According to data being gathered at the Mauna Loa Observatory in Hawaii, which has been monitoring atmospheric carbon dioxide since 1958, the CO2 concentration in the Earth’s atmosphere officially exceeded the 400 parts per million mark last week, a value not attained on Earth since humans were first human.

This ominous milestone comes at a time when the evidence that human activity is resulting in unprecedented climate change is now overwhelming. More important, perhaps, even if all greenhouse gas production ceased immediately, this elevated carbon dioxide level would persist in the atmosphere for thousands of years.

Indeed, even moving relatively quickly toward a carbon-neutral economy will still result in a net increase in CO2 in the atmosphere for the foreseeable future. But that is moot, because we are nowhere close to moving quickly in this regard anyway. Fossil fuel reserves have effectively increased, due to improved technologies for extraction, and investment in alternative energy sources has been limited due to artificially low prices on carbon-based energy. As a result, 2012 was likely another record year for human-induced CO2 production.

So in addition to undertaking dramatic global efforts to reduce present and future CO2 emissions, we need a strategy for addressing the carbon already up there. Recently, a broad group of geologists, planetary scientists, climatologists, social scientists, and physicists convened at the Origins Project at Arizona State University, which I direct, to explore such strategies. (Disclosure: Future Tense is a partnership of Slate, the New America Foundation, and ASU.) As an upcoming paper being prepared by 15 of the participants at the meeting will argue, we came to a broad consensus that there is an increasingly urgent need to seriously consider removing and sequestering CO2 directly from our atmosphere.

This effort should not be confused with ongoing efforts to capture CO2 and sequestering it at its source, for example, from outgoing flue gas from coal-fired power plants. That area is important, too, but it’s already being explored, and the technological demands are quite different.

Extracting CO2 from the atmosphere, even with its current level of 400 ppm, is very different—and in some ways more difficult—than extracting it from flue gas, where the CO2 concentration is much greater. But on the brighter side, extracting ambient CO2 from the atmosphere does not have to be anywhere near 100 percent efficient. Both of these factors imply different constraints on the extraction process that will affect its ultimate cost.