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Boreal and temperate forests have emerged as the main carbon sinks 

Tropical forests are no longer the world’s major global carbon sinks, as previously believed. Instead, boreal and temperate forests have taken up this role, according to an international research team coordinated by the French Alternative Energies and Atomic Energy Commission (CEA) and INRAE. 

Focus of the study 

Using a novel analysis technique for satellite images, the researchers meticulously mapped yearly fluctuations in global forest biomass over a span of a decade, from 2010 to 2019. 

The results showed that tropical forests, although expansive and ancient, are now almost carbon-neutral due to the degradation brought about by deforestation, fires, and droughts.

Biomass and carbon sequestration

A surge in plant biomass signifies a boost in carbon sequestration, a key process in counteracting the climate crisis. 

The balance of this biomass carbon hinges on a delicate equilibrium between gains from plant growth and forest expansion and losses stemming from deforestation, natural disturbances, and other factors. 

Keeping track of these stocks is crucial for deciphering and forecasting climate change impacts, as well as assessing human-induced ecosystem modifications.

Satellite imaging

The Soil Moisture and Ocean Salinity (SMOS) satellite, leveraging the L-band vegetation optical depth (L-VOD) techniques, was instrumental in estimating the above-ground carbon stocks at a global scale. 

However, challenges like radio interference from human activities and L-VOD’s sensitivity to vegetative water content posed hindrances. The team’s solution was an innovative double filter to offset these effects.

By employing this novel method and analyzing above-ground biomass data, the researchers were able to determine the overall biomass.

They meticulously calculated the spatial and temporal distribution of total biomass carbon from 2010 to 2019, creating detailed maps that charted the annual changes.

Carbon stock insights

The study revealed that terrestrial biomass carbon stocks experienced a surge of roughly 500 million tons of carbon annually from 2010 to 2019.

Surprisingly, it was the boreal and temperate forests that emerged as the prime contributors to this carbon sink, overshadowing tropical forests which have transformed into minor carbon sources due to deforestation and recurrent droughts. 

Old-growth tropical forests, where trees often surpass 140 years in age, are barely breaking even in terms of carbon. In contrast, the younger and middle-aged trees of the temperate and boreal forests, aged between less than 50 to 140 years, are proving to be the carbon champions.

Study implications 

This revelation challenges the established prediction models that painted old-growth forests as significant carbon sinks, neglecting the nuances of forest demography and the debilitating effects of deforestation on tropical forests.

The study emphasizes the importance of considering forest degradation and age dynamics in predicting future carbon sinks. 

The study is published in the journal Nature Geoscience.

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