Fig. 5.3 The impact of climate change on UK arable soil C stocks under the SRES A1F1 scenario (HadCM3 model, 2080s) using the RothC soil carbon model. PET, PRECIP, TEMP = changing only potential evapotranspiration, precipitation or temperature; PET + PRECIP — changing both PET and PRECIP; ALL = changing PET, PRECIP and TEMP (MODEL) simultaneously in the model, and summing values from runs changing single climate variables (SUM)

higher soil C stocks and drier conditions in lower soil C stocks, since increased NPP in wetter conditions could override any increase in respiration. In contrast, temperature appeared to control changes in C stocks under climate change globally. Finally, a recent inter-comparison of coupled climate-carbon cycle GCMs found a wide range of positive climate-carbon cycle feedback values, showing response of soil carbon to climate change to be highly uncertain (Friedlingstein et al. 2006).

5.3 Climate Change Threats and Opportunities for Greenhouse Gas Mitigation in Cropland Soils

5.3.1 Impacts of Climate Change on Cropland Greenhouse Gas Mitigation Options and Their Potential

Climate change could potentially act to reduce (or increase) GHG mitigation potential through different constraints as depicted in Fig. 5.1. Changes in temperature, moisture and CO2 concentrations could act to alter the biological potential and land suitability which, in turn, along with other climate-related factors could alter economically constrained potential. For example, by changing the demand for crops and agricultural products, future changes in environmental policy, such as adaptation and mitigation efforts, could alter the socially/politically constrained potential.

Falloon and Smith (2003a) used the RothCUK soil carbon model to investigate the impact of climate change on the carbon sequestration potential of alternative land-use and land management practices in the UK. This showed that while there was considerable potential for single land management option to contribute towards Kyoto Protocol emissions reductions targets (up to 0.8% of 1990 UK CO2 emissions, or 6.4% of the UK target), climate change could significantly reduce C sequestration potential of alternative land management options (Fig. 5.4), by up to 57% of the estimate without climate change, depending on land management and climate change scenario.

These simulations also found a non-linearity in the response of soil C storage, climate change and land management changes, since summing the results from runs considering either only climate change or land management changes did not equal the results of runs changing both factors simultaneously. These studies only considered the impact of climate change on soil carbon storage, and did not assess how crop responses to climate change might alter C inputs to soil and hence soil C sequestration. In light of likely future decreases in C inputs to croplands due to reduced yields and changes in harvest index as discussed above, a further reduction in soil C sequestration potential under climate change might be expected at the global scale.

Smith et al. (2005) assessed future changes in European cropland and grassland soil carbon stocks, using the RothC soil carbon model incorporating climate data from four global climate models under different IPCC SRES scenarios, with


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