Satellites reveal extent of rainforest carbon storage

For decades, scientists have struggled to measure exactly how much carbon rainforests store, hidden beneath their dense canopies. But that may be about to change. The European Space Agency has just launched a satellite which is capable of peering through the treetops to reveal what lies beneath. Mat Disney is a leading expert on remote sensing and forest structure at UCL and the National Centre for Earth Observation…

Mat - We've had satellites and we have satellites at the moment that can arguably do a better job in places here and there and in snapshots, but they can't do it consistently across this huge range. Southeast Asia, Congo Basin, Central Africa and the Amazon rainforest of course.

Chris - So talk us through where the satellite's going to be parked, for want of a better phrase, so that it can do that comprehensive analysis.

Mat - Okay, so it's going to be in an orbit, not parked, so travelling quite fast, but rotating round and round the Earth at an altitude of about 660 kilometres. It goes round the Earth about every 90 minutes or so, and with the Earth sort of rotating underneath it, that's what enables it to build up this picture over time of the tropical forest continent.

Chris - So it's going pole to pole, is it?

Mat - Not quite, but nearly pole to pole. So it's what we call a near polar orbit, so it's about 10 degrees off polar.

Chris - All right, so it's sort of tilted off the poles, and that means that the satellite goes round and the Earth goes round, and that's how you get that comprehensive general cover of everything. How will it be scrutinising the Earth below?

Mat - It's using this transmitter on board that sends out radar signals that are using what's called a P-band wavelength. So it's sending out a signal that's about as long as your forearm, and the key thing about P-band is that this signal can go through clouds, and it can go through the smaller bits of the top of the forest, so it doesn't get scattered off the leaves and small branches, and it can bounce off the base of the forest, and it can also then bounce off large trees, the big trees that are 50 or 80 metres tall, and contain a lot of the carbon. So it only really sees the big stuff, the big trees in the canopy, and that again is this kind of unique thing that this instrument is capable of doing that nothing else we've had so far can do.

Chris - Effectively then, it's weighing forests.

Mat - That's right. So by looking at how dense the forests are and how tall they are, then we can get a really, really good idea of how much material there is in there, and then how much carbon there is in there.

Chris - Different parts of the world have different sorts of forest. Can it deal with both with impunity?

Mat - It can to some extent. The type of orbit and the way it's been designed, it's been designed to focus specifically on the tropics, and even in the tropics, if you look at Southeast Asia, or you look at the Congo Basin, and you look at the Amazon, they're all tropical forests, but they're all different. So it can do those kind of big forests better than almost any of the other kind of instruments that we have. When you look at forests that are smaller in stature, so shorter and less dense, it does a reasonably good job, but there are other satellites that can do that job better. It's just that they can't do the really tall, dense forests, which is why we've decided to design and build this mission.

Chris - How long is it going to take you to acquire all the data? And once you've got it, what are you going to do with it?

Mat - It's going to take six months in the commissioning phase of the mission. Then it'll take six months to check that all the systems are working. Then it's going to take another 18 months before we get our first real global picture from this, and then hopefully the mission will carry on updating that for the next three years after it. As soon as we start getting data, we'll be able to get estimates of carbon over large spatial areas, which we just haven't been able to do before. We've been able to get very good estimates in very small locations, and then everything else is trying to fill in the gaps. The first thing we'll start to look at is building maps of how much carbon there is in these forests and comparing that with what we already know, which we know is patchy and uncertain. Once we have these consistent maps, we're going to be able to start looking at comparing one area of forest to another. Even within the Amazon, the different regions of the Amazon are behaving in different ways under climate change. We know that there are areas of the Amazon that have been very strong carbon sinks. They've been taking up carbon and storing it. Because of reductions in rainfall and increases in fire and disturbance, there's large areas of parts of the Amazon that look as if they're turning into a net source of carbon. Again, our pictures of that are very sparse and patchy. That's a really serious consequence of climate change, if that's true. Then more generally, we're going to be looking at things like deforestation, logging, degradation more generally. Where is that happening? How intensely is that happening? What are the consequences of that for the loss of carbon and for our ability to make political decisions to finance ways in which to reduce that deforestation and degradation?

Chris - Once you've got it up there and it's active, and you're starting to get measurements, how do you know it's actually accurate? How do you calibrate this instrument so you can say, yes, the measurement the satellite comes back with actually corresponds to this much tree cover or this much effective forest weight of carbon on the ground? Have you got some kind of standard bit you're going to look at and then compare what you know is there with what it says is there so you know it's right or you can correct accordingly?

Mat - What you're getting at is we have a satellite that's 600 kilometres up in space and it's sending out radar signals. Those are not measurements of biomass at all. It has to go through a whole process of models and these models are what we turn our observations into estimates of biomass. We absolutely need to make measurements on the ground to know that the satellite is doing a good job to validate that product. That means going to certain places. That's one of the projects that I've been involved with is collecting data from the ground using a whole bunch of other different techniques, but to measure individual trees in one hectare plots in places like Malaysia and in Gabon and in French Guiana. Then we're able to say exactly how much carbon is in this one hectare bit here. Once we get an estimate from the biomass mission, we can compare those and say, well, is it doing what it's supposed to be doing? That kind of ground-based measurement is really crucial to the success of a mission like this. It's no use in having consistent maps all across the globe if they're consistently wrong.