Last year brought glum news that Canada's forests are only a so-so defence against global warming.
Today it gets a little worse: We thought our forests were getting better at soaking up greenhouse gases, but they're not.
A study by Canadian, Chinese and European researchers shows that as the climate gets warmer, northern forests aren't soaking up extra carbon dioxide from the air after all. Forests may, in fact, become worse at storing carbon if climate trends continue.
Canada has always argued that our forests strongly "offset" some of the fossil fuels we burn. Our official position is that Kyoto-style climate plans should give Canada credit for the good work our forests do.
However, a series of studies over the past two years, and continuing today in the journal Nature, is calling that into question.
Forests soak up less pollution that we'd hoped. But even as Canada realized it had over-hyped the air-cleaning work done by forests, one apparent piece of good news emerged.
Scientists noticed that the global warming trend was waking up trees earlier each spring. As well, the trees were staying green longer into the autumn.
This longer growing season, they reasoned, meant that trees should work longer each year at building new branches and leaves -- the process that soaks up carbon from the air.
So, shouldn't that get rid of more carbon dioxide? No, says today's study by the Global Carbon Project, a multinational science network that includes Canada.
The study focuses on years of data-gathering - largely from Canadian forests - that record precisely how much carbon dioxide is in the air of a forest day by day, much as a weather stations charts changing temperatures.
In the past 20 years, two things have happened. The autumn in many forests of Canada, Europe and China has warmed by about 1.1 degrees. And the autumn forests are releasing carbon dioxide back into the air faster than they soak it up.
This trend is so strong, atmospheric scientist John Miller of the University of Colorado writes in Nature, that it "seems to largely cancel" the gains made through the earlier arrival of spring and its extra forest growth.
Shilong Piao and colleagues used satellite observations of vegetation greenery and biosphere modelling to explain this response to autumn warming: although plants' respiration (emitting carbon dioxide) and photosynthesis (storing carbon dioxide) are both stepped up, the respiration outstrips the photosynthesis to cause a net loss of carbon from plants into the air.
Piao works at the Laboratory of Sciences on Climate and the Environment, at France's national science agency. He says the trend overall may make forests worse at storing carbon.
"If warming in autumn occurs at a faster rate than in spring, the ability of northern ecosystems to sequester carbon will diminish in the future," he says in a written announcement of his results.
Support for the study came came from Fluxnet-Canada, Natural Resources Canada, the Natural Sciences and Engineering Research Council, and Canadian Foundation for Climate and Atmospheric Sciences.The Vancouver Sun
