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We’re transitioning to renewables; we are using the energy we generate with extraordinary efficiency; our industries are innovating with clean, green methods; we’re recycling, reusing and reclaiming. Our greenhouse gas emissions are slowing. But perhaps it’s not enough.
There is another final – some might say ‘last resort’ – set of tools in the decarbonisation toolkit: ‘negative emissions technologies’ – technologies that store or sequester more greenhouse gas emissions than they produce. These come in two main forms: nature-based solutions such as reforestation and afforestation, and more technological solutions such as direct air carbon capture and storage, enhanced weathering, biochar, and soil carbon sequestration.
As a 2020 report from the International Energy Agency argues, carbon capture, utilisation and storage technologies are a critical part of ‘net-zero’ goals because they enable key sectors to reduce their emissions directly, but also help to balance some of the more intractable emissions.
But carbon capture is a twin challenge. First, you have to capture the carbon dioxide, either directly from the atmosphere or from emissions sources. Then you have to put it somewhere that will store it securely for as long as possible.
The good news is this is already happening naturally. Around half of the excess carbon dioxide that is released into the atmosphere by human activity – the combustion of fossil fuels – is ‘drawn down’ again by natural processes: half by land-based processes – mainly plants – and half by the oceans. We can’t – and shouldn’t – seek to control these natural processes. But we can take advantage of them.
Ironically, higher concentrations of carbon dioxide in the atmosphere can actually increase plant growth; a phenomenon called carbon dioxide fertilisation. There is evidence that plants are already putting out more leaves during their growing season in response to increasing carbon dioxide availability. However, plants eventually adapt to the higher concentration of carbon dioxide, so the effect is limited. And as climate change brings warmer temperatures and more rainfall to some parts of the world, that could increase the length of the growing season there. But in others, higher temperatures and decreased rainfall could have the opposite effect.
The fact remains, though, that trees are carbon guzzlers. Around half the mass of a single tree is pure carbon. Given that forests cover 31 per cent of global land area – around 4 billion hectares – that’s a lot of stored carbon.
The problem is scientists don’t know exactly how much. At the moment, forest cover is mapped from space using satellites that can tell the difference between surfaces such as forest, grasslands and desert, for example. But they don’t show whether a tree in a forest is ten metres tall or 100 metres tall, and that is a critical piece of climate information. “If we don’t know how much carbon is even stored in the Earth’s forests, let alone how it’s changing with deforestation and whatnot, that’s a massive uncertainty for those climate models,” says remote sensing scientist Laura Duncanson from the University of Maryland.
This uncertainty has significant implications for how we assess the impact of ongoing deforestation, how we plan reforestation or what’s called avoided deforestation – not chopping down existing forests – and how we calculate the emissions credits associated with that reforestation or avoided deforestation. It’s knowledge that is critical to the concept of reducing emissions from deforestation and forest degradation (also known as REDD+).
Which is where GEDI – Global Ecosystem Dynamics Investigation – comes in. This Nasa project uses a technology called LiDAR, or Light Detection and Ranging: a pulsed laser shot from the International Space Station to measure the height of objects like trees. A beam, with a footprint measuring around 25 metres in diameter, is painted billions of times across the Earth’s surface, and the open-source data from those billions of samples can then be translated into a map of the Earth’s forests that will allow scientists to calculate forest carbon with far greater accuracy than ever before. “Instead of just saying, ‘Yes, we know there’s trees there,’ we actually have measurements of the physical structure of those trees that we can turn into estimates of carbon,” says Duncanson.
There are already numerous initiatives underway around the world to plant trees. For example, the Bonn Challenge aims to reforest 350 million hectares of degraded and deforested landscapes by 2030, and has already achieved 150 million hectares of reforestation in countries such as Brazil, Burkina Faso, India and Cameroon. One study has estimated that 0.9 billion more hectares of forests could be grown on existing viable land that isn’t already occupied by forests, agriculture or urban areas, and that these could store 205 gigatonnes of carbon – the equivalent of around one-quarter of the carbon dioxide currently in the Earth’s atmosphere.
