Drylands extent and environmental issues. A global approach

Drylands, a critical terrestrial system of the Earth due to low water availability, are known for their extensive global reach, estimated by most scientific sources at approximately 41% of the world's land area, or similar to 60 mil km(2). However, the analysis of the global dryland areas, using new climate data, suggests a total of similar to 45% of the Earth's terrestrial area, almost 7 mil km(2) more than initially estimated. This new spatial dimension involves a wide range of environmental issues, some of which have yet to be associated with these critical global systems. This paper primarily aims to accurately quantify the global, continental and national extent of drylands by using a high-resolution climate database presently available at global level. Also, based on relevant scientific literature, this approach attempts to briefly highlight the main environmental issues (natural and anthropogenic) of the major continental and regional dryland areas. In this respect, special attention was given to the land degradation processes (water and wind erosion, vegetation degradation, salinization, soil compaction and nutrient loss), as it is known to be the main environmental perturbation in almost all dryland systems. Research shows that, given the fact that Africa and Asia have the most extensive dryland systems on Earth (each of them has almost 23 mil km(2), or similar to 15% of the global land area), these continents are especially threatened by major environmental perturbations (desertification, in addition to other ecological and climatic disturbances such as drought, dust storms, heat waves, water stress, extreme rainfall events, wildfire, dzud, or disease emergence), which are currently affecting 46 African states (37% of the 126 states affected by aridity worldwide) and 38 Asian states (30%). Given this context, anthropogenic systems are indirectly severely threatened by the crisis generated by soaring poverty, food insecurity, population migration, and escalating conflicts and regional political instability. Moreover, in the current context of large-scale aridity identified at high latitudes, another critical threat reviewed was the cryosphere's destabilization, which can potentially accelerate climate warming by means of positive feedback mechanisms that can be triggered in the global climate system. In this respect, a major concern is attributed to permafrost melting that, against the background of a significant expansion in the terrestrial northern hemisphere (in Russia, Alaska and Canada), can generate a massive acceleration of climate warming due to the potential release of large quantities of carbon dioxide (CO2) and methane (CH4), which are currently stored in these frozen soils in the Arctic and sub-Arctic regions. (C) 2016 Elsevier B.V. All rights reserved.