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Microsoft recently announced that it aims to become carbon negative by 2030. What’s more, the company said that by 2050, it plans to have removed from the atmosphere all the carbon that it has emitted since it was founded in 1975. This is a significant commitment from an individual company, and it underscores the potential for approaches using negative emissions, or carbon dioxide removal, to play an important role in meeting international climate goals.
Carbon neutrality, or “net zero,” means that any CO2 released into the atmosphere from human activity is balanced by an equivalent amount being removed. Becoming carbon negative requires a company, sector or country to remove more CO2 from the atmosphere than it emits.
Meeting ambitious international climate goals may require global CO2 emissions to fall below zero in the second half of this century, achieving what is known as net negative emissions. In the Intergovernmental Panel on Climate Change (IPCC) Special Report on Global Warming of 1.5°C, published in late 2018, almost all the pathways analysed by the authors relied to some extent on carbon removal approaches in order to achieve net negative emissions after 2050.
This does not mean, though, that carbon removal is only a long-term solution: the technologies can also play an important near-term role in clean energy transitions. They can neutralise or offset emissions that are currently technically challenging or prohibitively expensive to address. This includes in some industrial processes, such as steel-making and cement production, and long-distance transport, like shipping and aviation.
It is important to note that carbon removal technologies are not an alternative to cutting emissions or an excuse for delayed action. But they can be part of the portfolio of technologies and measures needed in a comprehensive response to climate change.
There are multiple ways of removing CO2 from the atmosphere, most of which fall into three broad categories: (1) nature-based solutions, (2) measures that aim to enhance natural processes, and (3) technology-based solutions.
Nature-based solutions include afforestation and reforestation. These involve the repurposing of land use by growing forests where there was none before (afforestation) or re-establishing a forest where there was one in the past (reforestation). Other nature-based solutions include restoration of coastal and marine habitats to ensure they continue to draw CO2 from the air.
Enhanced natural processes include land management approaches to increase the carbon content in soil through modern farming methods. This can incorporate the addition of biochar (charcoal produced from biomass) to soils, where the carbon can remain stored for hundreds or thousands of years. Less developed approaches include enhanced weathering to accelerate natural processes that absorb CO2 (for example, by adding very fine mineral silicate rocks to soils) or ocean fertilisation in which nutrients are added to the ocean to increase its capacity to absorb CO2. Enhanced weathering and ocean fertilisation approaches require further research to understand their potential for carbon removal as well as their costs, risks and trade-offs.
Technology solutions include bioenergy with carbon capture and storage (BECCS) and direct air capture, which – as the name suggests – involves the capture of CO2 directly from the atmosphere. Both of these solutions rely on geological storage of CO2 for large-scale carbon removal and could play an important role in clean energy transitions. They are discussed in more detail below.
In pathways limiting global warming to 1.5°C with limited or no overshoot, the IPCC found that agriculture, forestry and land-use measures could be removing between 1 billion and 11 billion tonnes of CO2 per year by 2050. The potential amount of CO2 removal from BECCS ranged from zero to 8 billion tonnes per year by then. To put this in context, global energy-related CO2 emissions were 33 billion tonnes in 2018. Other carbon removal options are not included in the IPCC pathways because of their lack of maturity.
BECCS involves the capture and permanent storage of CO2 from processes where biomass is burned to generate energy. This can include power plants using biomass (or a mix of biomass and fossil fuels); pulp mills for paper production; lime kilns for cement production; and refineries producing biofuels through fermentation (ethanol) or gasification (biogas) of biomass.
BECCS enables carbon removal because biomass absorbs CO2 as it grows, and this CO2 is not re-released when it is burned. Instead, it is captured and injected into deep geological formations, removing it from the natural carbon cycle.