2024-01-12 10:30:47
A study published in Nature Geoscience reveals that the carbon stored by global forest biomass increased by around 500 million tonnes per year between 2010 and 2019. The largest contributors to this carbon sink are boreal and temperate forests, while tropical rainforests have experienced a slight loss of carbon.
Plant biomass plays an essential role in the climate by capturing atmospheric CO2. The carbon footprint of this biomass is the result of gains obtained through plant growth and increased forest cover, and losses linked to harvesting, deforestation, degradation and tree mortality. Monitoring the evolution of these carbon stocks is essential to better understand and predict the effects of current and future climate change as well as measure the direct impacts of human management on the environment. Until now, few approaches have made it possible to estimate these stocks at the global level and those used have led to disparities in predictions. An international research team, coordinated by the CEA and INRAE, published a study in the journal Nature Geoscience in which scientists describe how they managed to map annual changes in global forest biomass using a new method of analyzing satellite images.
This research work was carried out using data from the SMOS satellite of the ESA (European Space Agency) which records the emission of low frequency microwaves at the surface of the soil and makes it possible to probe the entire plant layer. , and not just the top of the canopy. A vegetation index called L-VOD (L-band vegetation optical depth) might thus be calculated and made it possible to globally estimate the above-ground carbon stocks of forests. Researchers have encountered difficulties in generalizing this method across the globe due to signal disruptions linked to radio frequency interference (RFI) and due to human activities, particularly in populated areas in the north, but also in due to the sensitivity of the signal to the water content of plants. Faced with this, they designed a double filtering method, relying in particular on a method of temporal decomposition of the signal (seasonal variations, trends, etc.), to eliminate these two effects. Finally, they calculated tree root biomass using a global map relating aboveground to belowground biomass.
Boreal and temperate forests contribute the most to carbon storage
Using this method, global maps of the annual evolution of plant biomass might be generated between 2010 and 2019. From these, regional carbon stock balances were calculated over areas covering 25 km.2, by attributing carbon gains or losses to each of them. It appears that the net carbon stocks of global plant biomass have been increasing at a rate of regarding 500 million tonnes of carbon per year. The two largest contributors to this carbon sink over the last decade are, on the one hand, the boreal forests with a gain of approximately 370 million tonnes of carbon, and on the other hand, the temperate forests, with a gain of around 130 million tonnes of carbon. In contrast, tropical rainforests experienced a slight net carbon loss of 70 million tonnes, which is believed to be linked to deforestation and their degradation linked to water stress, particularly in the southeastern Amazon. As for the biomass of arid regions and that of dry tropical forests, their carbon footprint has been described as neutral.
The particularity of this study is also to have taken into account the age of the forests. To do this, they were classified into eight different age classes using a global forest age map. Scientists have found that young temperate and boreal forests, i.e. less than 50 years old, and middle-aged forests, i.e. between 50-140 years old, are the largest contributors to carbon sink. Conversely, old-growth forests, which are older than 140 years, and which represent approximately 10% of the global forest area, have shown a net loss of their biomass over the past decade. These forests are mainly found in the tropics.
“These new results differ from those from current plant biomass prediction models which indicate that all old-growth forests are carbon sinks, poorly taking into account the demography of forest stands and the impacts of deforestation as well as the degradation of forests. tropical forests losing biomassdeclare the authors of this study. These results underline the importance of better taking into account the degradation suffered by forests as well as their age to better predict the evolution of these carbon sinks at the global level, and thus better guide climate change mitigation policies. »
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