In 2017 researchers from several universities used advanced laser-based technology to peer inside ice cores pulled from high in the Alps. They found the Black Death.
The ice-core record showed that during the past 2,000 years, the annual levels of lead in the atmosphere took a sudden dip only once. That period was 1349 to 1353, matching up roughly with one of the worst pandemics in human history: when the bubonic plague killed from a third to half of Europe’s population. All that death collapsed economic activity, including lead mining and smelting. Fewer tiny lead particles were floating in the air and settling onto Alpine glaciers, where snow compressed them into ice that cast each year’s record in a core.
A new pandemic now burns across the world. Recent studies show that various types of pollution have declined significantly as people have foregone driving, planes have stopped flying and factories have remained shut. Nitrogen dioxide emitted from vehicles dropped by 40 to 60 percent over cities in China, compared with a similar period last year. Carbon monoxide concentrations above New York City have fallen to half of their 2019 levels. Worldwide emissions of carbon dioxide are down by 17 percent since a year ago, and analyses suggest that 2020 will see the biggest year-on-year drop for those emissions, at around two billion metric tons, or 5.5 percent of 2019’s total.
But are these massive disruptions to our collective pollution output large enough to be captured in tree rings, ice cores and sediment deposits? Will the planet “remember” the COVID-19 crisis?
If we imagine a researcher 100 years from now drilling into the ice, the most likely marker to be found would be aerosols, says Paul Mayewski, director of the University of Maine’s Climate Change Institute and senior author of the Black Death paper. Aerosols are ultrafine particles that can float through the atmosphere for days or weeks before falling to the ground. Pollutant particles, such as those of lead, cadmium and sulfur, arise from factory and power plant smokestacks, vehicle tailpipes, mining and smelting operations, and other sources.
“Ice cores can reconstruct aerosols at monthly resolution in some cases, so the COVID-19 signal should show up,” says Christo Buizert, an Oregon State University paleoclimatologist who specializes in ice cores and abrupt climate change. With lockdowns across large portions of the industrialized world already two or three months and a global economic slowdown still ongoing, a drop in sulfur or cadmium ice-core deposits seems likely.
Another important aerosol that Buizert says could show up in ice cores is soot—specifically, particulate matter of 2.5 microns in diameter or smaller, known as PM2.5. These particles come primarily from coal and natural gas power plants, as well as vehicle tailpipes and cookstoves. And they aggravate human health around the world. PM2.5 levels over Wuhan, China, thought to be where the pandemic originated, dropped by 44 percent during the city’s lockdown. Meanwhile Delhi had a 60 percent reduction, and Los Angeles had a 31 percent decline.
Our 2120 paleoclimatologist could also likely find the pandemic in tree rings. As trees grow, they take up sulfur, nitrogen oxides and metals such as cadmium deposited from the atmosphere into soil and water. Scientists can use mass spectrometry to analyze how levels vary from one year to the next. The rings might offer an even better record than ice cores because trees are found much closer to cities and industrial centers than your average glacier. Studies show that even particles that remain aloft for only short periods of time can circulate fairly far. For example, fossil-fuel combustion in the U.S. and Europe is a primary source of soot particles that cover ice and snow in the Arctic.
Other markers of the pandemic might actually involve greater levels of certain materials than average instead of less of them. Kim Cobb, a paleoclimatologist at the Georgia Institute of Technology, thinks the growing mountain of plastic personal protective equipment, or PPE, that is being discarded could show up in sediment layers in waterways. “You’d probably see these in river deltas, in coastal sedimentary sequences and, I would imagine, some lake systems, especially if they’re adjacent to large cities,” she says. Many metric tons of plastics already find their way into these sediments, but the addition of billions of gloves, masks and other single-use items could create a pulse—a thicker and perhaps even distinct layer representing a plastics-rich cataclysm. “It would be a marker, a chronological layer, which would be such a fascinating thing for future geologists,” Cobb says.
In 3020 an intrepid researcher might still be able to discern that layer, given the long time many plastics require to degrade. A dendrochronologist might be in business with the aerosol record in some long-lived trees, as well. Ice cores would certainly retain their markers—if a few glaciers and ice sheets were still around.
Ice would tell the same tale 100,000 years from now. The oldest cores to reveal our past climate stretch into the millions of years. “Ice cores don’t lie,” Mayewski says. “They capture, to the best of their ability, everything that is transported in the atmosphere.”
In all these records, though, pandemic-related changes to CO2 emissions would be tougher to spot. Gases are exchanged between the atmosphere and the snow until it is compressed into ice. If the dip in emissions lasts only a few months before rebounding, that period is likely not enough time to leave a noticeable change. Of course, if the pandemic stretches longer than we all hope, the ice would indeed record the drop.
Perhaps humanity can view the decrease in the use of fossil fuels during the pandemic as an opportunity to really break away from them and intensively mitigate climate change. If that response happens, 2020 could end up looking like a turning point of sorts. Cobb says she imagines a scenario “where thousands of years from now, 2020 will mark the year of peak emissions —and therefore peak atmospheric CO2 concentrations—because we came to value science and our collective responsibility to one another on a small planet.”