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, based on Yatir measurements, an inorganic carbon precipitation rate is estimated to give an additional 216 g CO2 m−2 yr−1 globally, via calcite (CaCO3) precipitation in soil. This soil inorganic carbon (SIC) sequestration is 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. Measurements at Yatir show that drip irrigation to 18% average Soil Moisture (higher than the rainfed 12% SM) would double the organic carbon sequestration rate. In addition, the tree density could be increased, which would independently double the organic carbon sequestration rate. The potential total annual sequestration rate is then conservatively estimated as 20.0 Gt CO2 yr−1, divided between 14.0 Gt CO2 yr−1 (organic) and 6.0 Gt CO2 yr−1 (inorganic). This corresponds to 100% of the annual rate of atmospheric CO2 increase.
