The significant depletion of Lake Erie’s water volume in 2015, amounting to 290 cubic miles (1,200 cubic km), or 250% of its capacity, serves as a stark illustration of a larger global trend.
While some freshwater loss in lakes, rivers, and aquifers was anticipated between 2014 and 2016 due to an El Niño event and its associated increased evaporation and reduced precipitation, the concerning aspect, as detailed in *Surveys in Geophysics*, is the persistent lack of replenishment eight years later, despite subsequent cooling periods.
This near-decade of dwindling freshwater resources coincides with the nine warmest years on record, strongly suggesting a causal link to climate change.
“The striking series of high-temperature years since 2015,” notes Matthew Rodell, a NASA hydrologist and lead author of the study, “suggests a strong correlation with the observed terrestrial water storage decline. This is definitely a cause for concern.”
The extensive data documenting this widespread drying were collected by twin GRACE (Gravity Recovery and Climate Experiment) satellite pairs. A joint NASA, German Research Centre for Geosciences (GFZ), and French Centre National d’Études Spatiales (CNES) project, the first GRACE mission operated from 2002 to 2017. Its successor, GRACE Follow-On, launched in 2018 and is slated to continue operations into the 2030s.
These satellites don’t directly measure water levels; instead, they track variations in Earth’s gravity field, which are sensitive to changes in mass and density beneath the satellites’ flight paths. “The gravity field is uneven,” explains Rodell. “For instance, mountainous regions have greater mass, thus a stronger gravitational pull, resulting in a slightly higher weight at such locations.”
Similarly, rivers, lakes, and aquifers exert a stronger gravitational pull when full and a weaker pull when depleted. The GRACE satellites maintain a 124-mile (200 km) average separation, with slight variations in this distance responding to changes in the planet’s gravitational pull.
“The satellites measure this distance with micron-level precision, approximately the size of a red blood cell, every five seconds,” Rodell explains.
Rodell and his colleagues calculated a global freshwater loss of 290 cubic miles, resulting in an average water level decrease of 1 cm (0.39 in.) across all lakes, rivers, and aquifers. While the planet’s overall water budget (including oceans, seas, clouds, glaciers, etc.) remains constant, the location of this water is crucial, and less is readily available to the 8.1 billion people who depend on it.
Rising global temperatures accelerate surface evaporation and the atmosphere’s water-holding capacity, drying out soil and aquifers, and reducing sea and lake levels. The resulting precipitation tends to fall as intense, short-lived storms, hindering absorption into the parched earth.
“This extreme precipitation runs off the surface, failing to replenish the soil moisture,” explains Michael Bosilovich, a senior Goddard meteorologist and co-author of the study. Some water flows into freshwater sources, but a significant portion ends up in the ocean. “The crucial freshwater replenishment we once had is absent.”
For cities and agricultural areas reliant on aquifers, this creates a vicious cycle of depletion. Increased groundwater pumping to meet human needs, coupled with reduced rainfall and increased runoff, exacerbates the situation. This has been worsened by a series of regional, national, and continental droughts across the globe during the study period. Intense drying began in [Region], extending to Australia, Southeast Asia, South America, North America, Europe, and Africa. Notably, 13 of the 30 most severe droughts recorded by GRACE since 2002 occurred in or after 2015, likely intensified by climate change-driven evaporation.
“The series of major droughts worldwide largely explains the persistent decrease in terrestrial water storage,” says Rodell.
While at least six more years of GRACE Follow-On data are anticipated, offering further insights into the planet’s water resources, the study authors express pessimism. “Based on the GRACE data, we don’t see a reversal of this trend,” says Bosilovich.
“Past freshwater declines, such as those in the 1980s, showed recovery,” adds Rodell. “However, the current decline, characterized by its abruptness and persistence, lacks such recovery even after nine years.”