more on the CO2 challenge

So I was just playing with the numbers to try to get a handle on the size of the challenge of sequestering a billion metric tonnes of carbon dioxide per year. My first thought was "hey, how about building materials?" After all, we build a lot of buildings around the world. Could we store carbon dioxide in our walls, essentially by growing a fast-growing plant (like, say, bamboo) and using it as a construction material? In that case, all the technology's already there--all we'd need to solve is the (probably much harder) problem of changing building codes.

Well, maybe not. Or at least, maybe not enough. According to Bamboo Living, some bamboo species sequester up to 12 tons of carbon dioxide per hectare per year. I dropped that number down to 5 because I figured we wouldn't be lucky enough to have the species all the builders want to use absorb that much. Then I pulled in some numbers from the CIA World Factbook to see how much land would be available for this project. Here's what I wound up with:

annual tons CO2 per hectare that bamboo absorbs: 5
convert to annual metric tonnes CO2/hectare: 4.55
target metric tonnes CO2 per year: 1,000,000,000 tonnes/yr
hectares necessary: 220,000,000 hectares
square km per hectare: 0.01
square km necessary: 2,200,000 km^2

So we need 2,200,000 square kilometers of bamboo. Where shall we put it? How about the U.S., since it's the world's largest producer of carbon dioxide?

total land area of U.S.: 9,161,923 km^2
total arable land in U.S.: 1,650,062 km^2

Oops, looks like it won't fit. Just for comparison, let's look at the total arable land in the world:

total land area of world: 148,940,000 km^2
total arable land: 19,823,914 km^2
total permanent cropland: 7,015,074 km^2

So we'd be looking at a sizable chunk of the world's cropland.

That's a lot of carbon dioxide.