As we’ve writtenhere before, forests have gained a lot of attention in the climate change conversation because of their ability to suck carbon out of the atmosphere. Individuals can buy “reforestation” offsets on the Internet. There’s talk of including credits for carbon stored in trees and wood products as part of many proposed cap-and-trade systems. Cities and businesses are even planting trees as part of their efforts to slow climate change.

There’s no doubt that forests, and their tremendous ability to store carbon, can play a role in protecting the climate. But we have to be cautious about that role. Forest ecosystems are, by their nature, unpredictable-there’s simply no way to know how much carbon a forest will store over the long haul, Worse, climate change itself magnifies those uncertainties. If a warmer climate makes forest fires more frequent – as some people believe is possible – then a lot of “offsets” will simply go up in smoke. Or consider BC’s devastatingpine beetle infestation – an example of how ecosystem disruption can fell more trees than any chainsaw.

And there’s troubling news today that makes us more cautious than ever: A new global study by researchers at the University of Helsinki shows that trees are absorbing less CO2 than predicted, as the world warms and vegetation patterns shift.

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  • It turns out that changing growing seasons may shorten the seasonal window when vegetation is a “sink”— that is, taking in carbon—rather than extending that season. That means plants are spending less time as net carbon sinks, and more time as net carbon sources. Not good.

    The Guardian reported the study’s main points today:

    • The ability of forests to soak up man-made carbon dioxide is weakening, according to an analysis of two decades of data from more than 30 sites in the frozen north.
    • The finding means that more of the CO2 we release will end up affecting the climate in the atmosphere rather than being safely locked away in trees or soil.
    • The results may partly explain recent studies suggesting that the amount of CO2 in the atmosphere is increasing faster than expected.
    • If higher temperatures mean less carbon is soaked up by plants and microbes, global warming will accelerate.

    Climate scientist John Miller of the University of Colorado put it this way in his commentary on the study for the journal Nature: “We are currently getting a 50 percent discount on the climatic impact of our fossil fuel emissions” (That is, half of what we pump out is sucked up by the oceans and ecosystems on land.) “Unfortunately, we have no guarantee that the 50 percent discount will continue.”