资源环境科技发展态势分析平台

By Michael Le Page

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The most comprehensive analysis so far of how much warming is caused by aeroplanes has found that flying’s contribution to global warming nearly doubled between 2000 and 2018. Rapid growth is far outpacing efforts to reduce its contribution.

“It is growing so rapidly,” says David Lee of Manchester Metropolitan University in the UK. “It’s just astonishing.”

The study only goes up to 2018, before the big decrease in flying due to the coronavirus pandemic, but this is just a blip, says Lee. “It’s not going to make much difference in the long term.”

Flying has extremely complex effects on the climate. For instance, the soot from jet engines triggers the formation of contrails that, like clouds, can have both a warming effect by reflecting outgoing heat back down to Earth’s surface, and a cooling effect by reflecting sunlight back into space.

Similarly, nitrogen oxides from the engines increase the formation of ozone, a potent greenhouse gas, but also destroy methane, another potent greenhouse gas.

So the team, including Ulrike Burkhardt at the Institute of Atmospheric Physics in Germany, used computer models to improve on previous estimates of the overall effect. These suggest that contrails cause less than half as much warming as previously thought.

Even so, short-lived contrails still lead to more warming than the long-lasting carbon dioxide emissions from aircraft.

Overall, the team calculated that flying is responsible for 3.5 per cent of the global warming effect resulting from human activities. That’s less than previous estimates of around 5 per cent.

(You might also have read that flying is responsible for around 2 per cent of carbon dioxide emissions; this figure does not take into account the other ways in which flying causes warming.)

This contribution of 3.5 per cent has remained relatively constant since 2000, but only because other sources of warming have also increased rapidly. Over this time, the warming effect from flying has nearly doubled.

“It’s the growth that is the real feature,” says Lee. “It is rising quite dramatically.”

Switching to biofuels – such as palm oil – is not the answer, he says, because when the full impacts of growing crops for biofuels are taken into account it is not clear that biofuels reduce emissions much if at all.

However, using renewable energy to turn carbon dioxide from the atmospheric into synthetic kerosene could greatly reduce emissions. Switching to synthetic kerosene would make flying carbon neutral. It would also halve the warming effect from contrails, Lee says, because synthetic kerosene does not contain the aromatic chemicals that produce most soot.

“It’s feasible but we don’t know how to do it at scale,” he says. “And as long as it’s cheaper to dig it out of the ground it’s never going to happen.”

A major policy initiative is required, he says, such as setting a date beyond which the use of fossil kerosene will be banned.

Journal reference: Atmospheric Environment, DOI: 10.1016/j.atmosenv.2020.117834