Global aviation industry responsible for 3.5% of climate change caused by human activities, finds study
Aviation is one of the most important global economic activities in the modern world. Aviation emissions of carbon dioxide as well as non-carbon dioxide aviation effects result in changes to the climate system. Scientists have now estimated that global air travel and transport is responsible for 3.5% of all human activities that drive climate change. The findings reveal that two-thirds of the impact from aviation is attributed to non-carbon dioxide emissions and the rest from carbon dioxide.
The authors evaluated all of the aviation industry’s contributing factors to climate change, including carbon dioxide and nitrogen oxide emissions, and the effect of contrails and contrail cirrus – clouds of ice crystals created by aircraft jet engines at high altitudes. This was analyzed alongside water vapor, soot, and aerosol and sulfate aerosol gases – fine particles suspended in the air – found in exhaust plumes emitted by aircraft engines.
The five-year scientific effort was led by UK's Manchester Metropolitan University in collaboration with numerous academic and research institutions across the globe. The study has been published in the journal Atmospheric Environment.
According to co-author David Fahey, director of National Oceanic and Atmospheric Administration or NOAA’s Earth System Research Laboratories in Boulder, Colorado, the findings strengthen the scientific foundation for understanding the role of aviation in climate change. "Our assessment 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," writes Fahey.
What did researchers find?
For the analysis, the team calculated that the cumulative carbon dioxide emissions of global aviation throughout the industry’s entire history – defined as between 1940 and 2018 – were 32.6 billion tonnes. Approximately half the total cumulative emissions of carbon dioxide were generated in the last 20 years alone, attributed largely to the expansion of the number of flights, number of routes and fleet sizes, though it was partially offset by improvements in aircraft and jet engine technology, larger average aircraft sizes and increasing efficiency in the use of aircraft capacity to fit more passengers in the same space.
The authors estimate that the figure of 32.6 billion tonnes accounted for 1.5% of total carbon dioxide emissions ever at that point. When the non-carbon dioxide impacts were factored in, aviation emissions were calculated to be 3.5% of all human activities that drive climate change.
"Two-thirds of the impact from aviation is attributed to contrails, nitrogen dioxide, water vapor, sulfate aerosol gases, soot and other aerosols. The remainder is due to the cumulative heat-trapping effects of long-lived carbon dioxide emissions: 32.6 billion tonnes, or roughly the total global carbon dioxide emissions for the year 2010," the findings state.
The team emphasizes that the study is unique because it is the complete first set of calculations for aviation that uses a new metric introduced in 2013 by the Intergovernmental Panel on Climate Change (IPCC) called "Effective Radiative Forcing (ERF)". It represents the increase or decrease in the balance between the energy coming from the Sun and the energy emitted from the Earth since pre-industrial times.
Using this metric, investigators found that while contrail cirrus has the largest climate warming impact, it is less than half that previously estimated. Carbon dioxide emissions represent the second-largest contribution but unlike the effects of contrail cirrus, carbon dioxide’s effect on climate lasts for many centuries.
"Given the dependence of aviation on burning fossil fuel, its significant carbon dioxide and non-carbon dioxide 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" carbon dioxide emissions by around 2050," explains lead author David Lee, professor of atmospheric science at Manchester Metropolitan University and director of its Centre for Aviation, Transport and the Environment research group.
The authors, however, note that while the 2016 Paris Agreement on climate change does include domestic aviation in individual country's reduction targets, it does not address international aviation, which accounts for 64% of air traffic.
"It is unclear whether future developments of the Paris Agreement or International Civil Aviation Organization negotiations to mitigate climate change, in general, will include short-lived indirect greenhouse gases like nitrogen oxides, contrail cirrus, aerosol-cloud effects, or other aviation non-carbon dioxide effects. Aviation is not mentioned explicitly in the text of the Paris Agreement, which says total global greenhouse gas emissions need to be reduced rapidly to achieve a balance between man-made emissions and sinks of greenhouse gases in the second half of this century," explains Lee.
Lee says that estimating aviation's non-carbon dioxide effects on atmospheric chemistry and clouds was a complex challenge. "We had to account for contributions caused by a range of atmospheric physical processes, including how air moves, chemical transformations, microphysics, radiation and transport," he adds. The new study will allow aviation’s impact on climate change to be compared with other sectors such as maritime shipping, ground transportation and energy generation as there is now a consistent set of estimates.
The researchers emphasize that as the Covid-19 pandemic changes, aviation traffic is likely to recover to meet projected rates on varying timescales, with continued growth, further increasing carbon dioxide emissions. "Historical emissions of carbon dioxide also take many centuries to be removed. Therefore, reducing carbon dioxide aviation emissions will remain a continued focus in reducing future man-made climate change, along with aviation's non-carbon contribution," says Lee.