A peer-reviewed study published August 25 estimates that emissions traced to major fossil-fuel and cement producers contributed about one-third of climate-driven groundwater loss in California’s Central Valley between 2003 and 2024, linking reduced water supply and higher irrigation demand to emissions-related climate impacts.
Key Takeaways
- Researchers estimate emissions traced to the Carbon Majors accounted for about 34% of climate-driven groundwater loss in the Central Valley from 2003 through 2024.
- The study attributes declines in snowpack and warm-season streamflow across the western United States partly to Carbon Majors emissions since 1950.
- Those emissions were also associated with a 2.4% increase in irrigation demand across the study region over the past decade.
- Researchers estimated that total observed groundwater decline in the Central Valley reached 33.9 cubic kilometers between 2003 and 2024, with about 10.2 cubic kilometers attributed to climate-related factors.
- The researchers used climate and hydrological modeling to estimate the share of water-resource changes attributable to Carbon Majors emissions.
Study Measures Carbon Emissions’ Effects on Western Water Resources
Researchers estimate that emissions traced to major fossil-fuel producers and cement manufacturers since 1950 contributed to changes in water supply and demand across the western United States, including a measurable share of climate-driven groundwater loss in California’s Central Valley. The findings were published August 25 in Communications Earth & Environment.
The study examined emissions associated with the group known as the Carbon Majors. The research defined the group as 122 fossil-fuel producers and cement manufacturers whose emissions have accounted for a substantial share of global industrial carbon dioxide emissions. About 97% of the emissions attributed to those entities occurred after 1950, according to the study.
Researchers evaluated three main water-related measures across the western United States: April 1 snow water equivalent, warm-season streamflow and annual irrigation demand. They then examined how changes in those measures affected groundwater conditions in California’s Central Valley.
The analysis found that emissions from the Carbon Majors since 1950 accounted for between 40% and 64% of the changes attributed to human-caused climate change across the three water measures. The researchers reported a 15% decline in April 1 snow water equivalent, a 6% decline in warm-season streamflow and a 2.4% increase in irrigation demand attributable to those emissions across the western United States over the past decade.
California’s reliance on groundwater is also documented in previous coverage of state water scarcity and management, including groundwater depletion in Central Valley basins and the state’s efforts to manage aquifers.
Central Valley Groundwater Loss Shows a Climate-Driven Component
The Central Valley analysis focused on Sacramento, San Joaquin and Tulare basins. Researchers used late-summer terrestrial water storage as a proxy for groundwater conditions and modeled year-to-year changes using streamflow and irrigation-demand data.
The study estimates that observed groundwater decline across the Central Valley totaled 33.9 cubic kilometers from 2003 through 2024. Researchers estimated that about 70% of that decline was associated with overallocation, while about 30% was associated with climatic factors, including human-caused climate change and natural variability.
That produced an estimated 10.2 cubic kilometers of groundwater depletion associated with climatic factors during the study period. The researchers then estimated that human-caused climate change accounted for 6.5 cubic kilometers of the decline, while emissions from the Carbon Majors since 1950 accounted for 3.5 cubic kilometers.
The Carbon Majors estimate represented about 34% of the climate-driven groundwater loss identified by the study. The researchers also calculated that the emissions-associated decline represented about 10% of the total observed groundwater loss in the Central Valley between 2003 and 2024.
The study reported an uncertainty range of 1.9 to 4.7 cubic kilometers for the estimated groundwater reduction attributed to Carbon Majors emissions. Researchers also found that a second proportional method produced an estimate of 3.1 cubic kilometers, compared with 3.5 cubic kilometers under the study’s attribution approach.
Previous reporting on Central Valley agriculture challenges has also identified water scarcity as a major issue for the region’s agricultural sector, alongside irrigation efficiency and climate-related pressures.
Carbon Majors Emissions Reduced Snowpack and Streamflow
The study connected emissions-related climate effects to changes in the western United States’ seasonal water supply. Researchers estimated that Carbon Majors emissions since 1950 reduced April 1 snow water equivalent by 15% across the region over the past decade, equivalent to about 35 cubic kilometers of water per year.
California was among the areas with the largest absolute reductions in snow water equivalent. The researchers estimated that Carbon Majors emissions reduced April 1 snow water equivalent in California by about 5.1 cubic kilometers over the 2014–2024 period, representing roughly 18% to 20% of observed values for the basin.
Warm-season streamflow also declined. Across the western United States, researchers estimated a 6% reduction, or about 15 cubic kilometers per year, attributable to Carbon Majors emissions during the past decade. The estimate represented 47% of the reduction attributed to human-caused climate change.
