Switching from hydropower to fossil fuels during periods of drought has cost Western U.S. states about $20 billion over the past two decades, according to new findings from Stanford University scientists.

When reservoir levels and river flows succumb to dry heat, hydropower plants can no longer operate — meaning that utilities must kindle facilities that burn coal and natural gas to meet rising electricity demand.

But the researchers found that with such a shift comes dramatic consequences.

This sharp transition from hydropower to fossil fuels results in surging carbon dioxide emissions, methane leakages, air quality-related deaths and enormous financial expenses, the scientists recently revealed in the Proceedings of the National Academy of Sciences.

“The impact on greenhouse gas emissions, air pollution, and human health could represent a large and unaccounted-for cost of climate change,” lead author Minghao Qiu, a postdoctoral scholar in the Stanford Doerr School of Sustainability and Stanford Center for Innovation in Global Health, said in a statement.

Qiu and his colleagues calculated that the total health and economic damages caused by drought-driven fossil fuel power generation in the U.S. was equivalent to about $20 billion between 2001 and 2021.

Carbon emissions were the biggest contributor to these damages, costing about $14 billion, while deaths associated with fine particle pollution accounted for about $5.1 billion and methane leakages were responsible for just under $1 billion, according to the study.

To draw their conclusions, the authors conducted their calculations based on widely accepted estimates for costs of carbon and methane emissions.

They assessed the statistical value of a human life based on the Environmental Protection Agency’s “mortality risk valuation” — a measure that considers how much people are willing to pay for small reductions in air pollution-related risks of death.

The authors also used best available estimates to ascertain how much methane leaks to the atmosphere during the production, processing and transportation of oil and gas: 2.3 percent per unit of gas consumed.

Under extreme drought conditions, electricity generation from individual fossil fuel plants can skyrocket up to 65 percent relative to average conditions, mostly due to the need to compensate for lost hydropower, according to the study.

And the resultant pollution does not respect state lines.

More than 54 percent of this drought-driven, fossil fuel-based electricity production is transboundary, with drought in one region leading to net imports of electricity — and therefore increasing emissions from power plants in other regions, the authors found.

“This is not a local story,” Qiu said. “A climate shock in one place can have serious ramifications for a totally different geographic area due to the interconnected nature of many energy systems.”

The total monetized costs of excess death and greenhouse gas emissions are about 1.2 to 2.5 times greater than reported economic costs that directly result from reduced hydropower production, the researchers estimated.

In California alone, the drought-induced shift away from hydropower electricity production led to more than $5 billion in damages from 2012 to 2016, or 2.5 times the direct economic cost of switching to more expensive fossil fuels, according to the study.

Those states that are heavily dependent on hydropower for their electricity — such as California, Washington and Oregon — have suffered particularly dramatic effects from the switch to fossil fuel reliance during dry periods.

During future years of extreme drought, such shifts could end up responsible for up to 40 percent of total emissions released via electricity production, the scientists warned.

Previous research, they contended, has underestimated the toll droughts are taking on electricity systems by failing to account for factors beyond the direct economic costs of these disruptions.

Increasingly frequent extreme heat periods will therefore present a challenge to policymakers set on achieving net-zero emission goals in hero-dependent states, according to the authors.

While this specific study focused on the American West, the researchers stressed that many countries around the world are facing similar risks.

Higher-emitting coal-fired plants could end up replacing lost resources in some nations, as opposed to the comparatively lower-emitting fossil fuel — natural gas — that tends to stand in for hydropower in the West, the authors explained.

Meanwhile, other countries that lack excess generation capacity could endure blackouts if hydropower operations shut down, they warned.

“In these regions, drought’s interaction with the energy system can have a cascading series of negative impacts on emissions and health,” corresponding author Marshall Burke, an associate professor at the Stanford Doerr School of Sustainability, said in a statement.

Going forward, the scientists recommended that policymakers implement “more ambitious and targeted measures” to mitigate the both the emissions and health burden that stems from the electricity sector during drought.

“If we want to solve this issue, we need an even greater expansion of renewable energy alongside better energy storage, so we don’t need to tap into fossil fuels as much,” Qiu said.

“Ultimately, to limit future warming and the drought risks that come with it, we need to reduce our emissions,” he added.