This Thursday, the UK’s independent advisory body the Committee on Climate Change is anticipated to recommend a net-zero emissions target for the UK when it publishes its latest advice to government. But what does ‘net-zero’ mean and what are the challenges – from technological to moral – to achieving it, in the UK and worldwide?

It is clear from the science that the amount of carbon dioxide (CO2) in the atmosphere as a result of human activity largely determines the extent of global warming. This means that to prevent catastrophic climate change, CO2 emissions need to be reduced to zero. The science led to governments worldwide agreeing to achieve a balance between emissions and removal of greenhouses gases, in the Paris Agreement.

‘Net zero’ refers to achieving an overall balance between emissions produced and emissions taken out of the atmosphere. Like a bath with the taps on, an approach to achieving this balance can either be to turn down the taps (the emissions) or to drain an equal amount down the plug (removals of emissions from the atmosphere, including storage for the emissions such as ‘carbon sinks’).

In contrast to a gross-zero target, which would reduce emissions from all sources uniformly to zero, a net-zero emissions target is more realistic because it allows for some residual emissions.

This takes into account that some emissions are produced by ‘hard-to-treat’ sectors, such as aviation and manufacturing, where reducing emissions is either too expensive, technologically too complex or simply not possible.

In a net-zero scenario the residual emissions from these sectors are allowed as long as they are offset by removing emissions using natural or engineered sinks – gross negative emissions.

Negative emissions can be achieved in a variety of ways, by utilising nature or engineering. Most straightforwardly, this can involve planting more trees to absorb the CO2 in the atmosphere through photosynthesis – afforestation (planting new forests) and reforestation.

There are also negative emission technologies: some of the most commonly used involve carbon capture and storage (CCS). This works by capturing CO2 before it is released into the atmosphere, by removing carbon from the gases produced by burning fossil fuels, or using hydrogen or oxygen in the process. Once the CO2 has been captured, it is compressed into liquid state and transported so that it 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. The technology can capture up to 90% of CO2 released by burning fossil fuels in electricity generation and industrial processes such as cement production.

Combining these approaches, bioenergy with carbon capture and storage (BECCS is the process of growing plants, crops or trees, harvesting them for energy generation and then capturing the carbon given off so it can be stored underground.

In addition, Direct Air Capture (DAC) removes carbon dioxide directly from the air to convert it into oxygen and store the carbon. This has historically been used in closed environments where oxygen is not available, such as submarines and space craft, to remove CO2 from the air before concentrations become too high for humans. However, there are still real questions about how economically viable this technology currently is at scale, meaning it is not a silver bullet.

The fact that some emissions will remain poses a number of important questions for achieving net-zero.

What is the scale of residual emissions?

The Committee on Climate Change in its most recent assessment considers that even if existing technologies to decarbonise the energy, heating, transport, industry, agriculture and waste sectors are maximised, there will be around 130 million tonnes of carbon dioxide-equivalent (MtCO2e) a year of residual emissions that negative emission technologies will have to remove.

Industry will account for 24 per cent of this 130 million tonnes, aviation and shipping for 31 per cent, and greenhouse gases other than CO2 (chiefly from agriculture) for 36 per cent.

These residual emissions will need to be reduced to match the amount of emissions being offset by negative emissions strategies.

What proportion of the reduction in residual emissions should occur within the UK’s borders?

Another strategy that countries can use is to pay another country to offset its emissions. This is used especially if it is difficult to for the country to reduce some of its own emissions. By having another country plant more trees or use negative emissions technologies, this reduces the amount of emissions globally, so countries may wish to use this to achieve their net-zero target.

Some, including the chair of the Energy Transitions Commission, argue that this approach is not the right way to achieve net-zero and that the UK’s net-zero target should be measured solely by emissions reductions and negative emissions within the UK. After all, if every country offsets a proportion of their emissions reduction, it will be very difficult or perhaps impossible ever to reach net-zero on a global scale. Costa Rica is so far the sole country that has said it will its net-zero target without offsetting emissions abroad.

Given the UK’s history and legacy as a high-emitting country, there is an argument that it has a moral duty to decarbonise as much as feasibly possible domestically. The argument for this is that countries that knowingly sustain and promote the use of fossil fuels also bear responsibility for climate catastrophe and should take on more of the burden of preventing these effects.

The Paris Agreement also made it clear that wealthier countries must act to decarbonise their economies more quickly than developing countries, which are still growing their economies.

What is the net-zero deadline likely to be?

The timing of the net-zero target that will be announced by the Committee on Climate Change is the final key element. Four options have been considered: 2025, 2045, 2050 and any time thereafter.

The most ambitious target of 2025 is what the Extinction Rebellion movement is demanding. These protests have rightly helped to focus attention on the urgent issues of climate change, but 2025 is nevertheless perhaps an unrealistic target. Some of the technologies to deliver net-zero don’t yet exist, some that do can’t physically be built in time, and it takes decades for planted trees to grow. The cost of meeting this target would also be high, with the effects likely most keenly felt by low income groups. This means that the likelihood of this deadline being adopted is very low.

A target date of 2045 would match the ambition of Sweden and Norway, and, through setting a stringent target, demonstrate leadership on the international stage. This would promote the UK as a global forerunner for international diplomacy on climate change. However, it is also important to recognise that the UK is not even on track to hit its 4th or 5th carbon budgets, so expediting the target date while simultaneously increasing ambition seems unlikely to happen.

All these factors considered, the most realistic target is likely to be 2050, as the Financial Times has now alluded to. Further, this would require no change in primary legislation or formal revision of the 2008 Climate Change Act, whereas any deviation from 2050 is likely to necessitate the proposal of a new Bill or Act.

A version of this commentary was first published in Business Green.

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