Above: The A5 highway at Isle-Aumont, France, was deserted on Monday, March 23, 2019, the seventh day of a lockdown aimed at curbing the spread of the novel coronavirus in France. (Stephane de Sakutin/AFP via Getty Images)
In just a matter of weeks, the rapid spread of the novel coronavirus has pushed the world into a mammoth dual experiment in public health and economic policy. As the impacts of COVID-19 illness spike in country after country, governments are mandating business closures, quarantines, and “shelter in place” practices at a scale that few would have imagined.
Given that large parts of the world are shutting down large parts of their economies for weeks if not months, a new question emerges: what kinds of short- and long-term effects could this drastic slowdown have on the atmosphere—not only on air quality, but also on regional and global climate?
It’s obvious from satellite-based sensors that the sudden drop-off in industry and transport in some areas has literally cleared the air. The most vivid evidence is from measurements of nitrogen dioxide (NO2), a pollutant that can lead to harmful ground-level ozone. High-resolution NO2 data gathered in recent weeks by the Copernicus Sentinel-5P satellite is especially striking. NO2 concentrations plummeted in the Wuhan area of east-central China after the highly industrialized region was put under virtual lockdown. Similar drop-offs were detected over northern Italy after that hard-hit nation put in place severe restrictions on commerce and mobility. And some U.S. cities are seeing big reductions in NO2 this month, as measured by Sentinel-5P and reported by the New York Times on Sunday.
Sentinel-5P is also monitoring carbon monoxide (CO), which evolves over a longer time period. Potential reductions in CO may not be as dramatic as for NO2, in part because changes in emissions from power generation, transportation, industry, and residences are likely having different effects on different pollutants, according to Helen Worden (National Center for Atmospheric Research). Worden is the U.S. principal investigator for the U.S./Canadian MOPITT instrument on the NASA/Terra satellite.
Power plants and vehicles are key sources of NO2, Worden points out, which means factory shutdowns and stay-at-home orders would have a big impact on that pollutant. However, a large part of carbon monoxide (CO) in China is still from residential burning—which would presumably continue even if folks are staying home—so it is possible we won’t see as dramatic a drop in CO emissions.
Moreover, NO2’s short lifetime in the atmosphere (only a day or so) means that the impact of the coronavirus shutdown is immediately evident.
“When you see NO2, you’re basically seeing the sources of it,” Worden says. “With CO, the lifetime is a couple of months, and so you’re not only seeing the local sources—you’re seeing contributions from global transport of CO.”
Update: Worden and colleagues reported on March 26 that carbon monoxide values dropped by more than 30% in some parts of the corridor between Beijing and Wuhan for the week following Chinese New Year 2020 (January 24-30, when the coronavirus lockdown was in effect) versus the week following Chinese New Year 2019 (February 4-10).
Worden has been collaborating with Kevin Bowman (Jet Propulsion Laboratory) on a JPL study using “inverse modeling”—working backwards from the satellite measurements to figure out more precisely where pollutants have come from over the last several months. Bowman and colleagues are now writing up results from a study that quantifies changes in emissions during the coronavirus era.
“We’re also actively looking at the impact of Australian fires on the carbon balance. These were incredibly intense fires,” said Bowman. He believes there is great promise in analyzing short-fuse events like these using “chemical weather” inversion models that take into account the usual atmospheric conditions as well as pollutants and photochemistry (sunlight-driven effects, such as the formation of ground-level ozone).
“With the world changing at the speed we’re seeing, these inversion models are becoming critical to timely scientific and policy-relevant information,” he added.
Aerosols at work
If there’s an immediate effect on climate from the world’s response to COVID-19, it will be from changes in aerosols (or particulates)—the tiny airborne solids and liquids spewed out by factories, vehicles engines, and furnaces, including the pollutants above. Even though their atmospheric lifespan is only a matter of a few hours to a few weeks, these sun-blocking aerosols are continually replenished as we burn fossil fuels, and they can have a surprisingly strong cooling effect as a whole.
Computer models strongly suggest that the aerosols belched out from the multi-decade surge in industrial activity after World War II are a prime reason why air temperature held roughly steady worldwide—and actually dropped a bit in parts of the highly industrialized Northern Hemisphere—from the late 1940s to the 1970s.
Environmental laws in North America and Europe (the world’s industrial powerhouses at that time) began to tighten in the Seventies, and aerosol pollution began to decrease. Greenhouse gases weren’t addressed, though. The decline in globally averaged aerosol pollution, combined with ever-increasing concentrations of carbon dioxide and other greenhouse gases, fostered a rise in global air temperature that gained traction in the 1980s and has continued into the 2020s, with only slight variations modulating the trend from decade to decade.
The 21st-century explosion of industry in eastern and southern Asia, especially China, led to a regional increase in aerosols, one that’s been suddenly crimped by the coronavirus shutdown. If factories and transportation are slow to ramp up this spring and summer, it’s possible that an otherwise hot summer in parts of this region might be slightly hotter in the absence of the usual sun-blocking aerosols. Likewise, North America and Europe could see a slightly warmer summer than one would otherwise expect, although the effects there would likely be more muted because of previous aerosol reductions over the past 40 years.
