Dear Colleagues,

The United Nations has claimed that up to 70% of drylands suffer from desertification, therefore it is considered one of the main global environmental issues. In response to the question of how to tackle desertification the international community adopted the United Nations Convention to Combat Desertification (UNCCD) in 1994. Despite that 197 countries have ratified the UNCCD, little progress has been made to solve the problem. One constraining issue is the lack of scientifically robust methods for monitoring and assessing desertification. In this regard, the Millennium Ecosystem Assessment (2005) highlighted the lack of sufficient monitoring and assessment of desertification and land degradation and stated that “without a scientifically robust and consistent baseline of desertification, identifying priorities and monitoring the consequences of actions are seriously constrained”.

Remote sensing had been widely used and gradually became the foremost means to monitor and assess desertification due to its extensive spatial coverage, periodicity, systematic data acquisition at ever finer spatial, temporal and spectral resolutions. However, several challenges persist:

• Quantifying desertification impacts requires an established benchmark against which changes in the ecosystems can be assessed. The lack of reference situations or ecosystems in a pristine state against which actual desertification could be evaluated is one of the challenges for properly assessing desertification.

• The assessment and monitoring of desertification should ideally be based on the identification of appropriate indicators. Definitions of desertification tend to be holistics and, therefore, there is no agreement about the definition and application of indicators. As a consequence, candidate indicators cannot be easily translated into the electro-magnetic signals captured by space- and airborne sensors.  Methods of analysis may lead to contradictory answers in terms of which areas are degrading or improving. P Measurements of the electromagnetic signals associated with spatio-temporal changes in the structure and functioning of ecosystems would provide a robust solution, but require building consensus around a different definition of desertification.  
• Our knowledge on the relative contribution of human vs environmental causes of desertification is still far from complete. Long term studies spanning over different ecosystems would certainly contribute to disentangle the impacts of climate change from human activities such as agriculture, grazing and wood collection.

• Desertification is often interpreted as a process of persistent reduction or loss of biological productivity. This suggests the occurrence of critical thresholds that, once crossed, imply a significant reduction in the probability of recovery without management inputs. Characterizing these critical thresholds and the development of early warning indicators is key to improve desertification monitoring and decision making.

This Special Issue, "Remote Sensing of Desertification”, calls for papers representing advances in the application of remote sensing to overcome these important gaps in the monitoring and assessment of desertification. Therefore, studies combining remote sensing observations with field-based data over large spatial and temporal scales are among our priorities. Additionally, we envisage this special issue as an outlet to leverage aerial photography, UAVs, LiDAR, microwave, hyperspectral or multi-spectral satellite data to advance desertification science.

Prof. Massimo Menenti
Dr. Juan José Gaitan
Dr. Santiago R. Verón
Guest Editors

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• desertification
• remote sensing
• drylands
• monitoring
• assessment