Carbon removal techniques
Afforestation and Reforestation
The planting of new forest – and reforestation – the regrowing of former forest – remove carbon from the atmosphere. A single mature tree may remove several tonnes of carbon from the air over its lifetime. A forest may remove many thousands or millions of tonnes.
New and regenerating forests store carbon in the bodies of the trees themselves, in the soil and the wider forest ecosystem. How much carbon is stored depends on the conditions in each forest.
Recent studies have shown that old forests and ancient trees are still absorbing carbon today, so preserving existing forests also preserves their ability to remove carbon.
Many of the innovative groups working on forests are helping to address different challenges from more accurately measuring carbon drawdown to ensuring those looking after the forest are paid fairly.
Tree planting is considered an established carbon removal approach. However, the carbon removed is only as permanent as the forest. If the forest is felled, burnt, or collapses from climate impacts already in motion, much of the carbon stored will be released once again.
When biomass is carefully heated to several hundred degrees in a low-oxygen “pyrolysis kiln”, it leaves behind a stable, charcoal-like substance known as biochar that can store carbon for hundreds of years. There is growing evidence that some varieties of biochar can, once in the soil, perform a long list of useful jobs for plant roots and soil microbes, such as holding onto water and nutrients that would otherwise drain out of reach.
The important (and difficult) part is making sure the right kind of biomass gets pyrolysed in the right way and is then applied into the right kind of soil. If done right, this process can lock up for centuries a solid fraction of the carbon stored in the original biomass that would otherwise have decayed over a few years.
In practice, this means finding the right feedstock – like small pieces of wood, straw or corn husks – that don’t have other uses, then putting them through a kiln with specific settings (like temperature, length of time, and oxygen levels). This allows the user to customise certain features of the biochar and make sure it is well suited to whatever soil it gets added to.
The leading groups in biochar are doing the R&D to source and produce the best biochar possible for different uses. The next steps are generally to get more data from different experiments in field conditions to understand how stable and beneficial different biochars can be in reality.
Bioenergy with carbon capture and storage (Bio-CCS)
Like biochar, Bio-CCS works by taking carbon originally captured by plants and locking it up securely to prevent it from returning to the atmosphere. But where biochar stores carbon as solid charcoal, Bio-CCS systems first burn biomass to create energy and carbon dioxide, and then capture and store this carbon dioxide in some other form.
If done badly, Bio-CCS has been described as “throwing a forest into a power station then capturing and storing the emissions” ( unnamed individual, ~2018) . Crucially, Bio-CCS only counts as carbon removal if it results in more carbon being taken out of the atmosphere than would otherwise have been the case.
Done well, however, using sustainably-sourced types of biomass, and state-of-the-art energy production and emissions capture technology, Bio-CCS systems can complement other types of forestry and agriculture and draw carbon down from the air, all while generating low carbon energy.
Often, and particularly especially in energy system models, Bio-CCS has been imagined as a large and singular technology, a sweeping technological solution deployed at mass scale to mop up the excess carbon dioxide of the fossil fuel economy. There are other uses for land and biomass beyond generating electricity with carbon removal – growing food and nurturing biodiversity, for example – and many people have warned that relying on Bio-CCS could further threaten these systems on which we all depend.
In reality, Bio-CCS represents a whole family of approaches, where different bio-energy and CCS technologies come together. The challenge is to find combinations of technologies, business models and regulations that are suitable and sustainable in different places and for different types of biomass.
Leading Bio-CCS companies are doing the hard but valuable work to find these sustainable value chains where socially and environmentally positive sources and uses of biomass can produce sustainable energy and capture carbon in the process.
Blue Carbon / Marine Ecosystem Restoration
Carbon can also be removed by the plants and animals on the coasts and in the ocean.
Coastal wetlands, like tidal marshes, mangrove forests and seagrass meadows, trap and preserve carbon in mud and sediment, and can build up large stores of it over many years. More recently, kelp forests have also been recognised as an important carbon sink, absorbing carbon rapidly in shallow seas until some fraction of it sinks into the deep ocean.
Plankton are also in on the act, providing a ‘carbon pump’ that pulls carbon out of the surface ocean as the plankton multiply and grow and then buries it in the depths as they die and sink. Even whales contribute to carbon removal. They interact with the plankton carbon pump in the ocean through their diet. A recent report by the International Monetary Fund estimated that if whales were allowed to return to their numbers before whaling started, they would remove an extra 1.7 billion tonnes of carbon dioxide each year.
Direct Air Capture
Direct Air Capture (DAC) uses the same sort of technology found in spaceships and submarines to scrub carbon dioxide from the air in enclosed environments to keep it at safe levels for the inhabitants. DAC developers, however, wish to scale-up that technology to help keep Spaceship Earth’s carbon dioxide levels in balance.
