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Sign up for our email newsletter! Only the cold deserts are bigger: icy Antarctica's frozen desert spans about 5. The study authors originally set out to examine seasonal cycles of temperature and rainfall across Africa, consulting data spanning to But their attention was quickly drawn to trends of decreasing precipitation in the Sahel, a semiarid region linking the Sahara to the savannas of Sudan. By looking at this more closely, they hoped to discover how rainfall trends might be linked to the Sahara's growth over time, according to Thomas.
To a certain extent, many deserts' boundaries expand and contract seasonally, as conditions fluctuate between wetter or drier. But the researchers found that there has been "a strong expansion" of the Sahara within the 20th century, Thomas said. Depending on the season, the Sahara experienced growth of at least 11 percent, and it grew by as much as 18 percent during the driest summer months, according to data collected over roughly years. Therefore, the climate index is used to investigate long-term trend and decadal variability of the areal extent and boundary changes over SD and ArcTG in this study.
The results from both climate index and vegetation index are cross-validated, and the possible causes for their difference are discussed. The statistics for their areal extents and changes are summarized in Table 1. However, this general expansion is not constant in time. The Sahel areas experienced a dramatic change from wet conditions in the s to much drier conditions in the s, then partially recovered after the s.
A climate regime shift has been identified during the s 15 , Different from previous SD studies, which only identify one trend for the entire study period, the year , is identified in this study as turning points according to Eq. The Sahara Desert SD extent and boundary change based on climate and vegetation indices.
Observed and simulated e time series and f trend of SD extent defined by climate and vegetation indices.
The error bars in f indicate one standard deviation due to the LAI based non-vegetation criterion range of 0. Figure including maps in a—d are created by NCL version 6. The simulated climate indices properly reproduce SD extent and its changes during — Table 1. Meanwhile, the models properly reproduce SD shrinking rate during — A multi-model experiment has demonstrated the land use and land cover change LULCC contribution to the drought during the s, which should cause land degradation This anthropogenic effect is missing in this CFS simulation, which may lead to underestimation of the SD expansion rate during — In the future projection, the Sahel temperature is projected to be about 1.
Despite the projected increase in precipitation in the midst century, the warming-induced high evaporation dominates and makes the area drier and yields an SD expansion.
The heat stress on Sahel ecosystem is well represented in KTC and has important implication for the future projection. Meanwhile, the projected heterogeneous precipitation anomaly distributions result in different desertification risks for various Sahelian countries. Different from previous similar studies, in this study, we have also used vegetation indices derived from observation and a coupled climate-ecosystem model to assess the SD extend and its change, which provides a more clear geographic definition and can be used to cross-validate the results from the climate index.
This ecosystem model has been extensively evaluated for its performance on north American and global ecosystem variability and trend 15 , We employ a range of 0. In the western Sahel, no significant change is projected during —, different from the projection based on climate index.
As such, no assessment can be made based on the vegetation index. Some discrepancies are likely due to errors in satellite-derived LAI and simulated climate and vegetation variables over the sparse vegetation area The accelerated warming rate in the polar regions and intensive interactions between climate and vegetation, snow, and glacier have led to remarkable land condition changes in the ArcTG area in past decades Lloyd et al.
The shrinking rate accelerates after the s. The shrinking is accompanied by boundary retreat all over around the Arctic Circle Fig. Figures including maps in a—d are created by NCL version 6. Observed and simulated ArcTG extent c time series and d trends based on climate and vegetation indices. Figures including maps in a and d are generated by NCL version 6. The models generally reproduce the coverage of ArcTG and its changes based on climate index during — Table 1.
The lack of black carbon deposition and greenhouse gas emission in CFS may contribute to the discrepancies. In the Arctic, human-induced black carbon on snow is reported to accelerate the warming effect by enhancing surface radiative forcing The lack of greenhouse gas emission due to enhanced soil carbon respiration may also contribute to an underestimation of atmospheric warming 3 , The enhanced soil carbon respiration come from thawed permafrost, where microbial decay is increasing respiration CO 2 and methane fluxes to the atmosphere.
This in turn amplifies the rate of atmospheric warming and further accelerate permafrost degradation, resulting a positive permafrost carbon feedback. Meanwhile, the warming temperature and elevated atmospheric CO 2 concentration cause an enrichment of shrubs and trees in the Arctic forest-tundra ecotone and produce positive feedbacks.
