Bioenergy, the carbon cycle and climate change mitigation

Bioenergy, the carbon cycle and climate change mitigation – Download the factsheet

When biofuels are used for transport, heat or electricity, carbon dioxide (CO₂) is released into the atmosphere in the same way as when fossil fuels are used. However, these CO₂ emissions tell us little about the climate impact of bioenergy. Here we explain how bioenergy and fossil fuels differ in their interactions with the carbon cycle and how bioenergy can contribute to climate change mitigation by limiting the increase in atmospheric CO₂ levels.

Bioenergy and fossil fuels in the carbon cycle

Figure 1 gives an overview of the carbon cycle; the flow of carbon (in solid, liquid and gaseous forms) between the different pools, or reservoirs, where it is stored. Most of the Earth’s carbon is found underground in rocks and sediments. This is where fossil fuels are found, and carbon stored here is often referred to as fossil, or geologic, carbon. The rest of the carbon is found in the atmosphere, oceans, and land, where it is stored in plants, animals and other living organisms, soils, freshwater, and biobased products such as construction timber, paper, food and biofuels. This fast, or active, domain of the carbon cycle involves large exchanges of carbon between the different pools, and the time that carbon remains in a pool ranges from a few years to centuries, in contrast to the slow processes associated with geological carbon, where carbon turnover times are 10 000 years or more. While CO₂ emissions from fossil fuel use transfer carbon from geological storage into the atmosphere, harvesting and using biomass for energy returns carbon to the atmosphere that was recently removed by plant photosynthesis, and as new plants grow, carbon is removed from the atmosphere again. However, as shown in Figure 1, bioenergy systems can affect many carbon flows, which in different ways influence how much carbon is emitted and removed from the atmosphere. The net effect on atmospheric CO₂ levels is determined by the way in which all these carbon flows are affected by bioenergy systems.

The effectiveness of bioenergy in mitigating climate change depends on three main factors:

Figure 1: An overview of the carbon cycle and how it is affected by bioenergy and fossil fuel use. BECCS = bioenergy with carbon capture and storage. DACCS = direct air carbon capture and storage. These two carbon dioxide removal (CDR) options differ in that carbon is either removed from the atmosphere through photosynthesis (BECCS) or through technological means (DACCS). The natural exchange of carbon between geological reservoirs and the fast domain of the carbon cycle is relatively small and can be ignored when discussing human impact on the climate.

Anthropogenic CO₂ emissions

Atmospheric CO₂ levels, which remained below 300 ppm for millennia, have risen rapidly since the beginning of the industrial era (Figure 2). Fossil fuel use has been responsible for about two-thirds of the anthropogenic CO₂ emissions since then. In the last decade (2010-2019), it accounted for 86% of the anthropogenic CO₂ emissions. The remaining anthropogenic CO₂ emissions come from waste management, cement production and human land management and land use change, particularly deforestation to make way for pasture and cropland, which releases carbon from soils and vegetation into the atmosphere.

Figure 2: Atmospheric CO₂ concentration in the last 800 000 years and anthropogenic CO₂ emissions in the period 1870 to 2019. ‘Others’ comprise emissions from carbonates during cement manufacture and flaring to avoid methane emissions in oil and gas production and waste management. Source: left bottom diagram: NASA, 2024, The relentless rise of carbon dioxide. Right top diagram: Global Carbon Project, 2024, Global Carbon budget 2023.

Impact of bioenergy on atmospheric CO₂ levels

As many solid, liquid, and gaseous biofuels can replace fossil fuels, there is a significant opportunity to use bioenergy to reduce fossil carbon emissions. The use of fossil fuels in the bioenergy supply chain can diminish the mitigation benefits, but fossil supply-chain emissions can be avoided by switching to biofuels and other non-fossil alternatives. As explained above, the impact on atmospheric CO₂ levels also depends on the influence of bioenergy on carbon flows and carbon storage within the fast domain of the carbon cycle. Policies to promote bioenergy can reduce or increase carbon storage in ecosystems and biobased products, depending on how they influence land management and land use change, and the production and use of biobased products. This needs to be considered in assessments of bioenergy options, otherwise they will not fully reflect how bioenergy options will affect atmospheric CO₂ concentrations.

The CO₂ emissions from a bioenergy plant can be captured and injected into geological reservoirs (process known as bioenergy with carbon capture and storage, BECCS) or used to make new products (known as bioenergy with carbon capture and utilization, BECCU). Both of these can contribute to the carbon dioxide removal (CDR) from the atmosphere that is needed to achieve global and national net zero greenhouse gas emissions targets. In principle, BECCU is similar to product reuse or material recycling, in that it helps to keep carbon out of the atmosphere. BECCS provides longer-term carbon storage, but without the benefit of putting biogenic carbon to  productive use in society. Biomass conversion that produces biochar along with bioenergy provides CDR with shorter storage times than BECCS, but with potential for co-benefits. For example, when used as a soil amendment biochar can improve soil structure, water retention, and nutrient availability.

In summary, three main factors determine the impact of bioenergy on atmospheric CO₂ levels:

• the extent to which fossil carbon emissions are displaced
• the way in which the bioenergy system affects carbon flows and carbon storage within the fast domain of the carbon cycle,
• the extent to which CO₂ from bioenergy use is captured for geological storage or reused in new products, or biogenic carbon in bioenergy byproducts is retained in durable products such as biochar.

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Recommended reading

• Berndes, G., et al. (2018). Forests and the climate: Manage for maximum wood production or leave the forest as a carbon sink? Report from a conference held on March 12–13, 2018, in Stockholm Sweden, organized by the Royal Swedish Academy of Agriculture and Forestry, the Royal Swedish Academy of Sciences, and the Royal Swedish Academy of Engineering Sciences. KSLA-T 6/2018.
• Cowie, A., Carbon Accounting for Sustainable Biofuels, International Energy Agency, Paris.
• IEA (2024). Carbon Accounting for Sustainable Biofuels, International Energy Agency, Paris.
• IEA Bioenergy (2018). Is energy from woody biomass positive for the climate?
• Pelkmans, L., et al.(2024). The role of bioenergy in the energy transition, and implications on the global use of biomass. 
• IPCC Working Group III, Factsheet about Carbon Dioxide Removal.

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“Bioenergy, the carbon cycle and climate change mitigation” is the third in a series of factsheets prepared by IEA Bioenergy. These factsheets aim to inform and engage readers by addressing the key issues related to bioenergy, fostering greater awareness of its potential and challenges. Through these resources, IEA Bioenergy seeks to bridge the knowledge gap and promote the adoption of bioenergy solutions in the context of a global shift towards renewable energy.

Bioenergy, the carbon cycle and climate change mitigation – Download the factsheet

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