AIR TRANSPORT Climate change

Aiming higher

RAeS Past President and Emeritus Professor of Aerospace Engineering at Cranfield University, Professor IAN POLL FRAeS gives a personal view on how aviation could halve its impact on the environment almost overnight if the right actions were taken.

For the past 20 years, a rapidly growing airline industry has appeared to do little to mitigate its increasingly significant contribution to global warming, generating a hostile reaction from environmentalists. The result has been a wholly negative narrative that puts the future of air travel and its supporting industries at risk. To make matters worse, the recently proposed ‘solution’ of a combination of alternative fuels, advanced technology aircraft and a massive offset scheme is a slow-starting, long-term, high-risk and hugely expensive undertaking that addresses less than half of the problem. It makes aviation a hostage to fortune and, unsurprisingly, it has been greeted with a great deal of scepticism, leading to yet more criticism and pressure.

However, I believe there are financial and regulatory actions that could be taken today, incurring only marginal cost, requiring no new technology and whose environmental impact would be significant and immediate. These actions are ethical, fully aligned with the scientific consensus and impact all the mechanisms contributing to climate change. They have the potential to reduce aviation’s contribution to global warming by 50% in the short term and, in the longer term, make aviation an overall global cooler. This would create a positive narrative for aviation in which everyone wins.

Background

While most industries contribute to climate change by emitting greenhouse gases at ground level, aviation is much more complicated. Engine exhaust contains a cocktail of gases and aerosols and, uniquely, these are emitted in the upper troposphere and lower stratosphere. However, contrary to the assertions of most environmentalists and some climate scientists, these differences do not make aviation a bigger problem. Rather they create opportunities to deploy targeted actions that can reduce and, perhaps, even reverse aviation’s effect.

Environmental impact is related to the global mean temperature (GMT). Simply put, a warmer atmosphere holds more water, more water means more weather and, potentially, climate change.

The current, internationally agreed, target is to limit the increase in the GMT to 1.5C above pre-industrial levels. GMT is the temperature that gives a balance between incoming solar radiation and the Earth’s natural outgoing thermal radiation. It is not constant, varying continuously in response to changes in factors, such as total solar irradiance, Earth’s albedo and the levels of atmospheric greenhouse gases.

If there is a change in any of these, the balance is disturbed and the temperature starts to move towards a new equilibrium value. The degree to which any variation perturbs the balance is known as its ‘radiative forcing’ (RF). This characteristic can be estimated and used as a metric. For example, comparing present-day values gives an indication of a factor’s relative importance for GMT change, while comparisons with the past reveal the impact of anthropogenic activity. Importantly, RF values can be attributed to each element of an individual industrial activity. Global warming is extremely complex and using radiative forcing as a metric is far from perfect. However, it is good enough to identify the most important effects and to inform and guide policymaking.

The greenhouse gas most affected by anthropogenic activity is carbon dioxide, which is a product of fossil fuel combustion and, since the Earth’s natural sequestration processes take centuries, there is a progressive accumulation of CO2 2 in the atmosphere. This has serious long-term implications and it is the main reason why the reduction of CO2 2 emissions is so important.

For most industries, cutting carbon dioxide is the only way to reduce their contribution to global warming. However, aviation is different and, if it is to be allowed to achieve the greatest possible reduction in environmental impact in the shortest possible time, it must be viewed differently and treated differently.

Aviation’s climate impact

The environmental consequences of aviation go way beyond CO2 2. In the scientific literature, the impact of non-CO2 2 effects are usually weighed against that of CO2 2 by comparing the relative radiative forcing and, on this basis, aviation is found to have two important non-CO2 2 contributors.

The first arises through the presence of nitric oxide and nitrogen dioxide (NOX X) in the exhaust. While these compounds are not greenhouse gases, adding them to the atmosphere initiates chemical reactions that affect concentrations of the natural greenhouse gases, ozone and methane. The result is a net increase in radiative forcing, making a positive contribution to global warming. Climate effects of NOX X are complex and not fully understood. However, the impact is believed to be about half that of aviation’s CO.

