Major new study finds global aviation is responsible for 3.5 per cent of human-induced climate change
Mon 14 Sept 2020 – Although it is established that aviation’s contribution to climate change goes further than just emissions of carbon dioxide from jet engines, there have been considerable uncertainties over the impact of non-CO2 effects from, for example, contrails and contrail cirrus created by engines at high altitude. Attempts have been made to come up with multiplication factors to calculate aviation’s true climate impact but they have not been driven by the science. A major new international study, the first of its kind since 2009, has now calculated aviation is responsible for 3.5% of all human activities that drive climate change. The study is unique in that for the first time the calculations have been made using a new metric – effective radiative forcing (ERF) – that was introduced in 2013 by the Intergovernmental Panel on Climate Change (IPCC). Using the metric, the researchers found the impact from contrail cirrus is less than half previously estimated but is still the sector’s largest contributor to global warming.
The study, which is published in the journal Atmospheric Environment, was led by the UK’s Manchester Metropolitan University in collaboration with the National Oceanic and Atmospheric Administration (NOAA) and National Center for Atmospheric Research in the US, the Center for International Climate Research (CICERO) in Norway, the German Aerospace Center (DLR) and universities in the UK, US, Italy and China.
“Given the dependence of aviation on burning fossil fuel, its significant CO2 and non-CO2 effects, and the projected fleet growth, it is vital to understand the scale of aviation’s impact on present day climate change, especially in view of the requirements of the Paris Agreement to reach net-zero CO2 emissions by around 2050,” said David Lee, the study’s lead author and Professor of Atmospheric Science at Manchester Metropolitan University and Director of its Centre for Aviation, Transport and Environment (CATE) research group.
“But estimating aviation’s non-CO2 effects on atmospheric chemistry and clouds is a complex challenge for contemporary atmospheric modelling systems. It is difficult to calculate the contributions caused by a range of atmospheric physical processes, including how air moves, chemical transformations, microphysics, radiation and transport.”
Researchers evaluated all of aviation’s contributing factors to climate change, including CO2 and nitrogen oxide (NOx) emissions, and the effect of contrails and contrail cirrus, which are clouds of ice crystals created by aircraft jet engines at high altitude. This was analysed alongside water vapour, soot, and aerosol and sulphate aerosol gases found in the exhaust plumes emitted by aircraft engines.
The ERF metric represents the increase or decrease since pre-industrialisation times in the balance between the energy coming from the sun and the energy emitted from the earth, known as the earth-atmosphere radiation budget. The scientists undertook a comprehensive analysis of individual aviation ERFs to provide an overall ERF for global aviation for the first time.
Similar studies were conducted in 1999, 2005 and 2009 but this was the most current and most extensive, with lots of the details in the science having changed and matured, Lee pointed out.
“The new study means that aviation’s impact on climate change can be compared with other sectors such as maritime shipping, ground transportation and energy generation as it has a consistent set of ERF measurements,” he said.
The study found that contrail cirrus formation yields the largest positive (warming) ERF term, followed by CO2 and NOx emissions, with minor contributions from aerosols and water vapour.
Potential operational solutions suggested by the study to reduce aviation’s non-CO2 forcings, such as contrail formation, include re-routing aircraft or optimising flight times to avoid the more positive effects caused at night-time. Technology changes to reduce net NOx have also been proposed in other studies. However, the team notes in their study that re-routing can result in a longer flight path with more fuel burnt, producing more GHG emissions, and changes to combustor technology to reduce NOx emissions can also increase CO2 emissions.
Unlike direct emissions of non-CO2 greenhouse gases from sources such as the agricultural sector, aviation’s non-CO2 effects were not covered by the Kyoto Protocol, the Paris Agreement’s predecessor.
“It is unclear whether future developments of the Paris Agreement or ICAO negotiations to mitigate climate change in general will include short-lived indirect GHGs like nitrogen oxides, contrail cirrus, aerosol-cloud effects or other aviation non-CO2 effects,” said Lee. “Aviation is not mentioned explicitly in the text of the Agreement, which says total global GHG emissions need to be reduced rapidly to achieve a balance between man-made emissions and sinks of GHGs in the second half of this century.”
Co-author David Fahey, Director of the Earth System Research Laboratories at NOAA, said the latest assessment had strengthened the scientific foundation of the role of aviation in the climate system and established a framework for future assessments.
“It will aid decision-makers and the industry in pursuing any future mitigation actions while protecting this important sector from any inaccurate assertions concerning its role in the climate system.”
Speaking at last week’s ICAO CO2 Stocktaking virtual seminar, Lee said aviation (international plus domestic) was responsible for 2.4% of global CO2 emissions in 2018. He noted the percentage would have been higher at 2.8% under previous calculation methods but land-use change emissions are now included in the new global carbon budget methodology, so reducing aviation's previous share.
“However, annual emissions don’t really matter in terms of the long-term impact on climate,” he added. Cumulatively since 1940, the aviation sector had emitted 32.6 billion tonnes of CO2, the study calculated, around half of which had occurred in the last 20 years.
This cumulative impact over time was important to note, he said, because CO2 emissions had a very long lifetime in the atmosphere, a fraction of which can be for millennia. The IPCC had shown a robust approximately linear relationship between increases in cumulative CO2 emissions and global mean surface temperature change, added Lee, a contributor to the UN body’s research work.
Because of the longevity of CO2 emissions, the dramatic fall in air traffic due to Covid-19 would not make a big difference in terms of aviation’s general impact on the climate, he warned.
“As the pandemic changes, aviation traffic is likely to recover to meet projected rates on varying timescales, with continued growth, further increasing CO2 emissions. Therefore, reducing CO2 aviation emissions will remain a continued focus in reducing future man-made climate change, along with aviation’s non-CO2 contribution,” said Lee.
“The problem hasn’t gone away.”
Schematic overview of the processes by which aviation emissions and increased cirrus cloudiness affect the climate system (graphic: MMU):