Groundwater Depletion, Land Subsidence, and Salinization.

 Groundwater now supplies about 50% of domestic water use and over 40% of irrigation water worldwide, making many urban areas, food systems, and farming communities heavily dependent on aquifers that are being depleted faster than they can recharge17. Groundwater depletion continues accelerating, and the planet’s groundwater storage is declining. About 70% of the world’s major aquifers exhibit long-term declining trends, many of them effectively irreversible on human time scales due to compaction and loss of aquifer storage capacity9,18–21. In parallel, groundwater quality is being degraded by salinity, nitrate and pesticide contamination from agriculture, industrial and mining pollution, and naturally occurring arsenic and fluoride mobilized by deeper pumping, rendering parts of some aquifers physically present but economically and ecologically unusable. The consequences of unsustainable groundwater exploitation are already visible on the land surface not only through the increasing appearance of sinkholes. Globally, more than 6.3 million square kilometers, nearly 5% of the global land area, including 231,000 square kilometers of urban and densely populated areas housing nearly 2 billion people, almost 25% of the global population, are experiencing significant subsidence rates linked to excessive groundwater extraction. In some locations, subsidence rates reach 25 centimeters per year, damaging infrastructure, increasing flood risk, and further compromising deltas and coastal cities. Where aquifers are overdrawn in coastal zones, saltwater intrusion can render groundwater unusable for generations, if not permanently. Land and soil degradation further amplify these hydrological stresses. Over 50% of global agricultural land is already moderately or severely degraded, undermining soil moisture retention and accelerating the transition of drylands toward desertification. Global salinization alone has degraded about 82 million hectares of rainfed cropland and 24 million hectares of irrigated cropland (together bigger than the total land area of France and Spain combined), undermining soil fertility, contaminating local groundwater and surface water with salts, reducing yields in some of the world’s key breadbaskets, and directly threatening food security, health, and livelihoods at local, regional, and global levels.

Regional trends in water storage in the twenty-first century. The map shows how Terrestrial Water Storage (TWS) has changed over time in different parts of the world, based on satellite observations from the Gravity Recovery and Climate Experiment (GRACE) and its Follow-On mission (GRACE-FO). The observed trends reveal significant changes in the total amount of water stored on land, including groundwater, soil moisture,rivers and lakes, snow, and ice across the globe. Yellow, orange, and red areas are suffering from water depletion, i.e., negative TWS changes.

Degraded soils and vegetation not only reduce the capacity of landscapes to retain water during dry periods; they also alter the way they respond to rainfall. When soils are compacted, eroded, or crusted, their infiltration capacity can decline by up to 90%, so that even moderate storms generate rapid runoff and flash flooding, carrying sediments and pollutants downstream while root zones remain too dry to sustain crops between events. In many drylands and semi-arid regions, this “drought–flood” paradox— crop failure during prolonged dry spells followed by destructive flash floods on desiccated, degraded land—has become a defining characteristic of the Anthropocene’s water reality, rather than an exception. These land–water feedbacks also reinforce biodiversity loss and desertification. As vegetation cover is reduced and soils lose organic matter, habitats for soil biota and many plant and animal species are degraded or lost, further weakening ecosystem functions such as infiltration, evapotranspiration, and microclimate regulation mechanisms. In many dryland regions, this has contributed to the expansion of desertified areas, increased the frequency and intensity of sand and dust storms, and led to the loss of pastoral and agropastoral livelihoods that depended on more resilient rangeland systems.

Reported land subsidence rates and drivers across the globe. (A) Global map of main land subsidence drivers (colors) with mean (circles) and maximum (triangles) rates (shape sizes). (B) The primary causes of land subsidence rates at each location. Image source: Huning et al. (2024), Reviews of Geophysics