Does the new Duke study prove that tree-based offsets are no good?


I’ll spare you the suspense, and start with the answer: no, it does not.

Now the full story. A few weeks ago, I pointed out some of the well-known problems with carbon offsets from tree-planting projects. These include issues of permanence, timing, and measurability.

Coincidentally, only a few days later some researchers at the Nicholas School of the Environment and Earth Sciences at Duke University released some findings from a highly-regarded 10-year study of forest growth. According to accounts in the popular press, the main conclusion is that we can’t count on trees to soak up the excess carbon dioxide we’re putting in the atmosphere. The study got a smattering of attention from various green blogs, which generally took the result to be another nail in the coffin of tree-planting projects.

The problem with this reading of the Duke results is that the study doesn’t have anything to do with carbon offsets. If I’m reading it correctly (always a big if), the study is entirely concerned with whether forests will act as a natural corrective to global warming, even in the absence of any human-directed efforts to reforest the planet.

To understand why trees might act as a natural corrective, you have to consider the concept of feedback loops. Global warming is notorious for its many positive feedback loops, which are mechanisms by which climate change becomes self-reinforcing. For example, ice is a highly reflective substance, so glaciers help to cool the planet by reflecting sunlight back into space. A hotter globe causes ice to melt. Less ice means more sunlight hitting the earth. More sunlight means more warming, meaning less ice…and so on. There are many such positive feedback loops in global warming.

There are also some hypothesized negative feedback loops, mechanisms by which global warming could act as a brake on itself. One conjectured loop is the notion that a more carbon-rich atmosphere might stimulate plant growth. The plants would absorb some of the excess carbon, thus acting as a natural buffer against climate change. Such negative feedback loops are particularly popular among global warming denialists, who like to claim that climate change will fix itself if we just calm down and ignore it.

The Duke study casts doubt on this fantasy scenario. The problem, researchers found, is that plants can only make use of excess carbon in the atmosphere if they also have access to lots of water and nutrients. But global warming tends to cause drought, so it now seems likely that the excess carbon will be unavailable to plants.

This is a disappointing result, but it has little to do with tree-planting projects or avoided deforestation projects. The Duke study doesn’t imply that trees can’t sequester carbon. It just suggests that natural tree growth won’t keep pace with our emissions.

Of course, tree-planting projects still suffer from all of the problems I mentioned previously. The overall picture hasn’t changed one way or the other.

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  1. Dennis Beeson - August 22, 2007

    Windmill vs Tree:

    Which one cools the Planet……TREE

    Which one makes Rain………….TREE

    Which one Sequesters CO2………TREE

    Which one produces Oxygen……..TREE

    Which one clean the soil………TREE

    Which one purifies the Water…..TREE

    Which one takes less Fossil Fuels

    to Construct or Plant,,,,,TREES,TREES

    Which one would you rather look At~??


    Dennis Beeson / Wind Farmer now TREE FARMER~!!

  2. Adam Stein - August 22, 2007

    Hi Dennis,
    We agree that trees are great for all sorts of reasons. But the notion of “windmills vs. trees” is a very dangerously flawed way of approaching climate change. It’s pretty obvious that we can’t tree-plant our way out of our fossil fuel consumption.
    Some further points on this:
    Trees don’t make rain.
    Both windmills and trees reduce CO2 in the atmosphere — sequestration vs. avoided fossil fuel use is not a meaningful distinction.
    Both windmills and trees contribute to the level of oxygen in the atmosphere. Again, sequestration vs. avoided fossil fuel use is not a meaningful distinction.
    Cleaning the soil is good, but only windmills prevent the release of NOx, SOx, mercury, and other nasty pollutants by displacing coal use.
    Ditto for water quality.
    I have no idea whether trees or windmills takes less fossil fuel to build/plant, actually, but I can’t imagine this fossil fuel cost is very large compared to the downstream CO2 reductions.
    I personally find windmills to be quite pretty, although, granted, not nearly as pretty as trees. But one of the purposes of preventing global warming is to preserve the world’s beautiful places. Therefore, we have an obligation to find the most effective solutions.

  3. Dave C - August 23, 2007

    I still think we need to plant trees. Unless I am mistaken, carbon offsets do nothing to actually get rid of carbon dioxide that is presently in the atmosphere and the “major sinks” (i.e. ocean) are filling up. Carbon offsets assist us in limiting future creation of more carbon which is beneficial, but we still need to do something more to solve the problem.
    In addition, trees do A LOT MORE than just capturing carbon dioxide. Erosion reduction, habitat creation, wind breaks, and beautiful vistas are just a few other things trees give us.

  4. Adam Stein - August 23, 2007

    Hi Dave,
    You are sort of mistaken. You’re right that trees absorb carbon already in the atmosphere, whereas clean energy development prevents the emission of new carbon. So your assumptions are correct. The mistake is the conclusion that there is an environmental difference between these two forms of carbon reduction.
    In both cases, there’s less carbon in the air then there otherwise would be. In the former case, the carbon is sequestered in the form of a tree. In the latter case, it’s sequestered in the form of coal (or oil) that goes unburned. There’s no difference here, as far as global warming is concerned. The advantage with renewable energy is that the reduction takes place now, rather than over the next four decades.
    Trees do give us a lot more than carbon reductions. TerraPass customers are paying for carbon reductions, though, and so we seek the best sources of them.

  5. marvin - August 28, 2007

    Hey Adam,
    I went to the Duke site , and I am having trouble finding the study. Also do you know of a good place to find data on the CO2 absorption capacity of trees in general.

  6. Adam Stein - August 28, 2007

    Hi Marvin —
    I couldn’t find the study either, probably because the results were presented as part of a conference proceedings. My understanding of the results is based on various public discussions I’ve come across. A good set of quotes from the researchers themselves is available on Green Car Congress.
    – Adam