To calculate Desertification Risk, we use the variables [Copernicus Climate Data Store (CDS)]:
- Standardized precipitation index for 6 months cumulation period (SPI6)
- Warm spell duration index (WSDI)
Desertification is linked to a land degradation process, caused by two main factors: water scarcity and excessive heat stress. Therefore, by combining the two variables, we can estimate Desertification Risk.
To define risk levels, we consider the absolute value of the SPI6 index (annual average) and the return period RP (see Cold Wave Risk) for the WSDI index.
Our methodology for assessing Desertification Risk is conceptually very advanced because it is based on the principle of compound risks, where the combined impact of two or more extreme events is greater than the sum of their parts.
Specifically, combining water scarcity (drought) with persistent extreme heat (heat waves) is exactly what modern science indicates as the key driver of land degradation and desertification.
- United Nations Convention to Combat Desertification (UNCCD)
The main global authority in this matter is the United Nations Convention to Combat Desertification (UNCCD). Our methodology reflects their definition.
Article 1 of the convention defines desertification as 'land degradation in arid, semi-arid and dry sub-humid areas, attributable to various causes, including climatic variations and human activities'. Subsequent science, particularly IPCC reports, has clarified that the most impactful 'climatic variations' are precisely the intensification of drought and heat waves. Our approach, therefore, quantifies the two main climatic drivers of land degradation.
- Special Report on Climate Change, Desertification, Land Degradation, Sustainable Land Management, Food Security, and Greenhouse gas fluxes in Terrestrial Ecosystems (SRCCL)
The IPCC special report on land and climate change is the most authoritative source explaining why the combination of drought and heat is so destructive for terrestrial ecosystems.
Chapter 3 ('Desertification') and Chapter 4 ('Land Degradation') of this report explain in detail the biophysical mechanisms. They highlight how:
- Water scarcity (drought) closes plant stomata, reducing growth.
- Extreme heat exponentially increases evapotranspiration, drying soil much faster and causing direct thermal stress to vegetation.
The combination of these two factors (a 'hotter drought') leads to vegetation collapse and soil exposure to erosion much faster than either factor alone would.
Our method, which creates increasingly severe risk categories when low SPI and high WSDI occur simultaneously, is a perfect operational translation of these scientific conclusions.
- Future climate risk from compound events
This article is a fundamental review that defines the field of 'compound events' study. It explains why analyzing extremes in isolation (only drought or only heat waves) seriously underestimates actual risk. The article explicitly states that the combination of drought and heat waves is one of the most damaging compound events for agriculture and ecosystems.
Finally, the sources for individual indicators composing our method are:
- The Relationship of Drought Frequency and Duration to Time Scales
- Standardized Precipitation Index User Guide
In conclusion, our methodology combines the official definition of desertification (UNCCD) with the latest discoveries on impacts of compound climate events (IPCC, Zscheischler et al.), using globally standardized indicators (SPI and WSDI).