Dryland Expansion Research Articles

Desertification of Bengal Dryland Areas Possible under Projected Climate Conditions

Drylands are some of the most sensitive areas to climate change and human activities around the globe. Assessment of future climate trend scenarios provides valuable practical information for dryland management decision-making. According to Huang et al. (2017), more than 50% of global drylands will expand by this century, with a maximum (78%) of newly expanded dryland occurring in developing countries. To understand the potential for expansion of drylands and desertification, we examine critical predictor variables (temperature and precipitation) of Bengal dryland expansion to guide early actions to mitigate and prevent desertification. Using trend analysis of bias-corrected CMIP6 projected climate change data for temperature and precipitation (2022-2041), results indicate future dryland expansion is possible from increases in temperature and declines in monsoonal precipitation. Over the next two decades (2022-2041), Bengal dryland areas will be 0.1-0.5°C warmer, and rainfall will decrease by 2.57-13.43cm total during the monsoon period. Given these variables are critical predictors of dryland expansion because of their role in driving evapotranspiration and soil moisture deficits; we anticipate an increase in the population affected by water scarcity, land degradation, and desertification. Our work provides information critical for effective dryland management, biodiversity conservation, and land-use planning under future climate conditions.

Dryland self-expansion enabled by land-atmosphere feedbacks.

Dryland expansion causes widespread water scarcity and biodiversity loss. Although the drying influence of global warming is well established, the role of existing drylands in their own expansion is relatively unknown. In this work, by tracking the air flowing over drylands, we show that the warming and drying of that air contributes to dryland expansion in the downwind direction. As they dry, drylands contribute less moisture and more heat to downwind humid regions, reducing precipitation and increasing atmospheric water demand, which ultimately causes their aridification. In ~40% of the land area that recently transitioned from a humid region into a dryland, self-expansion accounted for >50% of the observed aridification. Our results corroborate the urgent need for climate change mitigation measures in drylands to decelerate their own expansion.

Spatio-Temporal Changes of Aridity in the Province of Naâma (Western Algeria)

Arid regions are characterized by the fragility of their ecosystems, which are highly vulnerable to climate change. The increase in aridity in these regions makes them more exposed to droughts. This article analyses the trend in aridity and the expansion of drylands in the Wilaya (province) of Naâma, which is an arid region in south-western Algeria, over the period of 1951 to 2020. Monthly rainfall data from the Global Precipitation Climatology Centre (GPCC), as well as monthly mean temperature and potential evapotranspiration (PET) data from the Climatic Research Unit (CRU), characterized by a spatial resolution in grid points of 0.5º, were used. The results showed an increase in annual aridity, leading to transformation of drylands in to arid lands. As a result of these changes an additional 10% area of semi-arid land to converted to arid zone

Interacting effects of surface water and temperature on wild and domestic large herbivore aggregations and contact rates

Abstract Earth's climate is rapidly changing, bringing forth questions of how domestic and wild animals will alter their behaviour in response to increasing temperatures and dryland expansion. Dwindling water availability will likely impact animal behaviour and water foraging, potentially increasing animal aggregations and interspecific contacts. These interspecific contacts are especially important for competition, predation and disease transmission among wildlife and domestic animals. In this study, we analysed interspecific wildlife and cattle contacts using two years of camera trap data at an experimental water manipulation site at a conservancy in central Kenya. We found that on average, the hourly probability of any interspecific contact was approximately 3.4 times higher at water sources versus drained water sources and 18 times higher than surrounding matrix areas, and that this relationship was amplified by dry and hot conditions. Species‐specific analyses revealed variation in the magnitude of responses across wildlife and domestic cattle, although all animals had approximately 2–3 times higher interspecific contact probability with other species at water in hot conditions versus other conditions. Notably, we observed the largest behavioural changes for relatively water‐independent species, such as giraffe, which had 3.6 times higher interspecific contact probability at water sources in hot versus other conditions. Synthesis and applications. These findings show how elevated temperatures that will become increasingly common with future climate changes can increase interspecific contacts around critical water resources. In mixed wildlife‐livestock systems, maintaining wildlife‐only water sources may be a practical management tool to mitigate human‐wildlife conflict and disease transmission at this interface, especially during dry and hot conditions.

