Potential Global Sequestration of Atmospheric Carbon Dioxide by Drylands Forestation

Abstract Drylands forestation offers the potential for significant long-term sequestration of atmospheric CO2. Here we consider sequestration of both organic and inorganic carbon by a planted semi-arid forest, based on carbon that originates from atmospheric CO2. Measurements at Israel’s Yatir forest give a sequestration rate of ~550 g CO2 m−2 yr−1 as organic carbon in the trees’ biomass. In addition, ~216 g CO2 m−2 yr−1 precipitates as calcite (CaCO3) in the soil due to a combination of microbial activity on organic soil carbon, and the formation of soil carbonic acid (H2CO3) that arises from the reaction of soil water with CO2 exhaled from tree roots. Significantly, low rainfall in drylands precludes dissolving precipitated calcite. Published estimates restrict the potential drylands surface available for sustainable forestation to ~4.5 million km2, only ~10% of the global drylands. The dominant limitation is the apparent lack of water. However, immediately under many drylands, there are paleowaters (fossil water) that had recharged underlying aquifers during prior wetter climatic regimes. Conservatively, including fossil water, at least ~9.0 million km2 is available for afforestation. Such an area may yield a potential total annual sequestration rate of ~7.0 Gt CO2 yr−1, divided between 5.0 Gt CO2 yr−1 (organic) and 2.0 Gt CO2 yr−1 (inorganic). This represents a respectable ~35% of the annual rate of atmospheric CO2 increase. However, considering the reduction in land surface albedo (reflectivity), the effective cooling would be ~5.0 Gt CO2 yr−1. Drylands reforestation would provide additional area for sequestration. Overall, our estimate demonstrates the efficacy of implementing a global land management policy of widespread afforestation and reforestation in drylands regions.