Abstract
The regions of the world where average precipitation is between one fifth and half of the potential plant water demand are termed ‘semi-arid’. They make up 15.2% of the global land surface, and the approximately 1.1 billion people who live there are among the world’s poorest. The inter-annual variability of rainfall in semi-arid regions is exceptionally high, due to intrinsic features of the global atmospheric circulation. The observed and projected climate trends for most semi-arid regions indicate warming at rates above the global mean rate over land, increasing evaporative demand, and reduced and more variable rainfall. Historically, the ecosystems and people coped with the challenges of semi-arid climates using a range of strategies that are now less viable. Semi-arid ecosystems are by definition water limited, generally only suitable for extensive pastoralism and opportunistic cropping, unless irrigation supplementation is available. The characteristics of dryland plant production in semi-arid ecosystems, as they interact with climate change and human systems, provide a conceptual framework for why land degradation is so conspicuous in semi-arid regions. The coupled social-ecological failures are contagious, both within the landscape and at regional and global scales. Thus, semi-arid lands are a likely flashpoint for Earth system changes in the 21st century.
Highlights
Regions with Semi-Arid Climates as a Distinct Global EntityThe Food and Agriculture Organisation Agro-Ecological Zone concept defines those parts of the world where the annual precipitation (P) sums to between one fifth and one half of the potential evapotranspiration (Ep ) as ‘semi-arid’ (i.e., 0.2 < P/Ep ≤ 0.5) [1]
This paper focuses on the ‘hot semi-arid regions’ (Mean Annual Temperature > 18 ◦ C, which serves as a separator between the neotropical drylands and the temperate to sub-arctic drylands)
The semi-arid interaction between the typically have a distinct drainage pattern, with soils arranged in a topographical sequence from feature ridge of temporal variability of the climate with the spatial variability of the landscape, is a central crest to valley floor, at a scale of a few kilometres
Summary
The Food and Agriculture Organisation Agro-Ecological Zone concept defines those parts of the world where the annual precipitation (P) sums to between one fifth and one half of the potential evapotranspiration (Ep ) as ‘semi-arid’ (i.e., 0.2 < P/Ep ≤ 0.5) [1]. The ecological landscape in semi-arid regions is often notably spatially patchy [8] This patchiness variability of rainfall, often linked to variable modes of the global climate system, such as the ENSO has many sources, operating at a variety of scales. The prominence of geomorphology in semi-arid lands is due to landscapes the absence of glaciation show little soil development and differentiation. The semi-arid interaction between the typically have a distinct drainage pattern, with soils arranged in a topographical sequence from feature ridge of temporal variability of the climate with the spatial variability of the landscape, is a central crest to valley floor, at a scale of a few kilometres While the people of semi-arid lands often have a strong identity and sense of place, their influence on larger-scale political processes is usually weak [18,19,20]
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