Global dryland aridity changes indicated by atmospheric, hydrological, and vegetation observations at meteorological stations

Abstract. In the context of global warming, an increase in atmospheric aridity and global dryland expansion under the future climate has been expected in previous studies. However, this conflicts with observed greening over drylands and the insignificant increase in hydrological and ecological aridity from the ecohydrology perspective. Combining climatic, hydrological, and vegetation data, this study evaluated global dryland aridity changes at meteorological stations from 2003 to 2019. A decoupling between atmospheric, hydrological, and vegetation aridity was found. Atmospheric aridity represented by the vapor pressure deficit (VPD) increased, hydrological aridity indicated by machine-learning-based precipitation minus evapotranspiration (P − ET) data did not change significantly, and ecological aridity represented by the leaf area index (LAI) decreased. P − ET showed nonsignificant changes in most of the dominant combinations of the VPD, LAI, and P − ET. This study highlights the added value of using station-scale data to assess dryland change as a complement to results based on coarse-resolution reanalysis data and land surface models.

Impacts of Climate Change on Vegetation in Kenya: Future Projections and Implications for Protected Areas

Climate change will cause substantial vegetation shifts across the world. Africa may face varying dynamics such as tree decline, savannization, and woody encroachment due to rising temperatures and rainfall changes. This study examines the potential effects of climate change on Kenyan vegetation and vegetation shifts for 2050 and 2100, employing a statistical model to predict vegetation state as driven by environmental variables, including temperature, soil moisture, livestock density, and topography. We evaluate the model by hindcasting it from 2020 to 2000 and then project future vegetation states for 2050 and 2100 under SSP 2–4.5 and SSP 5–8.5. In response to moderate emissions, a notable increase in arid-associated shrubland vegetation (53–58%) is forecasted, leading to the expansion of drylands at the expense of savannas, grasslands, and forests. Under high-emission scenarios, savannas are forecasted to expand (52–65%) at the expense of forested areas. Overall, dense forest cover declines across scenarios, affecting protected areas by promoting increased savanna cover and reducing forest area (40% to 50%). These projected shifts in major vegetation types would likely alter ecosystem functioning and associated services, impacting pastoralists and wildlife and raising biodiversity concerns. Protected areas in Kenya could lose 50% of their forests, highlighting the urgency of climate change mitigation. These findings offer a crucial foundation for future research and action on Kenya’s vegetation.

Dryland Endolithic Chroococcidiopsis andTemperate Fresh Water Synechocystis Have Distinct Membrane Lipid andPhotosynthesis Acclimation Strategies upon Desiccation andTemperature Increase.

An effect of climate change is the expansion of drylands in temperate regions, predicted to affect microbial biodiversity. Since photosynthetic organisms are at the base of ecosystem's trophic networks, we compared an endolithic desiccation-tolerant Chroococcidiopsis cyanobacteria isolated from gypsum rocks in the Atacama Desert with a freshwater desiccation-sensitive Synechocystis. We sought whether some acclimation traits in response to desiccation and temperature variations were shared, to evaluate the potential of temperate species to possibly become resilient to future arid conditions. When temperature varies, Synechocystis tunes the acyl composition of its lipids, via a homeoviscous acclimation mechanism known to adjust membrane fluidity, whereas no such change occurs in Chroococcidiopsis. Vice versa, a combined study of photosynthesis and pigment content shows that Chroococcidiopsis remodels its photosynthesis components and keeps an optimal photosynthetic capacity at all temperatures, whereas Synechocystis is unable to such adjustment. Upon desiccation on a gypsum surface, Synechocystis is rapidly unable to revive, whereas Chroococcidiopsis is capable to recover after three weeks. Using X-ray diffraction, we found no evidence that Chroococcidiopsis could use water extracted from gypsum crystals in such conditions as a surrogate for missing water. The sulfolipid sulfoquinovosyldiacylglycerol becomes the prominent membrane lipid in both dehydrated cyanobacteria, highlighting an overlooked function for this lipid. Chroococcidiopsis keeps a minimal level of monogalactosyldiacylglycerol, which may be essential for the recovery process. Results support that two independent adaptation strategies have evolved in these species to cope with temperature and desiccation increase and suggest some possible scenarios for microbial biodiversity change triggered by climate change.

