Thursday, January 3, 2008

Forests Soak Up Less and Less CO2

Canada's trees getting worse at filtering gases, researchers say

The Ottawa Citizen reports:

Last year brought glum news that Canada's forests were only a so-so defence against global warming. Now, we find out our forests are getting worse at soaking up greenhouse gases.

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 in the past two years and continuing today in the journal Nature call that into question.

Forests soak up less pollution that we'd hoped. Even as Canada realized it had over-hyped the air-cleaning work done by forests, though, 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 trees should work longer 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.

In the past 20 years, two things have happened. The autumn in many forests of Canada, Europe and China has warmed by 1.1 degrees. The autumn forests are also releasing carbon dioxide back into the air faster than they soak it up.

This trend is so strong, University of Colorado atmospheric scientist John Miller writes in Nature, that it "seems to largely cancel" gains made through the earlier arrival of spring and its extra forest growth.

Shilong Piao and colleagues at the Laboratory of Sciences on Climate and the Environment in France's national science agency said 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.

"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," Mr. Piao says in a written announcement of his results.