Forests serve as crucial carbon reservoirs, sequestering carbon in tree trunks, roots, branches, and leaves. Yet their ability to absorb carbon fluctuates due to human impact and climate change. Because annual carbon stock changes vary across regions and time, scientists stress the importance of continuous long-term forest monitoring.
Published in Earth System Science Data, the study evaluated a 15-year dataset from SMOS, focusing on vegetation optical depth (VOD) - a measure of how much vegetation blocks microwave radiation. VOD serves as an indirect yet dependable indicator of above-ground biomass, offering a way to estimate carbon stored in forests from 2011 through 2025.
Although SMOS was originally designed to measure soil moisture and ocean salinity, its single instrument - the L-band Microwave Imaging Radiometer - has proven useful in many unanticipated ways. Its sensitivity to vegetation opacity allows researchers to assess biomass and detect environmental events like droughts and floods from space.
"SMOS detects how microwave signals weaken through vegetation, which informs us about total mass, including both dry biomass and water," said Matthias Drusch, ESA's Land Surfaces Principal Scientist. While this method offers significant insights, interpreting the data remains challenging because the signal reflects both biomass and moisture.
Klaus Scipal, ESA's SMOS and Biomass Mission Manager, emphasized the need to refine the use of VOD as a carbon proxy. "Over the SMOS time series, you can spot major trends... but interpreting them isn't always straightforward," he said. "We need to be cautious about what we're actually seeing."
Paul Vermunt from the University of Twente, supported by ESA's Living Planet Fellowship, is working to validate satellite readings with ground-based measurements. "You need long time-series, but also a way to interpret them," he explained. "That's why we're combining satellite data with on-the-ground measurements."
Launched in April, ESA's Biomass mission complements SMOS by offering deeper insights into forest structure using P-band radar wavelengths, which penetrate more vegetation than the L-band used by SMOS. Biomass can capture fine-scale canopy details, especially in dense tropical forests.
"Biomass gives us detailed structural data, especially in the tropics," said Matthias. "But it doesn't cover the globe and lacks a long time record. If we want maps without gaps, we have to combine multiple satellites."
Klaus added, "Biomass has a much finer resolution than SMOS... But to see long-term trends, you still need SMOS. Together, they tell us more than either one alone."
The SMOS data on above-ground biomass aligns well with results from ESA's Climate Change Initiative, offering robust support for ongoing and future climate research. As Biomass expands the spatial resolution of forest data, SMOS continues to deliver essential long-term context to help scientists understand Earth's carbon dynamics.
Research Report:A long-term dataset of vegetation optical depth from SMOS
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