The observed vegetation index based on the products of CAVM treeline in the year of delineates the northernmost latitudes where tree species survive, which is defined as the geographical Arctic tundra and glacier southern boundary.
This is because the treeline dynamics are not only affected by the climate but also mediated by species-specific traits and environmental conditions such as permafrost thawing 34 , which deteriorates the local hydrological regime such as active layer depth and damages the root system that would prohibit tree establishment.
The treeline advance for the 20th century with various starting dates has been reported in a number of site measurements across the circumarctic forest-tundra ecotone 1 , 35 , 36 , indicating an Arctic shrinking in the past decades.
The species-specific traits and local environmental conditions may also contribute to the treeline advance. In fact, the site observations in the Canadian Shield did not find the treeline advance in 20 th century 1. In contrast, two sites in the Taymyr Peninsula, Siberia, had significant treeline advance 1. These site measurements seem to be consistent with our simulation. Further assessments with more data are needed to reduce uncertainty. In this paper, we assess landform changes in the Sahara-Desert and the Arctic tundra-glacier regions during —, according to both climate index with both precipitation and temperature and vegetation index.
In previous studies, only precipitation or NDVI was used to make an assessment in separate studies. Extensive evaporation caused by warming temperature has contributed to the SD expansion in the future scenario. The shrinking is accompanied by boundary retreat across the circumarctic.
CFS models tend to underestimate the ArcTG shrinking rate, mainly caused by missing anthropogenic process such as black carbon in snow. The CFS simulation without dynamic vegetation substantially underestimates the shrinking rate, suggesting the two-way vegetation-climate interaction produces positive feedback and enhances the ArcTG shrinking.
The discrepancies between the climate and vegetation indices reveal that the geographic changes are not only determined by the climate, but also affected by species-specific traits and local environmental conditions.
The land condition in these two regions have shown to have a substantial impact on climate, weather and ecosystems at continental and even, probably, global scales. The area with annual mean leaf area index LAI less than a threshold 0. A range from 0. The treeline is defined as the edge of the habitat where tree species can grow, and thus it is regarded as tree fractional coverage equal to zero to its north.
Treeline near the Arctic area is used to define the boundary of the geographic Arctic tundra-glacier area. The KTC defines five temperature base groups tropical, subtropical, temperate, boreal, and polar climates and one precipitation base group. Under these temperature and precipitation conditions Eq. It is significantly higher than the mm isohyet, which is widely used to define the SD boundary 6 , 7. Therefore, Eq.
About and stations around the southern and northern boundaries of SD, respectively, had contributed to CRUTS precipitation assimilation at the beginning of the study period This data set is only available for the year The Mediterranean region would experience a drastic transformation with warming of 2 degrees Celsius, according to one study , with all of southern Spain becoming desert.
Another recent study found that the same level of warming would result in "aridification," or drying out, of up to 30 percent of Earth's land surface.
When land becomes desert, its ability to support surrounding populations of people and animals declines sharply. Food often doesn't grow, water can't be collected, and habitats shift. This often produces several human health problems that range from malnutrition, respiratory disease caused by dusty air, and other diseases stemming from a lack of clean water.
In , the United Nations established the Convention to Combat Desertification UNCCD , through which countries have committed to Land Degradation Neutrality targets, similar to the way countries in the climate Paris Agreement have agreed to targets for reducing carbon pollution. These efforts involve working with farmers to safeguard arable land, repairing degraded land, and managing water supplies more effectively.
A similar effort is underway in northern China , with the government planting trees along the border of the Gobi desert to prevent it from expanding as farming, livestock grazing , and urbanization , along with climate change, removed buffering vegetation. However, the results for these types of restoration efforts so far have been mixed.
One type of mesquite tree planted in East Africa to buffer against desertification has proved to be invasive and problematic. The Great Green Wall initiative in Africa has evolved away from the idea of simply planting trees and toward the idea of " re-greening ," or supporting small farmers in managing land to maximize water harvesting via stone barriers that decrease water runoff, for example and nurture natural regrowth of trees and vegetation. All rights reserved. Where is desertification happening, and why?
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