1) Prof. Joel Kronfeld

A limitation of afforesting drylands is the scarcity of readily available freshwater. Yet, this is more apparent than actual. For, beneath drylands often lie aquifers that were replenished during previous climatic cycles, holding significant reserves of ancient “fossil water.” On the desert surface, where little to no recharge takes place today, there are spring travertines and freshwater lacustrine limestones. The dating of these sediments shows that recharge happened cyclically during pluvial periods of the Pleistocene. The Nubian Sandstone Aquifer System, the largest fossil water reserve, spans over 2M km² beneath the Sahara Desert and contains approximately 370K km³ of freshwater. Other notable examples of paleowater under drylands without surface water expression include the Continental Intercalaire aquifer, which lies beneath Algeria and Tunisia and spans an area of 600K km², along with its shallower counterpart, the Complexe Terminal aquifer, covering 35K km² in Tunisia. In the Arabian Peninsula, which contains some of the driest deserts, substantial fossil groundwater reserves are found in the Mega Aquifer System, covering 1.9 M km², as well as in the Saq aquifer, which spans 560K km². Similarly, in southern Africa, fossil waters are found beneath the Kalahari Desert, which spans 900K km², and the Karoo Basin, covering 200K km². In Australia, the Great Artesian Basin covers an area of 1.7M km² and is estimated to hold around 65K km³ of high-quality paleowater. Overall, fossil waters are estimated to be accessible for dryland afforestation across an area of 9M km².