“If I sound a little testy,” says David Hawkins, sounding testy, “it’s because I see 20 complaints a day about this idea, and I don’t think it’s very helpful at this point.” Hawkins, director of climate programs for the Natural Resources Defense Council, has been fighting a mostly uphill battle to get the American political system to deal seriously with global warming. Just in the past few months, he has seen signs of progress, including new power-plant regulations from the Environmental Protection Agency and a few cracks in the Republican Party’s monolithic opposition to action on climate change.

Leading up to the long-awaited global climate summit in Paris, starting today, both China and India have said they would be cutting their projected emissions of carbon dioxide, a major step in itself that incidentally undercuts the argument made by Sen. Marco Rubio, R-Fla., at the second presidential debate, a significant Republican talking point: that it’s pointless for America to regulate carbon emissions because the rest of the world won’t go along.

So at this critical juncture, the last thing Hawkins wants is a bunch of noisy zealots proclaiming that saving the world requires a multitrillion-dollar project not just to reduce emissions of carbon dioxide, but also to actually remove the carbon dioxide that’s already in the air. You read that right: to run some substantial fraction of the Earth’s atmosphere through a network of machines to extract carbon dioxide and put it back underground — in effect, putting the entire fossil-fuel industry, built up over a century and a half of furious industrial activity, into reverse. If so-called “net negative emissions” could be achieved easily, it would be the greatest thing that could happen to the environmental movement — and, for that matter, to the fossil-fuel industry, which would no longer be on the hook for causing global warming. But to Hawkins, the idea is at best a costly distraction from the more achievable and urgent goal of moving toward renewable, “net zero” energy sources. “We’ve got fully proven technologies that we know the costs of,” he says, “so our priority should be on making them attractive to private industry.”

So-called “carbon removal” is a step further even than the not-yet-perfected technology for “carbon capture” — scrubbing CO2 directly from power-plant flue exhaust, where it is relatively concentrated. Even that is difficult to do today at a reasonable cost — but carbon dioxide in the ambient air is 300 times more dilute, making the problem even harder (although, for technical reasons, not 300 times harder). But that’s exactly what a growing number of climate scientists are urgently calling for. The view has been gathering support at least since 2009 when scientists from the National Oceanic and Atmospheric Administration concluded that “the climate change that is taking place because of increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop.

Or, as Tim Kruger of Oxford University explained at a World Economic Forum conference last year, carbon dioxide levels in the atmosphere, now around 400 parts per million and rising, will remain well above the safe level of 350 by the end of this century even “if everybody on the planet dies.”