One of the few upsides of lockdown was that, if a nightingale sang in Berkeley Square, you had a chance of hearing it. City dwellers across the globe delighted in the silence, the freedom from the incessant rumble of traffic, and the joy of birdsong. How cleaner the air seemed, and how splendid the blue skies without their usual grid of aeroplane contrails!

Those memories might now be receding but for some scientists they were not just silver linings to a grim situation. The enforced experiment in which human activity was brought almost to a standstill created an opportunity to conduct studies that are normally impossible, in topics ranging from acoustics to atmospheric science to ecology. “We are being given an extraordinary experiment,” said climatologist Nicolas Bellouin of the University of Reading.

Take the UK-wide scheme called The Quiet Project, which enlisted a network of acoustic engineers and researchers to map out changes to the ambient soundscape during lockdown. It has been building up a database of sound measurements that should act as an invaluable resource for understanding our acoustic environment: how sound correlates with economic activity, say, and how it affects wellbeing.

The Quiet Project has been soliciting and archiving observations from members of the public. “We’re taking lots of technical measurements, but we’re also gathering a lot of soundscape data that reflect what people actually feel,” said Lindsay McIntyre, director of the consultancy company KSG Acoustics in Glasgow, who is one of the leaders of the project. “Everyone I speak to has got an opinion on how the changes in noise makes them feel.”

Urban noise – from traffic, subways, sirens, and neighbours – is known to cause stress, but we don’t know so much about its true social costs. Yet it’s not just about the negative effects of shattered peace: for many people, the hum of life and entertainment is a positive part of the urban experience. “Some people are really missing the vibrant sounds of the city, which is perhaps linked to concerns about lack of work and earning,” says McIntyre.

Often it’s a question of finding the happy medium. “You might like waking up when it’s quiet, but in the evening you might want the vibrancy,” said Stephen Dance, professor of acoustics at London South Bank University and another of the project’s leaders. The value associated with these positive and negative effects of sound has never really been measured, he said – but this has been a chance. Dance said that lockdown seemed to have lowered urban noise by about five decibels, mostly because of absent traffic. That’s a 60% reduction – certainly enough to be aware of.

This acoustic information could be useful for planners – for example, for cost-benefit analyses of how designated “tranquil areas” in cities can be preserved from development, or to plan for potential changes in the future. It should help to make better predictions of what things would be like if all vehicles were electric, say – which is possible in 10 years’ time. “You could quantify what you’d get to acoustically,” said Dance, “and put a value on it – which is a factor in the economic costs of making that change.”

A shuttered assembly line at a Volkswagen factory in Lisbon

Some of the noise created by human activities lies beyond the threshold of hearing, especially at very low frequencies of around a few oscillations per second (a few hertz or Hz). Vibrations like these are detectable using technologies attuned to seismic waves – trains and construction, for example, shake the ground at such frequencies.

Usually the seismic effects of human activity are a mere distraction or nuisance for seismologists. “We think of anthropogenic noise as obscuring the natural signals from the planet that we’re interested in, be they earthquakes or other signals,” said earth scientist Zach Eilon of the University of California at Santa Barbara. So researchers tend to install seismometers far from sources of anthropogenic noise. But some stations are installed within cities, perhaps to monitor the earthquake threat – and these, Eilon said, are now supplying useful information about how human activity leaves a seismic trace. There is now an international effort, involving around 100 seismologists worldwide and coordinated by Thomas Lecocq of the Royal Observatory of Belgium in Brussels, to collect lockdown data.

Eilon said their studies so far, using seismic data from China and Italy, allow them to identify different noise sources from their seismic frequencies. Foot traffic and factories or businesses cause vibrations of 1-8Hz, traffic on motorways is around 10-30Hz, and trains around 20-30Hz. Purely from the seismic trace, it’s possible in normal times to identify working hours, lunchtimes, nightlife, holidays and big sporting events. Lecocq said that there are even some seismometers installed in schools that register pupils’ footsteps – and that fell silent while schools were locked down.

“The really exciting thing about our research is what it can teach us about human behaviour,” said Eilon. “We have never had this sort of lockdown scenario where many countries and cities around the world are so drastically changing their societal patterns.” The changes in seismic noise that they have seen far outstrip the variation normally seen between weekends and weekdays, or even national holidays. By comparing these with data on mobility, changes in factory output, toll road use, and changes to train schedules, they can figure out how each societal activity creates seismic noise, so that in future seismic measurements can be used to measure what we’re doing.

