When the Navajo Generating Station in Arizona shuts down later this year, it will be one of the largest carbon emitters to ever close in American history.

The giant coal plant on Arizona’s high desert emitted almost 135 million metric tons of carbon dioxide between 2010 and 2017, according to an E&E News review of federal figures.

Its average annual emissions over that period are roughly equivalent to what 3.3 million passenger cars would pump into the atmosphere in a single year. Of all the coal plants to be retired in the United States in recent years, none has emitted more.

The Navajo Generating Station isn’t alone. It’s among a new wave of super-polluters headed for the scrap heap. Bruce Mansfield, a massive coal plant in Pennsylvania, emitted nearly 123 million tons between 2010 and 2017. It, too, will be retired by year’s end.

And in western Kentucky, the Paradise plant emitted some 102 million tons of carbon over that period. The Tennessee Valley Authority closed two of Paradise’s three units in 2017. It will close the last one next year.

“It’s just the economics keep moving in a direction that favors natural gas and renewables. Five years ago, it was about the older coal plants becoming uneconomic,” said Dan Bakal, senior director of electric power at Ceres, which works with businesses to transition to clean energy. “Now, it’s becoming about every coal unit, and it’s a question of how long they can survive.”

Coal plant closures have been a feature of U.S. power markets for the better part of a decade, as stagnant demand, low natural gas prices and increasing competition from renewables have battered the coal fleet.

In previous years, most retirements were made up of smaller and lesser-used units. That means the emissions reductions were less substantial.

In 2015, the United States closed 15 gigawatts of coal capacity, or roughly 5% of the coal fleet. That still stands as a record amount of coal capacity retired in one year.

Yet the emissions reductions were modest by today’s standards. The units retired in 2015 emitted a combined 261 million tons in the six years prior to their retirement, according to an E&E News review of EPA emissions data. On average, they annually emitted about 43 million tons over that period.

Contrast that to 2018, when almost 14 GW of coal was retired. Those units emitted 511 million tons of carbon between 2010 and 2015. Their combined average annual emissions rate was 83 million tons.

The trend figures to be even more dramatic this year.

Small plants are gone

The U.S. Energy Information Administration expects almost 8 GW of coal to retire in 2019, or a little more than half the capacity retired in 2015. Yet the units retired this year emitted more than their 2015 counterparts. Between 2010 and 2015, their combined emissions were 328 million tons, giving them an annual emissions average of 55 million tons.

Other factors are also at play in the retirement of coal’s behemoths. In some cases, federal air quality regulations or an exodus of customers may have contributed to the closure, said John Larsen, who leads power-sector analysis at the Rhodium Group, an economic consulting firm.

The Navajo Generating Station is a case in point. The plant had already planned to shut down a unit to comply with federal smog regulations. Two utilities with a stake in the facility had either divested from the plant or plan to do so. And the plant’s largest customer announced it could buy power on the wholesale market for less.

“You notice the average size of retired plants going up over time. There are not a lot of small plants left, period,” Larsen said. “Once you’ve cleared out all the old inefficient stuff, it’s logical the next wave would be bigger and have more implications for the climate.”

There are several caveats to consider. Units scheduled for retirement generally produce less in the years running up to their closure, meaning the plants that closed in 2015 once emitted more than they did near the end of their lives.

There’s also this: The vast majority of super-polluters have no closure date in sight. That’s because massive coal plants generally benefit from large economies of scale. Because they crank out power around the clock, their cost of generating electricity is relatively cheap.

“The coal plants remaining have generally installed all the environmental controls,” Larsen said. “There are no additional regulatory threats, or they are cost-effective in a world where gas is $2.50 per MMBtu.”

Another caveat: Coal plant closures don’t guarantee power-sector emissions reductions on their own. In 2018, power-sector emissions increased for the first time in many years because electricity demand rose, prompting natural gas generation to spike.

But if there is a notable trend with the current round of plant closures, it is this: The large coal plants closing today are in places like Arizona, Pennsylvania and Kentucky.

“You’re not seeing climate policy close these plants,” said Mike O’Boyle, director of electricity policy for Energy Innovation, a nonprofit that advocates for a transition to clean energy. “Coal plants are becoming more expensive to operate over time.”