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EXCITEMENT in the room was palpable on the morning of 5 December last year. The day before, the launch of SpaceX’s Falcon 9 to supply the International Space Station had been delayed for 24 hours. That followed the discovery on board of mouldy food – not bound for the ISS crew but to feed some mice set to join them. Now, a crowd had gathered at Kennedy Space Center in Florida for the rescheduled lift off. Stowed along with the mice and fresh feed were experiments, including a remote-sensing system called GEDI – pronounced like the Jedi in Star Wars.
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GEDI – the Global Ecosystem Dynamics Investigation – turns out to be a particularly precious cargo. It is a NASA mission designed to provide the first three-dimensional look at the world’s forests. Surprisingly, given our achievements in space, we still have only a vague idea of how much living matter is on Earth. We do know that trees make up the bulk of it. We also know that forestation and deforestation contribute to atmospheric carbon dioxide concentrations. So, the unprecedented information that GEDI gathers on trees will be essential for understanding climate change.
The SpaceX launch was just the beginning. GEDI is in the vanguard of a new wave of innovative sensors that will assess the world’s plant life and how it is changing – how much carbon, for instance, is lost to the atmosphere when trees are destroyed as a result of catastrophic events such as wildfires, hurricanes and logging. These eyes in the sky will be invaluable in efforts to protect and regenerate forests, too. At last, we are starting to get a holistic picture of our green planet – and what we risk losing if we don’t take action.
Understanding the flow of carbon between living matter and the atmosphere is crucial if we are to tackle global warming caused by carbon dioxide. But tracking carbon can be tricky. We know that CO2 emissions from human activities put about 35 billion tonnes of carbon into the atmosphere each year. But not all of it stays there. “About half of what we’re pushing up into the atmosphere disappears back into the land systems somewhere,” says Laura Duncanson at the University of Maryland, who is a member of the GEDI team. “Where that is and the processes that govern that, this is a massive scientific mystery that we’re trying to solve.” Until we do that, we can’t nurture this precious carbon sink.
What we do know is that the oceans absorb some 25 per cent of the carbon we emit. When it comes to accounting for the rest of the carbon that vanishes from the air, forests are the prime suspects. Yet our lack of knowledge about them was made clear last year when researchers attempted to quantify the total mass of life on Earth for the first time. Using measurements from hundreds of previous studies, they estimated that nature contains the equivalent of about 550 billion tonnes of carbon. Bacteria were expected to account for much of this, so it was a big surprise to discover that they don’t. Instead, land plants alone make up 80 per cent. And .
Trillions of trees
A 2015 estimate put the total number of trees on Earth at 3.04 trillion, including 1.3 trillion in tropical and subtropical forests, 0.66 trillion in temperate regions and 0.74 trillion in the boreal conifer forests encircling the globe below the Arctic. Despite this, our current knowledge of how much carbon is contained in forests is still so poor that estimates for the Amazon rainforest range from 60 to 93 billion tonnes, a difference that isn’t far off the world’s entire annual carbon emissions.
The most precise way to measure the carbon in a tree is to chop it down and weigh it, trunk, branches, roots and all. Of course, that would kill it, so instead we take field measurements of tree diameters and then use known densities of different woods to calculate total biomass. This method is time-consuming and expensive – and in practice turns out to be near-impossible, especially in the tropics where forests are dense and difficult to navigate.
Scanning the forests from the air or from satellites seems an obvious solution. But most Earth-observing sensors can take pictures only of the tops of canopies. GEDI is different. It uses lidar, a method that sends thousands of laser pulses towards Earth’s surface, which bounce back to space after hitting solid objects. By measuring the time it takes for a pulse to travel there and back, the distance can be calculated. So, laser beams penetrating a forest at different depths from the canopy down to the ground can be used to build a three-dimensional map of the forest.
“About half of the carbon we are pushing up into the atmosphere disappears into land systems somewhere”
GEDI isn’t the first lidar sensor in space, but it is different. While others are good at things like monitoring ice sheets, GEDI was conceived to give the most complete picture of the forest possible. “We’ve designed the instrument, its lasers, detectors, and other technology, to get through dense tropical forests,” says Ralph Dubayah at the University of Maryland, who is principal investigator on GEDI.
GEDI’s lidar uses near infrared light, which is reflected off leaves so that the canopy looks brighter than it does in the visible spectrum. Nevertheless, it has some limitations. For a start, because it is hitching a ride with the ISS, over its two-year mission, it will sample only a fraction of Earth’s surface and won’t collect data north of about 52 degrees latitude, therefore missing most of the boreal forest. In addition, because lidar uses light in the near infrared part of the spectrum, it can’t penetrate clouds. So GEDI can’t do this all alone. That is where other systems that use radar come in.
Radar systems send out microwave radiation, which passes through cloud and scatters when it hits a solid object. The sensor detects this backscatter and the patterns it produces can be analysed to form pictures of the landscape. Microwaves range from about 1 millimetre to 1 metre in length, and the wavelength a particular radar uses dictates what it can see. Shorter wavelengths detect smaller objects. Longer ones can penetrate the forest canopy to bounce off larger objects below, such as tree trunks and branches. Too long, however, and the wavelength gets scattered in Earth’s ionosphere – a layer of the atmosphere extending from about 60 to 1000 kilometres up that contains a lot of ions and free electrons.
What is needed is a “Goldilocks” wavelength, not too long and not too short. The one that is just right, at 70 centimetres, is known as the P-band. Unfortunately, that wavelength couldn’t be used for many years. Satellites carrying P-band radars were banned because they would interfere with Earth-based operations that use the same wavelength, including military defence systems. For example, missions including the German Earth observation satellite TanDEM-X operate at much shorter wavelengths, so they aren’t very sensitive to biomass.
