Desertification

IPCC SPECIAL REPORT ON CLIMATE CHANGE AND LAND (SRCCL) Chapter 3: Climate Change and Land: An IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystem

Climate, Not Conflict, Explains Extreme Middle East Dust Storm

The recent dust storm in the Middle East (September 2015) was publicized in the media as a sign of an impending ‘Dust Bowl.’ Its severity, demonstrated by extreme aerosol optical depth in the atmosphere in the 99th percentile compared to historical data, was attributed to the ongoing regional conflict. However, surface meteorological and remote sensing data, as well as regional climate model simulations, support an alternative hypothesis: the historically unprecedented aridity played a more prominent role, as evidenced by unusual climatic and meteorological conditions prior to and during the storm. Remotely sensed normalized difference vegetation index demonstrates that vegetation cover was high in 2015 relative to the prior drought and conflict periods, suggesting that agricultural activity was not diminished during that year, thus negating the media narrative. Instead, meteorological simulations using the Weather Research and Forecasting (WRF) model show that the storm was associated with a cyclone and ‘Shamal’ winds, typical for dust storm generation in this region, that were immediately followed by an unusual wind reversal at low levels that spread dust west to the Mediterranean Coast. These unusual meteorological conditions were aided by a significant reduction in the critical shear stress due to extreme dry and hot conditions, thereby enhancing dust availability for erosion during this storm. Concluding, unusual aridity, combined with unique synoptic weather patterns, enhanced dust emission and westward long-range transport across the region, thus generating the extreme storm

Modeling the Effect of Urbanization on Climate and Dust Generation Over Desert Cities

abstract: Understanding and predicting climate changes at the urban scale have been an important yet challenging problem in environmental engineering. The lack of reliable long-term observations at the urban scale makes it difficult to even assess past climate changes. Numerical modeling plays an important role in filling the gap of observation and predicting future changes. Numerical studies on the climatic effect of desert urbanization have focused on basic meteorological fields such as temperature and wind. For desert cities, urban expansion can lead to substantial changes in the local production of wind-blown dust, which have implications for air quality and public health. This study expands the existing framework of numerical simulation for desert urbanization to include the computation of dust generation related to urban land-use changes. This is accomplished by connecting a suite of numerical models, including a meso-scale meteorological model, a land-surface model, an urban canopy model, and a turbulence model, to produce the key parameters that control the surface fluxes of wind-blown dust. Those models generate the near-surface turbulence intensity, soil moisture, and land-surface properties, which are used to determine the dust fluxes from a set of laboratory-based empirical formulas. This framework is applied to a series of simulations for the desert city of Erbil across a period of rapid urbanization. The changes in surface dust fluxes associated with urbanization are quantified. An analysis of the model output further reveals the dependence of surface dust fluxes on local meteorological conditions. Future applications of the models to environmental prediction are discussed.Dissertation/ThesisDoctoral Dissertation Mechanical Engineering 201

