The complexity of carbon flow of the terrestrial biosphere: challenging questions and new scale of observations

In recent years, we have increased dramatically our knowledge of the carbon cycle and its flow throughout the terrestrial ecosystems. During the 1990-2021 period terrestrial biosphere provided a net sink of about 21% of carbon dioxide emitted by fossil fuel burning with the major part occurring in forests. The notion of an active carbon sink of the terrestrial biosphere is driving the current debate of climate mitigation and its contribution to achieve carbon neutrality by 2050 to limit global warming within 2°C as set out in the Paris Agreement. Current knowledge is based on an outstanding wealth of systematic observations from the atmospheric gaseous exchanges, space and aircraft observation of tropospheric CO2 concentration down to flux towers and soil and leaves respiratory and photosynthesis measurements. However, achieving a solid scientific background about the real effectiveness of terrestrial carbon to support policy targets require an in depth analysis of the capacity of the biosphere to sustain long term carbon sequestration throughout the century. Main challenges affecting flows of carbon are to what extent climate extremes in both space and time domain may pulse carbon emissions to become dominant compared to mean carbon uptake by the terrestrial biosphere, the role of disturbances by biotic events that are occurring at increasing temporal frequency and the role of forest management to substantially regulate the flow of carbon through the long living wood products and their fate (nature based solutions, material substitutions and renewable energy).

The classic definitions of GPP, NPP, RE (ecosystem respiration) should be expanded to include the stochastic nature of abiotic and biotic disturbance and the human role on forest management to be able to provide a complete picture on the potential role of terrestrial ecosystems in supporting carbon neutrality targets. The presentation will address the complexity of carbon flows through the comprehensive chain, analyse research gaps and emerging monitoring technologies needed to better monitor the terrestrial biosphere.