Classifying mega-droughts according to population, land-use, and economic exposure

In recent years, extreme droughts – both on spatial extent and duration – have become more frequent and tend to occur in all continents. Some examples are the consecutive droughts in California in the 2010s and the 2020s, the long-lasting drought in the La Plata Basin in South America in the 2020s, and the pan-European summer drought of 2022. Such droughts, often named mega-droughts, also have multiple impacts over a wide set of sectors, including health, agriculture, forestry, ecosystems, and infrastructures, to the point that they may cause permanent land degradation and disruption of life-needed facilities, as the provision of clean potable water.

In the first part of the study, we collected and ranked the largest drought events from 1901 to 2024, dividing them by country and macro-region, and using a scoring system that accounts for drought extent, severity, intensity, peak, extreme conditions, and other 15 parameters. To do that, we used a combination of climate datasets (ERA5, GPCC, CRU, and Berkeley Earth) and two meteorological drought indicators (the SPI and the SPEI) at multiple accumulation scales. In this presentation, we present the classification system (5-class plus a 0-100 score) and we detail on some specific events.

In the second part, we assigned, to each single event part of our new database, a set of 25 additional parameters regarding the exposure of population, land-use (forests, croplands, pastures, and urban areas), and macro-economic indexes (GDP, GDP per capita, GDP-PPP). We do that to answer a key question: are the drought usually considered the biggest in terms of hazard also the biggest considering the exposure of specific categories? We therefore present another classification scheme to incorporate population, land-use, and GDP exposure to drought events, showing that sometimes the drought hazard is not so effective in capturing how drought affects critical parts of the Earth.

We present new lists of mega-droughts divided by hazard, by exposure of single classes, and eventually all grouped under a single score that accounts for both hazard, and exposure. Regarding population, we also included a special section focusing on the exposure of segments at high risk, the young people (below 5 years old) and the old people (above 69 years old).