Past and future adverse response of terrestrial water storages to increased vegetation growth in drylands

The response of terrestrial water storages to dryland vegetation growth remains poorly understood. Using multiple proxies from satellite observations and model outputs, we show an overall increase (decrease) in vegetation growth (terrestrial water storages) across drylands globally during 1982–2016. Terrestrial water storages in greening drylands correlate negatively with vegetation growth, particularly for cropland-dominated regions, and such response is pronounced when the growth rate of vegetation productivity is high. Reduction in terrestrial water storage is dominated by precipitation and evapotranspiration variability rather by than runoff. We predict reduction in terrestrial water storage of 41–84% by 2100, accompanying expansion of drylands by 4.1–10.6%. Our findings, which indicate sustained adverse response of terrestrial water storage to vegetation growth in drylands, highlight the need for concerted planning for balanced ecological restoration, agricultural management, and water resource utilization that will affect 5.17 billion people, 64% of whom live in developing countries.

Using the dynamics of productivity and precipitation-use efficiency to detect state transitions in Eurasian grasslands

In the face of accelerated global dryland expansion and grassland degradation, signaling grassland ecosystem state transitions is an ongoing challenge in ecology. However, there is still a lack of effective indicators and understanding of the mechanisms of grassland ecosystem state transitions at the continental scale. Here, we propose a framework that links ecosystem function-based indicators and critical slowing down (CSD) theory to reveal grassland state transitions. Across precipitation gradients, we quantified the statistical characteristics and spatial patterns in ANPP and PUE dynamics (variability, asymmetry, and sensitivity to precipitation and temperature) in Eurasian grasslands. We show that the CVANPP, CVPUE, AANPP, APUE, SPUE-P, and SANPP-P of temperate steppes were significantly higher than those of alpine steppes, while the SPUE-T and SANPP-T were the inverse. In temperate grasslands, AANPP, APUE, and SANPP-P indicated the transition of typical steppes, and CVANPP, APUE, and SPUE-T indicated the transition from meadow to typical steppes. In alpine grasslands, APUE indicated the transition between alpine deserts and alpine steppes, and AANPP and SANPP-P indicated the transition between alpine steppes and meadow steppes. The interannual variability of precipitation strongly affected xerophyte proportion and demographic processes, which control state transitions in low-resilience grasslands. Community structures and limiting factors (nutrient, light, and/or temperature) regulate state transitions in high-resilience grasslands. Our results demonstrate that function-based indicators are predictive of impending state transitions of temperate and alpine grasslands, highlighting the complementation of ANPP and PUE dynamics that have the potential for predicting grassland ecosystem regime shifts and their underlying mechanisms.

Desertification dynamics in China's drylands under climate change

Much of drylands has been hit by land degradation during the recent decades. But whether and how desertification is related to climate change is poorly understood. Here, using the Normalized Difference Vegetation Index (NDVI) time series and climatic variables, we assessed the desertification dynamics over China's drylands from 1982 to 2016 and explored the climate change impacts, with a particular focus on the influence of the 2015/2016 El Niño event. We found a fluctuant downward trend of barren land area from 1982 to the early 2010s, but followed by a sharp increase afterwards. Decrease in precipitation combined with high temperature are major contributors to the barren land expansion. During the 2015/2016 El Niño, the concurrence of severe drought and heat stress gave rise to the most serious land desertification in the record. Hyper-arid and arid drylands are the predominant contributors to the abrupt barren land area increase during 2015/2016. Our findings, therefore, highlight the climate change impacts on dryland desertification processes. Future dryland expansion and accompanying drought stress may exacerbate the risk of land degradation in these regions.

Overestimated global dryland expansion with substantial increases in vegetation productivity under climate warming

Drylands are serviced as an essential component of the earth’s ecosystem. The potential changes in dryland areas are of great importance to the environment, but various debates remain as to whether and to what extent drylands are expected to expand. Here we employ a physically-based potential evapotranspiration (E P) model accounting for vegetation response to climate change to quantify potential changes in dryland areas, on the basis of a commonly used indicator, aridity index (multiyear mean E P over precipitation). Results show that by the end of this century, drylands will expand slightly by ∼5%, while vegetation productivity will increase by ∼50%. Elevated CO2 slows down the increase rate of E P that impedes the expansion of drylands, but greatly promotes vegetation growth with increases in both leaf assimilation and canopy foliage. These findings improve our understanding of the potential changes in dryland and their ecological impacts in a warmer climate.

