Potential Global Sequestration of Atmospheric Carbon Dioxide by Drylands Forestation

Drylands forestation, a component of earth sciences, ecology, and ecosystems, offers the potential for long-term sequestration of atmospheric CO₂. Israel’s Yatir Forest is a 28 km² planted semi-arid Aleppo pine forest successfully growing with no irrigation or fertilization and only 280 mm average annual precipitation. Yatir’s organic carbon sequestration (OCS) rate was measured as ~550 g CO₂ m^-2 yr^-1 (150 g C) carbon. This value was obtained using the data of a 15 year long monitoring program that combined eddy covariance (EC) flux measurements, as well as carbon stock counting inventories. In addition, based on Yatir’s measured inorganic carbon sequestration (ICS) rate, an additional 216 g CO₂ m⁻² yr⁻¹ globally may be sequestered via calcite (CaCO₃) precipitation in soil. The above OCS ad ICS rates are assumed here to be representative of global drylands. Part of this ICS is related to root and microbial exhalation of CO₂. A tree’s roots exhale CO₂ into the soil after some of the tree’s glucose (produced by photosynthesis) has been oxidized to supply energy for the tree’s cellular processes. Exhaled CO₂ combined with soil H₂O forms soil carbonic acid (H₂CO₃); and then bicarbonate (HCO₃^-) which combines with soil Ca²⁺ to form calcite. Another part of the ICS comes from soil microbes that use extracellular polymeric substances (EPS) to directly precipitate calcite. Low rainfall in drylands precludes dissolving the precipitated calcite. The potential maximal efficacy of global forestation for reducing global warming and ocean acidification depends on the maximal area available for sustainable forestation. The dominant limitation, particularly in the vast drylands regions, is the apparent lack of water. This would reduce the potential area for sustainable forestation to a published estimate of roughly 4.5 million km², ~10% of global hot drylands. However, in most drylands areas, plentiful water is available from immediately underlying local paleowater (fossil) aquifers. Using such water, until now not previously taken into consideration, conservatively yields a functional dryland forestation area of ~9.0 million km². Measurements at Yatir show that drip irrigation to 18% average Soil Moisture (higher than the rainfed 12% SM) would approximately double the OCS and ICS rates. 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. Significantly, this quantity removed from the atmosphere would also reduce ocean acidification. Note however that the transformation of bright high albedo deserts to darker forests could reduce the positive projected climate cooling effects attained by as much as ~25%. The effective reduction may be less, considering that increased forestation evapotranspiration would decrease surface temperature; and increase albedo via increased cloud cover. Our sequestration estimate demonstrates the global potential, the need for further measurements, and the need to begin implementing a global land management policy of widespread tree planting in drylands regions.