Preserving forests is critical to slowing global warming

Forests are an important sink for carbon dioxide (CO₂) emissions. An international team including researchers from IIASA analyzed decades of experiments to map the potential of forests to increase their biomass and continue to absorb CO₂ in the future.

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Trees are slowing global warming by absorbing about a quarter of all CO₂ emissions produced by humans. It is however likely that this function will not last forever in the face of unabated fossil fuel consumption and deforestation. Scientists have long wondered whether forests could become saturated and no longer store additional CO₂ from the atmosphere.

In their study [1], the researchers found reason to hope that trees will indeed continue to take up CO₂ at generous rates until at least the end of the century. Fertilized by rising CO₂ levels, plant biomass is expected to increase by 12% by the end of the century – absorbing an amount of CO₂ equivalent to six years of current fossil fuel emissions. The study highlights the importance of the partnerships trees forge with mycorrhizal fungi to help them take up the extra nitrogen and phosphorus they need to balance their additional CO₂ intake. The availability of theses nutrients and the type of mycorrhizal fungi employed, controls how much extra CO₂ can be taken up – a process known as the CO₂ fertilization effect.

Figure: Panels a and b show that soil nitrogen (N) and soil phosphorus (P) are key plant resources limiting the CO₂ fertilization effect on above-ground biomass. Panel c shows the projected increases in biomass due to the CO₂ fertilization effect of a 250 ppm increase in atmospheric CO₂.

Previously, many individual experiments, such as fumigating forests with elevated levels of CO₂ and growing plants in high CO₂ chambers, have provided critical data on the size of the CO₂ fertilization effect, but there has been no definitive answer globally. To more accurately predict the global capacity of vegetation to sequester CO₂ in the future, the researchers performed a meta-analysis (synthesis) of data from all elevated CO₂ experiments conducted so far – in grassland, shrubland, cropland, and forest systems – including ones they directed themselves.

The study’s findings emphasize the critical role of all large forests, in particular tropical forests such as those in the Amazon, Congo, and Indonesia, as regions with the greatest potential to store additional carbon. The potential for continued net CO₂ uptake of the forests, despite nutrient limitations, underline the value of protecting this resource to help curb global warming into the future.

[1] Terrer C, Jackson RB, Prentice IC, Keenan TF, Kaiser C, Vicca S, Fisher JB, Reich P B, et al. (2019). Nitrogen and phosphorus constrain the CO₂ fertilization of global plant biomass. Nature Climate Change 9 (9).