A carbon sink is anything that absorbs more carbon dioxide from the atmosphere than it releases.
European forests are currently a net carbon sink as they take in more carbon than they emit. In climate negotiations, this temporary reduction of carbon dioxide in the atmosphere is also known as negative emissions. 

Forest carbon sinks are not an excuse to delay action in reducing fossil fuel emissions. This is because carbon absorbed by trees is dynamic. Forest carbon moves between the atmosphere (as carbon dioxide) and the tree (as carbon) in a continuous cycle, known as the forest carbon cycle.
Carbon stored in fossil fuel is static, remaining trapped outside the atmosphere for thousands of years. This means that forests can never cancel out or ‘offset’ emissions from fossil sources but will help slow climate change, but not reverse it on its own.

Using forest carbon sinks to justify carbon dioxide emissions from fossil sources will increase concentrations of carbon dioxide in the atmosphere, making it impossible to meet the global goal of keeping international temperature rises to well below 2°C.

Despite the clear difference between fossil and forest carbon, United Nations climate negotiators often suggest that planting trees or reducing deforestation is equivalent to reducing emissions from burning fossil fuels. Until this myth is finally busted, schemes to reduce emissions from deforestation and degradation (REDD), the Clean Development Mechanism or LULUCF have the potential to do more harm than good.

While it is quite possible to keep coal in the hole and oil in the soil, no government or company can ever ensure that carbon will remain in trees. Forest fires, insect outbreaks, decay, logging, land use changes and the decline of forest ecosystems as a result of climate change are all hard or impossible to control.

The UK

This doesn’t mean we shouldn’t try to protect and restore forests, just that we need to do it at the same time as reducing fossil fuel emissions to zero. The UK plan is to treble tree-planting rates by 2024 with at least three community forests to be created as part of the new England Trees Action Plan, however this means most of the planting will still be of non-native trees on commercial conifer plantations in Scotland, which deliver fewer benefits for wildlife and biodiversity the Government’s targets for new woodland should be met by integrating UK sourced and grown native trees into our landscapes.