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NASA’s GRACE satellite record shows that the average amount of freshwater stored on land from 2015 through 2023 was about 290 cubic miles (1,200 cubic kilometers) below the 2002–2014 average. That is a comparison between two periods, not a measurement of water people consumed, groundwater alone, or a uniform loss in every region. The finding points to a sustained shift in Earth’s water storage, while leaving its causes and future course unresolved.
What NASA’s satellites found
A study reported by NASA found that global terrestrial water storage dropped abruptly beginning in May 2014 and remained substantially lower in the period examined. Across 2015–2023, the average was 290 cubic miles (1,200 cubic kilometers) below the 2002–2014 average. NASA hydrologist and study co-author Matthew Rodell compared the difference to 2.5 times the volume of Lake Erie. The study appeared in Surveys in Geophysics. NASA’s November 2024 account describes this as a difference in satellite-derived storage averages—not a tally of water used up.
The signal describes a global aggregate. It does not mean that every watershed or country lost water, or that the same amount of water was missing from each year. Thirteen of the 30 most intense droughts observed by GRACE occurred from January 2015 onward, according to NASA’s account of the study. That helps place the storage shift alongside severe droughts, but does not by itself explain the entire long-term difference.
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Terrestrial water storage is broader than groundwater. NASA’s GRACE freshwater-availability dataset defines the measure as groundwater, soil moisture, snow and ice, surface waters, and wet biomass. Its coverage is 2002–2016, with a 0.5-degree grid resolution; it is a related data product, not the full 2015–2023 comparison described in the 2024 report. NASA’s dataset information explains the components and coverage.
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Because the total combines multiple stores, a lower reading cannot be translated directly into a specific volume of depleted aquifers. Snowpack, lakes, soil moisture, and groundwater can move in different directions across places and seasons. Nor does a storage estimate show whether water is clean, accessible, or delivered to households. Water storage and public water access are related, but they are not the same measure.
How GRACE and GRACE-FO detect water changes
The GRACE and GRACE-FO missions use paired satellites to detect small changes in Earth’s gravity field. Water moving into or out of a region changes its mass, which changes gravity; repeated observations let scientists estimate changes in the amount of water stored on and beneath land. The missions measure changes in storage over time, rather than drilling into aquifers or directly measuring water quality and human access. NASA’s mission overview describes the satellite approach.
The original GRACE mission operated from March 2002 to October 2017. GRACE-FO launched in May 2018, continuing the record after a gap. Turning gravity observations into estimates for particular water components requires further analysis. Researchers may combine satellite data with ground measurements, water-extraction records, and models to estimate groundwater changes or assess aquifer stress; those estimates have their own assumptions and uncertainties.
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What may have contributed to the decline
NASA’s account says the decline began during a major drought in northern and central Brazil. Droughts followed across Australasia, South America, North America, Europe, and Africa. The 2014–2016 El Niño altered atmospheric circulation and rainfall patterns, and storage did not rebound after El Niño subsided. These events offer context for the shift, but the report does not establish one cause for the global signal.
NASA researchers discuss several possible mechanisms and pressures:
- Changed rainfall and drought: Prolonged dry conditions can reduce replenishment of soil, surface water, and groundwater stores.
- Heavy rainfall running off: NASA Goddard meteorologist Michael Bosilovich described how intense precipitation can run off instead of soaking into the ground and replenishing groundwater.
- Warming-related effects: Warmer air can hold more water vapor; longer dry intervals may dry and compact soils, while intense rainfall may generate more runoff than infiltration. These are proposed ways warming could affect storage, not proof that it alone caused the global change.
- Groundwater pumping and irrigation: Where rainfall or snow does not recharge stores, pumping during drought and expanded irrigated agriculture can add pressure.
NASA researchers said definitive attribution to global warming is difficult. Susanna Werth, a Virginia Tech hydrologist and remote-sensing scientist who was not affiliated with the study, cautioned: “There are uncertainties in climate predictions,” and “Measurements and models always come with errors.” Rodell said, “We don’t think this is a coincidence, and it could be a harbinger of what’s to come.” That is his interpretation, not a settled forecast. The report did not resolve whether global storage would rebound, hold steady, or fall further. NASA’s report discusses the proposed mechanisms and attribution limits.
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How the global finding differs from aquifer and basin studies
Regional groundwater research provides important context, but its results should not be added to or treated as the same measurement as the global terrestrial-storage comparison.
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|---|---|---|---|
| 290 cubic miles (1,200 cubic kilometers) lower average storage | Global; total terrestrial water storage, including groundwater and other stores | 2015–2023 average compared with 2002–2014; satellite-derived storage | A lower global average across periods, not groundwater loss alone or water consumed. NASA, 2024 |
| 16.5 cubic miles (68.7 cubic kilometers) decline in groundwater supply | Great Basin; groundwater | About two decades; a 2024 study in Geophysical Research Letters | A regional groundwater estimate. Record snow in winter 2022–23 did not reverse the longer decline. NASA’s 2024 account |
| 13 of 37 large aquifers described as depleted while receiving little to no recharge; eight overstressed and five highly stressed | Large aquifers assessed in separate research | Studies discussed by NASA in 2015; distinct definitions and methods | Historical study results, not a current aquifer inventory or the finding behind the 2024 global storage comparison. NASA, 2015 |
The Great Basin example also shows why a single wet season is not enough to infer that a longer groundwater decline has reversed. Meanwhile, GRACE drought maps have a different purpose: NASA says its data-assimilation maps depict wetness and drought associated with climatic variability, not proof that withdrawals exceed recharge. A drought indicator should not be read as a direct measure of unsustainable pumping.
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The finding is an early-warning signal about the amount of water stored across land, not a map of who has usable water or which communities face shortages. A global average can conceal local increases and declines; the mixed components of the measurement also mean it cannot identify the status of every aquifer. NASA’s separate aquifer work has noted uncertainty in estimates of how much water individual aquifers contain.
NASA’s 2024 coverage also cites a UNESCO report on possible social harms from water stress. Those consequences are associated with water stress, not directly demonstrated by the GRACE storage measurement. The satellite record can inform broader monitoring, but assessing local supply requires local data on water quality, infrastructure, withdrawals, recharge, and access.
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What remains uncertain
- Cause: Drought, El Niño-related circulation changes, groundwater use, irrigation, and warming-related processes may all contribute. The report does not isolate one definitive explanation for the global decline.
- Local aquifer volumes: GRACE detects changes in mass, not each aquifer’s remaining capacity. Ground observations and other methods are needed for more specific estimates.
- Future direction: The 2024 report did not determine whether storage would recover, stabilize, or continue falling.
- Human impact: Storage anomalies alone do not establish water safety, affordability, or access for people in a given place.
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