But planting trees isn’t quite the straightforward solution that it appears. “It’s this great concept: you plant a tree, you save the planet from climate change and it’s actionable, it’s super-easy to integrate into economic solutions, and we all love trees,” Duncanson says. “But the reality is that it absolutely cannot solve the entire problem.”
For starters, the magnitude of that carbon draw-down is uncertain: Duncanson says some papers have based their calculations on the maximum theoretical amount rather than an average. Trees can’t be planted just anywhere, and not all those areas earmarked for possible reforestation will prove to be suitable. Regional climate or soil conditions may be unfavourable, with the result that tree planting in a particular place, far from helping the environment, will fail or will have a negative impact on local ecosystems. In practical terms, there’s the question of where the seeds and seedlings for such a massive reforestation effort will come from, whether there is enough genetic diversity, and how many of those seeds can be harvested without compromising the survival of existing forests. Finally, trees take a long time to grow and larger trees, which are also the ones storing the most carbon, can take decades to reach maturity – decades we probably don’t have.
Despite these concerns and limitations, given the incredible number of ecosystem services that trees provide to humanity – clear air, water, soil stability, oxygen, shelter, food and building materials – reforestation can only improve our environmental conundrum, not worsen it.
Traditionally, reforestation and agriculture have not sat well together, both requiring land that has sufficient nutrients, rainfall and temperatures conducive to growth. And agriculture, of course, poses its own environmental challenges, being responsible (along with forestry and other land use) for around 23 per cent of anthropogenic greenhouse gas emissions (particularly methane and nitrous oxide). But the two activities are not mutually exclusive. Agriculture can work with reforestation to play a vital role in climate change mitigation – sequestering carbon – while delivering the added benefit of more nutrient-rich soils, less fertiliser use, less water use, increased production and better food and economic security.
One farming approach that delivers climate change mitigation, food security and economic security is agroforestry. “Agroforestry is basically mixing trees together with other crops in an integrated system,” says Delphine Clara Zemp, a researcher in the Faculty of Forest Sciences and Forest Ecology at the University of Göttingen in Germany. For example, timber trees can be planted alongside coffee or tea bushes, fruit trees alongside turmeric, a banana plantation intermingled with sweet potato, oil palms alongside coconut palms.
Growing trees and crops alongside each other can benefit crops by stabilising the local microclimate, providing shading, and buffering crops against extreme drought events. The trees can also enhance biodiversity by creating buffer zones around areas of natural vegetation. Some trees increase nutrient levels in the soil, for example by fixing nitrogen, which in turn increases the yields from nearby crops. Agroforestry also offers greater financial resilience for farmers who, instead of relying on one cash crop that may fail or plummet in price, have a range of crops that are harvested at different times. But while the local benefits of agroforestry are well attested, not much research has been done on the climate change mitigation impact of agroforestry – something Zemp would like to see change. “That’s why we only now start to quantify this and try to understand the potential.”
Low-carbon agricultural methods are also attracting interest because of what’s called ‘carbon farming’, whereby farmers can earn carbon credits by adopting methods that reduce emissions or sequester carbon, and then selling those carbon credits to others. Louisa Kiely, a farmer and head of Carbon Farmers of Australia, says that at least half of the farmers who contact her for advice about carbon farming are already looking at improving their soil health through regenerative farming techniques, but want to find out if there’s a way to make some money from it at the same time.
One of the most popular methods of carbon farming in Australia is “human-induced regeneration of a permanent even-aged native forest”, which essentially means allowing native trees to return. This can be achieved by keeping livestock out of areas of native forests, or by managing the timing and extent of their grazing so as to allow the native trees to regrow. It’s also about managing non-native plants, and ceasing to use any kind of chemical or physical methods of destroying native regrowth.
