Forests and carbon sinks: changing balances

Tropical forests, hitherto considered among the best carbon sinks, are no longer the planet's green lung. Deforestation, fires, droughts and agribusiness pressures have now made them virtually carbon neutral: they store more or less the same amount of CO2 in the soil and biomass as they emit. Temperate boreal forests, i.e. those found in the mid-latitudes between North America, Europe and north-central Asia, on the other hand, enjoy a better state of health and have become the planet's new green lungs.

In summary, this is the content of a study in Nature Geoscience in which a team of researchers, coordinated by the Laboratoire des Sciences du Climat et de l'Environnement of French governmental agency CEA, analysed changes in the carbon storage capacity of global forest biomass. Globally, the carbon stored in biomass increased from 2010 to 2019 at a rate of 0.50 ± 0.20 billion tonnes per year (Gt/year), in step with the global growth rate of CO2 that was observed in the atmosphere. The main carbon sinks were found to be the boreal and temperate forests, both because tropical forests are under greater stress from deforestation and agricultural exploitation and because of their age. The authors of the study found that tropical forests older than 140 years, which are deforested and degraded, have almost zero carbon emissions, while young (age < 50 years) and middle-aged (50-140 years) temperate and boreal forests are the best carbon sinks. This result runs counter to the widespread idea that the oldest growing forests are also the best in terms of CO2 storage capacity.

The authors point out that the models in use today show that all ancient forests are large sinks and largely ignore the impacts of deforestation and degradation on tropical biomass. Our results highlight the importance of forest demography in predicting the dynamics of future carbon sinks under conditions of climate change.