Comparing contrails made during periods when these changes are happening and periods when they are not should provide reliable data on how much the contrails are being reduced. “The perfect outcome”, says Sebastian Eastham, part of the team working on Blue Skies at Imperial College, London, “is that a satellite image robustly and reliably has fewer contrails in it, time after time after time.” As well as NATS and Imperial, the consortium behind Blue Skies consists of Contrails.org, Google (which, as well as its software expertise, is chipping £1.4m, or $1.9m, of hardware and computing time into the £5m budget), the University of Cambridge and Britain’s Met Office. If contrail avoidance is to go mainstream, the Blue Skies trial needs to show not just that days with avoidance measures in place have fewer contrails, but also that integrating the process into normal operations can be done safely and straightforwardly. That would open the way to more widespread, and eventually everyday use. Adoption does not have to be universal to be useful. The warming attributed to contrails is concentrated. By one estimate 80% of the temperature-raising effect on the climate comes from just 2% of flights. Awkwardly, you cannot know in advance exactly which 2% they are. But Marc Shapiro, who runs
Contrails.org, says that a system eliminating 70% of the world’s contrail warming might require changing the flight plans of fewer than 5% of flights. According to a recent paper by Jessie Smith of Cambridge and colleagues elsewhere, if aviation does not change its ways it will be responsible for about 0.01°C of total warming above preindustrial levels in 2050. If the world is seeing about 2°C of warming by then, that contribution comes in at 5%, with more than half of it induced by contrails. A 75%-effective contrail- avoidance programme that began in 2035 would reduce the total by about a third, though there is a lot of uncertainty. Fractions of fractions sound small, but Dr Smith and her colleagues argue that the effect is similar in size to that of any other single measure proposed to reduce warming as a result of air transport—and could be realised faster. The benefits are not free. Flying at lower altitudes increases the amount of fuel used, which is a cost to airlines, and the amount of carbon dioxide emitted, which is a cost to the planet. But neither cost is huge. Estimates from models suggest that avoiding ISSRs makes flights only slightly longer and increases the amount of fuel used by less than 2%. Using 2% more fuel on 5% of flights would increase the industry’s total fuel bill by a thousandth. In the American Airlines trial the difference in fuel use between the flights that were diverted and those which were not was undetectable. Similar calculations hold for warming. According to Olivier Boucher, a climate scientist who has worked on the matter for 30 years and now runs Klima, a small French company dedicated to it, “If you really focus on the most impactful flights, maybe you can save like 100 tonnes of carbon- dioxide equivalent for an extra tonne of carbon dioxide.” There are also opportunity costs. Some worry that focusing on contrails will distract attention from airlines’ carbon-dioxide emissions and efforts either to reduce them (by using sustainable aviation fuels, or SAFs) or offset them (by removing CO2 from the atmosphere and putting it into permanent storage). Eliminating contrails without attacking these points—or reducing
the amount of flying—provides a one-off gain but does not change the fundamental fact that, in the long term, more flying means more warming. Doubters argue as well that SAFs can be designed to produce fewer particles around which ice can form, thus reducing contrails as well as carbon dioxide. This is true. But steering clear of ISSRs looks like a far more cost- effective approach to contrail reduction. The paper by Dr Smith and her colleagues found that adding SAFs to a system which already practised avoidance resulted in at most 0.001°C of cooling. There is, then, no free lunch. But the idea that giving passengers a minute or two longer to savour the in-flight cuisine that they have paid for on a few of the world’s many many flights might result in some good is about as appealing as climate interventions get. ■ This article was downloaded by zlibrary from https://www.economist.com/science-and-technology/2026/08/19/contrail-free-flying- could-help-the-climate
Science & technology | Probing the firn How to study Antarctic ice without blowing it up The sound of giant tractors may replace dynamite Aug 20th 2026 THE PISTENBULLY 300 Polar Antarctic vehicle is a 8.5-tonne heavy-duty specialised monster built to withstand extreme polar conditions. It features wide tracks, massive pulling strength, powerful cranes and multi-purpose tools designed for the toughest ice and snow operations. No Antarctic base should be without one. Research led by Liu Guofeng at China University of Geosciences in Beijing, however, suggests the PistenBully may be capable of more than just fetching and carrying. As Dr Liu writes in the Journal of Glaciology, it is also able to do actual science. Antarctica’s ice sheet is the largest reservoir of freshwater on Earth, but it is shrinking. Just above the ice proper, but below the surface snow, is a transition zone 50-100 metres thick called the firn layer. This is where snow