Radar unlocked
However, in 2004, the blanket ban on using P-band radar in orbit was lifted. Since then, Shaun Quegan at the University of Sheffield, UK, and his colleagues have been developing a system based on the Goldilocks wavelength to measure biomass from space. In 2013, the European Space Agency finally selected this system – the Biomass satellite – for implementation. It is set to launch in 2022. The mission will systematically map forests where information is most urgently needed, including all the world’s tropical forest and most subtropical and boreal forest. Remaining defence embargoes will prevent Biomass from surveying the 22 per cent of Earth’s boreal forests that are in Canada and Alaska, however.
During the initial 14-month phase of its five-year mission, the sensor will operate in a tomography mode. Like a CAT scanner imaging the human body in multiple slices, Biomass will build up a three-dimensional picture of the forest. Because it uses an imaging system at a long wavelength, instead of the discrete sampling system of GEDI, it will create a different picture: a continuous map of the woody parts of the forest structure that would otherwise be obstructed by the canopy. “At these wavelengths, the leafy canopy becomes transparent and we can see right down to the ground,” says Quegan. Thereafter, the satellite will switch to using interferometry – extracting information from interference patterns – and visit the same locations every seven months. This will allow scientists to estimate canopy biomass and height on a more frequent basis to build a better understanding of tree mortality and regrowth.
There are other projects on the drawing board, too. These include the NISAR mission, the first radar imaging satellite to use dual wavelengths, which is due to launch in 2021. A collaboration between NASA and the Indian Space Research Organisation, it is designed to observe and measure a range of natural processes including tsunamis, earthquakes, ice-sheet collapse and ecosystem disturbances. Together with satellites already in orbit, information from GEDI, Biomass, NISAR and more will be combined to give the first wall-to-wall picture of the world’s forests. “Collaboration does seem to be quite a unique component of what’s happening with the biomass missions,” says John Armston at the University of Maryland, who works on GEDI.
All this means that in a few years, we will have a much better idea of where the missing carbon is going – or at least how much of it is being taken up by trees. The new satellites will also help reveal the varying exchange of carbon between trees and the atmosphere. Last year, for example, the hurricanes that hit the southern and eastern US felled vast swathes of forest, while wildfires that raged in California turned countless trees to ash, sending their stored carbon literally up in smoke. Such dramatic events are likely to become more common as the world gets hotter. In future, with an array of remote sensors, we will be able to properly assess the damage they have done. We will get a more accurate measure of how much forest is being lost through logging, too, because the new generation of imagers will be able to detect the thinning out of woodland as well as the entire clearance of patches that conventional imagers can see.
Remote sensors also have the potential to aid forest protection and regeneration. The 2015 Paris climate agreement included a programme called REDD+, designed to help poorer countries keep their forests intact by offering financial incentives to reduce carbon emissions caused by deforestation and degradation. “For REDD+ to work, it is crucial for people to be able to accurately and reliably monitor how much carbon emissions have happened,” says Jonah Busch, chief economist at the Earth Innovation Institute. To do that, they first need to know how much carbon is contained in their forests. At present, this is estimated using satellite images and region-dependent estimates for biomass. The new sensors will provide accurate measures, putting the project on a more empirical footing.
Worldwide, some are felled each year – and more than 3 trillion have been cut down since people began farming around 10,000 years ago. Now, increasing urbanisation means there is great potential for reversing this trend. Indeed, research published in July reveals that Earth could support enough additional trees to cut atmospheric carbon levels by 25 per cent – making this by far the best climate change solution available. Some big reforestation projects are already under way. China, for example, has planted an area a quarter of the size of the Amazon rainforest in the past two decades. With the new wave of remote sensors, we will be able to measure the success of such projects, and their impact on efforts to keep global warming in check. That truly will be a giant leap for humanity.
Turning carbon blue
Restoring ecosystems could be a crucial weapon in our efforts to avoid climate catastrophe. By drawing carbon out of the atmosphere, plants lock away the greenhouse gases that cause global warming. because trees contain so much carbon, and historic deforestation means there are large areas of land that could be restored to woodland. Coastal habitats offer a similar opportunity – the available area may be less, but the carbon payback is even greater.
Lined with mangroves, salt marshes and seagrass meadows, coastal ecosystems are repositories of “blue carbon”. While forests hold most of their carbon within woody biomass, coastal plants pull carbon out of the air and water and channel it through their roots deep into the ground – burying it indefinitely, provided the system stays healthy. As a result, coastal plants can absorb many times more carbon than trees covering the same area. A seagrass meadow, for example, contains anywhere from 10 to 40 times as much, 95 per cent of which is stored in sediment. The problem is, these aquatic ecosystems are disappearing. Mangroves and salt marshes are often removed to make way for coastal developments, and seagrasses are dying as pollution depletes oxygen levels in coastal waters. It is estimated that approximately one third of blue carbon sinks have already vanished.
made to restore coastal ecosystems. One of the longest-running projects is in Chesapeake Bay on the east coast of the US, where seagrass meadows increased by 8 per cent last year. Now, researchers at the University of Southern Denmark are trying to work out the , seagrasses are vascular plants with roots and flowers, so can be sown from seeds or planted as seedlings. The team tested various cultivation methods and found that planting seedlings was most successful because shifting shorelines and sediment accretion made it hard for seeds to get established. They want to see their techniques adopted on a massive scale. They point out that seagrasses have the potential to grow in coastal waters all over the world, except Antarctica.
So blue carbon sinks could play a significant part in efforts to curb global warming – not to mention beautifying our coastlines.
Article amended on 5 November 2019
We corrected the estimated number of trees in boreal conifer forestsĚýand the estimate of total human CO2 emissions.