Dust emission from croplands in the Free State, South Africa

The global dust load showed a large increase during the last century due to climate change and the expansion of vulnerable land, both of which are caused by human modifications. The increase in vulnerable land, both in size as in intensity, is mainly attributed to the increase in agricultural areas and agricultural intensification. Dust emission has both an onsite effect due to the degradation of the emitting area, and an offsite effect on human health and climate. The degradation of land is especially relevant for agricultural areas where crop yield can strongly diminish due to the depletion of clay, silt, and nutrients from soils. The semi-arid Free State province has been identified as the largest emitter of dust in South Africa, which is caused by the large-scale agriculture, climate, and soil type. Dust storms have the potential to reach the densely populated Gauteng province causing negative consequences on human health and well-being. Dust events in the Free State show a strong seasonality that is attributed to the agricultural practices that leave soils bare and vulnerable to erosion after harvesting. However, the large differences in dust events per year indicate that additional surface characteristics control the emissions from these harvested croplands. One of the primary potential controls that farmers have on the emissivity of the land is the management of soil crusts, but, the role of soil crusts on sandy soils is often not considered. Therefore, this thesis will address the possible role of soil crusts on the dust emission from the Free State croplands. This thesis examines the formation of crusts by rainfall, the dust emissions from cropland soils, and the surface characteristics that control this erosion. The main instrument used to measure the erodibility of a surface is the Portable In-Situ Wind Erosion Laboratory (PI-SWERL). The comparability of this portable instrument was assessed by a cross-comparison with a traditional straight-line wind tunnel. The threshold friction velocity of sandy surfaces was similar for the two instruments, whereas the threshold friction velocity of loamy sand indicated that the PI-SWERL is a more precise instrument that is capable of detecting the initial, small PM10 emissions from a surface. To determine the potential for crusts to form on the sandy cropland soils, rainfall experiments combined with shear strength measurements were performed on Free State soils. The results showed that significant crusts develop within 15 mm of rainfall, and shear strengths similar to those measured in the laboratory were observed in the field. PI-SWERL measurements showed that these experimental crusts can limit the PM10 emission flux from 10.53 and 3.87 mg m-2 s-1 Luvisol and Arenosol soils, respectively, to below 0.03 mg m-2 s-1 for both. The addition of abraders increased the emission from a crust to 0.43 and 0.31 mg m-2 s-1 for Luvisol and Arenosol, respectively. The strong effect of crusts on emissions have been compared to field measurements, which showed a similar potential to minimize dust emissions, but also are complex interaction on the surface that defines this influence. The average emission of crusted surfaces was 0.476 mg m-2 s-1 (standard II deviation = 0.348, min = 0.004, max = 1.401 mg m-2 s-1) at a friction velocity of 0.59 m s-1, whereby the presence of abraders showed a power relationship to the emission from these surfaces. The emission from loose surfaces ranged between 1.646 mg m-2 s-1 (standard deviation 0.980, min = 0.291, max = 5.974 mg m-2 s-1), with a linear relationship between the emission and the clay and silt content. The initial sensitivity of an agricultural field to wind erosion needs to be considered when assessing the surface conditions under which crusts could play a minimizing role in the emission of dust. This is controlled by the soil cover, such as vegetation or degrading crop straw and stubble that is left after harvesting. Four fields, with a range of soil cover and crust characteristics, have been measured in the field. The soil cover has been quantified using Unmanned Aerial Vehicle (UAV) image analyses. The erodibility of the soils was characterised by the horizontal sediment flux and the saltation threshold. The soil cover differed from 11% for a harvested groundnut to 66% for a harvested maize field, the latter being the most common crop type in the Free State. This data shows the high initial importance of soil cover on the wind erosion from a field, whereby the sediment flux from the maize field was 11 and 187 times lower than that of fallow and groundnut fields. Considering the relatively high sediment flux from the fallow and low soil cover fields, crust and abrasion management should be considered on such surfaces. Some surfaces showed a depletion of clay and silt, which is evidence of land degradation caused by dust emission. This depletion could eventually lead to a lower yield and the need for more fertilizers. Furthermore, an enrichment in certain allergens and pathogens has been found in the suspended dust from Free State croplands. This shows the relevance of minimizing dust, both for the emitting region and the offsite areas that dust eventually reaches. Future studies should investigate the relationship between the sediment flux and the PM10 flux since this relationship is not known and could differ per field and soil type. Furthermore, the influence of roughness needs to be assessed because roughness is generally known to decrease the emission from a surface. However, to create roughness, it is required to disturb existing crusts, making it uncertain if such activities could increase or decrease overall emissions. Lastly, the implications raised by this thesis are not only relevant for Free State but can also be considered for other cropland areas with strong seasonality in cover and moisture

Assessing Minimal-Input Restoration Strategies for Desert Soil and Vegetation Restoration

