Trees are nice, but not good offsets. Photo: iStockphoto

The first rule of offsets, according to Joseph Romm, is “no trees.” This is a pretty good rule, as these thing go. The TerraPass offset portfolio contains no tree-planting projects, despite the fact that most consumers love trees and the fact that tree-planting projects are typically cheaper than offsets from renewable energy projects.

So if trees are both consumer-friendly and cost-effective, why avoid them? There are lots of reasons, and Romm chooses to focus on one of the more minor ones: a recent study suggesting that trees outside of tropical zones actually cause a net increase in global warming by absorbing sunlight.

I say this is possibly the least convincing not because I have any particular insight into the quality of the study. Rather, it’s just generally true that a single study based on a novel computer simulation can at best be described as suggestive, not conclusive. We’re a long way off from fully understanding the interplay between vegetation and climate. Given the perverse policy implications of a study suggesting trees actually contribute to global warming, extreme prudence is warranted.

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Regardless, many more fundamental reasons exist to be wary of trees as a source of carbon offsets. To be fair, Romm does touch on some of these in a subsequent post.

The biggest one is timing. A carbon offset represents not just a specific amount of greenhouse gas reduction, but also a specific period in which the reduction takes place. One of the most basic principles of offset quality is that, other things being equal, you want to sponsor reductions that are taking place now, not at some far-off point in the future.

Unfortunately, trees grow rather slowly. And particularly when they’re small, they don’t sequester much carbon. The small print on tree-planting offsets typically indicate a 40-year maturity. If you buy a tree-based offset today, you’re sponsoring a reduction that won’t be complete until 2047, by which time we’ll either be living in hurricane-proof seaside bunkers in the Rockies or flying around in hydrogen-fueled jet cars.

A second concern with tree-based offsets is permanence. An offset is only an offset if the reduction is real and ongoing. Trees have an unfortunate habit of dying or being cut down. Particularly given the time frames involved, with all the attendant issues over land rights, it can be very tricky to say what will happen to an individual forest several decades down the road. Some offset companies claim to guard against this risk by padding their tree offset purchases, but such tactics don’t seem to guard against large-scale deforestation.

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There are additional problems with tree-planting projects, which I catalog below. But before delivering the whole list, I want to provide some perspective to this downbeat picture.

The first bit of perspective is that tree-planting projects make up an extremely small percentage of offsetting projects worldwide. For example, reforestation accounts for 6 out of 1,783 projects in the CDM pipeline. Consumers are disproportionately aware of trees because such projects make up a disproportionate share of the tiny voluntary market. As mentioned, marketers love these projects because they’re cheap and consumer-friendly. I wish this weren’t so, but it doesn’t really affect the worldwide market very greatly.

A second bit of perspective is that, despite these problems, it would be really great to bring tree-planting projects credibly into the carbon offsetting fold. Deforestation is the cause of 20 percent of anthropogenic global warming. And of course, deforestation is an environmental problem for dozens of reasons beyond just carbon emissions. Opening up another revenue stream to protect forests is an extremely worthwhile goal.

A final bit of perspective: the problems I have outlined, though serious, are not necessarily insurmountable. Organizations like the Pacific Forest Trust are trying to address issues of timing and permanence through novel offsetting protocols that rely more on forest preservation than just tree planting. Such protocols track biomass accumulation year by year, rather than front-loading future growth. The resulting offsets are far more expensive than typical tree-based offsets, but as the price of carbon rises, they will become economically viable.

So the first rule of offsets stands: unless you’re willing to do a lot of diligence, you’re best off avoiding trees. But let’s keep our fingers crossed for innovation in this area.


Postscript. For the sake of completeness, here are some further issues with tree-planting projects as a source of offsets. A third concern, after timing and permanence, is measureability. It’s fairly complicated to measure the amount of carbon absorbed by a forest; some planting practices can actually result in a net release of carbon from the soil. A fourth is the aforementioned sunlight absorption issue. A fifth is the possibility of “leakage,” which means that the new trees just displace deforestation, rather than reduce it. Sixth is a variety of well-documented quality issues with some tree plantations, such as monocultures of nonnative species, although these are addressable through proper project design.

In general, I find the fundamental issues of timing and permanence to be much more important than project-specific quality control issues, which hopefully will work themselves out in the longer term.

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