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Shorter Winters Weaken Forest Carbon Sinks

Posted by Anna Fahey
01/03/2008 06:05 PM
New study: trees are absorbing less CO2 than predicted.

As we've written here 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 devastating pine 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.

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."  



Comments
Posted by Dan 01/04/2008 10:49 AM
Let me point out, Eric, that my new home is also having serious issues with bark beetle infestation in our lodgepole and Englemann spruce stands.

Second, beetle epidemics have occured in the past. Nonetheless, it does appear as if man-made climate change is making the situation worse.

But the overall point with beetles is that increased chaoticality in weather and climate is expected from ongoing man-made climate change, making planning and investment that much more difficult. This increases the chances of economic instability, which can lead to social unrest, yada.

Couple this with the US acting as if Canada is bad when they are "dumping" softwood on the market. They're not "dumping" on purpose - what else are you going to do with millions of hectares of trees after you cut them down? Burn them (we see firewood bundles in the grocery stores with blue stains on them now)? Build a billion houses to sit empty?

Anyway, our society has hardly begun to discuss these issues. We're still flailing away.

We need some leadership across societies to step up and start tackling these issues. They aren't going away.

Posted by John Lee 01/06/2008 05:49 PM
The study may show that "trees are absorbing less CO2 than predicted", but I couldn't tell that from reading the Guardian's reporting.

That article told me the studied northern forest plots stopped sinking carbon into new growth sooner in the fall than expected: it didn't tell me why, and it didn't tell me what the actual annual growth volumes / carbon tonnages were in the study areas. In fact, from the Guardian's comment that the study " focused particularly on the date in autumn at which the forests switched from being a net sink for carbon into a net source" I feel that I can't be sure those were directly measured at all.

Maybe you can tell us more, Anna, or find a way to link directly to the study itself.

[ If the key finding is a surprise in the date when the carbon flow balance shifts, I'd want to know a lot more about local precipitation patterns before ascribing the early date to some unexplained mechanism linked to global warming.]
Posted by Anna 01/08/2008 09:35 PM
John Lee,

I poked around for the original study and couldn't find it online. Too bad the Guardian doesn't link to it directly. You're right -- there are important details missing in the write-up that leave us with a bunch of questions. The take-home, though, is that counting on forests as sinks is becoming more and more unpredictable in general -- whether it's because of growing seasons, fires, storms, or economic conditions.


Another point raised in the Guardian piece:


The precise effect the trend will have on future warming is hard to predict, said Colin Prentice of the University of Bristol. "Over a longer period of decades, models predict changes in vegetation structure, including tundra regions becoming forested, and the forests tend to take up far more carbon than the tundra. So I would be skeptical about reading any particular future implication into these findings."


Most importantly it may be a convincing reason not to put all -- or even a lot -- of our climate policy eggs in the carbon sink basket.

Posted by Dan 01/09/2008 11:08 AM
Here is the study.

I no longer have a sub., but reading the first para., the increased respiration is the reason for the decreased sequestration (metabolism used in respiration takes away potential for storage products [C, as in making sugars and starches, wood]). Results derived from a model and NDVI measurements.

HTH.

Posted by Dan 01/22/2008 04:27 PM
Here is another paper that finds tropical forests are absorbing less CO2 as well:

    Changes in growth were significantly associated with regional climate changes: at both sites growth was negatively correlated with annual mean daily minimum temperatures, and at BCI growth was positively correlated with annual precipitation and number of rainfree days (a measure of relative insolation). While the underlying cause(s) of decelerating growth is still unresolved, these patterns strongly contradict the hypothesized pantropical increase in tree growth rates caused by carbon fertilization.

    [...]

    While the cause(s) of decreasing tree growth rates is still poorly resolved, the patterns at BCI and Pasoh are most consistent with a decrease in productivity due to changes in
    regional climate including rising temperatures and increased cloudiness.


Although the implication that increasing CO2 in the atmosphere is not strong - land-use changes may be a factor as well - there is a strong implication that whatever cause, it is man-made.



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