The Mojave and Sonoran Deserts have been negatively impacted by anthropogenic disturbances. Considering that these ecosystems may recover on millennial timescales, research has shown that restoration techniques can be fairly successful in initiating long-term recovery processes in these sensitive environments. However, uncertainty remains as to which techniques are effective in different circumstances, such as in different climates or across different soil properties, and which techniques may best avoid unintended consequences, such as facilitating non-native plants. To reduce fugitive dust as a human health hazard, increase soil stability, and enhance wildlife habitat, further work is necessary to develop restoration techniques for disturbed desert landscapes. The aims of this thesis were to examine the impacts of severe disturbances on soils of the Mojave and Sonoran Deserts and to investigate the efficacy of target restoration techniques within these ecoregions. Studies were conducted in the field, laboratory, and greenhouse to determine how anthropogenic disturbances impact soil characteristics and test the effectiveness of the three implemented restoration techniques. The target restoration techniques chosen for this study span varying levels of effort and financial cost to better understand how effective minimal-input restoration strategies are in contrast to costlier, more intensive strategies. The minimal-input techniques examined here included vertical mulch (placing dead branches upright in the soil to simulate the appearance of dead shrubs), soil surface manipulations (such as surface de-compaction and contouring the soil to create water catchments), outplanting, and seeding with litter. My research analyzes the effectiveness of vertical mulch treatments, surface de-compaction, and seeding with litter in the Dead Mountains Wilderness Area located 18 km northwest of Needles, CA in the Mojave Desert. I analyzed the influence of vertical mulch, water catchments, and outplanting in four distinct study sites south of Joshua Tree National Park along the Devers Palo II Transmission Corridor from Indio, CA to Blythe, CA. I conducted laboratory analyses of soil conditions at each of the sites. Before establishing restoration treatments in both regions, soil conditions were characterized by a lack of natural recovery of native perennial vegetation, and lower vegetation cover in disturbed sites in comparison to undisturbed sites. Among the treatments at the Dead Mountains site, vertical mulch yielded the highest plant cover, soil moisture, soil stability, and lowest compaction in the Dead Mountains sites. During the wetter year of the survey, the surface de-compaction treatment had similar, less apparent results, indicating that surface de-compaction may be an alternative to vertical mulch if managers do not require vertical mulch structures to prevent public use of disturbed areas. These trends were not mirrored in the Devers Palo II Transmission Line sites, which had highly variable data, potentially due to the soil characteristics of each of the four sites. Each site had distinct bulk density, soil texture, pH, electrical conductivity, and C/N ratios that may have caused variability in the soil and plant responses to restoration treatments. The sites with the highest clay, silt, and organic matter had the highest plant cover and soil moisture whereas the site with the most mobile, well-drained soils had the lowest. Soil accumulation was highest in the vertical mulch treatments among all sites. Outplanting was largely unsuccessful due to the seedlings dying within four months of planting but may have had legacy effects, such as de-compacting the soil, inputting nutrients, and forming vertical mulch. These findings suggest that soil conditions may have been a stronger driver of soil and vegetation variation than restoration treatments. The collected data suggest that the effects of vertical mulch surpass visual effects to include ecological ones. Vertical mulch and, to a lesser degree, soil de-compaction are a viable restoration treatments to reduce soil erosion and increase plant cover. However, the degree of restoration success depends upon soil conditions, indicating that a contextual understanding of study sites is necessary for overall success. This thesis can help inform restoration activities within arid lands, which are increasingly threatened by human-induced disturbances

Global assessment of sand and dust storms

The specific objectives of the assessment are to: 1) Synthesise and highlight the environmental and socio-economic causes and impacts of SDS, as well as available technical measures for their mitigation, at the local, regional and global levels; 2) Show how the mitigation of SDS can yield multiple sustainable development benefits; 3) Synthesize information on current policy responses for mitigating SDS and 4) Present options for an improved strategy for mitigating SDS at the local, regional and global levels, building on existing institutions and agreements

Quantifying the Impact of Dust Sources on Urban Physical Growth and Vegetation Status: A Case Study of Saudi Arabia

Recently, dust has created many problems, including negative effects on health, and environmental and economic costs, for people who live both near to and far from sources of dust. The aim of this study is to evaluate and quantify the impact of dust sources located inside Saudi Arabia on the physical growth and vegetation status of cities. In order to do so, satellite data sets, simulated surface data, and soil data for Saudi Arabia from 2000 to 2021 were used. In the first step, a dust sources map of the study area was generated using multi-criteria decision analysis. Land surface temperature (LST), vegetation cover, soil moisture, precipitation, air humidity, wind speed, and soil erodibility factors were considered as effective criteria in identifying dust sources. In the second step, built-up land and vegetation status maps of major cities located at different distances from dust sources were generated for different years based on spectral indicators. Then, the spatiaotemporal change of built-up land and vegetation status of the study area and major cities were extracted. Finally, impacts of major dust sources on urban physical growth and vegetation were quantified. The importance degrees of soil erodibility, wind speed, soil moisture, vegetation cover, LST, air humidity, and precipitation to identify dust sources were 0.22, 0.20, 0.16, 0.15, 0.14, 0.07, and 0.05, respectively. Thirteen major dust sources (with at least 8 years of repetition) were identified in the study area based on the overlap of the effective criteria. The identified major dust sources had about 300 days with Aerosol Optical Depth (AOD) values greater than 0.85, which indicates that these dust sources are active. The location of the nine major dust sources identified in this study corresponds to the location of the dust sources identified in previous studies. The physical growth rates of cities located 400 km from a major dust source (DMDS) are 46.2% and 95.4%, respectively. The reduction rates of average annual normalized difference vegetation index (NDVI) in these sub-regions are 0.006 and 0.002, respectively. The reduction rate of the intensity of vegetation cover in the sub-region close to dust sources is three times higher than that of the sub-region farther from dust sources. The coefficients of determination (R2) between the DMDS and urban growth rate and the NDVI change rate are 0.52 and 0.73, respectively, which indicates that dust sources have a significant impact on the physical growth of cities and their vegetation status.Institutional Fund ProjectsPeer Reviewe