nature

Carbon storage, carbon sink

hiilivarasto, hiilinielu

Forests’ invaluable ability to sequester carbon dioxide from the atmosphere has put it in the centre of heated political debates. To fight the climate crisis, we need to reduce emissions, and on the other hand, to sequester the already released CO2. These two actions are very strongly linked – if we fail to reduce emissions, we need to sequester more. With many future uncertainties it would not harm us to make sure we are doing more than enough – to apply the so-called precautionary principle.

The most crucial aspect is the time frame for climate actions. According to Sampo Soimakallio, a researcher in the Finnish Environment Institute, maintaining carbon sinks can have significant consequences during the next decades. We are near tipping points that can lead to large and often irreversible changes. What we do during the next decades will be the determining factor when it comes to achieving the goals of the Paris climate agreement.

Why do we sometimes talk about carbon sink, and other times about carbon storage?

A forest can be a carbon storage and a carbon sink. Carbon storage means the carbon sequestered and stored by the trees and the soil.

When the storage is growing, the forest is also a carbon sink. It’s sequestering more carbon than emitting. This is most often the case when forests are growing, even though they release small amounts of carbon naturally.

Carbon sinks reduce the amount of carbon dioxide from the atmosphere.

Let’s compare what happens depending whether the forest is left to grow or if it’s cut down. In the second scenario the forest is cut down, and stops being a carbon sink. Logging residues such as tree stumps hold a part of the carbon storage, but the carbon is released gradually when they decay. The remaining forest is now a source of carbon.

The carbon from the logged trees continue to be stored in products made from wood and wood-based raw materials throughout their life cycle.

The duration of the wood-based products’ life makes a big difference. Around 70% of wood is used for short-term products, such as energy use or cellulose, from which the carbon is released in under five years from the logging.

Products higher on the value chain that last longer, for example wood construction, can also serve as a carbon storage longer. Nevertheless, these products don’t reduce the CO2 concentration of the atmosphere, and producing them has reduced forest carbon sinks.

The logged forest’s growth is speeding up and after 15–20 years the forest turns from a source of carbon back to a carbon sink. A young forest is a very effective sink, although the storage is still smaller than that of the old forest. It has released more carbon than the unharvested forest.

It takes decades until the forest that was logged has stored the same amount of carbon that it had before.

In the meantime, the forest left to grow is still holding on to its carbon and continues to grow, if it hasn’t faced any natural disasters. Even though it’s not such an effective sink anymore, the carbon storage is very valuable.

This summarises the big challenge of wood use: logging more wood now will increase the amount of CO2 released into the atmosphere for a few decades.

Sources:

SYKE blog
SYKE policy brief