Earlier this year, when the fascist responsible for the El Paso massacre cited ecological degradation as part motivation for his killing spree, many considered him entirely deranged.

Eco-fascism sounds oxymoronic, a mashup of irreconcilable philosophies.

Yet, while eco-fascists violently oppose contemporary environmentalists, they often appropriate ideas from the past of the environmental movement.

In the United States, the first conservationists – men like Teddy Roosevelt – were wealthy big-game hunters trying to preserve creatures they liked to shoot.

These elite enthusiasts for “the manly sport with the rifle” blamed impoverished arrivals from eastern and central Europe for “pot hunting” – that is, for killing game for food or money rather than for fun. But they also saw immigrants as polluting America’s racial stock, making an explicit parallel between “lesser races” and the invasive species threatening native animals and plants.

On that basis, Julia Scott from the Daughters of the American Revolution could speak at the National Conservation Congress of 1910, urging attendees not only to protect flora and fauna but to conserve “the supremacy of the Caucasian race in our land”.

You can get a sense of the centrality of eugenic theory to the early movement from the career of Madison Grant, the most important environmentalist of his generation.

We can thank Grant for saving the American bison, the bald eagle, the pronghorn antelope, the Alaskan bear and the fur seal, as well as, according to his biographer, contributing to the preservation of elephants, gorillas, koalas and many other charismatic species.

But Grant also campaigned for the racist Emergency Quota Act of 1921 and the Immigration Restriction Act of 1924, and wrote the bestselling The Passing of the Great Race, or The Racial Basis of European History, an impassioned defence of white supremacy that Hitler described as his personal “bible”.

The book’s popularity in Germany was not coincidental. Grant, like many of his colleagues, derived his environmentalism from a conservative Romanticism, the philosophical source for Nazi concepts about the importance of Lebensraum (living room) and Blut und Boden (blood and soil).

Modern eco-fascists still draw on the same Romantic contrast between the sublime hierarchies of nature and the supposedly effete degeneracy of modernity.

They also exploit the legacy of a tendency influential in environmental circles in much more recent times.

In 1968, Paul Ehrlich published a book called The Population Bomb, in which he argued that ecological destruction – and, indeed, almost all social problems – could be attributed to overpopulation.

As the historian Thomas Robertson notes, few signs of a mass environmental movement existed when The Population Bomb first appeared. Yet, “by the spring of 1970, Americans could hardly pick up a magazine or a newspaper without seeing mention of ecology and the environment”. That year, the first Earth Day attracted an astonishing 20 million people to environmental teach-ins, a result attributable at least in part to Ehrlich.

Today, his influence – and that of population theory more generally – has waned considerably, not least because the rate of world population growth has slowed substantially while his predictions of ever-worsening famines in the 1970s proved spectacularly wrong.

But progressive environmentalists also recognised the succour populationism provided to the extreme right.

The attribution of ecological destruction to demographic growth obscures the social relations through which, for instance, a mere 20 fossil fuel companies can be linked to more than one-third of greenhouse gas emissions in the modern era.

Worse still, arguments that (in theory) blame all people often (in practice) target particular people: usually the poor and the oppressed.

The Population Bomb itself opened with Ehrlich describing how he’d understood the case for population control “emotionally” during a visit to Delhi where, he said, the streets “seemed alive with people” and the “dust, noise, heat and cooking fires gave the scene a hellish aspect”. At the time, Delhi housed some 2.8 million while the population of Paris stood at about eight million – and yet it would be difficult to imagine Ehrlich reacting with equivalent disgust to crowds on the Avenue des Champs-Élysées.

Other populationists came to oppose not just fertility but also immigration, on the basis that, if the teeming masses from poor nations moved to the rich world, they’d adopt a western resource-heavy lifestyle. As a result, as academics Sebastian Normandin and Sean A Valles argue, America’s modern anti-immigrant movement “was built and led by – and in some cases is still led by – a network of conservationists and population control activists”.

The vast majority of greens now disavow the environmental populationist John Tanton, who, according to Susan A Berger, constructed many of the anti-Mexican organisations relied upon by Donald Trump as he electioneered for his border wall during the 2016 poll.

Yet the argument that ordinary people, rather than social structures, should be blamed for climate change still circulates – and invariably pushes in rightwing directions.

Think of the El Paso shooter and the document in which he justified his racial massacre.

“If we can get rid of enough people,” he wrote, “then our way of life can become more sustainable.”

The glibness with which he advocates environmental murder marks the El Paso perpetrator as distinctly fascist.

Fortunately, genuine fascists remain on the political margins in the English-speaking world. Nevertheless, we can increasingly imagine the space that eco-fascism might occupy. When asylum seekers fleeing floods and famines arrive on Australian shores in their millions, how will politicians respond?

Unless there’s a radical shift in the political culture, they will, presumably, rely upon the strategies perfected during decades of bipartisan anti-refugee campaigning, deploying the familiar arsenal of boat turnbacks, naval patrols and offshore detention, albeit on a much, much larger scale.

In other words, they’ll embrace an approach already advocated by European racist populists like Marine Le Pen’s National Rally, who posits massively intensified border policing as a “realistic” response to the inevitability of the environmental disaster.

Just as the stop-the-boats campaigners of recent decades depict refugee advocates as indifferent to drownings at sea, the supporters of “green” border policing will denounce climate activists for sabotaging “practical” responses to climate change with their utopianism.

It’s not difficult to imagine “eco-authoritarianism” or what Naomi Klein calls “climate barbarism”: a politics centred on the state making “our way of life” sustainable as the environment disintegrates. Future governments committed to this project will be able to draw upon the vast array of coercive powers they’ve acquired over the past decades: draconian anti-protest laws; secret trials and imprisonment; the deployment of the army to quell civil disturbances; and so on.

Eco-fascism represents a related but distinctive tendency. It will emerge not through the state but as a political movement, with people like the El Paso perpetrator violently defending climate privilege against immigrants, environmentalists and progressives.

We’re nowhere near that point yet. But it’s a lot less unimaginable than it should be.

Jeff Sparrow’s new book, Fascists Among Us: Online Hate and the Christchurch Massacre, is out now through Scribe