed land in Europe during this century due to climate change. By contrast, also including projected changes in land use resulted in small overall decreases. Felzer et al. (2005) projected increases in carbon storage on croplands globally under climate change up to 2100, but found that ozone damage to crops could significantly offset these gains.

Finally, recent studies show the importance of identifying potential synergies between land-based adaptation and mitigation strategies, linking issues of carbon sequestration, emissions of greenhouse gases, land-use change and long-term sustainability of production systems within coherent climate policy frameworks (e.g., Smith et al., 2005; Rosenzweig and Tubiello, 2007).