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Science corner: why carbon sequestration isn’t any better than avoided emissions


Update: This post has engendered a fair amount of confusion, for which I am mostly to blame. So let me provide some necessary context. Tree-planting projects are a popular type of carbon offset, but they unfortunately suffer from a large number of quality problems and do not in fact make for very good offsets.

However, one recurring argument made in favor of tree-planting projects is that they perform carbon sequestration rather than preventing new emissions. That is, trees absorb carbon that is already in the air, which some see as a useful complement to reducing fossil fuel use.

This notion rests on a misconception. In reality, there is no meaningful distinction between carbon already in the air and carbon not yet emitted. You should instead use other metrics to gauge offset quality, such as measurability, permanence, timing, and additionality. None of these metrics tend to favor tree-planting projects.

OK, on to the original post…

* * *

I’m on a tree kick lately, in case you didn’t notice. Today I’m going to set aside the controversial stuff and instead deliver some straight-up knowledge. Specifically, I’m going to address the common misconception that offsets from carbon sequestration (e.g., tree-planting) are somehow different in kind than offsets from clean energy (e.g., wind).

The misconception stems from a seemingly sensible intuition that sequestration deals with carbon that’s already up in the air, whereas clean energy deals with carbon that hasn’t yet been emitted. It’s the difference between cleaning up a mess you’ve already made vs. preventing a new mess from occurring, right?

Well, kinda sorta, but the distinction isn’t environmentally meaningful. Assuming the offsets are of equal quality, the end point in both cases is exactly the same. There are a couple of ways of explaining why this is so. I’ll mention two.

The first is to consider the fact that clean energy can be thought of as a type of carbon sequestration. When a wind turbine spins, it is in effect sequestering carbon in the form of coal, which otherwise would have been burned for electricity. This is an admittedly strange way of thinking about what a wind farm does, but it’s perfectly valid. Trees sequester carbon in the form of wood. Wind turbines sequester it in the form of coal. It’s not particularly relevant whether the carbon was formerly blowing around in the air or buried beneath our feet. All that matters is that the carbon is kept out of the atmosphere.

The second way is to consider the bucket analogy. You’re filling a bucket with water. You want a drink. You can fill your cup by dipping it into the bucket or you can fill it with water flowing from the tap. The end result is the same: the bucket is less full by a cup.

For those who like numbers, the bucket analogy can be turned into a simple algebra problem. There are ten tons of carbon already in the atmosphere, and every year enough coal is burned to add one more ton. This year, you can either switch from coal to wind energy (catching one ton of carbon at the tap). Or you can continue burning coal and also plant a bunch of trees (scooping one ton of carbon from the bucket). In either scenario, you’ve got ten tons of carbon in the atmosphere at the end of the year. QED.

Incidentally, the same logic applies to the misconception that trees clean the air of pollutants but wind turbines don’t. Trees are often credited with absorbing or filtering pollutants such as nitric oxide, sulfur dioxide, and carbon monoxide. But where do you suppose these pollutants come from? Right: from burning fossil fuels. So wind farms — by displacing the burning of fossil fuels — have a similarly salutary effect on air quality.

And this logic further applies to energy efficiency measures, such as installing a compact fluorescent light bulb. To paraphrase Gertrude Stein (no relation), a ton is a ton is a ton of CO2, which is why carbon offsets from different project types can be freely traded.

Photo available from flickr user Peasap under Creative Commons license.

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