Commentary: Reducing air travel by small amounts makes big difference in climate action
A decline in aviation can partly reverse some warming – but air traffic is quickly rebounding to pre-pandemic levels, says a researcher.
OXFORD: Just before the pandemic, aircraft engines were burning 1 billion litres of fuel a day. But then the number of daily civil aviation flights fell from 110,000 to less than 50,000 during 2020, on average.
With the easing of travel restrictions, air traffic is increasing back towards its pre-pandemic peak.
Most world leaders and delegates will have flown to Glasgow to attend COP26 – the 26th annual UN climate change summit – in person.
But as they haggle over emissions targets to limit global warming to 1.5 degrees Celsius, and not 3 degrees Celsius or more, aviation is unlikely to be included in them, given the lack of low-carbon alternatives to long-haul flights.
But it should be. In new research, my colleagues and I calculated that if the aviation sector continues to grow on its present trajectory, its jet fuel consumption will have added 0.1 degrees Celsius to global warming by 2050 – half of it to date, the other half in the next three decades.
Aviation is responsible for 4 per cent of the 1.2 degrees Celsius rise in the global mean temperature we have already experienced since the industrial revolution.
Without action to reduce flights, the sector will account for 17 per cent of the remaining 0.3 degrees Celsius left in the 1.5 degrees Celsius temperature target, and 6 per cent of the 0.8 degrees Celsius left to stay within 2 degrees Celsius.
Airlines effectively add more to global warming than most countries.
AVIATION'S LARGE WARMING FOOTPRINT
At the current rate, the world will have warmed by 2 degrees Celsius within three decades. To quantify how different activities contribute to warming, scientists measure carbon emissions.
This is because how much the Earth warms is proportional to cumulative carbon emissions in the atmosphere. This is a very good approximation in many cases, but it is inaccurate for emissions caused by aeroplanes travelling at altitudes of up to 12km.
As well as carbon dioxide (CO2), aircraft engines emit nitrogen oxides, water vapour, sulphur and soot, causing contrail cirrus clouds and other complicated chemical reactions in the atmosphere.
The sum of these so-called non-CO2 effects adds more warming on top of the CO2 emissions. So the total warming footprint of aviation is between two and three times higher than a conventional carbon footprint.
While a large share of a flight’s CO2 emissions remain in the atmosphere for many thousands of years, the non-CO2 effects diminish over time, vanishing within about ten years. So any growth in aviation, measured in global jet fuel consumption, has an amplified impact as both CO2 and non-CO2 effects add up.
But a decline in aviation can partly reverse some warming, as the non-CO2 effects disappear over time until only the CO2 effects remain.
Think of the non-CO2 effects like a bathtub – it fills up when the taps are turned further and further, despite a slow outflow down the plughole. But the same bathtub will eventually empty if the taps are gradually turned down.
The non-CO2 effects of flights on the atmosphere will slowly disappear if fewer and fewer flights are taken, so that aviation’s contribution to warming eventually levels off.
In that situation, the increase from continued CO2 emissions would balance the fall in non-CO2 effects, and although aviation would still contribute to climate change, the total warming from both would remain constant over time.
How much would aviation need to shrink to level off its influence on global warming?
Our calculations show that flying does not need to stop immediately to prevent aviation’s contribution to global warming expanding. Flying has already caused 0.04 degrees Celsius of warming to date.
But with a yearly decrease of 2.5 per cent in jet fuel consumption, currently only achievable with cuts in air traffic, this warming will level off at a constant level over the coming decades.
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WHEN DO WE REALLY NEED TO FLY?
COVID-19 had a huge impact on the aviation sector. Air traffic is still approximately 10 to 20 per cent below pre-pandemic levels but is rebounding quickly.
Politicians should shift subsidies from flying to more sustainable modes of transport, such as train journeys. And there is much more that can be done.
Lockdowns and the shift to remote working made many people rethink the necessity of flying. People resolving to fly less can contribute considerably to reducing the number of unnecessary flights.
Combining in-person and virtual attendance in hybrid meetings wherever possible is a great way to support that shift.
Reducing the space that business classes take on aeroplanes is another way to cut the number of flights, as it allows more passengers to travel on one flight.
Not allowing airport expansions could also have a big impact. The UK’s Climate Change Committee, an expert body which advises the UK government, has recommended not expanding airports to align the sector with climate targets. Yet the expansion of Heathrow airport is currently planned to go ahead.
Sustainable aviation fuels, and hydrogen or electric planes, are being developed, but none of these technologies are currently available at the necessary scale.
At the moment, there is little chance of the aviation industry meeting any climate targets if it aims for a return to its pre-pandemic rate of growth.
Milan Klower is Postdoctoral Researcher in Weather and Climate Modelling at the University of Oxford. This commentary first appeared on The Conversation.