It may be odd to focus a political movement on a relatively obscure bit of science, but a world-wide push to limit concentration of atmospheric carbon dioxide to 350 parts-per-million made a big splash last week, with rallies and gatherings all over the planet focusing on drilling this number into the public consciousness. The number comes from work done by (among others) NASA's James Hansen, looking for potential climate "tipping points." 350ppm for CO2 is a safe limit—get too much beyond it, and the dangers multiply.
It's an audacious goal, for reasons of both communication and science.
In terms of communication, while a simple meme like "350" or "350ppm" fits nicely on protest signs and bumper stickers, it's a term without much context for the vast majority of the populace. In and of itself, that's not a problem; however, it can make a visceral connection to the concept more difficult. Activists adopting the 350 meme will need to match rhetoric with education, to make the number meaningful. Again, not impossible, but likely an ongoing challenge.
The scientific audacity with the 350 meme comes from a single, simple fact: current concentration of atmospheric CO2 is roughly 385ppm. That is, we already exceed the 350 limit, and most climate scientists say we'll be hard-pressed to keep from going over 450ppm by the middle of the century. And carbon dioxide takes centuries to cycle out of the atmosphere—even if we stopped all anthropogenic sources of CO2 right this minute, we'd still see too-high concentrations for years to come.
(Even more troubling: even if we stopped all anthropogenic carbon sources immediately, we'd still see continued warming for at least decades, possibly longer, simply from the thermal inertia of the oceans. Absent a radical step, we're guaranteed to see at least another degree or two of warming, no matter what we do.)
If this sounds like I think the 350 movement is a bad idea... I don't. I rather like the simplicity of the meme, and the target is—if difficult—smart. It's not saying "let's keep things from getting too much worse," it's saying "let's make things better." That's the kind of goal I like.
But getting back to 350ppm requires more than a rapid cessation of anthropogenic sources of atmospheric carbon. It requires an acceleration of the processes that cycle atmospheric CO2. Planting trees is an obvious step, but it's slow and actually doesn't do enough alone. We'll also need to bring in more advanced carbon sequestration techniques, such as bio-char. The combination of the two would likely bring down atmospheric carbon levels, given enough time.
Unfortunately, we may not have enough time.
If efforts to eliminate carbon emissions continue to happen at a pace most generously described as "leisurely," we will almost certainly face a situation where we approach and even pass critical tipping point concentrations. Ocean thermal inertia means that climate benefits from emission cessation won't be seen for decades. There's a very real scenario where finally get it right, both cutting out anthropogenic emissions and sequestering megatons of carbon via plants and bio-char ... and still face terrible environmental consequences, simply because we didn't act fast enough.
That's where we start to talk about much more radical, and potentially dangerous, steps. Geoengineering to hold temperatures down is one; to meet the 350ppm goal, we will likely also start looking at large-scale methods to sequester carbon, such as with triggered algae blooms.
350ppm is an audacious goal, but one worth striving for. But its challenge comes not just in the effort to eliminate anthropogenic carbon emissions around the world—a massive endeavor alone—but also in figuring out how to remove the extra carbon already there. I hope that the 350 leaders have thought through the implications of what that means.