The coronavirus-related economic downturn may have set off a sudden plunge in global greenhouse gas emissions, but another crucial metric for determining the severity of global warming - the amount of greenhouse gases actually in the air - just hit a record high.
According to readings from the Scripps Institution of Oceanography and the US National Oceanic and Atmospheric Administration (NOAA), the amount of carbon dioxide (CO2) in the air in May 2020 averaged more than 417 parts per million (ppm).
It is the highest monthly average value ever recorded, up from 414.7 ppm in May of last year.
Carbon dioxide levels are the highest they've been in human history, and likely the highest in three million years. The last time there was this much CO2 in the atmosphere, our early human ancestors lived in Africa, global average surface temperatures were significantly warmer, and sea levels were 50 to 80 feet higher.
The continuing rise in CO2 concentrations in the atmosphere may sound surprising in light of recent findings that the pandemic and the associated lockdowns had led to a steep drop in global greenhouse gas emissions, peaking at a 17pc decline in early April.
But the CO2 that winds up in the atmosphere is driven not only by human emission levels, but also through processes on land (especially forests) and in the oceans that fluctuate on a yearly basis.
According to a Scripps announcement, CO2 reductions of the order of 20-30pc would need to be sustained for six to 12 months in order for the increase in atmospheric CO2 to slow in a detectable way.
"The build-up of CO2 is a bit like trash in a landfill. As we keep emitting, it keeps piling up," said Ralph Keeling, who directs Scripps's carbon dioxide monitoring programme, and whose late father, Charles David Keeling, began measurements at the Mauna Loa Observatory in Hawaii in 1958.
"The crisis has slowed emissions, but not enough to show up perceptibly at Mauna Loa. What will matter much more is the trajectory we take coming out of this situation," he said.
The rate of increase of this long-lived greenhouse gas in the atmosphere is accelerating.
In the 1960s, the annual growth rate was about 0.8 ppm per year. It doubled in the 1980s, and eventually climbed to 2.4 ppm per year during the past decade.
Multiple lines of evidence show the cause of this increase is greenhouse gas emissions from human activities, said Pieter Tans, who leads NOAA's Carbon Cycle Greenhouse Gases Group.
The Scripps researchers found the average atmospheric CO2 concentration for May 2020 was 417.16 parts per million.
Using different calculations to the Scripps team, NOAA researchers found the peak monthly value was slightly lower, at 417.1 ppm.
The annual high typically occurs in May before CO2 levels temporarily ebb as trees and plants in the Northern Hemisphere absorb vast quantities of the planet-warming gas during the summer growing season. Although CO2 levels exhibit a seasonal cycle, the overall upward trend is clear.
The rate of increase in the past 12 months was slower than from the same period in 2018-2019, but natural factors such as El Niño events in the tropical Pacific Ocean and changes in terrestrial carbon sinks, such as forests, can have a large influence on this from year to year, Mr Tans said.
El Niño can contribute to drought and extreme heat in large areas of the world, Mr Tans added, noting they can weaken the ability of forests and other lands to soak up carbon dioxide, leading to a higher natural contribution to atmospheric CO2 levels.
The combination of record fossil fuel use and mild El Niño conditions between May 2018 and 2019 can explain the above average increase in atmospheric CO2 of 3.5 ppm that year.
Wildfires and deforestation can add to the increase in atmospheric CO2 as well, though their influence ultimately is overwhelmed by the global burning of fossil fuels.
According to Rob Jackson, an expert with Stanford University and the Global Carbon Project, emissions from wildfires were up in 2019 and 2020, contributing to the May peak, as was Brazil's due to deforestation and burning.
This year's May CO2 peak marked an increase of about 2.4 ppm compared with a year ago. The 2010 to 2019 average rate of increase is precisely the same at 2.4 ppm per year, according to NOAA.
The decline of El Niño during the past year may help explain why the increase in the last year was not as large as the previous, Mr Tans said.
Because atmospheric levels of CO2 are cumulative, they will continue to increase until net emissions are cut to zero.
They will not decrease until human activities and natural ecosystems are removing more greenhouse gases than are going into the air.
Molecules of CO2 can remain in the atmosphere for up to 1,000 years.
Scientists warn that we're on course to reach 450 ppm by mid-century, when levels would need to stop increasing to have a decent chance of meeting the goals in the Paris climate agreement, which seeks to limit climate change to well below 2 degrees Celsius above pre-industrial levels by 2100.
Kim Cobb, a climate scientist at Georgia Tech, said the new findings underscore the need to act now.
"It is a reminder that climate change is not on pause in any way, shape or form," she said.
Mr Keeling said it would take a sustained drop in emissions, rather than a sudden decline related to the coronavirus pandemic, to show up more clearly in measurements of atmospheric CO2.
"What really matters here is setting a new trajectory," he said.