Carbon emissions are worsening the western US water crisis, new study finds

A new study has linked decades of carbon emissions to a significant share of the worsening water crisis across the western United States, finding that emissions from major fossil fuel and cement producers have reduced snowpack and warm-season river flows while increasing irrigation demand.

The research, published August 25 in Communications Earth & Environment, examined emissions traced to the group known as the “Carbon Majors” since 1950.

Researchers found that these emissions account for roughly 40% to 64% of water-supply and demand changes attributed to human-caused climate change across the western US.


The findings come as the region faces shrinking mountain snowpack, earlier snowmelt, rising water demand and growing dependence on groundwater, particularly during prolonged dry periods.

Carbon emissions linked to shrinking snowpack

Snow in the western US is more than a winter landscape feature—it acts as a natural reservoir, storing water in the mountains before releasing it during the warmer months.

The study found that emissions from the Carbon Majors since 1950 have contributed to a 15% decline in April 1 snow water equivalent across the western US during the 2014-2024 period.

That represents an estimated 35 cubic kilometres of water per year that would otherwise have been stored in snowpack. Researchers said this decline represents about 42% of the total snowpack reduction attributed to human-caused climate change.

California and the Pacific Northwest recorded some of the largest absolute reductions. In California, the estimated loss was about 5.1 cubic kilometres, while the Pacific Northwest saw a reduction of approximately 27 cubic kilometres during the study period.

Rivers are also carrying less water when demand is highest

The research found another major change in the timing and availability of water.

Warm-season streamflow, measured from April through September, has declined by about 6% across the western US because of emissions traced to the Carbon Majors since 1950.

That amounts to roughly 15 cubic kilometres less water each year, according to the researchers. The decline represents about 47% of the warm-season streamflow reduction attributed to human-caused climate change.

The timing of runoff has shifted as well. Researchers estimate that the centre of the annual runoff season has moved approximately 5.6 days earlier on average across the region, with some mountainous areas experiencing shifts of as much as 30 days.

That creates a difficult mismatch: water is arriving earlier, while agriculture and other users need substantial supplies during the hotter, drier months.

Farmers are facing higher irrigation demand

The problem is not limited to declining water supplies.

As temperatures rise, crops and landscapes lose more water to evaporation, increasing the amount of water needed for irrigation.

The study estimates that Carbon Majors' emissions since 1950 have increased irrigation demand by 2.4%, equivalent to around 0.89 cubic kilometres of additional water each year across the western US.

That increase represents about 57% of the irrigation-demand rise attributed to human-caused climate change.

The largest increases were identified in heavily irrigated agricultural areas, including California's Central Valley and parts of Washington, Oregon and Idaho.

California's Central Valley faces a deeper groundwater problem

The study also examined how changes in surface water supply and irrigation demand could affect groundwater.

The researchers focused on California's Central Valley, one of the country's most important agricultural regions and an area that has experienced substantial groundwater depletion.

Between 2003 and 2024, the Central Valley lost an estimated 33.9 cubic kilometres of groundwater. Researchers estimated that about 30% of that loss was associated with climate-related factors, with the remainder largely linked to long-term water overallocation.

Of the climate-driven portion, the study estimates that emissions from the Carbon Majors were responsible for approximately 3.5 cubic kilometres of groundwater loss.

That amounts to about 34% of the groundwater depletion attributed to climate factors, or roughly 10% of the region's total observed groundwater loss during the period studied.

The water supply-demand gap is getting wider

Several western US river basins are particularly exposed because water demand rises at precisely the time when surface-water supplies are declining.

Researchers identified the Klamath, Sacramento and San Joaquin basins in California, along with the Middle Columbia basin spanning parts of Oregon and Washington, as areas where both streamflow reductions and increased irrigation demand are especially pronounced.

Without changes in irrigation methods, cropping patterns or water storage, the growing gap can force greater reliance on groundwater.

That creates a potentially difficult cycle: less surface water can mean more pumping, while a warmer climate simultaneously increases the amount of water crops require.

Why the study focused on 'Carbon Majors'

The researchers were not simply examining climate change in general. They specifically sought to estimate how much of the observed change could be attributed to emissions from major fossil fuel producers and cement manufacturers.

The study notes that around 70% of global industrial carbon dioxide emissions since 1854 can be traced to 122 Carbon Majors, with approximately 97% of those emissions occurring after 1950.

The scientists used a source-to-impact attribution method to distinguish the effects associated with those emissions from broader human-caused climate change.

They then modelled the resulting changes in snowpack, streamflow, irrigation requirements and, ultimately, groundwater.

Researchers caution against oversimplifying the numbers

The study also examined a simpler method that estimates impacts based on the proportion of overall emissions attributable to the Carbon Majors.

While the two approaches produced broadly similar results, they were not identical.

The researchers found that the simpler proportional method could overestimate declines in snowpack and warm-season streamflow while underestimating increases in irrigation demand.

This is particularly important in areas where temperatures are close to the point at which precipitation shifts from snow to rain.

The authors therefore caution that attribution estimates should account for the nonlinear ways in which warming affects different parts of the water cycle.

A warning for the future of the western US

The study paints a picture of a water system being squeezed from both directions.

Mountain snowpack is declining, rivers are delivering less water during the warm season and agricultural demand is increasing. At the same time, groundwater reserves are being drawn down to compensate for shortages.

The researchers estimate that emissions from the Carbon Majors since 1950 have contributed roughly half of the changes in western US water supply and demand that can be attributed to human-caused climate change.

The scale of some of those changes is striking.

The study says the annual snowpack reduction associated with Carbon Majors emissions is roughly comparable to the maximum storage capacity of Lake Mead, while the additional irrigation demand is equivalent to about one-third of California's annual residential water use.

For a region already struggling with drought, groundwater depletion and competing demands for water, the research suggests that the effects of past carbon emissions are not an abstract climate statistic.

They are increasingly visible in the amount, timing and availability of water across the western United States.