The Goldilocks effect: Weighing the world's forests

Diagram of satellite monitoring trees from space

A view from space: the BIOMASS satellite is the first in the world to be able to measure how much wood is locked in the world's forests

26 November 2018 by Roland Pease

It is an old complaint: not being able to see the wood for the trees. But the grumble is being smashed thanks to a new dimension from a European Space Agency (ESA) mission soon to launch.

The question: working out how much wood is growing on Earth. The problem: most of it is hidden from view by leaves in the forest canopy. It seems surprising. Forests are clearly visible from space and satellites have been monitoring them for decades. But they give only a two-dimensional view of the area covered by leaves. The canopies hide the trunks and branches beneath, where the mass mostly resides. That makes an accurate measure of the total mass of wood a daunting task.

Uncharted territory

Shaun Quegan, National Centre for Earth Observation scientist and professor of mathematics at the University of Sheffield, however, was undaunted. And in four years, BIOMASS, the satellite he has been pushing for more than a decade, should change the way we view the Earth's forests.

Once it is unfolded after launch, the satellite's 12-metre antenna will act as three eyes in one, capable of measuring the mass in the world's forests, the height of the trees, and the internal forest structure. It will also lead to better insight into rates of habitat loss and the impact that this has on biodiversity.

Shaun says:

With this mission we will see things about the forests we have never seen before.

Particularly important, will be the ability to track forest degradation. If someone cuts down all the trees from a site, they leave scars that can be seen with conventional imaging, though we still wouldn't be able to weigh them. On the other hand, if someone thins an area they can remove large quantities of living wood without leaving a visible mark.

Mathew Williams of the University of Edinburgh, another of the key players in the project, says:

You can lose as much as half of the forest biomass this way without detecting it with conventional satellites.

Where does the carbon go?

The Amazon rainforest, the 'lungs of the Earth', shows how poor the current state of knowledge is. Estimates of its biomass are so uncertain that they range from 60 to 93 billion tonnes of carbon. And it is not only the total mass that matters. Forests have a huge influence on the concentrations of greenhouse gases, by taking carbon dioxide (CO2) out of the atmosphere through photosynthesis and putting it back via felling and burning.

Mathew says:

The carbon fluxes in both directions are enormous. We put around 10 billion tonnes of carbon into the atmosphere as CO2 each year, but only around four or five billion tonnes remain there.

We know oceans absorb around 40% of that missing carbon and we think plants absorb the rest. But we don't know exactly where it's going, or how it's changing, so it's not a solid basis for climate science.

New space technology

With experience working on satellite missions, combined with 16 years building NERC-backed expertise through leading the multi-institution NERC Centre for Terrestrial Carbon Dynamics and later the carbon cycle theme of the National Centre for Earth Observation, Shaun has many of the skills needed to make the mission succeed.

Professor Shaun Quegan

At the centre of the mission is a radar using what the insiders call a P-band wavelength. These 'Goldilocks' waves are just right - not too long and not too short - allowing them to penetrate through the forest canopy and to reflect off the larger, woody mass beneath. They also don't get too scrambled when passing through the ionosphere, a region of charged particles extending upwards from around 80km above the Earth's surface.

As this is the radar band that early warning systems use to spot incoming missiles and track space debris, any space-borne P-band radar would blind those defensive capabilities. It was only in 2004 that international agreement was reached to allow P-band to be accessed by satellites for secondary use, where it does not conflict with the military.

These early warning systems are focused on the north, over the Atlantic and Arctic, so Shaun saw the opportunity to instead operate the satellite over the tropics, the critical and least understood region of the world's forests. But the earlier ban meant there was no experience of using P-band radar in space. A trial system, AirSAR, flown on NASA planes in the early 1990s had shown the way; analysis of the data by Shaun and other researchers convinced them of the power of the technology. ESA has since funded further airborne campaigns that proved the technology over French Guiana, Gabon and Indonesia.

But this is a space-based mission, and that means BIOMASS has to peer through the interfering effects of charged particles in the ionosphere. Shaun established that, of the two key effects, one could be corrected, while the other could be avoided by putting BIOMASS in a special orbit, which crosses the equator at dawn and dusk when the ionosphere is at its most stable.

Getting space on the satellite

BIOMASS had to beat off 25 rival collaborations competing to build ESA's seventh explorer mission. Noting the fearsome responsibility of seeking half a billion Euros funding, Shaun recalls:

We were up against some of Europe's best. Most of these forests are remote and impenetrable. You can't just walk in and count and measure the trees. But with BIOMASS you can.

For Shaun, BIOMASS's capacity to see the wood, the trees, and details of the world's most important forests, is what makes it so significant.