Another way to increase soil carbon is to change how the soil is handled. Ploughing or tilling, which cuts up and turns over the top 15–25 centimetres of soil, breaks apart and releases much of the soil’s stored carbon into the air, as well as killing many of the important soil micro- organisms. With growing awareness of the consequences of these methods both for soil fertility and for greenhouse gas emissions, many farmers are moving to low-till or no-till methods of farming, where seeds or seedlings are planted directly into the soil without disturbing it as much. In addition, these methods deliberately leave more crop waste on the soil surface, which not only returns those nutrients to the soil but also reduces both the release of carbon into the air and erosion from wind and rain. No-till agriculture also uses around one-third the fuel of conventional tilling, and improves water storage in the soil.
It’s not a one-size-fits-all solution. The amount of carbon sequestration is influenced by soil type and climate, and one analysis has suggested no-till methods may store less carbon in cool, dry climates, for example. However, in the Canadian prairies, the area of agricultural land farmed with no-till methods increased from five per cent in 1991 to nearly 50 per cent in 2006, and no-till cropping is now practised across 21 per cent of all cropland in the US.
A mangrove forest at low tide is a magical place. This ever-changing buffer between land and sea is a place of strange creatures and plants that have evolved to thrive in the extremes of wet and dry. The surface is a rich ooze of fine dark mud that bubbles and crawls with life, marine snails map their journey on its canvas, and shellfish cluster on every inch of exposed rock.
This is the home of blue carbon. Mangrove forests, along with seagrass meadows and tidal salt marshes, grow on hundreds, even thousands, of years of stored carbon. In this wet, salty, low-oxygen environment, organic matter such as leaves breaks down slowly into the carbon-rich sediment. Because those sediments are water-logged or underwater most of the time, the carbon is sequestered for far longer than in a terrestrial ecosystem, where exposure to air means a much greater proportion of the carbon is returned to the atmosphere as carbon dioxide.
Blue carbon is sometimes described as ‘boutique carbon’. Mangroves, salt marshes and seagrass meadows provide a host of vital ecosystem services from which humanity has benefited enormously. They limit coastal erosion, protect against extreme events such as storm surges and tsunamis, improve water quality, support tourism, and are habitat and nursery for many of the seafood species that humans eat. Preserving them brings a host of benefits beyond simple carbon sequestration.
In blue carbon environments, carbon is almost entirely stored in sediments rather than within the structure of a plant, which means the carbon storage capacity over these coastal ecosystems is almost limitless. Sediment cores taken from seagrass meadows in the Mediterranean found some sediments were more than 3,000 years old, while other studies have dated sediments in seagrass meadows at more than 6,000 years old. In contrast, the turnover of carbon in a terrestrial forest might be measured in decades, and occasionally centuries.
The idea that coastal ecosystems might be sequestering significant amounts of carbon was first floated in 1981, in a paper that suggested these carbon sinks might represent a significant and unaccounted-for element in global carbon budgets. Today, blue carbon is recognised as one of the most intense carbon sinks on the planet: around half of all the carbon sequestered in ocean sediments despite covering less than two per cent of ocean area.
The problem is that coastal ecosystems are being decimated. Mangroves are disappearing at a rate of around 2 per cent per year – their loss accounts for around ten per cent of emissions from global deforestation. The global area of tidal marsh has halved, and around 30 per cent of seagrasses have been lost. Blue carbon is under threat.
At Edith Cowan University in Western Australia, PhD student Cristian Salinas has been modelling the impact of the loss of Australia’s coastal seagrass meadows that has taken place since the 1950s. He has calculated that the destruction of around 161,150 hectares of seagrass has released the equivalent of five million cars’ worth of carbon dioxide each year, accounting for around a two per cent increase in the annual carbon dioxide emissions associated with land use change in Australia. “It’s not just that you are losing all the carbon that was buried there, it’s that’s also you are losing the capacity that these seagrasses were providing to sequester new carbon,” Salinas says.
