Since 2005, the amount of atmospheric CO2 absorbed by the continent's trees has been slowing, researchers reported. Writing in Nature Climate Change, they said this was a result of a declining volume of trees, deforestation and the impact of natural disturbances. Carbon sinks play a key role in the global carbon cycle and are promoted as a way to offset rising emissions. Writing in their paper, the scientists said the continent's forests had been recovering in recent times after centuries of stock decline and deforestation. The growth had also provided a "persistent carbon sink", which was projected to continue for decades.
However, the team's study observed three warnings that the carbon sink provided by Europe's tree stands was nearing a saturation point. "First, the stem volume increment rate (of individual trees) is decreasing and thus the sink is curbing after decades of increase," they wrote. "Second, land use is intensifying, thereby leading to deforestation and associated carbon losses. "Third, natural disturbances (eg wildfires) are increasing and, as a consequence, so are the emissions of CO2." Co-author Gert-Jan Nabuurs from Wageningen University and Research Centre, Netherlands, said: "All of this together means that the increase in the size of the sink is stopping; it is even declining a little. "We see this as the first signs of a saturating sink," he told BBC News.
Many of Europe's forests are reaching an age where growth, and carbon uptake, slows down
Sinking feeling
The carbon cycle is the process by which carbon - essential for life on the planet - is transferred between land (geosphere and terrestrial biosphere), sea (hydrosphere) and the atmosphere. Carbon sinks refers to the capacity of key components in the cycle - such as the soil, oceans, rock and fossil fuels - to store carbon, preventing it from being recycled, eg between the land and the atmosphere. Since the Industrial Revolution, human activity has modified the cycle as a result of burning fossil fuels and land-use change. Burning fossil fuels has resulted in vast amounts of carbon previously locked in the geosphere being released into the atmosphere. Land-use change - such as urbanisation and deforestation - has reduced the size of the biosphere, which removes carbon from the atmosphere through photosynthesis.
Dr Nabuurs explained that saturation referred to the point where the natural carbon sinks were unable to keep pace and absorb the additional atmospheric carbon being released by human activities. He said emissions had risen a lot over the past decade, primarily through the rise of emerging economies in countries such as China, India and Brazil. The researcher's conclusions appear to contradict the State of Europe's Forests report in 2011 that showed forest cover in Europe had continued to increase. The report said trees covered almost half of Europe's land area and absorbed about 10% of Europe's annual greenhouse gas emissions. But Dr Nabuurs said that the rate of afforestation was slowing, adding that a sizeable proportion of forests were mature stands of trees, which were mainly planted in the early part of the 20th Century or in the post-World War II period.
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BBC News - European forests near 'carbon saturation point'
