‘A 7% drop – we’ve never seen this since world war two,’ says Corinne Le Quéré, professor of climate change science at the University of East Anglia, UK. Le Quéré is talking about an unprecedented global decrease in carbon emissions in modern times, the result of the coronavirus pandemic.1 The data Le Quéré and other atmospheric scientists have gathered over the last year are both shocking and insightful. They show a world that was profoundly altered by a virus that has killed more than 2.5 million people to date, while giving a glimpse of what a future with cleaner air might look like – one that could save many lives.
As Covid-19 swept across the globe, countries started to restrict people’s movement to stop the spread of the disease. With the lockdown measures came major reductions in air pollution from vehicles, planes and factories. Photos of clear blue skies over Delhi in India – a city usually covered by smog and pollution – circulated in media outlets around the globe.
For air pollution scientists the pandemic has been an opportunity unlike any other, possibly the biggest unintentional atmospheric chemistry experiment in history. ‘There have been some of these unwanted experiments in the past – for example, September 11 shut down the whole airline industry,’ says analytical environmental chemist Cora Young from York University in Canada. ‘But in terms of a global event, nothing like this has ever happened before.’
Clearing the air
‘As air quality researchers, we are always wondering if we will benefit or not from reducing emissions – nobody really knows,’ says Fei Liu, an atmospheric scientist at Nasa’s Goddard Space Flight Center in the US. ‘Before the pandemic, we just did simulation. This time, it really happened.’
Liu recalls talking with her supervisor one day after lunch when she heard the news of Wuhan’s lockdown. At 2am on 23 January 2020, Chinese authorities informed the city’s 11 million residents that from 10am Wuhan would be quarantined. ‘That was big news, to seal off such a huge city,’ Liu recalls. The lockdown came two days before Chinese New Year, a peak season for travel.
Liu and her colleagues immediately started collating data from spectrometers on board two different satellites, each of which provides daily global atmospheric data with a spatial resolution as fine as 5.5 × 3.5km. ‘At the very beginning, we barely got any useful observations, because it was very cloudy,’ she recalls. ‘But then we started to see the huge reduction of nitrogen dioxide over China.’
Liu’s team saw nitrogen dioxide signals disappear along major motorways, as well as from areas dominated by steel, iron and oil industries, and fossil fuel power plants.2 Overall, levels over China dropped by almost 50%. While some reductions aren’t unusual around the Lunar New Year holidays, 2020’s decline was 21% larger than the previous five years.
Nitrogen dioxide – and other nitrogen oxides, jointly classed as NOx – is one of the four pollutants that cause the most damage to human health. The others are sulfur oxides (SOx), ground-level ozone and particulate matter (PM) – tiny particles made from sulfates, nitrates, ammonia, sodium chloride, black carbon, mineral dust and water. ‘NOx is a very good indicator of human behaviour and economic activity because the primary source is fossil fuel combustion, in vehicles or coal power plants,’ Liu explains.
In very high concentrations, nitrogen dioxide is acutely toxic, causing airway inflammation. But long term exposure to it and other pollutants has much wider health implications, ranging from stroke and heart problems to lung cancer and chronic respiratory diseases. Given the increased Covid risks for people with some of these existing conditions that became apparent early last year, scientists wondered whether places with more severe air pollution saw more Covid deaths.
‘There’s a statistically strong link between high particulate matter levels specifically, and Covid deaths and Covid spread – one of them is a study of 110 provinces in Italy3,’ explains air quality researchers Rima Isaifan from Hamad Bin Khalifa University in Qatar. Many other analyses conducted in several countries in early 2020 similarly showed an association between pollution and Covid mortality.
But firm evidence that people were more likely to fall ill with severe Covid if they lived in a more polluted place remained elusive. The hastily collected data made it difficult for researchers to account for timing of the epidemic curve, differing lockdown restrictions and underreporting of Covid deaths.4 The UK Office for National Statistics found that as Covid deaths rose and lockdown was introduced, the correlation between pollution and mortality lessened.
