The amount of carbon activated by a change from tundra to forest outweighs that soaked up by the new trees. Photo: Ross Duncan
IN A surprise finding, researchers have shown that as trees start to grow closer to the North Pole, replacing once-barren tundra, they release more greenhouse gases than they absorb.
The study has global implications for measuring the speed of global warming, because it had previously been thought that when forests colonise the frozen Arctic they might act to slow dangerous climate change by soaking up extra carbon dioxide from the air.
Instead, as temperatures rise and plants take root further north, the barren soils are ''primed'' by new growth and start to release long-held stocks of carbon. The amount of carbon activated by a change from tundra to forest outweighs that soaked up by the new trees, leading to a net increase in the amount that warms the atmosphere.
''We suggest that, as more productive forest communities colonise tundra, the decomposition of the large [carbon] stocks in tundra soils could be stimulated,'' the researchers wrote in a paper to be released today in the journal Nature Climate Change.
''Thus, counter-intuitively, increased plant growth in the European Arctic could result in [carbon] being released to the atmosphere, accelerating climate change.''
A team of eight researchers from Scotland and England braved frigid conditions in northern Sweden to gather and measure soil samples from patches of forest and tundra.
They carefully peeled the soil into centimetre-thick layers and measured the carbon and organic content of each, using the traces of radioactivity from atomic weapons testing in the 1950s as a marker to distinguish older carbon deposits from those that had been more recently absorbed.
A senior climate change researcher at the CSIRO, Michael Raupach, said the study could be used to refine estimates of how fast the world would warm, but more detail was needed to judge the scale of the phenomenon.
''It's very interesting, it will have ramifications, and it's a process that hadn't been expected, but it's very difficult to say what those ramifications will be on a global scale,'' Dr Raupach said.
Between 1982 and 2010, the amount of vegetation in the Arctic had increased by 8 per cent, the report said.