The challenge is therefore to protect what is left, and to create new blue carbon ecosystems. Fortunately, interest in this boutique carbon has grown considerably in recent years, and it has been discussed with increasing intensity at successive UN climate change conferences since 2015. That there is now real money to be made in blue carbon suddenly makes it more attractive, says Salinas. “This is a way to protect and restore these ecosystems.”
Efforts to incorporate blue carbon into carbon inventories are under way in countries including Madagascar, Costa Rica, Australia and Indonesia. Blue carbon has even attracted the attention of technology giant Apple, which is working with Conservation International to restore a mangrove forest in Colombia that is expected to draw down around 1 million tonnes of carbon during its lifetime. That said, it’s a race against time to save and restore these precious coastal ecosystems while we can.
The term ‘synthetic forest’ might suggest some science fiction-inspired vista of metallic or translucent trees topped with mechanically waving silicon leaves, tended by an army of sterile-suited workers, the whisper and rustle of wind through foliage replaced with a mechanical hum.
It’s a futuristic aesthetic but, unfortunately for artists and dreamers, nothing like the real thing. A synthetic forest will mostly likely be row after row of long, narrow, rectangular constructions, their surfaces pocked with rows of giant fans that pull air through as fast as possible to find those 400 or so molecules of carbon dioxide in every million molecules of atmosphere.
Just as trees make use of the carbon in carbon dioxide to build their structure, so too can humans. Ever since carbon dioxide became Climate Enemy Number One, scientists have therefore been trying to work out how to make the most of the excess. When conversations started about the idea of capturing carbon from the atmosphere, the focus was on how to use that process to take care of the emissions generated by burning coal, to create so-called ‘clean coal’. “It’s a terrible phrase,” says Jennifer Wilcox, professor of chemical engineering at Worcester Polytechnic Institute. But now, as the coal industry staggers towards its demise, carbon capture has taken on a new meaning. “So we have this view of carbon capture being a 2.0 version, where it’s not about decarbonising coal, it’s really looking at deep decarbonisation or carbon capture and storage,” Wilcox says. It’s about not just capturing carbon from existing processes, such as steel or cement manufacture, but actively removing it from the atmosphere and either storing it or putting it to use.
The first challenge is how to capture it in such a way that we don’t create more environmental problems than we started with. Direct air capture of carbon dioxide can be done in a number of ways, but the basic principle is that the carbon dioxide is brought into contact with a solid or liquid material – potassium hydroxide, for example – that it binds to chemically. That material is then processed, usually with high levels of heat, to extract the captured carbon dioxide and purify it. This last step takes a lot of energy, which ideally is sourced from renewable sources so that few or no additional emissions are created by the process. There are already several companies that have developed direct air carbon capture technologies, such as Carbon Engineering in Canada, Climeworks in Switzerland and Global Thermostat in the United States.
Once that carbon dioxide is captured and purified, what can be done with it? It can be buried, and the best place to do that is in rocks that are very good at absorbing carbon dioxide, such as those high in magnesium. In Iceland, for example, CarbFix has developed a method for injecting carbonated water into underground seams of basalt rock, where the carbon dioxide reacts with the basalt and is literally turned to stone. Unfortunately, burying carbon isn’t necessarily profitable. One estimate is that the cost of direct air capture on a commercial scale is around $600 per tonne, although that may come down to around $200–$300 per tonne in the next few years. Another issue, if the direct air capture plant isn’t co-located with these mineral resources, is that the liquid carbon has to be transported there. “There’s plenty of storage in the Earth,” says Wilcox. “It’s just that it’s not everywhere. And so the question is, how much are you willing to pay for the transportation costs?” Burying the carbon, while it achieves the end goal of removing it permanently from circulation – at least on a geologic time frame – can’t necessarily be a profitable venture unless the carbon credits are priced highly enough to make it worthwhile.