The steep decline in NOx and PM levels in locked down cities begged another question. Are the Covid quarantines preventing deaths by providing cleaner air? Worldwide, outdoor air pollution kills an estimated 4.2 million people per year. People living in low- and middle-income countries bear the brunt of this – 91% of all premature deaths related to air pollution occur here, in particular in south-east Asia and the western Pacific countries. ‘Compared to 7% or 8% annually over many years that air pollution has been killing people, I thought that [the pandemic] might have saved more people than it had killed so far, at that time,’ says Isaifan, who conducted a study on the topic in early 2020 when Covid mortality was an estimated 3.4%.5
One team of scientists estimated that lockdowns in China and the associated reductions in NOx and PM saved an estimated 12,000 lives, most of them by avoiding pollution-related cardiovascular diseases.6 Others, however, pointed out that such numbers need to be approached with caution. Difficulties such as accounting for the weather’s effect on pollution and obtaining an accurate mortality baseline might limit the study’s conclusions.7
Despite the abrupt decline in NOx during the pandemic’s early days, not every place ended up with cleaner air. ‘For some regions due to their specific chemical condition they found no improvement and sometimes even worse air quality,’ says Liu. In Delhi, one of the world’s most polluted cities, the air became visibly clearer – yet, Young recalls, she was shocked at the dramatic increase in ground-level ozone during lockdown.
Atmospheric reactions are complex. They depend not only on the chemical species but also on their concentrations. While NOx are emitted directly, ozone is a secondary pollutant that forms from photochemical reactions of primary pollutants. A delicate network connects ground-level ozone, NOx and volatile organic compounds (VOCs), which includes benzene, acetone, chloroform and many more.
‘There’s two regimes: one is where the NOx is limited, so the amount of NOx is what drives the ozone production,’ explains Leigh Crilley, an atmospheric chemist at York University. This means when NOx levels go down, ozone levels go down. But many polluted cities are NOx saturated – their atmospheric chemistry operates in a VOC limited regime where less NOx does not mean less ozone.
NOx acts as a sink for hydroxyl radicals, one of the reactive oxygen species that drive photooxidation reactions. With less NOx around, hydroxyl radicals speed up oxidation of VOCs, which in turn drives ozone production. This relationship causes the weekend effect in cities, with higher ozone levels when traffic NOx go down. ‘That’s what we saw in Delhi, it was VOC limited because it wasn’t responding to changes in NOx,’ says Crilley.
Data from almost 500 air monitoring stations in 34 countries showed a similar trend globally.8 On average, there was a 34% NOx and 15% PM decrease during strict lockdowns from January to the end of April 2020, compared with 2015 to 2019 averages. At the same time, global average ground-level ozone concentrations increased by 86%. Given ozone’s detrimental effect on people’s lung function, the question of whether lockdowns produced cleaner air remains difficult to answer. And some changes might seem more dramatic than they really are – when taking into account the weather.
‘While it’s true that the meteorology is reasonably consistent year on year, on a day to day basis, you can’t predict meteorological change,’ explains Crilley. The many studies that don’t account for local weather aren’t wrong, he says, but they can only ‘give you an idea of the change, but not what caused the change’.
In a preprint, Crilley, Young and their colleague Yashar Iranpour tapped remotely into data collected at an air monitoring station in India. One location in locked down Delhi showed median NOx and PM concentration decreases of 75% and 57%, respectively, when compared with the same period over the last three years. However, comparing the lockdown concentrations to levels predicted by a meteorologically adjusted model which took account of specific atmospheric conditions in 2020 showed much less impressive reductions of 20% for NOx and 8% for PM.
This much smaller than expected drop indicates that traffic is only one of the reasons for Delhi’s air pollution problem. There are many sources – industry and burning crop residues – that remained mostly unchanged during lockdown.
Covid’s carbon footprint
Polluting processes also drive carbon dioxide emissions. However, the greenhouse gas is often missing from discussions around air pollution as it doesn’t directly affect human health in the way other air pollutants do. Nevertheless, ‘it certainly is a compound that is detrimental to the functioning of the environment in general’, says Le Quéré.
The decrease in road transport and industrial production during the lockdowns had an immediate effect on carbon emissions. In April 2020, when most countries had implemented their strictest quarantine measures, carbon dioxide levels fell by 27% compared with the previous year. As soon as lockdowns were lifted, emissions shot back up. ‘The flexibility was quite remarkable,’ says Le Quéré.
