Great Green Walls: Hype, Myth, and Science

Visions of planting walls of trees to block the expansion of the desert have long been promoted but never fully realized. The green wall myth persists today even though it is premised on outdated understandings of desertification. We review the history of the idea of green walls and focus on two sets of contemporary initiatives to assess their outcomes: peri-Saharan programs (Algeria's Green Dam and Great Green Wall in sub-Saharan Africa) and China's Three Norths Shelterbelt Program. This review reveals a mixed record of technical success with low rates of tree establishment, particularly in drier areas and monocultures of fast-growing trees vulnerable to disease. While there is evidence for reduced wind erosion in some areas, afforestation is also associated with reduced soil moisture and lowering of water tables. Social impacts include increased water scarcity for people and livestock in some cases, and resource enclosures that particularly work against pastoralist livelihoods. Expected final online publication date for the Annual Review of Environment and Resources, Volume 48 is October 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.