For many environmental leaders, "clean energy" is a moving target.  What was acceptable last year is no longer so, and the question is why.

In terms of carbon emissions, natural gas is twice as clean as coal.  That was once good enough to earn it lukewarm praise from many green supporters.  Now, with the U. S. Energy Department projecting that gas will be the fastest-growing global fuel source through 2040, environmental leaders have turned against it.  They now claim that hydraulic fracturing causes catastrophic damage to the earth, air, and water, and that natural gas was never that clean to begin with.

What's behind this change of heart?  Maybe it's the need to continue the struggle.  Admitting that natural gas is clean and that America has enough of it to power the country for a century -- where would that leave the leaders of environmental groups that now raise hundreds of millions in donations?  It might leave them having to make a living like everyone else.

Maybe the environmental pros are more interested in their own survival than that of the planet.  To maintain their constituency's support and the generous donations that go with it, environmental groups must always protest something.  And to expand their donor base, they must expand their protest.  If coal, oil, and natural gas are "dirty," so are hydro and ethanol, and, soon enough, wind, solar, and biomass may be as well. There is literally no end to the anti-growth agenda of the environmental left.

Maybe it's just that the leadership of the green revolution, like every other revolution out there, expect their followers to do without while the leaders themselves bask in luxury.  What's important, it seems, is that the struggle continue and the salaries of the environmentalist pros continue to be funded.  That would explain the sudden shift in environmentalist attitudes toward natural gas -- once the sanctioned "bridge fuel" and now the latest pariah.

On its website, the Sierra Club includes links not just to "Beyond Coal" and "Beyond Oil," but also to "Beyond Natural Gas."  As portrayed on this site, natural gas is a dirty and environmentally harmful fuel.  The argument seems to be that it is better to forego use of natural gas entirely, as well as of coal and oil, unless all damage to the environment can be averted.  But since there will always be some disturbance resulting from oil and gas drilling, best not to use any.

Unfortunately, the same argument could be made, and is being made, for other forms of energy.  If the environmental movement has its way with carbon-based sources, it will soon turn its sights on so-called clean energy as well.  In fact, it already has.

Is there any feasible energy source that has the full backing of a national environmental group?  Not hydro, nuclear, or ethanol.  And, increasingly, not solar, wind, or biomass, either.  An AP story by Dina Cappiello focusing on windmill-related golden eagle deaths may be the harbinger of things to come.  Not just golden eagles, but hundreds of thousands of other birds are killed each year by wind-power installations.  It's hard to imagine the endangered "lesser prairie chicken" (yes, there is such a creature) fluttering about amidst a large wind installation.

For now the green position on wind energy is "conflict avoidance."  Site the windmills where birds don't fly, and shut them down if a lone bird or bat comes near.  But it seems unlikely that the industrial-scale installations necessary to power the entire U.S. economy -- necessary if carbon is to be replaced altogether -- could ever be sited so as to avoid all damage to birds, bats, and other wildlife.  Better not to use any.

And if windmills lose their clean energy standing, will solar be far behind?

The problem with solar is that in most locales, the sun shines only about a third of the time.  Carpeting hundreds of square miles with solar installations to power the grid one third of the time does not make sense.  Solar, like wind, requires a carbon-fueled backup system.  The requirement of a full-scale backup system that can immediately be switched on and off makes wind and solar more than twice as expensive as coal or gas alone. 

Nor is solar without environmental impact. Beneath the perpetual darkness of miles of solar panels, the desert becomes an entirely different habitat. 

Then there is the need for thousands of miles of high-voltage transmission lines that may be a lot more damaging to the environment than an oil and gas pipeline network.  While greens are still gaga over "rooftop solar," how long will it be before environmentalist groups turn against solar altogether?    

Having already raised questions about ethanol, environmental groups will eventually turn against other alternatives.  For the professional organizer, the M.O. is protest or die.  And as long as everything is not restored to its pristine condition, before the arrival of humans, there will always be something to protest.

To any impartial judge, that goal would seem simply mad.  A totally pristine environment is a chimera, but that is what makes it so attractive to environmental left.  As long as human beings occupy the earth, they will be wreaking ecological havoc, as far as the professional environmentalist is concerned.

The rest of us (those who make a living actually producing a good or service) understand that cheap and reliable energy is the foundation of our national prosperity.  We celebrate the fact that there are now over seven billion human beings on the earth, each of them filled with wondrous potential and each entitled to a shot at happiness.  We admire the remarkable technological advances of human civilization, many of them dependent on carbon-based fuels.  We recognize that human beings have never lived as well as they do today.  And we don't want to return to the Stone Age.

Someday, perhaps, the marks who donate to environmental groups will come to their senses, and the profession of environmental leader will die off.  But 'til then, the environmental pros will be very much alive, and well compensated to boot.

Jeffrey Folks is the author of many books on American politics and culture, including Heartland of the Imagination (2013).