Aviation’s greenhouse gas emissions could consume around half the carbon budget available to the UK in 2050, even if the sector’s emissions growth is constrained.

If aviation emissions increase with rising demand for flights, the sector could claim as much as two-thirds of the budget for 1.5C, Carbon Brief analysis of the latest official climate advice shows.

The numbers make for awkward reading as the government approves a new runway at Heathrow, which it says is needed to meet ever-rising demand for air travel.

Paris Agreement

The Paris Agreement on climate change, due to enter force on 4 November, pledges to limit warming to “well below” 2C and, if possible, no more than 1.5C above pre-industrial temperatures.

In order to meet this aim, countries must make careful use of the very limited remaining carbon budget. That budget could be used up within five years, leaving the world reliant on unproven negative emissions technologies in order to draw CO₂ out of the atmosphere.

Paris means the UK must raise its existing climate ambition: it will have to reach net-zero emissions, whereas its current legislated target is to cut emissions 80% below 1990 levels by 2050.

The Committee on Climate Change (CCC), the government’s official advisers, says it is too early to set a date for reaching net zero. However, the CCC notes that the 1.5C goal of Paris implies UK reductions of “at least 90% below 1990 levels by 2050”.

It gives a range of 86-96% for cutting emissions by 2050, if the UK takes an equal per capita share and if the world aims for at least a 50% chance that 1.5C will be avoided. Note that other ways to divide the burden of cutting emissions would probably entail more drastic cuts for the UK, while, arguably, a 50% chance of exceeding the 1.5C limit is risky.

Setting aside these questions, the middle of the CCC’s range – a 91% cut – would give the UK a carbon budget of 72 million tonnes of CO₂ equivalent (MtCO₂e) in 2050. This is close to the limit of what the CCC believes to be possible using currently known technologies and options.

It thinks the maximum plausible cuts to UK emissions in 2050 would reach 92% below 1990 levels, or 64MtCO₂e. It’s worth adding that this builds in significant use of negative emissions, including biomass with carbon capture and storage (BECCS), as well as afforestation.

Flight forecasts

Where do aviation emissions fit into this? In its most recent forecasts of demand for air travel, the government said that even without a new runway at Heathrow, UK airports would serve 445 million passengers per annum (mppa) in 2050. This is more than twice the 211 mppa served in 2010.

The Department for Transport (DfT) said UK aviation emissions, including international flights departing from UK airports, would reach 47MtCO₂e by 2050 without airport expansion. With new runways, passenger numbers could rise to 480mppa, the DfT says. Carbon Brief estimates this would translate into emissions of 51MtCO₂e in 2050.

This figure is more than two-thirds (71%) of the 72MtCO₂e mid-range carbon budget for 2050 implied by the CCC, if the UK is to play its part in meeting the ambition of the Paris Agreement. It is also nearly a third (32%) of the budget for 2C, assuming the UK sticks with its 80% by 2050 target.

Graph: UK greenhouse gas emissions including the UK share of international aviation. Historic data runs through to 2014, the latest year for which aviation figures are available. The shaded area shows projected linear progress towards an overall 91% cut in 2050, with aviation capped to 2005 emissions. Source: Department for Business, Energy and Industrial Strategy and Carbon Brief analysis. Chart by Carbon Brief using Highcharts.

However, the CCC has said that UK aviation emissions should be limited to no more than 2005 levels, if the UK is to meet its 2050 carbon targets as cheaply as possible. This would mean a cap of 37.5MtCO₂e for UK-based air travel. That 37.5MtCO₂e cap would be equivalent to more than half (52%) of the allowable 1.5C-compatible carbon budget in 2050.

The CCC says the 37.5MtCO₂e cap can be met if plausible increases in aircraft efficiency and use of lower carbon fuels is accompanied by demand growth of no more than 60% above 2005 levels.

Note that this cap includes additional room to grow compared to today’s levels because emissions fell from 37.5MtCO₂e in 2005 to 31.9Mt in 2010, partly as a result of the financial crisis. They had reached only 32.9MtCO₂e in 2014, still more than 10% below the 2005 cap.

Airports Commission: aviation emissions to double by 2020

For its part, the Airports Commission led by Sir Howard Davies, said that demand for air travel would grow by closer to 100% to 2050. This would breach the CO₂ cap for aviation and would entail the UK buying overseas carbon offsets to balance the books.

Note: The UK will participate in an international carbon trading scheme to limit aviation emissions at 2020 levels, agreed by 191 countries in Montreal on 6 October. This will cover 85% of air traffic.

However, the CCC continues to oppose the use of international offsets, saying that “UK targets should focus on domestic effort”. In a letter to the CCC, Davies said that UK aviation emissions could be constrained to the 37.5MtCO₂ cap, but only with an extremely high carbon price.

Writing in the Telegraph this week, Davies says that climate goals mean it would be a mistake to allow both Heathrow and Gatwick to expand: “Allowing two proposals to continue could mean neither is built, as it would be impossible to argue that both runways could be fully used in the next twenty years while meeting our legislated climate change commitments. So the decision could be challenged in the courts.”

It’s worth adding that Davies suggests Birmingham airport might be expanded in future. His comments also relate to the UK’s existing climate targets, rather than the tougher goals likely to result from Paris.

Global problem

The UK is already responsible for an above average share of international air travel, a position it presumably wishes to retain as it goes out into the world without EU membership.

Aviation emissions are among the most difficult to tackle, along with those from farms and factories. That’s why the new aviation climate deal is based around emissions offsets.

At a global scale, aviation could consume a quarter of carbon budget for 1.5C, recent Carbon Brief analysis showed. If the UK wants new runways, it must also take responsibility for the emissions those flights generate.

Dr Simon Evans is policy editor for Carbon Brief, covering climate and energy policy. He holds a PhD in biochemistry from Bristol University and previously studied chemistry at Oxford University. He worked for environment journal The ENDS Report for six years, covering topics including climate science and air pollution.

This article was originally published by Carbon Brief (CC BY-NC-ND).