is compressed into ice. It is also where meltwater from above is captured in pores and refrozen. Knowing the firn layer’s exact thickness and understanding how it behaves is crucial to predicting what will happen, over the long term, to the ice sheet and thus to the world’s sea levels. Visualising this subsurface melange is difficult, though. Current practice, borrowed from oil- and gas-prospecting, is to use an “active source”. This is geologist-speak for dropping a stick of dynamite down a drill hole and listening to the explosion’s sound, propagated through the rock—or, in this case, ice. The listening is done by sensors called geophones. Since sound travels at different speeds through water, ice and snow, a computer analysis of when and how powerfully the echoes arrive at different geophones permits construction of a high-resolution image of the firn layer. This works, but is not ideal. First, someone has to drill the hole down which to drop the dynamite. That is not always possible in polar conditions. Then, the explosive must be purchased, brought to Antarctica and the actual dropping carried out. Moreover, detonating dynamite in sensitive environments is not to be done lightly. An alternative source of sound would thus be good. And, after hearing the din created by a convoy of PistenBullys, Dr Liu thought they might be the very thing. To this end, he and his colleagues experimented with geophones already in place in the Larsemann Hills of East Antarctica. They commandeered some PistenBullys and arranged for them to be driven around the hills. They found that if the vehicles were sent along both sides of a line of geophones, parallel with the sensors, they got the effect they were after. Since the Larsemann Hills’ firn had already been mapped the old-fashioned way, Dr Liu was able to compare his machine-created images with those obtained using dynamite. They were pretty much identical. He thus seems to have devised a way of mapping the firn’s structure that is simpler than blowing it up, but equally good. ■ This article was downloaded by zlibrary from https://www.economist.com/science-and-technology/2026/08/19/how-to-study-antarctic- ice-without-blowing-it-up
Science & technology | Dinosaur gastroliths Rock-solid evidence for the origins of birds Stones in dinosaur stomachs may help understanding of avian evolution Aug 20th 2026 How do you know what a dinosaur ate? Guts rot away after death, so the details of dinosaur digestion remain obscure. The best indicators are teeth. Sharp, pointed teeth probably ripped chunks of flesh off prey for swallowing whole. Robust grinding surfaces suggest lots of chewing was involved. Yet many dinosaurs belonging to a group called the theropods, which gave rise to birds, lost their teeth altogether and evolved beaks. This has made it hard to work out what they ate and how they digested it. There is, however, a second clue. Lots of dinosaurs had stones, known as gastroliths, in their stomachs. These are assumed to have assisted with digestion, and thus have their own tales to tell. And modern birds and
crocodilians (which are both, like dinosaurs, members of a larger evolutionary group called the archosaurs) often have gastroliths, too. Takasaki Ryuji at Okayama University of Science, in Japan, decided to investigate. In a study published in Paleobiology, he and his colleagues examined the gastroliths of 46 modern archosaur species (42 birds and four crocodilians) and compared them with those from 16 species of dinosaur. The birds included herbivores, omnivores and carnivores. The crocodilians were all carnivorous. The shapes of the stones varied. As might be expected, if an animal had a highly muscular stomach (ducks and geese), its gastroliths tended to have been worn into the sorts of rounded shapes displayed by beach pebbles. Weaker stomachs (seabirds and crocodiles) contained more angular stones. Looking at the dinosaurs, Dr Takasaki and his colleagues found that ornithischians (think Triceratops), a group which their teeth suggest were herbivores, retained angular gastroliths. These creatures, this implies, relied mostly on their jaws to grind up their food, leaving their stomachs to apply merely the finishing touches. Long-necked sauropods (think Diplodocus) also had angular gastroliths. That is more of a puzzle, for these animals had pencil-like teeth using which, it is assumed, they stripped the leaves off tree branches that less cervically elongated animals were unable to reach. The explanation, Dr Takasaki suspects, is that like those of modern ungulates, sauropods’ stomachs depended mainly on fermentation to break food down. Stones in the guts of theropods tell yet another story. Tarbosaurus, a hypercarnivore similar to Tyrannosaurus, and similarly endowed with sharp, pointed teeth, had angular gastroliths. However, theropods with beaks (of which science recognises at least four groups) had rounded ones. Indeed, Dr Takasaki saw a clear relation between tooth loss and stone roundness, as the work of grinding up food was transferred from jaws to stomach. That shift may have helped make possible the evolutionary success of birds. Teeth are heavy, as are the jaw muscles those teeth require to do their job. This puts a lot of weight near an animal’s front. Shifting the job of grinding,