About a year ago, on March 26, 2012, Sandra Steingraber, an environmental writer and activist against natural-gas fracking, wrote a public letter titled "Breaking Up with the Sierra Club." Breakups are never easy, and the letter, published on the website of the nature magazine Orion, was brutal from the start: "I'm through with you," Steingraber began.

The proximate cause of the split was the revelation that between 2007 and 2010 the nation's oldest environmental organization had clandestinely accepted $26 million from individuals or subsidiaries associated with Chesapeake Energy, a major gas firm that has been at the forefront of the fracking boom. "The largest, most venerable environmental organization in the United States secretly aligned with the very company that seeks to occupy our land, turn it inside out, blow it apart, fill it with poison," Steingraber wrote. "It was as if, on the eve of D-day, the anti-Fascist partisans had discovered that Churchill was actually in cahoots with the Axis forces."

In 2010, the club's new executive director, Michael Brune, stopped taking Chesapeake Energy's cash. Brune also made the decision to come clean with the revelation and express regret for his predecessor's lack of better judgment. "We never should have taken this money," Brune wrote in response to the breakup letter.

But to Steingraber and many others, the betrayal had been done.

"I call them gang-green," says Maura Stephens, an activist based in Ithaca, New York, who spearheads several anti-fracking groups, including Frack Busters and the Coalition to Protect New York. "There are a lot of so-called environmental groups that were started with noble ideals-for example the ideals of John Muir-but who no longer live up to their mission. … They do good work on some level, but on this [fracking] they are selling us out."

The eco-infighting over natural gas is just one example of internecine strains that appear to be intensifying in the green movement. When it comes to prescribing ways to address the planet's ecological challenges, environmentalists increasingly find themselves at odds with each other. In a way, greens' predicament is a measure of their own prescience. For at least 40 years, they have been warning about the consequences of overpopulation, the risks of industrial pollution, and the loss of wilderness and wildlife habitat due to human encroachment. Few heeded the warnings in time to halt the first effects of large-scale global pollution and resource depletion, and now the consequences of ignoring the warnings have come to pass. Many global fisheries are on the brink of collapse; nearly half of the planet's land is dedicated to feeding a global population that will soon reach nine billion; freshwater scarcities in some regions are becoming acute; and, most frighteningly, we appear intent on wrecking the global atmosphere, the ecosystem on which all other ecosystems depend.

Environmentalists have found themselves being taken seriously, and it has proved to be something of a curse. As they are asked to come up with solutions for the cascading eco crises, internal divisions are becoming more obvious. The biggest divide may be between those who would do anything to cut carbon emissions and slow climate change-going so far as to support natural gas and nuclear fuel, or even supporting geo-engineering and other controversial ideas-and conservationists who don't want to trade one earth-damaging practice for another.

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"I feel like the community has splintered," says Chris Clarke, a writer in Joshua Tree, California, and a co-founder of the group Solar Done Right, which has battled the construction of utility-scale solar stations in the Mojave Desert that involve destroying vast stretches of wilderness. "Some people are unwilling to call themselves 'environmentalists' because 'environmentalist' has now come to mean climate-change mitigation at any cost."

Some environmentalists say the divisions have been fueled by gadflies looking to appear contrarian for the sake of minor celebrity. "I think, bluntly, that part of this is [happening] because there's some value to the post-environmentalists in hippie-punching," says Alex Steffen, a self-described "bright green" futurist who is the author of a new book, Carbon Zero. "Just saying, 'Oh, those guys are wrong'-since there are a lot of people who want to think that traditional environmentalists are wrong-is a great way to sell books and get speaking gigs."

It's true that some of the noise seems calculated for effect. But it would be dangerous to wave off the differences of opinion. A careful look at the environmental movement reveals a profound gap among people who share a worry about the state of Earth. There is a real split over what should be considered a smart survival plan for billions of people on a finite planet. That split, if it's not navigated constructively, threatens to sap the environmental movement's political muscle just when it is needed most to achieve its goal: keeping the planet healthy enough to maintain our civilization.

In a sense, today's differences are just a new variation on a century-old dispute. In the late 1800s and early 1900s, American environmentalists fell into two distinct camps. The first, led by Sierra Club founder John Muir, was part of the larger Romantic movement that viewed wild areas as pristine places that needed to be saved from the scourge of humanity's hand. The second, led by the founding head of the U.S. Forest Service, Gifford Pinchot, thought of nature more like a garden-something to be tended by man. Natural resources, in Pinchot's view, should be mindfully stewarded to conserve them for future generations.

