Forests remove huge quantities of carbon dioxide (CO2) from the atmosphere every year but how this CO2 uptake will be affected by climate change is uncertain and the subject of research projects around the world.
The net amount of CO2 taken up by forests around the world is currently estimated at 9 billion tonnes each year, which is equivalent to around one quarter of total greenhouse gas emissions. Without this carbon sink, the global temperature rise we have seen so far would have been even greater. The land carbon sink is also an important component of plans to reach net-zero emissions as enhancing it could help compensate for sources of greenhouse gases which are hard to eliminate. The rate of carbon uptake varies enormously between individual forests, depending on age, location and other factors. However, even mature forests will continue to take up carbon for years.
Projections of future carbon sinks are usually made with complex climate models and many models predict that the land sink will get stronger in the future. The main reason is a ‘fertilisation effect’ on photosynthesis from increased levels of CO2 in the atmosphere. However, there is much variation between different model results and there are reasons to believe that the level of fertilisation occurring in these models could be optimistic. One important detail is that not all models currently cater for the fact that there may not be enough nutrients (particularly nitrogen and phosphorus) available for the trees to take full advantage of the additional CO2.
To try and shed more light on what will happen in reality, scientists around the world (including the UK) are conducting experiments which involve exposing vegetation to a higher CO2 concentration and monitoring its response. A recent paper reported on such a CO2 enrichment experiment in a mature eucalyptus forest in Australia. The scientists reported that the trees took up more carbon and produced more sugars when exposed to enhanced levels of CO2. However, instead of storing the additional carbon for the long term, much of it was lost through increased respiration by the root system and soil.
This may lead to concerns for policy makers, foresters and climate scientists around the fate of mature forests and their ability to contribute to the mitigation of climate change. The response of this Australian forest was within the bounds of what scientists thought was theoretically possible, but it is was the low end of possible responses. It is important to consider that different forests will perform in different ways, and further research will help to address the uncertainties in the models. The results of the case study do not mean we should give up hope of forests playing an important role in climate change mitigation. What it does mean is that we cannot rely on existing forests alone to take up more carbon as CO2 levels rise. Ongoing research across the world will help us to better understand the fate of carbon storage in forests under a changing climate. But, most importantly we must minimise our impact on the climate by reducing greenhouse gas emissions as far as possible.