Yet another study undermines the seemingly obvious concept that trees are inherently good for what ails the planet, climate-wise. Carbon-offset vendors take note: you could be making things worse. They're still needed in the Amazon, of course, but not so much in Ontario.
Tom Gower et al write in Nature that forest fires in northern boreal zones are helping turn forests into net carbon emitters instead of the sponges most people think they are. The paper, "Fire as the dominant driver of central Canadian boreal forest carbon balance," also puts the blame for the change in fire patterns on climate change:
We find that the carbon balance of this region was driven by changes in fire disturbance from 1948 to 2005. Climate changes affected the variability, but not the mean, of the landscape carbon balance, with precipitation exerting a more significant effect than temperature. We show that more frequent and larger fires in the late twentieth century resulted in deciduous trees and mosses increasing production at the expense of coniferous trees.... [and] climate and hydrological changes have the potential to affect disproportionately the carbon dynamics of these areas.
In simpler terms, Gower told the Globe and Mail: "The warmer climate has increased fire frequency and extent....Those wildfires have caused this transition in the boreal forest from a carbon sink to a carbon source."
So, at least in Canada, Siberia and other northern boreal forest ecozones, planting more trees is probably not a good idea if you are trying to offset your carbon emissions. (Organizers of the 2010 Olympics in Vancouver/Whistler are trying to make the games carbon-neutral, so they should probably be staying abreast of this sort of thing.)
This isn't the first evidence that the "more trees are better" approach is always a good idea, though. Back in 2000, Richard A. Betts of the Hadley Centre for Climate Prediction and Research in the UK warned, in Nature again, of the problem of albedo (reflectivity) that comeswith measuring the net effect of forests on atmospheric carbon-dioxide levels:
...in many boreal forest areas, the positive forcing induced by decreases in albedo can offset the negative forcing that is expected from carbon sequestration
Researchers at Duke University have also produced some solid data on the subject. At the time, policy analysts advising government negotiators trying to work out national greenhouse-gas emissions reductions targets loved the idea of getting credit for managing forests responsibly. When I put the problem of albedo to Canadian advisers, my question was treated as if it had come from the Twilight Zone.
It took about five years for Canada to give up on the idea of getting credit for forests. Let's hope that it doesn't take as long for those who are still promoting simplistic solutions to the carbon problem to accept what the science is telling them.
Forests, at least, are something we can control. By comparison, the news that the oceans are also turning into carbon sources is far more worrisome. Recent studies of both the northern and southern oceans show they're no longer absorbing as much carbon dioxide as most models assumed. David Archer at Real Climate opens a straightforward explanation of the science of ocean carbon sink/source research with this sobering line:
There are uncertainties and caveats associated with each study, but taken as a whole, they provide convincing evidence that the hypothesized carbon cycle positive feedback has begun.
I don't understand how trees could, over the long-run be net emitters of CO2. When a tree burns, all the CO2 released in the process is CO2 that was originally taken from the atmosphere in the first place, isn't it? There is no other source of carbon for trees, right?
I would think that it would follow that whatever carbon is locked up in the mass of trees must be carbon that was taken out of the atmosphere. It's hard to see how it could be otherwise (unless trees get a significant portion of their carbon from sources other than CO2).
"I don't understand how trees could, over the long-run be net emitters of CO2."
I think you're right, in the long term. However if you're losing more biomass in fires per annum than you're gaining through growth, then (in the short-term) I guess you have a source.
As to albedo - I can see what they're getting at, but it must depend on how much snow you get. Scotland's wintertime albedo was probably a lot higher 30 years ago than it is now, but this has nothing to do with our trees - we're just simply not getting much snow these days.