And because seismic data – unlike phone tracking, say – can’t identify individuals, there are no problems about violations of privacy. Data like this could even be helpful for controlling the pandemic, for example by showing how well people are adhering to social distancing and travel restrictions.

“Several locations in the US were already showing increases in seismic noise in early May as citizens became less compliant with lockdown,” said Eilon. He and his colleagues also saw differences between Italy and China, probably because public transport was not shut down in the former case.

Turin residents sing Bella Ciao from their balconies for a Liberation Day flashmob in Italy. But neighbourly noise in lockdown may not always be welcome.

The findings are also revealing more about very low-level earthquake activity. The signals from small earthquakes are generally in the same frequency range as these human sources, said Eilon – so “that noise can prevent us from seeing below a certain threshold of earthquakes. Basically, we are probably missing a lot of the small earthquakes that we might otherwise detect because they are getting drowned out.” But these might become visible in the lockdown data. “This is exciting because it can tell us more about the state of stress and movement in the crust,” Eilon said.

As well as the cities and the skies, another environment that became less noisy was the oceans, thanks to suspensions of shipping. Low-frequency noise from ships’ engines can travel over thousands of kilometres. “Most ocean basins have a persistent background hum generated by ships”, said David Barclay, an oceanographer at Dalhousie University in Canada.

This noise can disrupt the communications of marine animals that rely on acoustic signals, such as whales and dolphins. “Fish, marine mammals and crustaceans use sound to hunt, forage, communicate, mate and navigate,” Barclay said. “High levels of sound can physically harm and kill marine animals, or cause permanent or temporary hearing loss, and can trigger behavioural responses like flight, stress and disruption of feeding.”

He and his co-worker Dugald Thomson have studied how such noise changed during lockdown, both in the open ocean (in the north-east Pacific) and in an inland waterway in the Strait of Georgia outside the port of Vancouver. They saw a small drop in the weekly average noise at frequencies of about 100Hz in the open ocean, and a larger decrease in the strait. “We would like to understand which factors – ship traffic density, speed, tonnage, type of ship – are playing the biggest role,” Barclay says – information that could help to plan ocean transportation so it is less disruptive to marine life.

An empty train carriage in Tokyo, 2 May.

“This is an opportunity for marine biologists to make observations under far-from-normal conditions – or that would have been normal 50 years ago,” he said. “I would say that every marine-related biologist or ecologist has at least one key research question that they can address during this period of relative pause.”

An improvement in air quality was seen over many urban areas as pollution from traffic fell . While nitrogen dioxide levels plummeted, other changes in air quality were more modest, Bellouin said. Transport is just one source of small particles called aerosols, for example, and “other sources, like domestic heating or agricultural fertilisers, have not changed” – not to mention natural sources like wildfires and desert dust. But that’s useful, Bellouin said, because it allows these different contributions to be identified and measured.

Aerosols have an impact on climate. They reflect sunlight back into space, can act as the seeds of droplet formation for clouds, and can increase the reflection from those clouds. Overall, they generally have a cooling influence – but some aerosols, like soot, warm the air by absorbing sunlight strongly. “The links between aerosols and the climate are many and complex,” said Samset. “They’re difficult to pin down and are still a source of major uncertainty when we try to project what will happen to the climate over the coming decades.”

Usually it’s hard to disentangle the climate effects of aerosols from those due to rising emissions of greenhouse gases. But the lockdowns might have helped us do that. “What Covid-19 gives us is a rapid, short change in one of these influences,” said Samset. “Aerosols drop out of the air in a matter of days, so we should be able to see the effects if we look closely.

Researchers are now planning to simulate the climate with and without the lockdown, to see if their models can match the observations. “This will help us improve our weather and climate models,” said Samset. “These lessons can then be applied to bigger questions of the overall influences of aerosols on climate and society.”

These opportunities are not completely without precedent. The eruption of the Eyjafjallajokull volcano in Iceland in 2010, for example, stopped air travel for a week, not only reducing aeroplane noise but also that from road traffic close to airports. And the 9/11 attacks in New York reduced international traffic by sea as well as air. Still, said Barclay, “It’s hard to imagine any other voluntary scenario where we would have the opportunity to study the marine habitat under these conditions.”

“We’re all hoping that we can make the best of a bad situation and learn something useful,” said Samset.

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