Optimising carbon sequestration in arid and semiarid rangelands

Destocking degraded rangeland can potentially help climate change mitigation by re-sequestering emitted carbon. Broad-scale implementation has been limited by uncertainties in the magnitude, duration and location of sequestration and the profitability relative to the existing grazing land use. This paper employs a novel methodology to assess potential rangeland sequestration and its profitability, using 31 Mha of rangeland in New South Wales, Australia as a case-study. This approach combines remotely sensed data and modelled estimates of various components. Remotely sensed, synthetic aperture radar data were used to determine woody biomass of minimally degraded forest (benchmarks) and neighbouring more-degraded forest, followed by sequestration modelling using non-linear growth rates based on woody thickening and slow-growing plantations, scaled to the benchmarks. Livestock concentration and livestock-based farm profits were modelled. We compared sequestration and grazing net profits, for a carbon price of AUD$10 Mg⁻¹ CO₂-e, at different growth stages for different levels of forest attrition. We found that broad-scale destocking with subsequent C re-sequestration was initially unprofitable compared with grazing. However, after 50 years, with full costing of C emissions, the returns were similar for the two alternatives of continued grazing or re-sequestration, for areas with biomass below benchmark levels. Reforestation of recently deforested land represents the most profitable option with profitability increasing with growth rate. Emissions of soil organic carbon, set in motion by climate change over the next century, were calculated to be the largest of all sources. Emissions from biomass, induced by climate change, will be higher where vegetation cannot adapt. The secondary effects of climate change will reduce re-sequestration and grazing profits, possibly limiting the carbon stored by re-sequestration projects.