David Roberts strongly objected to a critique of offsets and especially of credits for tree planting. The critique was originally made in the comments section of a post on a “carbon neutral” Super Bowl.

Bruce Sterling chimed in, noting that nobody can compete for purity with the dead. This is first rate irony, but unless the intention is that no one should ever criticize false solutions, no matter how wrongheaded, it only has bite if the solutions critiqued actually work. Tree planting may do all sorts of good things, but outside the tropics, it is not a significant way to fight global warming.

Let’s start by looking at the Livermore study (PDF) showing that tree planting outside of tropical zones does not result in net cooling. The key here is albedo — reflectivity of light. Trees are darker than the grass they replace. (If soil is wet and fertile enough for trees but does not have them, odds are there will be grass or other ground cover.)

So, more sunlight is converted into the long-wave radiation that greenhouse gases trap. A number of commentators on the Super Bowl post sneered at the idea that albedo can significantly affect global warming. But it is a known feedback, and one of the ways the melting of the icecaps speeds climate chaos. At the low level of potential sequestration we are talking about it is significant.

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In the tropics, conditions are different. In the intense sun of that environment, tree photosynthesis is far superior to grass photosynthesis; also, trees transpire more than grass. (Transpiration is a process whereby plants emit water vapor — as part of the circulation of nutrients, but also for cooling purposes.)

There are still problems with the tree plantations that are the most common form of tree planting in the tropics. In terms of global warming, these problems include the fact that the trees are often replacing other trees, and they are often harvested. In terms of other ecological effects, these plantations are usually monocultures, reducing biodiversity. In terms of human effects, local inhabitants are often thrown off their land to make way for these plantations. However, not all tropical tree planting is in such plantations; tree planting can make real contributions to tropical life. (Even then, carbon credits are not the right way to encourage tree planting: more on this later.)

Now is this rock solid consensus science? No; though I will note it was peer reviewed before publication, and a year after publication has stood up to all criticism so far. But I would compare critiques made of the net energy of corn ethanol. Even if corn ethanol produces a tiny amount of net energy, the difference between net input and output is so small that you would be much better off investing the money in other things. (Cellulosic ethanol is an entirely different story.) At the moment, the argument that planting trees outside the tropics results in net warming has not been refuted. But even if it is, I suspect the refutation won’t show any large net sequestration. I predict that any successful rebuttal will show such a small net sequestration per tree that you can gain much greater reductions by investing money in expensive PV electricity generation. In fact, I will bet a Super Bowl t-shirt on that, if anyone is interested in a wager.

Note, by the way, that this is not an argument for cutting down existing trees, which contain decades or centuries of stored carbon. It is not even an argument against planting trees outside the tropics. (Net heating is very small and can easily be compensated for by other types of savings.) It is an argument against planting new trees outside the tropics with fighting global warming as the main goal. It is simple realism not to do what doesn’t work, or attribute some virtue to a process that it does not possess.

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Offsets in general are a really poor idea, but even where trees do sequester carbon, offset credits from trees are an especially bad idea. First, most tree offsets (including those purchased for the Super Bowl) are planted after purchase. That means you emit, now, and the offset occurs over the decades that follow. Even if the sequestration is calculated correctly, feedback from carbon emissions ensures that this results in a net loss. That is, you emit X amount of carbon. This results in Y additional feedback. But you only offset X.

Worse, the offset numbers can’t be right. The problem is that carbon fixation in plant matter varies a lot — between species, between the same species in differing micro-climates and soil, between the same trees at differing times. You really can’t predict how much carbon a tree sequesters. If you can’t put a number on it, it still might be a good idea to encourage it. But if you can’t put a number on it, it is a really bad idea to use like a medieval indulgence, an excuse for emitting carbon elsewhere.

This is a problem with offsets in general. With offsets, even production of renewable energy, you can’t know how much would have been done in any case. This is known as the “additionality” problem, because no one knows how much additional reduction you are gaining. Further, since we don’t have the ability to visit alternate worlds and see what would have happened, we will never know. So when you grant someone credit for a quarter ton of emission reductions, and someone buys that and uses it to emit a quarter ton of CO2, you never know whether you have really broken even or whether you have actually increased net emissions. Both buyer and seller have a strong incentive to assume that the result is a wash; we have no certain way to detect whether their guess is correct or not. So emission credits are the ultimate contradiction — a market mechanism trying to work without feedback; price signaling that does not convey information.

I have problems with emission trading in general. But when someone has a target they must comply with, and can generate credits only by exceeding that target, at least you’re selling something measurable. In comparison, project-based offsets are a nightmare; there is really no feedback beyond the market collapsing from too much counterfeit currency.

Tree planting and encouraging renewables are both good things. But they don’t offset carbon emissions, and should not be hyped as doing so by an environmental magazine. Oranges are healthy and good tasting. But they are not a significant protein source. Your local nutritionist can expect criticism if they say otherwise.

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