Flight diversions are intended to reduce contrails and slow global warming
Minor adjustments to flight routes for a small proportion of global flights could significantly reduce global warming. This result of several studies is now supported by a new study, which even comes to the conclusion that these changes could also be implemented quite cost-effectively.
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Airplanes produce a lot of carbon dioxide when they burn their fuel, contributing to climate change. Jets also release heat, water vapor and particulate matter, which can create thin clouds in the sky known as contrails in particularly cold, humid and icy areas of the atmosphere. If many flights pass through such areas, the contrails can form cirrus clouds that float above the earth like a blanket and absorb the radiation emitted from the surface.
This cirrus-forming phenomenon could account for about 35 percent of aviation’s total contribution to climate change. According to some estimates, this corresponds to around one to two percent of total global warming. But because only a small proportion of all flights, namely two to ten percent, cause a good 80 percent of the contrails, there is growing hope that simply rerouting these flights could significantly reduce their climate-damaging effects. The result would be a potentially effective, inexpensive and rapid way to curb warming.
Diversions lead to higher fuel consumption
Last summer, the umbrella organization “Breakthrough Energy” founded by Bill Gates, which is intended to promote innovations in the field of clean energy and the fight against climate change, Google Research and the airline American Airlines announced some promising results from a research collaboration. They used satellite imagery, weather data, software models and AI prediction tools to guide pilots over or under areas where their planes were likely to produce contrails. American Airlines used these instruments on 70 test flights over six months. With success: satellite data showed that the diversions had reduced the total length of contrails by 54 percent compared to flights that were not diverted.
Implementing such a strategy would of course come with costs. Diversions lead to higher fuel consumption and correspondingly cause more greenhouse gas emissions. Because of the higher costs, airlines are unlikely to implement such measures voluntarily. Unless they are more cost-effective than expected.
This is exactly what a new study published in the journal “Environmental Research: Infrastructure and Sustainability” has shown. She studied the problem by pairing commercial flight route optimization tools with models that simulated nearly 85,000 American Airlines domestic and international flights in various weather conditions last summer and this winter. The result of these simulations: On average, a 73 percent reduction in the warming effect of contrails would increase fuel costs by just 0.11 percent and total costs by just 0.08 percent. Only 14 percent of the simulated flights had to be adjusted to avoid the formation of warming contrails.
“Clearly there is a trade-off between additional fuel consumption and reducing harmful contrails. This is one of the biggest challenges in solving the climate problem,” said Marc Shapiro, co-author of the study and leader of the contrail team at Breakthrough Energy. “But we show in this study that the additional fuel consumption is much lower than we expected.”
Airlines could also use such a commercial route tool to make decisions that balance their financial and climate goals, Shapiro says. For example, they could allow some contrail-forming flights if the cost of adjusting routes would be particularly high.
Other research groups and airlines are also exploring this concept through projects, including a collaboration between Delta and the Department of Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT). (MIT Technology Review is part of MIT but is editorially independent).
“The cheapest and fastest way”
There are other approaches to reducing the formation of contrails, such as switching to other fuels or developing more powerful electric or hydrogen aircraft. However, studies so far suggest that rerouting flights could be one of the simplest ways to significantly reduce warming caused by contrails. “So far it looks very promising that this is the cheapest and fastest way to reduce the impact of aviation on the climate,” says Steven Barrett, head of the MIT department.
Short-term advances in aviation are even more important because it will likely be a long time before scalable, affordable methods for reducing emissions from heavy fuel oil use can be developed and implemented, he adds. However, further model studies and real experiments are needed to prove that so-called “contrail avoidance” works as effectively as hoped.
For one thing, according to Barrett, researchers still need to test, refine and develop systems that can reliably predict when and where contrails will form – even in changing weather conditions. There are also some thorny issues that still need to be resolved, such as the fact that cirrus clouds can not only increase but also reduce warming by reflecting short-wave solar radiation.
The loss of this cooling effect would have to be taken into account when calculating the net benefit, or perhaps avoided. For example, according to Shapiro, the first strategy could be to schedule flights only in the early evening and at night, which would eliminate the complication of sunlight reflection.
Furthermore, any reduction in warming from contrail avoidance must more than offset the additional warming from increased greenhouse gas pollution. This question becomes even more difficult when considering whether short-term or long-term warming is more important: avoiding contrails provides an immediate benefit, but the additional carbon dioxide can take decades to have its full warming effect, which then affects hundreds to thousands can last for years.
Open questions
The new study at least found that reducing contrails reduces net warming over both 20- and 100-year horizons, although less so in the latter scenario, even when additional greenhouse gases are taken into account. But this also needs to be investigated in further studies. Another open question is whether airspace restrictions and traffic bottlenecks could limit airlines’ ability to regularly reroute necessary flights.
As a next step, Breakthrough Energy hopes to work with airlines to explore some of these questions by expanding flights and real-world observations. But even if further studies show that this is a quick and cost-effective way to reduce warming, it is not yet clear whether airlines would take such measures without regulators forcing them to do so. Even though fuel costs are small in percentage terms, they could quickly add up across a fleet and over time.
Still, the study’s authors believe they have shown what contrail avoidance could provide “massive immediate climate benefits at a lower cost than most other climate measures.” In their view, this approach “should become one of the main focuses of aviation in the coming years.”
(jl)
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