The split between those who esteem nature for its intrinsic value and those who want to protect it for its instrumental value persisted through the years. Some 21st-century environmentalists-most prominently the leaders of The Nature Conservancy-now talk almost exclusively about environmental protection in terms of preserving ecosystem services. We should invest in nature and protect natural infrastructure because humans benefit from them: Wetlands blunt hurricanes, forests suck up carbon dioxide, clean rivers bring us water. At the same time, some environmentalists have been re-energized by a nascent grassroots movement to recognize legal rights for natural systems, an effort inspired by the new constitutions of Ecuador and Bolivia that grant nature formal rights.

Opposing opinions on what constitutes appropriate use of modern technology also divides some putative eco allies. An instinctual techno-skepticism has formed an undercurrent in environmental thought-at least since Silent Spring and the backlashes to the nuclear disaster at Chernobyl and near disaster at Three Mile Island. As worries intensify about unchecked greenhouse-gas emissions, however, some greens are rethinking their posture toward once-verboten technologies. James Hansen, the NASA climatologist who twice has been arrested at the White House while opposing the Keystone XL pipeline, has said, "Next-generation, safe nuclear power is an option which we need to develop." Nuclear power is anathema to many other environmentalists, but the British writer George Monbiot reversed his long-standing opposition two years ago and wrote in The Guardian, "Abandoning nuclear power at a time of escalating greenhouse gas emissions is far more dangerous than maintaining it."

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The use of genetically modified organisms also highlights this divide. Even as most rank-and-file environmentalists remain suspicious of them-with their vibe of Promethean overreach and their control by monopolist corporations like Monsanto-some self-identified greens say GMO technologies are the only way to feed a growing population. In a speech earlier this year, Mark Lynas, another British environmentalist, told the Oxford Farming Conference, "The risk today is not that anyone will be harmed by GM food but that millions will be harmed by not having enough food."

Another rift involves the geographic scope of individual environmentalists' concerns. Ever since Henry David Thoreau set up a shack on Walden Pond, environmentalism has been animated by a love of place. A righteous parochialism was the spark that inspired scores of successful environmental campaigns: a desire to protect this river, this forest grove, this mountaintop. On the other hand, environmentalism has also been animated by a planetary consciousness from the moment the Apollo mission beamed back images of a tiny blue marble floating in space. For a generation these two ideals were in chorus, exemplified best by the greenie bumper sticker: "Think Global, Act Local." But in the era of global climate change, a love for the local and a concern for the global might be in conflict.

This is best illustrated by the controversies over putting giant solar installations in the Mojave Desert and building a wind farm off of Martha's Vineyard. One person's blueprint for clean energy infrastructure is another person's unthinkable desecration of a beloved place. While some environmentalists argue that we have to pave parts of the desert with solar panels in order to save other parts of the desert from a four-degree Celsius temperature rise, others see that as heresy.

"I think the important split is actually between people are who thinking in planetary terms and people who are not," Steffen, the futurist, told me. "The key to intelligent planetary thinking is to recognize that goal number one is to be promoting the stability of planetary systems, and then figuring out goal number two: how to get the greatest set of interesting possibilities for humanity into that constraint. And I worry that this debate between 'old environmentalists' and 'post-environmentalists' or whatever totally misses the larger point. The only kind of conservation worth having is one that starts at those larger systems, talks about what is necessary to maintain their stability, and starts scaling down from there into the particularities of political contexts, and specific places, and technological systems."

Achieving those goals could get increasingly difficult, however, if the movement is publicly split, as has happened with the issue of hydrofracking for natural gas.

The Sierra Club, under the leadership of its previous executive director, Carl Pope, wasn't the only prominent environmentalist organization heralding natural gas as a bridge fuel that could take our energy system from carbon-intense coal to renewables like wind and solar. (When burned, gas emits about half as much carbon dioxide as coal.) Among the most vocal proponents of natural gas today are Ted Nordhaus and Michael Shellenberger, founders of the Oakland-based liberal think tank the Breakthrough Institute. Nordhaus and Shellenberger ticked off greens in the early aughts with the essay "The Death of Environmentalism," which urged green groups to rethink the core assumptions of their political strategy. The pugnacious pair is often bashed for their rhetoric, but the two are genuine in their hawkishness on the climate and their commitment to global equity.

"As we look ahead to the human-development challenge, we're going to need other kinds of low-carbon and zero-carbon energy," Shellenberger says. "If we have everything riding on solar and wind, then we have all of our eggs in one basket."

