Opinion | Tuesday, 8th December 2020

The tricky tale of mitigating aviation’s climate impact

Are emissions trading schemes the answer?

By Professor David Lee and Dr Bethan Owen, from Manchester Metropolitan University’s Aviation and Climate Research Group

Never before has climate change been so high on the world’s agenda. In the midst of the coronavirus pandemic, the Government is still keen to outline its strategy for tackling global warming.

Prime Minister Boris Johnson has said he wants to cut emissions by 68% by 2030 based on 1990 levels and is due to host a virtual climate summit with world leaders on December 12.

One of the important keys to unlocking this ambitious target is understanding the role of aviation in contributing to climate change. Aviation has taken a huge hit from the pandemic as flights across the world ground to a halt and with it, a huge drop in the industry’s emissions.

But with a return to some form of air travel likely in 2021 and aviation keen to return to business as usual once it’s safe to do so, it’s just as important as ever to plan on how to mitigate aviation’s climate impact.

It has been long recognised that aviation’s emissions play a role in climate change. The sector’s emissions of carbon dioxide (CO2) represent 2.4% of annual global emissions and have been growing at 2.5% per year over the last 20 year - although this ramped up to around 4% per year over the period 2010 to 2018.

Warming

But the impacts don’t stop at CO2. Aviation has additional warming effects from emissions of nitrogen oxides (NOx) and soot particles that play a role in forming contrail cirrus - blankets of high-level clouds that warm the atmosphere. Overall, aviation contributes 3.5% to the drivers of climate change and these non-CO2 effects are currently warming at three times the rate of CO2 emissions.

What can be done to help aviation reduce its impact on the climate – and are there any stumbling blocks?

Emissions trading schemes are one such tool, a cost-effective way of reducing greenhouse gas emissions. Caps can be set by governments or blocs, such as the European Union (EU), on the top level of emissions that should be allowed. They can then set permits than allow aviation companies to emit emissions up to the level that each permit allows. They can be used – ‘spent’ – or bought and sold with other companies, ‘traded’. The overall idea is that it allows emissions to be managed to a designated target while the firms involved can buy and sell their permits based on their business need at any given time, and the cap progressively tightened up over time.

Here at Manchester Met, our research and analysis has played a role in framing international policy around such schemes. We were part of the EU’s design team of the emissions trading scheme for aviation for CO2.

Shortly before the EU’s aviation Emissions Trading Scheme was initiated in 2012, the question arose whether we could incorporate other emissions, such as NOx. At the time, the answer was ‘no’ because of uncertainties in the science.

The EU legislation obliged a later review of whether non-CO2 climate impacts of aviation could be tackled as well.

Last month, the EU published a report, for which the science and technology teams were led by us, answering to this legislative obligation. Along with a high-profile team of scientists and policy experts across Europe, the report sets out the constraints and options on how aviation’s non-CO2 impacts might be attempted.

So, what’s the answer? The answer is this: it’s tricky.

There are still significant uncertainties in the non-CO2 impacts. Recent research shows that the contrail cirrus signal is roughly half of what we used to think it was, using a better metric of its impact on climate change. Nonetheless, contrail cirrus is the largest component of aviation’s overall impact on climate change.

Contrail cirrus is formed from tiny particles of soot, emitted from aircraft engines, on which water can condense and freeze to form ice crystals in certain parts of the atmosphere depending on the meteorological conditions. Sometimes, the formation of these aircraft induced-cirrus clouds can be extensive and easily observable from the ground and from space.

It is potentially possible to avoid some of the formation of contrail cirrus by flying around, or more likely, over or under these contrail cirrus-forming regions. This is because it is the conditions of the atmosphere that dictate whether or not these clouds are formed and areas of ‘contrail cirrus-forming regions’ tend to be wide (10-100 km) but very shallow (1 km or less).

Calculation conundrum

However, the report highlights some problems with this approach. Firstly, with avoidance, extra CO2 emissions are often involved which makes for a very difficult comparison of the trade-off between the two effects. Contrail cirrus lasts for hours but the effects of extra CO2 can last for many thousands of years. Secondly, our ability to predict with sufficient accuracy in time and space, where these aircraft induced cirrus clouds might happen is not yet robust enough.

Some have suggested simply putting a ‘multiplier’ on the emissions of CO2 traded, to account for the non-CO2 impacts. This seems like a very attractive solution – the climate impact of aviation is ‘more’ than just its CO2 emissions, so why is not such a simple solution easy to implement?

The problem with this is that there are a range of legitimate ‘metrics’ by which emissions-equivalency to CO2 can be calculated, with very different results (from 1.1 times to 3.0 times as much).

Also, all the metrics involve a choice of what timescale over which effects are calculated (remember the very long timescales of CO2?) – this also introduces quite a wide range of legitimate answers (from one to four times as much). This large range of legitimate answers is hard for both policymakers and airlines – who are sensitive to costs – to stomach. This variation, along with the underlying scientific uncertainties of non-CO2 impacts (eight times larger than those for CO2) don’t necessarily make this an attractive option.

So, is there no way of quickly and easily incorporating aviation’s non-CO2 impacts into current legislation?

It is unlikely to be done quickly and easily. However, in the longer term there are some bright spots on the horizon.

Arguably, the ‘big prize’ is actually reducing CO2. This could be feasibly done by using liquid hydrocarbon fuels that are ‘sustainable’ in nature. Biofuels are a possibility, these have been controversial but seen as a necessary part of the path forwards. Synthetic fuel, manufactured by powering its production from renewable energy and CO2 captured directly from the atmosphere, is a possibility but is a long way off.

The advantage of bio and synthetic fuels is that they represent a win-win situation on CO2 and non-CO2. The tiny soot particles that ultimately form the particles on which contrails are formed largely come from the aromatic content of fossil fuels (a subset of ring-shaped hydrocarbons). Bio and synthetic fuels have zero aromatic content, so potentially, if we have truly carbon-free aviation, then we reduce the contrail cirrus cloud formation and its warming effect.

So why does this not happen? The answer is simple – as long as there is no policy or regulation to disincentivise fossil fuels for aviation, and it’s cheaper to dig oil out of the ground, then nothing will happen.

It seems that at least some answers are clear. They just require policy action and investment.