Bringing forests back has long been though to help combat climate change. Trees absorb the most common greenhouse gas, carbon dioxide, helping to pull the heat-trapping gas from the atmosphere, the logic goes. This is why forests are usually expected to act as carbon sinks. But a surprising new study of tree growth in the Arctic tundra suggests that, in some circumstances, reforestation in the far north could have the opposite effect, making climate change worse rather than better.
As the Arctic warms, birch forests are quickly re-establishing themselves across the tundra regions of Eurasia, from Finland to western Siberia. But as these trees and other plants grow, their roots help to stimulate the decomposition of material in the soil.
This is a huge problem because Arctic soils store enormous amounts of carbon, more than is currently present in the atmosphere, so releasing that carbon could accelerate climate change.
Scientists from the University of Exeter measured the carbon in plants and soil, comparing frozen tundra to nearby birch forests. They found that although vegetation in the forests stored a two-fold greater amount of carbon, the soil in the forests contained far less carbon, enough to outweigh the benefits from plant growth, according to a paper published in Nature Climate Change. They also analyzed carbon released from the soil and concluded its release was prompted by faster decomposition.
“It shows that the encroachment of trees onto Arctic tundra caused by the warming may cause a large release of carbon to the atmosphere, which would be bad for global warming,” said Dr. Gareth Phoenix of the University of Sheffield’s Department Animal and Plant Sciences and one of the researchers. “So, where before we thought trees moving onto tundra would increase carbon storage it seems the opposite may be true.”
And trees are returning to the Arctic region at an astoundingly fast rate. Another team of scientists found that across 100,000 acres of northern European and Russian tundra, vegetation has been spreading faster than expected, according to a study also published in the journal Nature Climate Change.
These researchers analyzed satellite images and interviewed locals who herd reindeer in the area. They concluded that growth has come strikingly fast across as much as 15% of the region. Some trees in the frigid region grew over six feet within 40 years.
“It’s a big surprise that these plants are reacting in this way,” said lead researcher Dr. Marc Macias-Fauria of Oxford University’s Department of Zoology and the Oxford Martin School, in a press release. “Previously people had thought that the tundra might be colonized by trees from the boreal forest to the south as the Arctic climate warms, a process that would take centuries. But what we’ve found is that the shrubs that are already there are transforming into trees in just a few decades.”
The researchers also found that Arctic forests cause warming via another mechanism. In previously frozen areas, trees trap heat because green vegetation absorbs the sun’s rays more than white reflective ice.
The Arctic region is warming at a much faster rate than much of the rest of the planet, according to a string of recent papers. If overall global temperatures rise 2 degrees Celsius, the Arctic will see even more pronounced changes of between 3 and 6 degrees, researchers now believe.
These local effects are clearly visible. Already, ships are able to navigate areas that not long ago were impassible because of dense sea ice. The Northeast Pass, a shortcut through the Arctic Ocean near Russia’s northern coast, is predicted to be ice-free again this summer, allowing ships to travel through, according to scientists with the Alfred Wegener Institute for Polar and Marine Research. Last year ships were able to use the passage for an unusually high number of weeks. This year huge areas of thin ice suggest that summer melting is likely to clear the pass early as well, the researchers found.
The melting ice is a major reason that warming is most dramatic in the Arctic region. Just like in the forests, as white ice is replaced by black ocean waters, the amount of heat absorbed rises, magnifying local impacts.