But carbon dioxide and carbon are useful materials, and a growing number of companies are looking at how to make money from them. Carbon can be used in carbonated beverage production, for example, to create synthetic liquid fuels such as syngas or make plastics such as ethylene. It can be used to make carbon-negative concrete: for example, injecting carbon dioxide – captured from industrial processes – into wet cement as it is curing can improve its strength and sequester carbon dioxide at the same time. A form of cement manufactured from magnesium oxychloride – which is made from byproducts of magnesium mining – mixed with fly ash from coal combustion is not only stronger and more fast-setting than conventional cement, but the magnesium oxychloride also actively absorbs carbon dioxide. Even the aggregate – the sand, gravel and rock that is bound together by the cement – can be replaced by rocks made from sequestered carbon dioxide captured from industrial processes.
Carbon capture doesn’t have to involve technology or even organic processes. Enhanced weathering is the low-tech speeding-up of the natural chemical interaction between carbon dioxide in the atmosphere and surface minerals, which has been sequestering carbon dioxide from the Earth’s atmosphere long before humans, or even life, emerged on the planet. That process can be accelerated by making more of those surface minerals – particularly those rich in calcium and magnesium – exposed to the atmosphere; for example, by digging them up or crushing them finely. One industry where that process happens all the time is mining.
In the early 2000s, the waste heap of an old asbestos mine in Quebec was found to be sequestering significant amounts of carbon dioxide, as the magnesium-rich minerals in the waste heap reacted chemically with carbon dioxide in the atmosphere to produce magnesium carbonate. One study estimated that that single mine heap was sequestering around 600 tonnes of carbon dioxide per year, roughly equivalent to the emissions from 118 passenger cars in one year.
Magnesium-rich mineral waste is a byproduct not only of asbestos mines, but also of diamond, platinum and nickel mines. Those waste heaps could therefore represent a huge, untapped carbon sequestration potential. “If we get enough of those reactions occurring, then we can sequester the amount of CO2 that’s being emitted by the mine or in some cases sequester even more potentially than the mine is emitting,” says Anna Harrison, an environmental geochemist and assistant professor at Queen’s University in Ontario, Canada. For example, one assessment of the Mount Keith nickel mine in Western Australia found the amount of carbon dioxide being sequestered by the mine tailings represented around 11 per cent of the mine’s total annual greenhouse gas emissions.
It’s taking advantage of a naturally occurring process that normally happens on geologic timescales. But in a mine waste heap, the rock is ground up into rubble, so much more surface area of the magnesium-rich rock is exposed to air, and the reaction happens much faster. And once the carbon dioxide is locked up in the carbonate minerals, it’s there for a very, very long time. It’s also possible that the reaction could be speeded up even more, because the main rate-limiting factor is the supply of carbon dioxide getting to the rock. “They’re just deposited as this big mass of fine-grain material that’s partially filled with water, and then the CO2 from the atmosphere only seems to react with maybe the upper ten to 15 centimetres of that tailings pile,” Harrison says.
Carbonated minerals can also be used in agriculture as a way of returning carbon to the soil and also helping to balance the acidity of agriculture soils. One study estimated that enhanced weathering used in this manner could sequester about the same amount of carbon as agricultural soil carbon sequestration methods.
Carbon mineralisation could deliver carbon-neutral, or even carbon-negative, mines. The diamond company DeBeers is already trialling it at some of its mine sites because kimberlite, which diamonds are often found in, is high in magnesium, making it well suited to carbonisa- tion. So well suited, in fact, that a typical diamond mine could produce enough kimberlite to offset ten times its own emissions.
“Carbon removal is an opportunity to go back,” says Jennifer Wilcox. That doesn’t mean back to pre-Industrial Revolution carbon dioxide levels, or even pre-1970 levels, because we’ve gone too far for that now. We are beyond the point where negative emissions technologies alone, without any other reductions in carbon emissions, could save us. But these technologies and practices can buy us time to get our backs off the 412-parts-per-million wall.
“We’ve got to take that back out, if we want to get back down to reasonable levels of CO2 concentrations in the atmosphere,” says Wilcox, but we also have the rates at which we keep dumping today and in the future. We need to remove the carbon, and we also need to avoid putting more up there in the first place: “We’ve got to do it all.”
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