Land‐Atmosphere Coupling Constrains Increases to Potential Evaporation in a Warming Climate: Implications at Local and Global Scales

AbstractThe magnitude and extent of runoff reduction, drought intensification, and dryland expansion under climate change are unclear and contentious. A primary reason is disagreement between global circulation models and current potential evaporation (PE) models for the upper limit of evaporation under warming climatic conditions. An emerging body of research suggests that current PE models including Penman‐Monteith and Priestley‐Taylor may overestimate future evaporation for non‐water‐stressed conditions. However, they are still widely used for climatic impact analysis although the underlying physical mechanisms for PE projections remain unclear. Here, we show that current PE models diverge from observed non‐water‐stressed evaporation across site (>1,500 flux tower site years), watershed (>10,000 watershed‐years), and global (25 climate models) scales. By not incorporating land‐atmosphere coupling processes, current models overestimate non‐water‐stressed evaporation and its driving factors for warmer and drier conditions. To resolve this, we introduce a land‐atmosphere coupled PE model by extending the Surface Flux Equilibrium theory. The proposed PE model accurately reproduces non‐water‐stressed evaporation across spatiotemporal scales. We find that terrestrial PE will increase at a similar rate to ocean evaporation but much slower than rates suggested by current PE models. This finding suggests that land‐atmosphere coupling moderates continental drying trends. Budyko‐based runoff projections incorporating our PE model are well aligned with those from coupled climate simulations, implying that land‐atmosphere coupling is key to improving predictions of climatic impacts on water resources. Our approach provides a simple and robust way to incorporate coupled land‐atmosphere processes into water management tools.

Desertification vulnerability under accelerated dryland expansion

AbstractGiven accelerated dryland expansion, studies on aridity‐driven desertification are of great importance for advancing our understanding of land degradation mechanisms and developing possible mitigation measures. However, the mechanism by which increased aridity drives ecosystems toward desertification and the impacts of aridity on desertification vulnerability both in space and over time remain unclear. Here, we collected eight key ecosystem structure/functioning data (including soil, vegetation, and animal) for threshold identification using piecewise linear models, and then linked them to biome shifts to explore the process of aridity‐driven desertification. The results show that ecosystems along aridity index (AI, 1 − precipitation/potential evapotranspiration) gradients experience three vulnerable stages featuring different threshold and biome responses: a low vulnerability stage (AI: 0.5–0.65; mainly featuring a soil fertility loss and a forest decline), a moderate vulnerability stage (AI: 0.65–0.75; mainly featuring accelerated non‐forest vegetation degradation) and a high vulnerability stage (AI: 0.75–1; mainly featuring rapid overall land degradation). According to further simulations, areas with low to high vulnerability account for 31% of the global terrestrial area, which is projected to increase by 18.8% (21.2%) by the end of the 21st century under the greenhouse gas emission scenario SSP2‐4.5 (SSP3‐7.0). Given increased vulnerability and population growth, Africa and East Asia are exposed to great desertification risk and ensuing contradiction between humans and land. These findings can provide a theoretical and practical reference for ecosystem management and policy development, which are critical for dryland ecological security and human welfare in areas threatened by desertification.

Greenhouse Gas Emissions Drive Global Dryland Expansion but Not Spatial Patterns of Change in Aridification

