A recent study (pdf) by my old friends from the forestry department at OSU finds that when you add up the gains and losses, ecosystems in Oregon stored about 8 million tons of carbon per year between 1996 and 2000. The forests west of the Cascades, in particular, were prodigious carbon sinks. (A carbon sink is basically something that removes more carbon from the atmosphere than it releases; it’s mostly being stored in trees and soil in this case). 

In case you’re wondering, 8 million tons is a lot of carbon storage.  In fact, it’s enough to offset about half of the state’s total fossil fuel emissions.

Which raises a huge question: given the huge amounts of carbon that Northwest forests can capture and store, what role should they have in climate policy?

  • I still haven’t decided how I feel about this issue.  Forests have gained a lot of attention in the climate change debate because of their ability to suck carbon out of the atmosphere.  You can support reforestation projects by buying offsets on the internet.  Land managers want to sell credits for carbon stored in trees and wood products as part of a cap-and-trade system.  Some cities are even planting trees as part of their efforts to slow climate change.  Forests store so much carbon that policymakers ought to pay attention to them. But as this study shows, integrating forest protection or reforestation into, say, a cap and trade system carries huge risks.

    To explain what I mean, let’s start by looking at one of the figures from this study (Bear with me, I promise you will be smarter at the end of this paragraph.) The top line represents yearly “Net Primary Production” or NPP, over 20 years.  NPP is basically the amount of carbon trees take out of the atmosphere through tree growth.  The middle line represents “Respiration” (Rh), or the carbon losses from decomposition of dead needles and wood.  The bottom line represents Net Ecosystem Production, or NEP.  Stated simply, NEP = NPP minus Rh. Carbon storage equals uptake minus releases. 

    So when NEP is positive the state’s ecosystems are a carbon sink (taking carbon out of atmosphere) and when it’s negative they’re a carbon source (releasing carbon to the atmosphere).  And as this graph shows, the losses are fairly constant while the gains change a lot from year-to-year. How much forests grow each year determines whether the state is a source or sink.

    carbon sink chart_280

    As someone who’s worried about over-reliance on forests as a major tool of climate policy, these three things concern me most:

    • Oregon was a big carbon sink some years and a source in others.  For example, the state was a net source of carbon in 2002 for two reasons: the notorious Biscuit Fire (which caused huge carbon losses all by itself) and dry weather (which slowed tree growth).
    • The size of the carbon sink is closely correlated with precipitation, especially in the spring and summer.  When it’s really dry, trees don’t grow as much (Makes sense, right?)  But most regional models of climate change predict that Oregon’s summers will get warmer and drier. That suggests that Oregon‘s forests may be a smaller carbon sink in the future, or even perhaps a source of carbon emissions. Moreover, drier weather conditions can also lead to more fires, which further reduce carbon stores.
    • According to this study and others, the best way to store carbon in forests is to avoid logging them (or at least cut them less frequently), because wood products store only a portion of the carbon stored in standing forests. Great, so we should stop logging or increase rotation times in the U.S and Canada, right?  But there’s a problem here too: unless we make meaningful changes in our consumption of products, preserving our homegrown forest will probably come at the expense of forests in the developing world.

    If forest carbon offsets are going to have an important role in climate policy, the storage they provide will need to be permanent, predictable, and real.  But if you buy the argument above, none of that is guaranteed.  In a warmer world, even old-growth forests aren’t necessarily permanent; year-to-year variations in growth, as well as forest fires, make forest carbon sinks unpredictable; and storage may not even be real, if protecting our forests means that others are cut in their place.

    Don’t get me wrong:  I think managing forests for increased carbon stores has a place in our climate policy puzzle.  I just find little reason to think that Northwest’s forests should be a central part of that equation.