There are two sources of NOX X. The first, prompt NOX X, is linked to the amount of nitrogen in the fuel. The second, thermal NOX X, is linked to the chemistry of air at the high temperatures generated during combustion. Being related to fuel burn, NOX X is reduced if fuel consumption is reduced. However, the development of more efficient engines has meant increasing combustion temperatures and, hence, more issues with thermal NOX X. Therefore, whatever fuel is used, NOX X is likely to be problematic and, even if additional CO emissions were to be eliminated altogether, aviation might still make a substantial contribution to GMT through its NOX X emissions.

The second contribution comes through contrail and contrail-induced cirrus cloud. Under certain meteorological conditions aircraft produce contrails. Jet exhaust is hot, humid and contains soot. If the air temperature is below about –40C, exhaust water vapour condenses on the soot particles and the droplets freeze. The resulting ice crystals reflect sunlight and a contrail appears. If the atmosphere is dry, only water from the engine freezes and, since the ice sublimes quickly, the contrail extends for just a few kilometres. However, if the aircraft is flying through air that is supersaturated with respect to ice (ISSR), the contrail will not only contain ice from the exhaust water vapour, but also water vapour from the surrounding atmosphere. In this case, a ‘persistent’ contrail forms, with its characteristic very long, twin parallel white lines caused by the ice crystals being trapped in the aircraft’s trailing vortex system. Persistent contrails can be hundreds of kilometres long, with an average lifetime of several hours. As the contrail ages, it may develop into contrail cirrus, the twin parallel lines disappear but a lenticular cloud, or clouds, remain.

A persistent contrail is a major event, involving the formation of tens of millions of tonnes of ice. Unsurprisingly, this perturbs the Earth’s radiative energy balance and it does this in two ways. Firstly, during the day, the ice crystals reflect incoming solar radiation back into space, giving a large cooling effect. Secondly, the same ice crystals absorb and trap some of the outgoing thermal energy. This is generally more effective at night and is strongly warming. Superposition gives a net warming effect and, although there is some uncertainty surrounding the precise values, the climate impact of persistent contrails and induced cirrus is believed to be about 1.7 times that of aviation’s CO.2

Simply put, the current level of understanding is that aviation’s total contribution to global mean temperature rise has three components – 1/3rd from CO2 2, 1/6th from NOX X and half from contrails.

What is happening right now?

With the full impact of aviation in mind, governments, pressured by the environmental lobby, are insisting that aviation contributes its ‘fair share’ to global CO emissions reduction and airlines are now committed to a number of measures. These involve the introduction of ‘sustainable’ fuels and offsetting through ICAO’s global, Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). In addition, airlines are looking to manufacturers to produce radically different, hightech options for future aircraft.

Currently, aviation relies upon kerosene, a near ideal energy source and elements of the gas turbine engine have been designed around it. This intimate relationship between engine and fuel presents unique challenges and the notion that kerosene can be easily replaced by another substance is naïve. Notwithstanding the stringent, safety-based fuel certification process, other important questions arise.

Is it absolutely carbon-neutral? Is it really sustainable? Is its manufacture ethical? Can its production be scaled up to meet the global demand? Will aviation be in competition with other users? Can it be made available everywhere in the world? And is it affordable?

None of these have been answered to everyone’s complete satisfaction. The alternative fuel approach also offloads responsibility to an industry that hardly exists today. Global demand for kerosene is about 350 megatonnes per annum, whereas current production rates for aircraft usable alternatives is barely 1% of this figure. By 2050, with the expected market growth, fuel use could be over 750 megatonnes per annum. Therefore, a whole new high-tech, high-capacity, high-energy and high-cost industry will need to be developed virtually from scratch. Who will provide the funding for this?