Trees are terrific in every way but one: They make lousy carbon offsets. That was the point of the First Rule of Carbon Offsets. But a number of comments at the Grist blog, and some media queries, have led me include two rare exceptions: certified urban trees and certified tropical forest preservation. The word “certified” is key in both cases.
For these two rare cases, I would allow trees to comprise no more than 10% of an overall offset portfolio (which should be heavily weighted toward efficiency, renewables, fuel switching, and perhaps carbon capture and storage). Also, their offset value should probably be discounted over time (because urban trees are unlikely to be permanent and tropical forest accounting is quite uncertain).
Let’s start with urban trees. I am a big fan of those — I have coauthored a Technology Review article and blogged on how shade trees in particular reduce the urban heat island, providing direct cooling as well as reduced air conditioning use. A good article on urban trees as offsets is here.
I would especially support urban trees that were 1) planted as shade trees and 2) part of an overall heat island mitigation strategy that included lighter color roofs. That said, I am unaware of any tree offset program that actually focuses on urban trees — primarily because they tend to be more expensive to plant and more expensive to maintain and monitor then trees outside of cities, which can be planted in large number in a small space (rather than individually over a large city).
The tricky part of urban tree planting is to set up a certification system that ensures these trees are permanent — and not, say, cut down by some landowner expanding their house or lost in a storm. I expect these will be rare offsets.
Now to tropical forest preservation, which is clearly both important and difficult. These are rare offsets for two reasons.