The Science behind Net Zero

Achieving Net Zero is the key to limiting global warming and related climate change impacts, but what is Net Zero (or net zero carbon) and what do we have to do to achieve this? Here we explain the methods needed, including the urgent need for carbon capture and storage. While there are differences and error bars on the estimates for the numbers in this article, some estimates with their sources are included for illustration.

To recap, certain naturally occurring gases in the atmosphere, including carbon dioxide (CO2) and water vapour (H2O) but also methane (CH4) and others, make our planet habitable by trapping heat in the atmosphere, which is called the greenhouse effect. However we are now seeing the increase of greenhouse gases (GHGs), due to human activities, beyond the level at which the mean global temperature can be stabilised, which is leading to global warming and undesirable climate change effects. We are already seeing weather extremes worldwide, which can be attributed to this increasing energy and other changes in the atmosphere, as well as rising sea levels and loss of habitats, due to rising temperatures. The scientific consensus is that this recent warming is due to various human activities – including largely the burning of fossil fuels (since the industrial revolution) but also the intensification of agriculture, deforestation and industrial processes. Population growth, combined with increased per capita consumption, is exacerbating the challenge of controlling GHG emissions and warming. We have already reached 8 billion people on the planet and the global population is projected to top out at about 10.7 billion in the 2080’s .

Achieving net-zero means that any GHG emissions created by human activities must be balanced by taking the same amount of emissions out of the atmosphere  (net-zero actually refers to a balance between all generated and removed GHG emissions, not just carbon dioxide, but this is usually expressed in tonnes of carbon dioxide equivalent – CO2eq). The world emits around 50 billion tonnes of GHGs each year.

There are many ways to remove carbon from the atmosphere – for example, planting trees, which absorb CO2 and release oxygen. However, globally, we are removing trees faster than we replant them and we can’t plant enough tress to fic the problem. Other parts of the natural system also absorb carbon dioxide and we lose biodiversity at our peril. Net-zero targets can be set up at a national scale or for a city or region or for a business. Science-based targets are stringent emission reduction targets that must be aligned with the aims of the Paris Agreement (aiming for a global temperature rise less than 2C). In 2019, the UK government became the first major economy to pass a net zero emissions law.  This includes a target that will require the UK to bring all its greenhouse gas emissions to net zero by 2050.

As well as trying to take CO2 out of the atmosphere, it makes sense to reduce the amount of CO2 that we are putting into the atmosphere in the first place. Apart from domestic use of energy, many of our industrial processes such as the production of raw materials like steel, cement/concrete, glass and paper are very energy-intensive. Agriculture – especially the raising of animals – generates carbon dioxide and methane so there are also initiatives to reduce the amount of meat we eat, but we cannot realistically expect to limit our consumption to solve the problem.

Emissions by sector are as follows:

  • Energy (electricity, heat and transport): 73.2%
  • Direct Industrial Processes: 5.2%
  • Waste: 3.2%
  • Agriculture, Forestry and Land Use: 18.4%

So how do we decarbonise?

  1. Cut energy consumption – insulate homes and become more efficient in our domestic, industrial and transport energy use. These are important steps we can all take, but not enough on their own.
  2. Use renewable energy to generate electricity (solar, wind, hydropower, hydrothermal, tidal) and stop burning fossil fuels and deforestation. We will still need the use of e.g. nuclear energy to provide the energy base load as many renewable sources of energy are intermittent. We are doing well in the UK with respect to increasing our use of solar and wind power for electricity but we have a long way to go with the decarbonisation of heat, transport and land use.
  3. Cement and steel are the essential ingredients of buildings, cars and other infrastructure, but the industries creating them are among the dirtiest on the planet. Cement and steel production contribute 6.5% (2.3 billion tonnes) and 7% (2.6 billion tonnes) of global CO2 emissions respectively. This is partly due to the huge quantities in which these materials are used: concrete is the second-most-consumed product on the planet, after clean water. They are also carbon-intensive manufacturing processes: the chemical reactions involved, and the burning of fossil fuels to deliver the extreme temperatures required, all release CO2. While we can introduce efficiencies and new processes it is not possible to completely cut these sources of emissions
  4. As a result of the limitations in all the above, the technology of Carbon Capture and Storage (CCS) is vital to achieving net zero.

