Potential Global Sequestration of Atmospheric Carbon Dioxide by Drylands Forestation

Drylands forestation offers the potential for significant long-term sequestration of atmospheric CO$_2$. We consider sequestration of organic and inorganic carbon by a planted semi-arid forest, based on carbon that originates from atmospheric CO$_2$. Measurements at Israels Yatir forest give a sequestration rate of $\sim$550 g CO$_2$ m$^{-2}$ yr$^{-1}$ as organic carbon in the trees biomass. The inorganic carbon precipitation rate gives an additional 216 g CO$_2$ m$^{-2}$ yr$^{-1}$ globally, via calcite (CaCO$_3$) precipitation in soil. This sequestration is due to a combination of microbial activity on organic soil carbon, and the formation of soil carbonic acid (H$_2$CO$_3$) that arises from the reaction of soil water with CO$_2$ exhaled from tree roots. Published estimates restrict the potential drylands surface available for sustainable forestation to $\sim$4.5 million km$^2$, only $\sim$10$\%$ 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 $\sim$9.0 million km$^2$ is available for afforestation. Measurements at Yatir show that drip irrigation to $\sim$18$\%$ average Soil Moisture (higher than the rainfed $\sim$12$\%$ 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 estimated as 20.0 Gt CO$_2$. Measurements at Israels Yatir forest give a sequestration rate of $\sim$20.0 Gt CO$_2$ yr$^{-1}$, divided between 14.0 Gt CO$_2$ yr$^{-1}$ (organic) and 6.0 Gt CO$_2$ yr$^{-1}$ (inorganic). This corresponds to $\sim$100$\%$ of the annual rate of atmospheric CO$_2$ increase.