The coronavirus pandemic’s 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 National Oceanic and Atmospheric Administration (NOAA), the amount of CO2 in the air in May 2020 hit an average of slightly greater than 417 parts per million (ppm). This is the highest monthly average value ever recorded, and is up from 414.7 ppm in May of last year.

Carbon dioxide levels are the highest they’ve been in human history, and probably are the highest in 3 million years. The last time there was this much CO2 in the atmosphere, global average surface temperatures were significantly warmer than they are today, 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 17 percent decline in early April.

But the total amount of CO2 that winds up in the atmosphere is driven not only by human emission levels, but also through processes on the land surface (especially forests) and in the oceans that fluctuate on a yearly basis.

According to a Scripps news release announcing the findings, CO2 emissions reductions on the order of 20 to 30 percent 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 buildup 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 program, 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 than 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. Though CO2 levels exhibit a seasonal cycle, the overall upward trend is clear.

The rate of increase from last May to this May was slower than it was in the comparable to 2018 to 2019 period, 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, Tans said.

El Niño events can contribute to drought and extreme heat in large areas of the world, Tans says, 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 emissions 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 land emissions 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, 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 is going into the air.

Molecules of CO2, a global warming agent, 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, where 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 3.6 degrees (2 Celsius) above preindustrial levels by 2100.

Kim Cobb, a climate scientist at Georgia Tech, says 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.

Keeling says 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.

Jackson, who conducted the analysis of emissions declines due to the pandemic with colleagues from the Global Carbon Project, said the findings are not surprising given how the planet’s carbon cycle works.

“This result is disappointing but expected in an atmosphere that is both big and variable,” he said via email.

“We estimated that fossil carbon emissions dropped 8 percent [during] January through April, from 12 billion metric tons in 2019 to 11 billion in 2020,” he said. “A billion tons is a lot, but not so much that we can find it with statistical confidence in a noisy signal.”

It’s important not to take the wrong message from the current finding, Jackson emphasized. “We shouldn’t fall into the trap of thinking that the effect of covid-19 on emissions is trivial or that climate progress is impossible,” he said. “It isn’t. Short-term changes in anything are hard to detect in something as big as the atmosphere.”