California recorded one of the largest absolute reductions. The study estimated that Carbon Majors emissions were associated with about 4 cubic kilometers, or 13%, less annual warm-season streamflow in California during the period examined.
The researchers also found that runoff timing shifted. Across the western United States, the average timing of runoff occurred about 5.6 days earlier under the emissions-related changes, with shifts reaching as much as 30 days in some mountainous areas.
A separate California report on El Niño and winter water conditions describes the role of snowpack, rainfall and runoff in seasonal water availability, providing additional context for the study’s analysis of western water resources.
Higher Irrigation Demand Added Pressure on California Water Supplies
The study also examined water demand rather than only changes in water availability. Researchers estimated that Carbon Majors emissions contributed to a 2.4% increase in annual irrigation demand across the western United States over the past decade, equivalent to about 0.89 cubic kilometers per year.
California’s Central Valley was among the areas with the largest absolute increases in irrigation demand associated with the emissions examined. The study estimated an additional 0.36 cubic kilometers of annual irrigation demand in California over the 2014–2024 period, representing a 1.9% increase.
The timing of these changes is relevant to the groundwater analysis. Irrigation demand reaches its highest levels during warm months, when runoff is lower. Researchers found that reduced warm-season streamflow combined with higher irrigation demand widened the seasonal gap between available surface water and agricultural requirements in several California basins.
The Sacramento, San Joaquin and Klamath basins in California ranked among the areas with high changes in both warm-season streamflow and irrigation demand. The researchers said these basins contain substantial irrigated agriculture and experience conditions in which changes in water supply and demand can increase reliance on groundwater.
The study used the relationship between streamflow and irrigation demand to model groundwater changes. Lower streamflow can reduce water available for irrigation and groundwater recharge, while higher irrigation demand can increase groundwater withdrawals.
Researchers Quantify Emissions-Related Groundwater Impacts
The researchers used a source-to-impact attribution method to separate water-resource changes associated with Carbon Majors emissions from those associated with overall human-caused climate change. The approach incorporated global climate models, high-resolution meteorological data and hydrological modeling.
The analysis first connected changes in global temperature associated with Carbon Majors emissions to local meteorological conditions. Researchers then developed counterfactual scenarios excluding changes attributed to those emissions and used the resulting data to model snowpack, streamflow and irrigation demand.
For groundwater, the researchers modeled a Central Valley hotspot using changes in cold-season and warm-season streamflow along with annual irrigation demand. The model had an R² value of 0.63 for predicting year-to-year changes in total water storage anomalies, which the study used as a proxy for groundwater anomalies.
The study also compared its attribution approach with a proportional method that estimates impacts based on the share of emissions associated with the Carbon Majors. The researchers found that the two approaches produced similar results for some measures but differed for others because water systems respond nonlinearly to changes in temperature.
For the Central Valley groundwater estimate, the proportional approach produced a cumulative Carbon Majors-associated decline of 3.1 cubic kilometers, compared with 3.5 cubic kilometers under the attribution method. Researchers said that difference was small relative to the uncertainty range surrounding the groundwater estimate.
The study therefore distinguishes between total groundwater depletion and the portion attributed to climate-related factors. Its estimate does not assign all Central Valley groundwater loss to emissions. Instead, the analysis identifies a climate-driven component and estimates the share of that component associated with emissions traced to the Carbon Majors.
Frequently Asked Questions
How much California groundwater loss does the study attribute to carbon emissions?
The study estimates that emissions from the Carbon Majors since 1950 accounted for about 34% of climate-driven groundwater loss in California’s Central Valley between 2003 and 2024. The estimated emissions-associated groundwater decline was 3.5 cubic kilometers.
What did the study find about Central Valley groundwater?
Researchers estimated total observed groundwater decline of 33.9 cubic kilometers in the Central Valley from 2003 through 2024. They estimated that about 10.2 cubic kilometers was associated with climatic factors, with 3.5 cubic kilometers attributed to Carbon Majors emissions.
Which emissions were examined in the California water study?
The research examined emissions traced to 122 fossil-fuel producers and cement manufacturers identified as the Carbon Majors. The analysis focused on emissions since 1950.
How did carbon emissions affect California’s water supply?
The study associated Carbon Majors emissions with reduced snowpack and warm-season streamflow, along with increased irrigation demand. In California, the researchers estimated reductions in snow water equivalent and streamflow and an increase in irrigation demand over the past decade.
When was the California groundwater emissions study published?
The peer-reviewed study was published August 25, 2026, in Communications Earth & Environment.
