Afforestation is an important approach to mitigate global warming. Its complex interactions with the climate system, however, makes it controversial. Afforestation is expected to be effective in the tropics where biogeochemical and biogeophysical effects act in concert; however, its potential in the large semi-arid regions remains insufficiently explored. Here, we use a Global Climate Model to provide a process-based demonstration that implementing measured characteristics of a successful semi-arid afforestation system (2000 ha, ~300 mm mean annual precipitation) over large areas (~200 million ha) of similar precipitation levels in the Sahel and North Australia leads to the weakening and shifting of regional low-level jets, enhancing moisture penetration and precipitation (+0.8 ± 0.1 mm d −1 over the Sahel and +0.4 ± 0.1 mm d −1 over North Australia), influencing areas larger than the original afforestation. These effects are associated with increasing root depth and surface roughness and with decreasing albedo. This results in enhanced evapotranspiration, surface cooling and the modification of the latitudinal temperature gradient. It is estimated that the carbon sequestration potential of such large-scale semi-arid afforestation can be on the order of ~10% of the global carbon sink of the land biosphere and would overwhelm any biogeophysical warming effects within ~6 years.Afforestation is considered as climate change mitigation strategy 1-3 , but it is also associated with potential climate feedbacks 2,4 and rarely considers the importance of semi-arid regions 5 . Attempts to explain the records that indicate the "greening of the Sahara" 6 to 9 thousand years ago 6 , showed that the "top-down" effects of changes in the earth-system that ultimately result in land cover changes, such as changes in the sea surface temperature (SSTs) of the Atlantic, Indian and Pacific Ocean basins, can be associated with changes in land-ocean circulation and teleconnection that influence moisture transport and local precipitation over semi-arid monsoon regions such as the Sahel 7,8 . The resulting changes in land cover can consequently lead to changes in surface temperature gradients and, in turn, to changes in the characteristics of local atmospheric circulation, such as the African easterly jet (AEJ) and the intensity of the monsoon westerly (MW) winds in regions such as the Sahel 9-11 . Similar processes to those described for the Sahel were also indicated for the Asian monsoon in Northern Australia 12,13 , including the development of the Australian Easterly Jet (AUSEJ).In contrast to the "top-down" effects noted above, locally driven, "bottom-up" climatic effects triggered by first changing the land-cover in semi-arid regions, such as desertification in the Sahel, were demonstrated by the pioneering work of Charney 14 , Ornstein 11 and others 15,16 . Inverse feedbacks also operated on re-forestation in such regions (e.g. 11,[17][18][19] ). While these pioneering studies dealt with some aspects of land cover and climate in t...
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