Recent droughts across the West have belted drinking-water supplies, withered crops and fanned deadly wildfires.

They’ve also squeezed hydroelectric facilities, with the less obvious side effect of hampering efforts to reduce greenhouse gas pollution.

A new study out of Stanford University finds that 10 percent of the total carbon dioxide spewed from California, Oregon, Washington and Idaho for power generation this century is the result of states turning to fossil fuels when water was too sparse to spin electrical turbines at dams.

The authors of the paper warn that as global warming causes more dry spells, more greenhouse gases loom. This poses an obstacle for places like California that are trying to make emissions cuts to address the growing threat of climate change.

“There has been some sense and some published work showing that droughts affect emissions. But what hasn’t been clear is exactly how significant these droughts are,” said Noah Diffenbaugh, a climate researcher at Stanford and co-author of the recent study. “Meeting these state-level climate mitigation goals is made a lot more difficult.”

From the Columbia River to Lake Oroville, hydropower has long been a pillar of the Western energy grid. In California, about one-fifth of the power has historically come from dams. Since hydroelectricity is cleaner than coal and natural gas, it’s helped limit the discharge of greenhouse gases, which are heating the atmosphere and driving such climate problems as extreme weather and rising seas.

As prolonged dry spells have drained rivers and reservoirs, however, the generation of hydropower has dropped. During California’s recent five-year drought, production was down nearly half in some years, according to the California Energy Commission.

The Stanford study found that switching to fossil fuel-generated electricity during dry periods was responsible for the release of an additional 100 million tons of carbon dioxide across 11 Western states between 2001 and 2015. That’s the equivalent of adding about 1.4 million cars to the highways.

California contributed more than half of the added pollution, or about 51 million tons of carbon dioxide, according to the study. That’s about 7.4 percent of the state’s total emissions from the energy sector.

In September, Gov. Jerry Brown ratcheted up the state’s climate targets, signing a bill that requires a 60 percent drop in energy-generated heat-trapping gas by 2030, over 1990 levels. The goal is zero emissions by 2045.

Diffenbaugh said meeting such objectives will require having not only clean primary sources of energy but alternatives that are nonpolluting.

While the power that states turn to during droughts varies, it’s generally much dirtier than hydropower, according to the study. California has made up for its loss of electrical turbines this century mostly with natural gas, which has less emissions than coal but still produces greenhouse gases. Other places, like Colorado, switched to coal. Washington and Oregon used a little of both.

“Overall in the Western U.S., the backup energy sources have not been low- carbon,” Diffenbaugh said.

The Stanford study, published this month in the journal Environmental Research Letters, also notes that drought-time power portfolios have produced more sulfur dioxide and nitrogen oxides. Both are pollutants that can cause respiratory illness and contribute to smog.

Pacific Gas and Electric Co., which provides the bulk of Northern California’s electricity, has been ramping up its arsenal of clean power sources. A recent surge in solar energy helped offset the loss of hydropower during the five-year drought, so much so that carbon dioxide emissions from PG&E power stayed mostly flat in the dry years, according to company records. Emissions even hit a new low in 2016.

“It’s really in the last four or five years that we’ve seen a significant increase (in solar),” said Alvin Thoma, a director in PG&E’s power generation department and the incoming chair of the board for the National Hydropower Association. “All else equal, we would have had an increase in carbon dioxide” without solar’s jump.

Thoma said he expects climate change to continue threatening the supply of hydropower, but he anticipates that clean energy supplies will also grow. About 70 percent of PG&E’s electricity portfolio, including hydropower, is now low-emitting. Renewable sources alone, such as solar, make up about a third of production.

Peter Gleick, an environmental scientist at the Pacific Institute water think tank, has long advocated for a greener mix of power sources. His organization, like Stanford, has noticed the uptick in greenhouse gas emissions that has come with less hydropower.

“The problem has been that for really the last 12 or 13 years, California has mostly been in drought,” Gleick said. “If this is a long-term trend, then this is a long-term problem, and I’m afraid it’s a long-term trend.”

While California has been relatively quick to replace hydroelectricity with nonpolluting sources of energy, not all states have, Gleick said. He hopes to see producers of clean power export more of their supplies.

“We need to expand the grid in the Western United States so that when California has extra solar we can send it to Wyoming or wherever, so they don’t have to burn coal, or when Texas has a lot of wind, they can send it somewhere,” he said.

“A better grid,” he added, “would really help send nonfossil energy where it’s needed, when it’s needed.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander