Accelerated dryland expansion under climate change

  • Huang J
  • Yu H
  • Guan X
 et al. 
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Drylands are home to more than 38% of the total global population and are one of the most sensitive areas to climate change and human activities 1,2 . Projecting the areal change in drylands is essential for taking early action to prevent the aggravation of global desertification 3,4 . However, dryland expansion has been underestimated in the Fifth Coupled Model Intercomparison Project (CMIP5) simulations 5 considering the past 58 years (1948–2005). Here, using historical data to bias-correct CMIP5 projections, we show an increase in dryland expansion rate resulting in the drylands covering half of the global land surface by the end of this century. Dryland area, projected under representative concentration pathways (RCPs) RCP8.5 and RCP4.5, will increase by 23% and 11%, respectively, relative to 1961–1990 baseline, equalling 56% and 50%, respectively, of total land surface. Such an expansion of drylands would lead to reduced carbon sequestration and enhanced regional warming 6,7 , resulting in warming trends over the present drylands that are double those over humid regions. The increasing aridity, enhanced warming and rapidly growing human population will exacerbate the risk of land degradation and desertification in the near future in the drylands of developing countries, where 78% of dryland expansion and 50% of the population growth will occur under RCP8.5. Drylands are defined as regions where precipitation is counter-balanced by evaporation from surfaces and transpiration by plants (evapotranspiration) 3 . Because most dryland soil is relatively in-fertile and the vegetation cover is sparse, dryland ecosystems are substantially more fragile 1 . Desertification and degradation are per-vasive in drylands owing to global warming and the effects of rapid economic development, population growth and urbanization 8 . There are also some studies indicating that the increasing hydro-climatic intensity will become a predominant signature of twenty-first-century warming, which leads to shorter, less frequent, and less widespread precipitation events and an increase in the length of dry spells 9 . These trends may induce the expansion of drylands and further increase the fraction of the population that is affected by water scarcity and land degradation 1,4 . Knowledge of how climate change will affect the extent of drylands in the future is essential for their protection and for adaptation strategies 10 . The CMIP5 has generated projections using several emissions scenarios 11 and has provided a crucial reference for maintaining drylands as renewable resources. This study verifies CMIP5 simulations and bias-corrects the projections using historical observational data to provide a clear understanding of the spatial and temporal evolution of drylands in the future. The results may motivate decision makers to respond early and effectively to mitigate the pending global desertification. The aridity of a region is generally measured by the aridity index (AI), which is the ratio of total annual precipitation to potential evapotranspiration (PET). Under this quantitative indicator, drylands are defined as regions with AI < 0.65 and are further divided into subtypes of hyper-arid (AI < 0.05), arid (0.05 ≤ AI < 0.2), semiarid (0.2 ≤ AI < 0.5) and dry subhumid (0.5 ≤ AI < 0.65) regions 3 . The observational data used here are from the Climate Prediction Center (CPC; refs 12,13). The simulation data are from 20 global climate models of CMIP5 (ref. 11; Methods). As the ensemble mean of these CMIP5 models (CMIP5-EM) can filter the uncertainty from inter-model variability and is the best representation of the response to imposed external forcing, it is better at predictions than any individual member

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  • Jianping Huang

  • Haipeng Yu

  • Xiaodan Guan

  • Guoyin Wang

  • Ruixia Guo

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