Remember when groups like the Sierra Club used to be about keeping the air breathable, water drinkable and trees huggable? We miss those days.

Evidently having run out of worthier targets, Sierra Clubbers now have trained their sights on your furnace, your water heater and your stove. If you thought the battle against climate change was only about fighting fossil-fueled power plants and gasoline-powered automobiles, guess again. It turns out natural gas-fired household appliances are next on the enemies list.

As reported Wednesday in The Gazette, more than two dozen environmental groups have petitioned the U.S. Environmental Protection Agency to ban the use of natural gas in homes nationwide. They claim it results in “deadly pollution from heating appliances.”

The Sierra Club-sponsored petition contends gas-fired home furnaces, water heaters, clothes dryers and stoves emit enough nitrogen dioxide and carbon dioxide that they must be regulated as “new stationary sources” of air pollutants like power plants and factories. As noted in The Gazette’s report, the petition would force the EPA to set standards that effectively would ban the use of natural gas in all new construction within a year after the regulation is implemented. And current gas appliances would have to be replaced with electric ones when they break down.

“The data is incontrovertible,” the petition states, “heating appliances in residential and commercial buildings contribute significantly to pollution that endangers public health and welfare.”

Even if we accept such sweeping policy pronouncements, what would the petitioners have us do about it? Natural gas-fired heat, for example — whether via forced air or radiant, hot water — pretty much warms all residences in Colorado as well as in the rest of the country. Replacing it with the likes of heat pumps, which only function well in temperate climates, isn’t efficient or even technologically possible in cold winters like Colorado’s.

Attempts to replace them also would prove exorbitant. Requiring all new homes to be built for electric appliances, and existing ones to be retrofit for them — a goal toward which a new Colorado law already is steering us — is costly, as well.

To all of which, the environmental lobby seems to be saying: too bad. Get with the program.

Yet, the premise of that epic upheaval is itself dubious. The activists’ case against residential natural gas seems to toggle back and forth, as needed, between its purported role in climate change and its supposedly dire effect on humans. You get the impression even the environmental groups themselves are unsure the data supports either claim.

A lot of the latest hyperventilation by activists over gas stoves, for example, stems from a Stanford University study released earlier this year that got a lot of press — but didn’t seem to say as much as environmentalists said it did.

Geologist and geochemist Daniel Tormey wrote in February in the Washington Times: “The study spawned countless stories with similar headlines, many of which followed the (press) release’s lead and drew a connection between gas stoves and worsened health. Except the study didn’t really show that … the study itself makes very minor connections to health issues.”

He concludes, “The body of research on this issue points less to the costly and drastic measure of replacing all of your gas appliances and more to ensuring proper ventilation in your kitchen.”

Pretty underwhelming — and of course not very helpful to the Sierra Club when it passes the hat at its next fund-raiser.