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        Land degradation is a systemic global problem, but the scale of the problem is disputed, with global estimates of degraded areas ranging from <10 to >60 million km2 . Changes in vegetation in drylands are predominantly caused by two factors: (i) anthropogenic climate change, which includes both changes in water availability driven by trends in precipitation and increases in temperature, as well as increased water use efficiency (carbon gain per unit of water lost) in response to rising atmospheric CO2; and (ii) land use practices, including grazing, cropping and deforestation. Unsustainable land use is considered the primary negative driver of dryland degradation. The impact of climate change on drylands is also generally thought to be negative, with some studies suggesting that anthropogenic forcing has already increased arid areas.

         Despite evidence for land use-induced degradation and the studies that find increased aridification over drylands, satellite estimates of vegetation greenness show a significant global increase since 1980. The key drivers of this global increase in apparent vegetation productivity are the vegetation’s response to rising CO2, increases in rainfall and temperature and land use. Model simulations which prescribe land use, attribute almost all of the trend in satellite-derived greening to CO2 fertilization, while satellite-derived models that do not account for CO2, explicitly find either climate or land use as the dominate factor. Neither approach explicitly accounts for rapid ecosystem change in their proportioning of the relative contributions of each driver. This can lead them to miss or underestimate rapid changes driven by processes like extreme fires, deforestation, reforestation, changes in agricultural policy, etc. Disentangling the roles of climate (temperature and precipitation), CO2 and land use thus remains a key challenge.

A.L. Burrell; J.P. Evans; M.G. De Kauwe. Anthropogenic climate change has driven over 5 million km2 of drylands towards desertification. Internet:<www.sciencedirect.com>  (adapted)