Overall, when compared with 2019, the world’s carbon footprint fell by 2.6Gt in 2020 – the equivalent annual emissions of the whole of Europe. ‘The Covid effect really hit home in terms of demonstrating the size of the action needed to tackle climate change,’ says Le Quéré. The Paris Agreement’s ambition to keep global average temperature increases to well below 2°C above pre-industrial levels requires carbon cuts of 1–2Gt per year throughout the 2020s.
‘We achieved [7% reduction] in one year, but I don’t like how we achieved it,’ says Devyani Singh, an energy and climate policy expert at Harrisburg University of Science and Technology in the US. ‘I feel that narrative will be taken on by the wrong side – fossil fuel lobbyists who were always saying that if we reduce emissions, it’s going to impact the economy.’ But that certainly doesn’t have to be the case, she points out. Finding solutions that can be sustained year on year while supporting the economy will be ‘the big thing’, adds Le Quéré.
During the pandemic, renewable energy proved to be resilient while electricity generation from coal fell by a record 8.3% in the first half of 2020. Yet Covid recovery plans in many countries contradict their climate commitments, a report by the London-based consultancy Vivid Economics found. Out of 30 countries analysed, which included the G20 economies, the positive impact of green policy decisions outweighed the environmental negatives from damaging stimulus packages in just 10.
Still, climate policies have been shown to reduce carbon emissions, though change has been slow. According to a 2020 analysis, the 1800 climate laws introduced worldwide between 1999 and 2016 have amounted to emissions savings of 38Gt of CO2 within that time – one year’s worth of global carbon output.9 ‘At this point, there is no silver bullet, we need to do everything and all of it – and we need to do it now,’ says Singh. Carbon taxes, offsets, a managed wind-down, transport electrification and clean energy all need to go hand in hand, she points out.
For the business world, the pandemic was a wake-up call, says Maria Carvalho, a climate policy consultant at sustainability solutions company South Pole. ‘Covid helped people understand that existential threats can really affect your business.’ This means investors started putting more pressure on companies to work towards environmental targets.
A South Pole poll of over 120 executives showed that about 61% set a net zero carbon target or a target in line with emission reductions required by the Paris Agreement. ‘But it’s not enough to set targets, you really do need a roadmap and an action plan behind it,’ says Carvalho. ‘We’ve only seen 48% of those with net zero targets have set milestones to get there.’
The way the pandemic has changed how many people work might go a long way towards cutting urban air pollution. ‘A lot of transport emissions is from idling cars stuck in traffic,’ says Carvalho. Now might be the time to encourage fewer trips to offices, commuting by bike, on foot or by public transport and more flexible hours to avoid traffic congestion.
Yet data seems to indicate that emissions are rebounding already and that that the profound change in 2020 wasn’t more than a blip on the curve. ‘The way that we are going to reconstruct at the end of the Covid-19 crisis is going to have a huge impact on climate change,’ Le Quéré says.
Correction: The affiliation of Leigh Crilley was updated on 22 April 2021.
1 C Le Quéré et al, Nat. Clim. Chang., 2021, 11, 197 (DOI: 10.1038/s41558-021-01001-0)
2 F Liu et al, Sci. Adv., 2020, 6, eabc2992 (DOI: 10.1126/sciadv.abc2992)
3 L Setti et al, BMJ Open, 2020, 10, e039338 (DOI: 10.1136/bmjopen-2020-039338)
4 C Copat et al, Environ. Res.,2020, 191, 110129 (DOI: 10.1016/j.envres.2020.110129)
5 R Isaifan, Global J. Environ. Sci. Manage, 2020, 6, 275 (DOI: 10.22034/gjesm.2020.03.01)
6 K Chen et al, The Lancet Planet. Health, 2020, DOI: 10.1016/S2542-5196(20)30107-8
7 H Achebak et al, The Lancet Planet. Health, 2020, DOI: 10.1016/S2542-5196(20)30148-0
8 A Torkmahalleh et al, Aerosol Air Qual. Res., 2021, 21, 200567 (DOI: 10.4209/aaqr.200567)
9 SMSU Eskander and S Fankhauser, Nat. Clim. Chang., 2020, 10, 750 (DOI: 10.1038/s41558-020-0831-z)
Air quality scientist Pallavi Pant from the Health Effects Institute in Boston, US, put together an extensive reading list containing hundreds of peer-reviewed and press articles on the topic of Covid and air pollution.