by Barbara Grady, Yale Climate Connections
September 22, 2026
As the Western United States grapples with severe drought, leading cities to scramble for water and farmers to abandon some crops, a new scientific study has found that 122 major oil, gas, coal, and cement companies and state-owned operations are partially responsible.
The study found that climate change from all climate-warming greenhouse gas emissions over the last 70 years has resulted in a 36% decline in annual average mountain snowpacks, the major source of surface water in Western states – and that about 40% of that decline is linked to the climate pollution caused by just 122 companies.
The annual loss attributable to those 122 companies is equivalent to the amount of water held by Lake Mead, the nation’s largest reservoir, at full capacity.
The authors of the peer-reviewed paper, “Carbon emissions exacerbating the Western US water crisis,” also found that global warming from greenhouse gas emissions has led to a 13% decline in average annual streamflows from mountains to the lower-lying areas that depend on them, and to a 4.3% increase in the need for agricultural irrigation. The study attributed about half the streamflow declines and roughly half the increased need for irrigation to the warming caused by the 122 companies. In other words, those companies’ products resulted in a 6% fall in streamflows in the region, according to the study.
Nine scientists from the University of California at Merced and the Union of Concerned Scientists contributed to the study, published August 25 in the Nature magazine affiliate Communications Earth & Environment.
The West relies on snowpack
Summers are dry in the Western U.S.
Precipitation occurs in winter, with much of it stored as snowpack in the mountains. The West’s multibillion-dollar agriculture industry and the reservoirs supplying cities and towns in 11 states depend on melted snow carried by rivers, streams, and water conveyance systems in late spring and summer. If the snowpack is scant or if snow melts early, water shortages can occur in summer when water demand is highest.
The West’s historic megadrought has persisted for two decades. With less surface water available, farmers have had to pump more groundwater for irrigation, depleting aquifers so much that the ground is sinking in many agricultural areas. Some farmers in the Southwest have had to abandon crops or fallow fields for lack of sufficient water. Corpus Christi, Las Vegas, Los Angeles and Phoenix are just some of the cities facing severe water shortages.
Tying drought to specific companies
Emily Williams, lead study author and a postdoctoral research scientist at the University of California at Merced, said in an interview that the study allowed scientists to disentangle how much worse water scarcity became over the decade 2014 to 2024 because of climate-change-induced atmospheric warming, as well as how much worse it became specifically because of the warming emissions generated by 122 companies and their products.
“It lets us get beyond how much more intense climate change is becoming to look at specific impacts like snowpack decline as well as specific sources” of emissions, she said. Her team found impacts on water resources “not just from climate change but because of emissions traceable to specific sources.”
The 122 companies cited in the study are the world’s biggest producers of climate-warming greenhouse gas emissions cumulatively up to 2024; they are all coal, oil, gas, and cement producers. That list, known in climate science as the carbon majors and compiled by other scientists, is based on publicly reported coal, oil, gas, and cement production data along with scientific formulas for calculating emissions per unit of product produced. Since 2024, the list has grown to include 178 entities.
Chevron, ExxonMobil, BP, and Shell are the fourth-, fifth-, eighth-, and ninth-biggest cumulative emitters of greenhouse gases in the world, according to the most recent version of the list. The world’s three highest emitters are state-owned coal, oil, and gas companies from the former Soviet Union, China, and Saudi Arabia (Saudi Aramco). U.S. coal producers Core Natural Resources and Peabody Energy are also in the top 25 carbon majors.
The study authors based their calculations on emissions since 1950, rather than earlier, because that was the decade when major oil and gas companies began to suspect that use of their products would cause changes to the Earth’s climate. Engineers at ExxonMobil who studied the issue determined that burning large amounts of fossil fuels would affect planetary climate systems. After its engineers shared their conclusions with management in the 1970s, ExxonMobil at first expanded that research to learn more and then – a decade later – pivoted to hiding the scientific findings and denying the existence of climate change, as reported in the Pulitzer Prize finalist series “The Road Not Taken” by Inside Climate News. Other major oil companies did the same.
The study authors wrote that “roughly 70% of global industrial carbon dioxide (CO2) emissions since 1854 are traceable to 122 fossil fuel producers and cement manufacturers (“Carbon Majors”), and ~97% of these emissions occurred after 1950.”
U.S. oil companies remain silent
When asked on August 27 for a comment on the water crisis study, Chevron – the largest U.S. emitter since 1950 – sent an automatic thank-you note but did not respond to the question by publication.
Chevron’s second quarter earnings report, released August 12, highlighted record-breaking oil production. The report said the company “achieved new U.S. upstream production record of nearly 2.1 million barrels of oil equivalent per day and record U.S. refinery throughput of over 1 million barrels per day” as well as “competitive growth” in operations “around the globe.” Chevron also has embarked on developing oil resources in Venezuela, which has large deposits of heavy crude – a variety of oil that’s highly polluting to process.
A media relations receptionist for ExxonMobil, the second-largest U.S. emitter, politely pledged to convey the reporter’s question to the media relations team, but the company did not respond.
The methodology that Williams and her eight colleagues used involved overlaying observational data of recorded temperatures, precipitation, relative humidity, and hydrologic conditions annually over the decade 2014 to 2024 in seven water basins across the Western states with global climate models. Then they introduced emissions data in three variable scenarios to disentangle the effect of emissions from the carbon majors. One scenario included all human-caused emissions since 1950, the second included no anthropogenic emissions, and the third scenario excluded emissions traced to the 122 companies.
“By comparing these scenarios and using pattern scaling, we calculated how much of the observed changes in snowpack, streamflow, and irrigation demand are attributable to climate change and specifically to emissions from the Carbon Majors,” the authors wrote.
Governments seek damages from polluters
The study is part of a growing body of what’s called attribution science. As scientific consensus has solidified around climate change resulting from atmospheric warming caused by the burning of fossil fuels, emissions of methane, and destruction of forests, scientists are looking more deeply at the impacts of that warming, how to minimize future warming, and the specific sources of the emissions.
Similar studies have attributed the size of areas burned in intense wildfires, specific measures of sea level rise, and the increased likelihood and intensity of extreme heat events to emissions from the carbon majors.
Read: Scientists agree: Climate change is real and caused by people
“We have a choice of how much worse climate change is going to get, and that is a function of how much emissions we allow,” Williams said. She said some changes to the planet’s climate system are already irreversible, but we do have a choice to avoid others and the worsening of some that are already baked in.
Many states and cities around the world are taking the biggest climate polluters to court in hopes of getting financial resources to help them address damages from extreme hurricanes, wildfires and floods whose intensification can be linked to climate change.
Two states – New York and Vermont – have passed climate superfund laws requiring that the biggest historical emitters help pay for climate damages incurred in their states. Many other states, including California, are considering such legislation. New York’s law was recently struck down in court, but the state also plans to appeal the decision. Meanwhile, several states are suing major fossil fuel companies over the damage they allege was wrought by their emissions.
“As climate impacts become increasingly severe, communities will be asking for more action” to help recover from damage and adapt to climate changes, said Carly Phillips, another author of the water study and senior research scientist at the Union of Concerned Scientists.
Communities, states and small countries are bearing an unfair share of the costs of the impacts of emissions, Phillips contended, compared with the companies that ignored the looming crisis for decades and even lobbied Congress against taking action sooner.
“Given the outsized contribution of certain entities to the problem – 122 companies is a very small number – there should be some accountability,” she said.
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