Tropical forests have a limit on how much carbon they can store

Tropical forests are among the last large Wilderness in the world and biodiversity hotspots. Unfortunately, they are rapidly disappearing as a result of fires, deforestation and land degradation. In the current climate crisis, the crucial role of tropical forests as a carbon sink is often emphasised. Tropical forests are the largest absorbers of the carbon in the atmosphere and thus have a great ability to decrease CO2 concentration in the atmosphere. In fact, higher CO2 concentrations even promote their growth, enabling them to become an even larger sink. However, there is a limit to how much carbon they can absorb.

The carbon absorption rate depends on the equilibrium between new tree establishment and tree growth, and tree mortality on the other side. When trees die, they start emitting carbon during their degradation. Therefore, to understand the future role of tropical forests as a carbon sink, it is important to also predict how tree mortality might change in the future. A recent study by Hubau et al. (2020) examined how the tropical carbon sink might change in the future due to changes in this balance.

Tropical carbon sinks dwindling

The results of the study showed that both in African and in Amazonian rainforest, the carbon sinks will significantly decrease in the future. In the Amazon, the carbon sink will in fact reach zero by 2035. This means that after then, the Amazonian forest will become a carbon source. The decline in carbon absorption rate is due to other effects of the climate crisis. The higher temperatures and the more frequent and prolongued droughts put greater stress on trees, resulting in higher mortality. As these changes were stronger in Amazonia than in Africa, the African tropical carbon sink will persist longer.

While the study only considered climate change as a possible control of the carbon sinks, other confounding factors might lower the carbon sinks further. The tree growth is limited by the phosphorus concentration in the soil. Therefore, even though higher carbon concentration promotes tree growth, this can only take place until there is enough phosphorus available. In addition, high tree density and competition for light and water may also decrease the tree growth, reducing the available carbon sink.

We need to decrease carbon emissions even further

In the combat against the climate crisis, many stakeholders rely on the ability of tropical forests to keep absorbing carbon. For example, the current calculations of the needed decrease in carbon emission to prevent 2°C warming all take into account a stable tropical carbon sink. However, given the tropical sinks will only decline, it is urgent to decrease our carbon emissions even further. At the same time, continuing strongly protecting forests to prevent further emissions is essential.