The concentration of the heat-trapping gas topped 420 parts per million, while the planet has warmed more than two degrees

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A webcam image from the Mauna Loa Observatory on Monday. (NOAA Global Monitoring Laboratories)

April 5, 2021 at 8:59 p.m. UTC

For the first time in recorded history, the concentration of atmospheric carbon dioxide, or CO2, was measured at more than 420 parts per million at the Mauna Loa Observatory on the Big Island of Hawaii. It’s a disconcerting milestone in the human-induced warming of the planet, around the halfway point on our path toward doubling preindustrial CO2 levels.

The research station, at an elevation of 11,135 feet on the summit of a picturesque volcano, has been monitoring the weather and chemistry of the atmosphere continuously since the 1950s. Its location allows it to sample some of the purest air available, providing scientists an untainted representation of how humans are irreversibly influencing climate systems.

When the station began collecting CO2 measurements in the late 1950s, atmospheric CO2 concentration sat at around 315 parts per million. On Saturday, the daily average was pegged at 421.21 parts per million — the first time in human history that number has been so high. Previously, it had never exceeded 420 parts per million.


“We’re completely certain that the increase in CO2 is warming the planet,” Kate Marvel, a climate scientist at NASA, wrote in an email. “I’m even more certain CO2 causes global heating than I am that smoking causes cancer. The world is already more than 2 [degrees Fahrenheit] warmer than it was before the Industrial Revolution.”

In addition to the temperature increase, a warmer atmosphere supports more instances of drought in some areas and flooding in others, along with stronger hurricanes and typhoons and the potential for more storms to rapidly intensify in dangerous, unpredictable ways.

Halfway to doubling preindustrial CO2

Carbon dioxide emissions are a product of electricity production, transportation and industry. The United States alone emits more than 5 billion metric tons of CO2 annually — the weight of 13.2 million fully loaded Boeing 747s, or roughly 68,000 Washington Monuments.

About half of emitted CO2 remains in the atmosphere and induces warming, while more than a quarter is absorbed into the oceans, where it acidifies the water and disrupts marine ecosystems.

There is special significance in reaching and surpassing a concentration of 416 parts per million. It means we’ve passed the midpoint between preindustrial CO2 levels, around 278 parts per million, and a doubling of that figure, or 556 parts per million.


The record of 421 parts per million reached Saturday is just a single point and occurred as CO2 levels are nearing their yearly peak. But the levels over the past two months, of more than 417 parts per million, signal that the annual average concentration is likely to exceed 416 parts per million.

“Although ‘halfway to doubled CO2’ [is] not of any physical significance, it can nevertheless be considered a milestone that highlights how much humans have already altered the composition of the global atmosphere and increased the amount of a gas that warms the global climate,” wrote the U.K. Met Office.

What a doubling of CO2 means

The anticipated doubling of atmospheric CO2, which is likely by the year 2060, has been connected to a predicted three-degree or greater warming of the planet.

A study released last year found that doubling CO2 levels will probably lead to a temperature rise between 4.1 and 8.1 degrees Fahrenheit (2.3 and 4.5 degrees Celsius), ruling out more modest warming scenarios.


Even if greenhouse gas emissions were to plummet overnight, the planet would continue warming for years to come.

“CO2 accumulates in the atmosphere,” Zeke Hausfather, a climate scientist at the Breakthrough Institute in California, wrote in an email. “The amount of warming that the world is experiencing is a result of all of our emissions since the industrial revolution — not just our emissions in the last year.”

He noted that’s why CO2 levels at Mauna Loa continued climbing and setting records despite a brief, dramatic reduction in global emissions stemming from the coronavirus pandemic. Emissions have since returned to near pre-pandemic levels.

Data obtained from glacial ice cores indicates that modern-day CO2 levels are higher than at any point in at least the past 800,000 years.

Worrying trends with other greenhouse gases

Methane, also a significant driver of the warming climate, has shown a “huge and unprecedented increase” too, wrote the British Antarctic Survey. Agriculture, specifically the raising of livestock and manure management, is a primary source of methane in the United States, followed by petroleum and natural gas production, as well as coal mining and landfills.


Carbon dioxide is roughly eight times more abundant in the Earth’s atmosphere, but methane is more effective at heating the atmosphere.

Sulfur hexafluoride, a greenhouse gas that results from the production of insulators used on electrical grids, also reached all-time records of 10 parts per trillion. While its concentration remains orders of magnitude more dilute than that of most other major greenhouse gases, its rate of increase in the atmosphere has doubled since 2003.

Sulfur hexafluoride is also thousands of times more potent — a single molecule can cause 23,900 times more warming than a molecule of CO2. And a single molecule of sulfur hexafluoride can stick around in the atmosphere for more than three millennia.

The implications

At the midpoint toward doubling CO2 levels, the planet has already warmed more than two degrees (1 to 1.5 degrees Celsius). Last year rivaled the planet’s hottest year in modern records.


The World Meteorological Organization recently said that there is at least a 1 in 5 chance of global average warming temporarily exceeding 2.4 degrees (1.5 Celsius) by 2024, which the Paris climate accord sets out to avoid.

At a speech at Columbia University in December, U.N. Secretary General António Guterres said nations need to cut emissions by at least 6 percent annually through 2030 to avoid surpassing this 1.5-degree threshold.

“We can’t avoid climate change — it’s already here,” NASA’s Marvel wrote. But “it’s still possible to escape the worst with smart policy that recognizes the scale of the threat and the need for quick action.”