Abstract Drylands play an essential role in Earth’s environment and human systems. Although dryland expansion has been widely investigated in previous studies, there is a lack of quantitative evidence supporting human-induced changes in dryland extent. Here, using multiple observational datasets and model simulations from phase 6 of the Coupled Model Intercomparison Project, we employ both correlation-based and optimal fingerprinting approaches to conduct quantitative detection and attribution of dryland expansion. Our results show that spatial changes in atmospheric aridity (i.e., the aridity index defined by the ratio of precipitation to potential evapotranspiration) between the recent period 1990–2014 and the past period 1950–74 are unlikely to have been caused by greenhouse gas (GHG) emissions. However, it is very likely (at least 95% confidence level) that dryland expansion at the global scale was driven principally by GHG emissions. Over the period 1950–2014, global drylands expanded by 3.67% according to observations, and the dryland expansion attributed to GHG emissions is estimated as ∼4.5%. Drylands are projected to continue expanding, and their populations to increase until global warming reaches ∼3.5°C above preindustrial temperature under the middle- and high-emission scenarios. If warming exceeds ∼3.5°C, a reduction in population density would drive a decrease in dryland population. Our results for the first time provide quantitative evidence for the dominant effects of GHG emissions on global dryland expansion, which is helpful for anthropogenic climate change adaptation in drylands. Significance Statement In the past decades, global drylands have been reported to show changes in space and time, based on atmospheric aridity (i.e., aridity index defined by the ratio of precipitation to potential evapotranspiration). Using two detection and attribution methods, the spatial change patterns of atmospheric aridity between 1990–2014 and 1950–74 are unlikely to be driven by greenhouse gas (GHG) emissions, whereas the temporal expansion of global drylands (i.e., 3.67% from 1950 to 2014) is principally attributed to GHG emissions (contribution: ∼122%). Quantitative evidence from the detection and attribution analysis supports the dominant role of greenhouse gas emissions in global dryland expansion, which will increase the population suffering from water shortages under future warming unless climate adaptation is adopted.

How has aridity changed over West Africa in the past four decades?

Notable dryland expansion from changes in global aridity has been documented in previous studies, particularly with the increase in global mean temperature. Here, we examined the changes in the aridity of the western region of Africa over the past four decades. Using the aridity index (P/PET) as an indicator, the Climate Research Unit (CRU TS) data was used to identify drying and wetting areas of the region. The result showed that intense warming of 0.018 °C per year has occurred which was more evident in the last two decades. The warming in the past two decades caused a significant increase in PET. The first two decades were the driest compared to the last decades, particularly the first decade which also had major drought occurrences across the region. The aridity index increased by 0.86 (x10−3) per year within the past four decades, with the greatest change at the semi-arid zone. Result showed that precipitation exerts dominant control on aridity changes in the region despite a significant increase in PET. This was evident in the expansion of the wet areas between 2000 and 2019 compared to the 1979–1998 period.

Sinkhole Flooding and Aquifer Recharge in Arid to Dry Sub-Humid Regions: A Systematic Review in the Perspective of Climate Change

On the one hand, Sinkhole Flooding (SF) is an essential hydrological process to recharge karst aquifer in arid to dry sub-humid regions. On the other hand, the increase of rain extremes is one of the major consequences of global warming, together with the expansion of drylands. Thus, appropriate runoff regulation in endorheic karst basins in order to reduce the risk of flooding and improve the quantity and quality of the water drained by sinkholes will be more and more crucial. With these premises, a systematic review of SF cases study was performed by using Web of Science (WoS) engine to infer the hydrological properties for modeling the water management in regions actually or potentially affected by water scarcity. Hydrological models are essential to manage the consequences of climate change on karst water resource, however the review shows that providing the tools necessary for reliable modeling is still challenging. Finally, due to the intrinsic vulnerability of the karst aquifers, pollution reduction and wastewater recycling policy will play a key role in the next decades.

Soil Salinity and Sodicity in Drylands: A Review of Causes, Effects, Monitoring, and Restoration Measures

Soil salinization and sodification are common processes that particularly characterize drylands. These processes can be attributed either to natural conditions or anthropogenic activities. While natural causes include factors such as climate, lithology, topography, and pedology, human causes are mostly related to agricultural land-use, and specifically, to irrigated agriculture. The objective of this study was to thoroughly review this topic, while highlighting the major challenges and related opportunities. Over time, the extent of saline, sodic, and saline-sodic croplands has increased, resulting in accelerated land degradation and desertification, decreased agricultural productivity, and consequently jeopardizing environmental and food security. Mapping and monitoring saline soils is an important management tool, aimed at determining the extent and severity of salinization processes. Recent developments in advanced remote sensing methods have improved the efficacy of mapping and monitoring saline soils. Knowledge on prevention, mitigation, and recovery of soil salinity and sodicity has substantially grown over time. This knowledge includes advanced measures for salt flushing and leaching, water-saving irrigation technologies, precision fertilizer systems, chemical restoration, organic and microbial remediation, and phytoremediation of affected lands. Of a particular interest is the development of forestry-related means, with afforestation, reforestation, agroforestry, and silvopasture practices for the recovery of salt-affected soils. The forecasted expansion of drylands and aggravated drying of existing drylands due to climatic change emphasize the importance of this topic.

