Satellites capture dramatic shifts in Earth's forests

Forests act like natural storage units for carbon. Trees soak it up and hold it in their trunks, branches, roots, and leaves. But this ability to store carbon isn’t fixed. Climate shifts and human activity can throw it off balance. What’s happening in one forest might not match what’s going on in another.

That’s why scientists need a consistent, long-term way to track how forest biomass is changing over time.

That’s exactly what researchers from the European Space Agency (ESA) have been working on. In a recent study, they used a satellite originally designed to measure soil moisture and ocean salinity to estimate forest biomass over a 15-year period. The key data point they used? Something called vegetation optical depth, or VOD.

Soil satellite spots forest trends

Launched in 2009, ESA’s Soil Moisture and Ocean Salinity (SMOS) mission was never designed to track forests. Its main tool – the Microwave Imaging Radiometer – was supposed to map soil moisture on land and salinity in oceans.

But over time, scientists noticed something else. The radiometer was also picking up signals from vegetation.

The VOD measurement from SMOS tells us how much vegetation is in the way of the microwave signal. In simple terms, the denser and wetter the vegetation, the more the signal gets blocked.

That’s why VOD can be used to estimate how much biomass is above ground. This includes both dry material like trunks and branches, and water stored in leaves and stems.

“SMOS is able to detect how the microwave radiation signal from its instrument gets weaker when it passes through vegetation,” said Matthias Drusch, Land Surfaces Principal Scientist at ESA. “That tells us about total mass: dry biomass plus water content. It’s not direct, but it’s very useful.”

Unraveling forest satellite data

Tracking VOD over time gives a picture of what’s happening in forests across the globe, but making sense of it isn’t always easy.

“Over the SMOS time series, you can spot major trends – big droughts, floods, or shifts in vegetation structure – but interpreting them isn’t always straightforward,” said Klaus Scipal, SMOS and Biomass Mission Manager at ESA. “Because the signal includes both biomass and water, we need to be cautious about what we’re actually seeing.”

That’s where long-term analysis and ground data come in. Paul Vermunt, a scientist from the University of Twente in the Netherlands, emphasized this point.

“You need long time-series, but also a way to interpret them,” he said. “That’s why we’re combining satellite data with on-the-ground measurements.”

“The idea is to link what we see from space with what’s happening inside forests and even individual trees.”

Biomass adds fine detail

In April of this year, ESA launched another satellite called Biomass. This one is equipped with a radar that uses longer radio waves than SMOS. While SMOS works in the L-band, Biomass operates in the P-band.

“Biomass gives us detailed structural data, especially in the tropics,” said Drusch. “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 – and that’s only possible if we understand the uncertainties in each.”

Scipal added, “Biomass has a much finer resolution than SMOS, so it helps us zoom in on the structural details. But to see long-term trends, you still need SMOS. Together, they tell us more than either one alone.”

Tracking forests across decades

The SMOS dataset now spans from 2011 to 2025 and provides consistent global observations. That’s invaluable for tracking how forests are responding to climate change and human pressures.

When paired with the more detailed view from Biomass, scientists can monitor both the big picture and the small shifts happening inside forest ecosystems.

“SMOS allows us to look on long-term trends on a global scale – and Biomass is now zooming in on spatial details and specifically at structural changes in the canopy,” said Scipal.

The findings from the SMOS study also line up well with other climate monitoring datasets, including those from ESA’s Climate Change Initiative. With more satellites coming online and better tools to interpret their data, researchers are building a stronger, clearer picture of the world’s forests – and how they’re changing.

The full study was published in the journal Earth System Science Data.

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