A longtime researcher in climate and air quality, Drew Shindell (Duke University), elaborated on these possibilities in an email:
“Aerosols have decreased over East Asia, and now over Europe and North America too (though there are fewer there to begin with), so most of the Northern Hemisphere mid-latitudes will see a drop. As the climate response is slow, I can't see the slightly earlier drops in emissions in East Asia having an effect before the others. And climate response is also spread out in the zonal (east-west) direction. So my best guess would be a very minor warming in the Northern Hemisphere midlatitudes for the next few months, a warming that is likely to be within the seasonal noise and thus undetectable in observations.”
WU co-founder Jeff Masters, who worked on air quality matters for his doctoral degree, called out another regional angle in an email: "The nation-wide shut down in business just ordered in India, if sustained, has the potential to bring about a noticeable increase in the summer monsoon rains, since aerosol particles emitted by India have been shown to decrease the intensity of the monsoon."
One other regional effect of the shutdown might be a reduction in the amount of pollution blowing toward the Arctic from northern midlatitudes. Aerosols deposited on ice and snow can darken its surface, increasing its ability to absorb sunlight and potentially leading to an enhanced melt-off once spring arrives. A multiyear research project, Dark Snow, is focused on quantifying the impact on Arctic melt from soot produced by industry and wildfires, along with mineral dust and microbes.
The project hasn’t yet detected any signals related to the coronavirus effects, according to Dark Snow PI Jason Box (Geological Survey of Denmark and Greenland). He expects that systems such as AERONET, a global network of photometers that measure sunlight blockage from aerosols, would eventually pick up on a signal of reduced pollution. “However, I expect the signal to be lower than the variability, so one would perhaps need to average many stations,” Box said in an email.
The long view
It remains to be seen how much the world’s output of greenhouse gases—carbon dioxide in particular—will be blunted by the massive global response to the novel coronavirus. We already know that economic downturns can trim emissions slightly from one year to the next. The carbon dioxide emitted globally from fossil fuels dropped by about 1% from 1991 to 1993 with the dissolution of the Soviet Union and the subsequent regional turmoil. Likewise, fossil-fuel-based CO2 emissions dropped by about 2% from 2008 to 2009 as the Great Recession took hold. In both cases, the drops were quickly swamped by subsequent increases.
Already, it’s been estimated by Carbon Brief that CO2 emissions from China have dropped by 25% since the coronavirus outbreak began early this year. If so, that would represent roughly a 6% drop in global emissions. If at least some of the world’s other major emitters experience a similar economic hit, then a global emissions drop on the order of 10% would be conceivable.
The bad news is that even a big reduction in emissions means that we’re still increasing the amount of carbon dioxide in the atmosphere, albeit at a slower rate. Think of it this way: if you trim the amount by which you overspend your household budget each year, your total debt is still adding up. Even a massive slowdown in global economic activity won’t be enough to produce a major effect on global CO2 concentrations, which have increased every year in the 60-plus years they’ve been measured atop Mauna Loa, Hawaii. NOAA reported in February that the CO2 concentration averaged 407.4 parts per million in 2018, an increase of 2.5 ppm from 2017.
At the Scripps Institution of Oceanography, which has conducted Mauna Loa observations since they began, researcher Ralph Keeling recently estimated that a 10 percent decline in global emissions sustained for a full year—a drop far larger than anything observed on Mauna Loa to date—would shave a mere 0.5 ppm from atmospheric CO2 concentrations.
“A minor relative cooling [is possible] due to the drop in CO2,” said Shindell, “but that's happened before with economic downturns and is likely temporary and therefore small. It'd be interesting, though, if we have a lasting effect on emissions, for example if teleworking becomes more accepted and widely practiced and so transportation emissions never return to their prior levels. Not sure if that'll happen of course, but that could have a long-term effect big enough to see.”
Indeed, an economic slowdown might open a window of opportunity for activists and innovators to push for greener policies and technologies that can be picked up as the economy eventually recovers.
“History has shown that carbon dioxide levels typically resume their climb quickly as normal economic activity rebounds,” noted Scripps media specialist Rob Monroe, who summarized Keeling’s take on the matter in a March 11 post at the Keeling Curve website.
“If there is any benefit of the coronavirus event in terms of slowing the pace of climate change, it could be the changing of people’s travel and work habits in ways that lead to sustained reductions in fossil fuel use. Only those kinds of long-term systemic reductions will change the trajectory of carbon dioxide levels in the atmosphere.”
It’s worth remembering that the mid-2010s saw global gross domestic product (GDP) increase by about 3% in each year from 2014 to 2016 while emissions held virtually flat. We now know that economies can grow without expanding their carbon footprint—and as many activists have pointed out, GDP is only one measure of people’s health, happiness, and well-being. Once we emerge from the immediate public health crisis posed by COVID-19, we may look at old economic assumptions—along with so many other things—in a new light.
