Plants take in carbon dioxide and store the carbon in their leaves, stems and roots. When the plant dies, this carbon is converted to soil organic matter. Ultimately it’s mineralized to carbon dioxide and returned to the atmosphere. The amount of carbon that a plant absorbs, the amount of time it spends in the plant and the time taken to decay in the soil all depend upon climate conditions.

Right now many plants and trees keep hold of their carbon for a significant amount of time, acting as a carbon sink. As much as one-fifth of emissions from fossil fuel burning and cement production are stored in this way each year. In particular, the cool temperatures at high northern latitudes mean that forests in these locations are excellent carbon sinks. But can they carry on mopping up our mess as temperatures rise?

To answer this question, Christoph Müller from the Potsdam Institute for Climate Impact Research in Germany and colleagues modelled the response of vegetation to differing amounts of climate change and land-use change, and measured the impact on plant carbon storage. They found that a carbon reversal (where vegetation emits carbon rather than stores it) can occur under a wide range of climatic conditions, and could happen as early as 2030. "This was the most extreme case, bringing together high greenhouse gas emissions, high climate sensitivity (warming per emitted greenhouse gas unit), strong land-use change and high tree mortality in the boreal north," said Müller.

The model also showed that boreal forests – currently a significant carbon sink – are the most vulnerable to heat stress, and subsequent carbon reversal. "If forests cannot tolerate the heat and associated drought, as recently observed in boreal ecosystems, then the terrestrial land carbon sink is in trouble," said Müller. But the team’s model also showed that carbon reversal is not inevitable. Under low emissions scenarios, plants and trees will continue to mop up our carbon throughout the 21st century.

However, if climate change does become sufficient to reverse the land carbon sink, turning it into a carbon source, climate change would be amplified, making it even harder to manage. "If the land carbon sink is reduced or even reversed to a net source, we would not only have to annihilate our own emissions, but also have to compensate for the emissions from natural ecosystems," said Müller, who published his findings in Environmental Research Letters (ERL).

All of which stresses the urgency of reducing anthropogenic carbon emissions as soon as possible, and protecting the vegetation that is helping to damp down the impact of climate change at present. "This needs to be considered when promoting large-scale bioenergy production as a mitigation option," said Müller.

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