Let's review. Carbon concentrations in the atmosphere are fluttering around 385 parts per million right now, a level that's already forcing uncomfortable changes—species hopping to higher ground or dying off, polar ice caps and glaciers melting. There's general agreement among IPCC types that we shouldn't climb past 450 ppm, or else we're just inviting disaster into our living rooms and offering him a tall stiff drink. At the same time, some scientists, like NASA's James Hansen, think we need to reverse course and go back down to 350 ppm, that we've already passed the danger zone.

Is that doable? After we've replaced every coal plant with [clean electricity generator of your choice] and swapped out every car for a [magic emissions-free model], is it even possible to suck carbon out of the air? Reforestation can help, but we'll likely need more than that. Biochar is promising, and conceivably that's part of the answer. But what about gizmos, gadgets—technology! The basic problem here is that it's incredibly difficult and energy-intensive to take a stack of a million little dots and pluck out the 385 carbon ones. (By contrast, it's much easier to scrub out the carbon from flue gas, which is how carbon capture at coal plants works—the main headache there is securing the carbon deep underground; capturing it, though, is fairly simply.)

But that doesn't mean it's impossible. Robert Kunzig and Wallace Broecker of New Scientist recently reported on three budding new technologies that could help us scrub carbon out of the air. They're all still swaddling infants, but there's plenty of interest from investors. In theory, if we could invent air scrubbers that can pluck CO2 out of thin air, then we could hook them up to anything—car tailpipes, airplanes, heating systems—and filter out the carbon from mobile and point sources where it's usually impossible to capture carbon. Alternatively, because CO2 mixes in the air rapidly, and levels are the same pretty much everywhere in the world, we could just hook up a scrubber over a suitable sequestration site and turn on the vacuum. Voila!

The problem is that this stuff's still extremely pricey. One scrubber model involves a field full of sun-tracking mirrors that heat a tube that removes carbon from the air. It's all very nifty, but with its price tag, if you're going to line a desert with solar concentrators, you may as well just put them to work generating carbon-free electricity. A second scrubber idea would only be profitable if carbon cost about $100/ton—far beyond what carbon costs in Europe right now, although we could eventually hit that price if we get a tight emissions cap. (In the meantime, they may well fill a niche role taking carbon from the air and selling it to greenhouses.)

So it's unlikely that this will ever be the quickest and easiest way to avert a climate fiasco. No sense getting deluded that this is a shortcut to avoid mitigation. That said, the day may well come when we've de-fossil-fuelized the global economy and reduced our emissions to negligible levels yet still need to shovel carbon out of the air to avoid destabilizing the climate. In that case, we'll have to give scrubbers like these a serious look.

--Bradford Plumer