No projected global drylands expansion under greenhouse warming

Drylands, comprising land regions characterized by water-limited, sparse vegetation, have commonly been projected to expand globally under climate warming. Such projections, however, rely on an atmospheric proxy for drylands, the aridity index, which has recently been shown to yield qualitatively incorrect projections of various components of the terrestrial water cycle. Here, we use an alternative index of drylands, based directly on relevant ecohydrological variables, and compare projections of both indices in Coupled Model Intercomparison Project Phase 5 climate models as well as Dynamic Global Vegetation Models. The aridity index overestimates simulated ecohydrological index changes. This divergence reflects different index sensitivities to hydroclimate change and opposite responses to the physiological effect on vegetation of increasing atmospheric CO2. Atmospheric aridity is thus not an accurate proxy of the future extent of drylands. Despite greater uncertainties than in atmospheric projections, climate model ecohydrological projections indicate no global drylands expansion under greenhouse warming, contrary to previous claims based on atmospheric aridity. Model projections of future drylands distribution using a proxy based on atmospheric aridity show expansion under climate change, but may not be an accurate representation. An alternative index based on ecohydrological variables such as water limitation shows no global expansion of drylands.

Expansion of drylands in China with an additional half a degree warming

AbstractTo contribute a more ambitious target of Paris Agreement, that is, limiting global warming to 1.5°C instead of 2.0°C above preindustrial levels, it is essential to quantitatively reveal regional impacts of an additional 0.5°C warming to inform climate policymaking. Based on bias‐corrected simulations of four models from the half a degree additional warming, prognosis and projected impacts (HAPPI), we examine how dry–wet condition may change in China at 1.5 and 2.0°C warming levels, focusing on the projected changes in the area of drylands. Results showed that China is projected to be dominated by a decrease Surface Wetness Index (SWI), due to a larger increase in potential evapotranspiration (PET) than precipitation, implying a drying signal, in both 1.5 and 2.0°C warming levels against a present‐day scenario. Correspondingly, an expansion of drylands is expected, the larger expansion with the higher warming level, with changes approximately 0.77 ± 2.02% in 1.5°C level, and 2.29 ± 2.69% in 2.0°C level, although there exists considerable intermodel variability. For an additional warming of 0.5°C, there is high confidence of the projected decrease of SWI and expansion of drylands, approximately 1.52 ± 0.77%. That is to say, limiting global warming to 1.5°C is suggested to robustly reduce the likelihood of emerging aridification compared to 2°C, and China might obtain considerable benefits in term of socio‐economic impacts related to the reduction of aridification. Thus, it is necessary for continued efforts to limit global warming to 1.5°C.

Potential shifts in climate zones under a future global warming scenario using soil moisture classification

Climate zones fundamentally shape the patterns of the terrestrial environment and human habitation. How global warming alters their current distribution is an important question that has yet to be properly addressed. Using root-layer soil moisture as an indicator, this study investigates potential future changes in climate zones with the perturbed parameter ensemble of climate projections by the HadGEM3-GC3.05 model under the CMIP5 RCP8.5 scenario. The total area of global drylands (including arid, semiarid, and subhumid zones) can potentially expand by 10.5% (ensemble range is 0.6–19.0%) relative to the historical period of 1976–2005 by the end of the 21st century. This global rate of dryland expansion is smaller than the estimate using the ratio between annual precipitation total and potential evapotranspiration (19.2%, with an ensemble range of 6.7–33.1%). However, regional expansion rates over the mid-high latitudes can be much greater using soil moisture than using atmospheric indicators alone. This result is mainly because of frozen soil thawing and accelerated evapotranspiration with Arctic greening and polar warming, which can be detected in soil moisture but not from atmosphere-only indices. The areal expansion consists of 7.7% (–8.3 to 23.6%) semiarid zone growth and 9.5% (3.1–20.0%) subhumid growth at the expense of the 2.3% (–10.4 to 7.4%) and 12.6% (–29.5 to 2.0%) contraction of arid and humid zones. Climate risks appear in the peripheries of subtype zones across drylands. Potential alteration of the traditional humid zone, such as those in the mid-high latitudes and the Amazon region, highlights the accompanying vulnerability for local ecosystems.