Carbon Capture and Storage

Carbon capture and storage (CCS) is the process of capturing and storing carbon dioxide before it is released into the atmosphere. This technology can capture up to 90% of CO2 released by burning fossil fuels in electricity generation and industrial processes such as cement production. At the moment, CCS is the only technology that can help reduce emissions from large industrial installations. When combined with bioenergy technologies for power generation (so-called BECCS – bioenergy with carbon capture and storage), CCS even has the potential to generate ‘negative emissions’, removing more CO2 from the atmosphere than emitted. Many scientists and policymakers argue that using CCS is crucial if the world is to limit temperature rise to under 2C. The International Energy Agency states that a tenfold increase in capacity is needed by 2025 to be on track for meeting that target and the Global CCS Institute estimates that 2,500 CCS facilities would need to be in operation by 2040 worldwide, each capturing around 1.5 million tonnes of CO2 per year.

CO2 can be captured using different methods: mainly post-combustion, pre-combustion and oxyfuel. Post-combustion technology removes CO2 from the flue gases that result from burning fossil fuels. Pre-combustion methods – carried out before burning the fossil fuel – involve converting the fuel into a mixture of hydrogen and CO2. Oxyfuel technology produces CO2 and steam by burning fossil fuels with almost pure oxygen. Post-combustion and oxyfuel equipment can be fitted to new plants or retrofitted but pre-combustion methods would require very large modifications to existing plants and are therefore more suitable to new-build power stations.

Once the CO2 has been captured, it is compressed into a liquid state and transported by pipeline, ship or road tanker to a deep storage facility. CO2 can then be pumped underground, usually at depths of 1km or more, to be stored into depleted oil and gas reservoirs, coalbeds or deep saline aquifers, where the geology is suitable.

CO2 could also be used to produce commercially marketable products. This is commonly known as carbon capture storage and utilisation (CCSU). The most well-established form of CO2 utilisation is enhanced oil recovery (EOR), where CO2 is injected into oil and gas reservoirs to increase their extraction. Other forms of CO2 utilisation are under investigation, including using CO2 in concrete or plastic materials or converting it into biomass e.g. by feeding CO2 to algae, which are then harvested and processed into biofuel for transport.

The first large-scale CCS project began operating at Sleipner in Norway in 1996. As of September 2022, a new report has said that the number of CCS facilities under development worldwide has grown 44 percent year on year to 244 million tonnes per annum in the past 12 months. There are now 196 projects in the pipeline with 61 new CCS facilities added in 2022 alone. The first UK commercial-scale CCS plant will be the Drax BECCS facility in Yorkshire (in early development), which aims to capture 8 million tonnes of carbon per year from the UK’s largest power station by 2027. More than 10 other plants are in development, aiming to become operational in the mid- to late-2020s.

Are there any drawbacks to carbon capture and storage?

Overall, the capture process is expensive, due to high deployment and energy costs. A plant with CCS uses more fuel than one without, to extract, pump and compress the CO2. The cost of CCS varies significantly between processes: where CO2 is already produced separately in concentrated streams, for example in fertiliser manufacturing, the cost is lower, but for other processes, such as cement production and power generation, the cost is much higher. However, research and development efforts are aiming to reduce the cost, and the price of avoiding a tonne of CO2 has already declined significantly. In the UK, the Government-commissioned 2016 Oxburgh Report argued that ‘CCS is essential for lowest cost decarbonisation’.

Possible environmental and climate change damages could be caused by CO2 leakages from storage sites if they are not adequately selected, managed and monitored. A Princeton University study, however, considers this risk to be low.

To summarise – the UK government has committed to net zero by 2050 (China aims to reach net zero by 2060). Without CCS most experts think this is unachievable. Presently, however, global efforts to reduce emissions, including investment in CCS, are still grossly inadequate. Achieving net zero by 2050 requires strong policy action. The pathway to wide-scale CCS deployment will require private sector financing, but governments must play a key role in creating an enabling environment for private sector investment.


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