Direct Air Capture plants are designed to do this by using large arrays of fans to efficiently move air over large ‘contactors’ that extract carbon dioxide. Contactors typically use either a liquid solution or solid chemical resin that the carbon dioxide can dissolve into or stick to as it passes by. After enough carbon dioxide has been trapped by the contactor, it is removed and undergoes ‘regeneration’, discharging nearly pure carbon dioxide to be used or stored safely.
Carbon dioxide is very highly diluted in the atmosphere, so this is harder than it sounds. One trick is to make the contactor material ‘sticky’ enough to capture a good fraction of the carbon dioxide passing over it, but not so sticky that it becomes hard to regenerate so that the carbon dioxide can be processed and stored.
Another challenge is to make the fans and contactor surface efficient enough to draw vast volumes of air through the system without using too much energy. Finally, there is the question of where to put the carbon dioxide once it’s been purified. For example, carbon dioxide can be carefully injected into underground reservoirs, or mixed with certain types of rock where it turns to stone in a few years.
Direct Air Capture is also being described as an ‘air mining’ technology, It closes the carbon cycle and uses carbon dioxide from the air as a building block for a range of uses, from materials to clean fuels. Some of these pathways – generally those that don’t involve burning the product e – can achieve carbon removal too if the carbon dioxide gets locked away. But even uses where the carbon dioxide is re-released to the atmosphere, like ‘air-to-fuels’ technologies, could be very important sources of clean energy in future.
Today, the leading Direct Air Capture groups are building their first pilot plants at commercially relevant scales. The purpose of these demonstrations include showing how the technology can operate at scale, e.g. how much energy and water is needed and how reliable the machines are. Another aim is to give more data to the debate around how much DAC costs and – as with other carbon removal approaches – how much people are willing to pay for it.
Soil Carbon Drawdown
Plants and animals are, of course, carbon-based life forms that ultimately derive their carbon from the atmosphere. Plants draw in carbon dioxide during photosynthesis and animals then eat the plants.
The sheer diversity of how ecosystems capture, cycle and store carbon is truly wonderful. Find a thriving biosphere, and you will find endless forms of life cycling carbon in and out of the atmosphere.
The tragedy is that as wildlife goes extinct and ecosystems start to collapse, their ability to cycle carbon out of the air is also lost. But the opportunity to restore these ecosystems back to health – and draw down greenhouse gases from the atmosphere in the process – is significant.
We’ve talked earlier about how trees and forests store carbon in their trunks, branches and roots. But globally, the world’s soils hold between twice and four times as much carbon as the world’s trees. Farmers and scientists worldwide are working on ways to store more carbon in the soil, often through smarter and more regenerative ways of farming. But as one of our favourite land restorers once said : ‘soil science isn’t rocket science… it’s actually a lot more complicated!’
The leading groups in soil carbon are working to bring together state of the art measurement, monitoring and market-moving technology to encourage landowners to store more carbon in their soil, and reap the economic benefits in the process.
As with other areas like forestry, critical to keeping the carbon stored is protecting and preserving the ecosystems and soils in question. So whilst the Earth might not wish to ‘bet the farm’ on these solutions, they still have an important role to play and many benefits to offer.
Carbon dioxide in the air reacts with specific minerals in the rocks and the carbon products dissolve in rainwater. The minerals and the carbon eventually end up in the oceans and, over millennia, in chalk and limestone sediments on the ocean floor. Many billions of tonnes of carbon dioxide are removed from the atmosphere this way.
Enhanced weathering aims to accelerate that process enough to make a difference to the atmosphere in decades, not millions of years. For the most part, this tends to involve mining and grinding up rocks that contain the most effective carbon-removing minerals – especially basalt and a rare rock type called peridotite – and spreading them around to catch carbon dioxide from the air.
Another form of enhanced weathering could also be done in the ocean. For example, if we can develop a low-carbon way of generating lime (the mineral, not the fruit) from limestone, some research suggests we could selectively add it to the oceans to both capture carbon dioxide and directly address the problem of ocean acidification.
The leading groups in enhanced weathering are working on how to put these minerals to use safely, effectively and economically. In addition to the ocean methods, proposals on land range from grinding up rocks and adding them to farmland, to changing the type of rocks used in coastal defences, where the waves crashing on the shore do a lot of the hard work. But one challenge common to all methods is the sheer amount of rock required to capture carbon dioxide at scale: more than a tonne of rock is needed for every tonne of carbon dioxide taken out of the air.
The big goals right now are to find viable pathways to do it in the real world, and to accurately measure how much carbon is actually getting removed in the process. A bit more attention and help by some progressive mining companies would also help.