A recent forecast by from the U.S. Energy Information Administration contains a stunning punchline: This year, natural gas is expected to supplant coal as the No. 1 source of electricity in the United States. Gas will supply 33.4 percent of power, and coal will contribute 32 percent, the EIA projected in the March installment of its Short-Term Energy Outlook.

“This would be the first time that natural gas has generated more power than coal on an annual basis,” the agency says. Natural gas beat out coal during several months last year, but beating it out for the year is another matter and far more momentous.

The projection came out earlier this month, but it was highlighted by the EIA last week, along with the following chart showing just how dramatically the way we get electricity has changed over the past 60 or so years, with the current transition representing just one of many:

Still, it is fair to say this latest change has taken everyone by surprise. The Environmental Protection Agency, in the Clean Power Plan released last year, projected that natural gas would hit 33 percent of generation and that coal would sink to 27 percent, in 2030 under the policy. Instead, that natural-gas target has more or less already happened today, even though the Clean Power Plan itself hasn’t even been implemented and remains tied up in court.

The reason, clearly, is the shale gas boom which has dramatically driven down natural-gas prices. Here’s a figure from the EIA, showing what occurred:

The price of natural gas — in dollars per million British thermal units, or BTUs — has been below $2 for much of this year.

“The energy system tends to move really slow, with a lot of inertia, except when it doesn’t,” Richard Newell, a Duke University energy expert and a former administrator of the EIA, told me last week. And when it comes to the natural-gas boom unleashed by fracking, “that was a case when it really didn’t,” Newell said.

The EIA also says that coal generation will decline by 3 percent this year, because of continuing coal plant retirements, even as renewables will continue to grow. (2016 is expected to be a new record year for installations of U.S. solar.)

The overall effect of these changes will be a slight decrease in the United States’ greenhouse-gas emissions from the use of energy (their biggest source), the agency projects. They declined 2.4 percent in 2015 and should drop another 0.3 percent this year.

Natural gas burns cleaner than coal, when it comes to carbon dioxide emissions. But considerable controversy remains over just how much fugitive gas emissions to the atmosphere, due to drilling operations and other leaks, undermine this benefit. (Natural gas, comprised chiefly of methane, releases carbon dioxide when burned. But if methane wafts into the air without that happening first, it has a much greater warming effect than carbon dioxide over short time frames.)

Democratic presidential candidates Bernie Sanders and Hillary Clinton have split markedly on the subject of natural gas. Clinton has defended natural gas as a lower-carbon “bridge fuel” into an era dominated by renewables. But Sanders has called for a ban on fracking, the technology behind the unconventional gas boom, because of its environmental consequences.

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