Assessment of aviation non-CO2 climate effects warns of trade-off dangers

A new assessment by a group of UK climate scientists into potential solutions to limit non-CO2 emissions produced by aircraft concludes there is no ‘silver bullet’ and concerted efforts and commitments are needed to better understand the complex chemistry trade-offs involved before making policy decisions. The findings, published in the Royal Society for Chemistry’s journal, Environmental Science: Atmospheres, are the result of a two-year study by Manchester Metropolitan University (MMU), Imperial College London and the universities of Oxford and Reading. Aviation is responsible for around 2.5% of global CO2 emissions caused by human activity but due to the amount of non-CO2 emissions it produces, the sector is responsible for around 4% of the increase in global mean temperatures. The effects have been known for many years but the science remains inconclusive, although recent trials claim contrail avoidance technology and route planning, plus the use of sustainable aviation fuels, may provide a partial solution.

However, the new paper’s lead author David Lee, Professor in Atmospheric Science at MMU and a lead author on landmark papers on aviation and the climate, cautions that reducing the impact of non-CO2 emissions on the climate is not straightforward. “Practically all routes forward with conventional liquid hydrocarbon fuels involve trade-offs, mostly at the expense of emitting more CO2, whether it be technological or operational efforts,” he said.

“These trade-offs and uncertainties mean that there are no simple silver bullets or low-hanging fruit to solve the problem. What is often forgotten is that while the non-CO2 climate impacts of, for example, an individual flight are short lived, a substantial proportion of the emitted CO2 persists for a very long time, literally tens of millennia. This means it is a difficult balancing act if reducing non-CO2 emissions leads to an increase in CO2 emissions.”

Added co-author Keith Shine, Regius Professor of Meteorology and Climate Science at the University of Reading: “Given the many uncertainties in the size of aviation non-CO2 climate effects, it is premature to adopt any strategy that aims to decrease non-CO2 climate effects but, at the same time, risks increasing CO2 emissions. We must be mindful that aviation affects local air quality as well as climate. Sometimes measures that improve one will be to the detriment of the other.”

Aviation non-CO2 emissions that affect climate include nitrogen oxides (NOx), aerosol particles (soot and sulphur-based) and water vapour. Aerosols and water vapour have small direct radiative effects but are also involved in the formation of contrails and contrail cirrus, currently the largest non-CO2 effect on  climate, but which comes with large uncertainties. According to the paper, these non-CO2 effects on climate are quantified by scientists with low confidence, compared to that of CO2, which is quantified with high confidence.

Reducing the occurrence of persistent contrails through navigational avoidance of cold ice-supersaturated regions has currently very low confidence as a mitigation measure, says the paper, because of the challenges in making accurate meteorological forecasts on the time and space scales required for operational implementation on an individual flight basis. “Robust statistics are needed to assess whether diversions would have avoided contrails, and these are not yet available,” it adds. The evidence also remains weak for the use of SAF in reducing contrail cirrus forcing, says the paper.

“It is clear that there is an appetite amongst some stakeholders for non-CO2 mitigation of aviation effects on climate, but we have serious reservations over recommending definitive courses of action until there is better quantification of the actual effects, and further studies of the trade-offs between non-CO2 reductions versus potential CO2 increases,” conclude the authors. “It is realised that this represents a serious barrier to technology development and policy making but there are no short cuts, and the underlying danger is of either nugatory and expensive efforts which are not easily reversed or making matters worse in terms of the total climate effect of aviation.”

The researchers argument for more work to be performed on the complex trade-offs “in order to urgently search for solutions” has been recognised by the UK government, which recently announced, through the Natural Environment Research Council, a £10 million ($12.5m) research programme to help inform policy decisions in this area.

“This is a very welcome and much needed development by the government,” said Professor Lee. “Some of our previous research that was used by the IPCC (Intergovernmental Panel on Climate Change) has vitally informed the government on the scale of the problem. We have endeavoured to keep the UK Department for Transport informed of our work while we prepared this assessment, to inform the shape of future research needed.”

Photo: University of Reading

Christopher Surgenor

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