Nordhaus adds: "Look, we have two billion people who don't have access to anything other than wood and dung [for energy]. Assume a world of nine billion people. Now assume that we have perfect economic redistribution from rich to poor, and everybody makes $15,000 a year. And then just do the math on global energy use-it still triples. You can't meet that all with renewables."

But since the fracking boom began in earnest, a larger, anti-fracking grassroots has emerged. Small towns in the East that were unaccustomed to the thrum of the fossil-fuel industry have been shocked to find themselves surrounded by trucks and heavy machinery and with compressors in their back lots whirring all night long. Some homeowners had their wells contaminated with flammable methane. Places like Ohio and Arkansas that weren't used to seismic activity started to experience earthquakes when underground wastewater injections stimulated geologic faults. Today, the movement against gas fracking has become a cause célèbre (Yoko Ono and Mark Ruffalo have an "Artists Against Fracking" group) and is one of the most invigorating issues among grassroots environmentalists. At February's Forward on Climate rally near the White House, easily a fifth of the placards in the crowd of 35,000 had to do with gas drilling.

"No sensible person would ever be a proponent of shale gas," anti-fracking activist Maura Stephens says. "The number of people whose water is contaminated, I can't even count. And the number of people who have been given a gag order and been given shut-up money is incredible. The whole idea is to do the harm and then mitigate."

"Of all the forms of fossil-fuel extraction, fracking is the only one that is wrapped up in a green myth," says Sandra Steingraber, who wrote the letter against the Sierra Club. "The demand for energy is not some inexorable thing like gravity. We control that. And it's plain to me that we could reduce our energy use by half and entirely run our economy on renewables."

Nordhaus and Shellenberger have a nearly opposite worry: that the intensity from partisans like Steingraber and Stephens has forced some big green groups to retreat from gas. The World Resources Institute, a D.C.-based environmental research organization, is an example of that shift. As recently as early 2012, the organization was expressing qualified enthusiasm for gas as a "potential game changer" that "should be part of America's low-carbon energy mix." But when asked recently to comment on the gas controversy, Jennifer Morgan, director of the institute's climate and energy program, chose her words carefully. "It's an extremely fraught and tough discussion," Morgan told me. "I think we recognize both the risks-and the risks are significant-and the potential opportunity."

And, of course, the Sierra Club has retreated from natural gas under its new executive director. Last week at a conference in Santa Barbara organized by The Wall Street Journal, Michael Brune warned that fracking's greenhouse-gas emissions might be worse than coal due to leaks of methane, a potent heat-trapping gas. The club also has launched a new section on its website: "Beyond Gas."

Whether the question is shale-gas development, nuclear power, utility-scale solar and wind, or GMO crops, the core of the debate among environmentalists comes down to what's realistic. That, of course, is the same dilemma that confronts any political movement, whether on the right or on the left. But environmentalists' conundrum is especially complicated because it involves a system beyond our control: Earth.

Nordhaus and Shellenberger say their pragmatism is grounded in what is politically possible given a range of shitty options. In the other camp, Steffen, Steingraber, and Stephens also claim the mantle of pragmatism, one based on geophysical necessity. The existential threat of climate change has become a sort of projection screen: Either it confirms that we are locked into business as usual, or it's proof that we need to make a societal 180-degree turn in how we relate to the planet.

"Those of us who are calling ourselves the latter-day abolitionists, our idea of what's possible is grounded in physical and natural laws. How much water and land and resources do we need to feed ourselves?" Steingraber says. "My hope that is that we can help people imagine, have a vision of a future when blasting gas out of the ground to make our tea kettles whistle is just barbaric, which it is." It's a view Nordhaus and Shellenberger call naïve.

It's clear that, much of the time, environmentalists are arguing past each other. Beyond any debates over strategy or technology, the various factions of greens harbor completely different ideas about human nature and the planet's capacity to hold us. While some eco-policy wonks appear to have internalized the notion that there are no alternatives to our modern, energy-dependent ways, the environmental grassroots remain committed to encouraging a change in consciousness that will prompt a new, less resource-intense mode of living. It's as if the environmental movement is playing three-dimensional chess, but with the players operating on totally different planes.

Such differences of opinion aren't necessarily a bad thing. Political movements often benefit from some degree of ideological tension. The differences only become a political liability because our environmental situation urgently needs a solution. Carbon emissions continue to rise, the number of humans continues to grow, and Earth isn't getting any bigger.

The environmental movement has a surplus of good ideas for how to manage ecological problems. It's got plenty of smart and passionate people. The one key asset it doesn't have is time to sort its issues out.