For the first time in millions of years, Earth’s atmosphere contained an average of 430.2 parts per million (ppm) of carbon dioxide, the result of humans burning fossil fuels.

The number, recorded in May at the National Oceanic and Atmospheric Administration (NOAA)’s Mauna Loa Observatory in Hawaii by scientists from University of California San Diego’s Scripps Institution of Oceanography, represents an increase of 3.5 ppm from May 2024.

“Another year, another record,” said Ralph Keeling, director of the Scripps CO2 Program, in a press release from Scripps. “It’s sad.”

The monthly @keelingcurve.bsky.social atmospheric carbon dioxide concentration clocked in at 430.2 parts per million in May for 2025, a 3.5 ppm increase over May 2024’s measurement of 426.7 ppm. Read more about the work led by Scripps Oceanography & @noaa.gov: bit.ly/43tMyeP

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— Scripps Institution of Oceanography (@scrippsocean.bsky.social) June 5, 2025 at 3:17 PM

Meanwhile, scientists with NOAA’s Global Monitoring Laboratory reported a 430.5 ppm average.

Ralph Keeling’s father, Scripps scientist Charles David Keeling, in 1958 started monitoring concentrations of atmospheric carbon dioxide at the observatory’s NOAA weather station. Keeling was the first scientist to recognize that levels of Northern Hemisphere carbon peaked in May. He also discovered that the levels fell during the growing season, only to rise again when vegetation died in the fall.

Keeling documented the fluctuations in the planet’s carbon dioxide levels in what is now known as the Keeling Curve. This record helped him recognize another pattern: Carbon levels were rising with each passing year.

Exceeding the threshold of 400 ppm was unimaginable just decades ago, reported NBC News. It translates to more than 400 million molecules of carbon dioxide for every one million of gas in Earth’s atmosphere. The milestone was first reached in 2013, and now scientists are warning that carbon levels could reach 500 ppm in the next 30 years.

Keeling said the last time atmospheric carbon levels were so high was likely over 30 million years ago.

“It’s changing so fast,” Keeling told NBC News. “If humans had evolved in such a high-CO2 world, there would probably be places where we wouldn’t be living now. We probably could have adapted to such a world, but we built our society and a civilization around yesterday’s climate.”

The Mauna Loa Observatory sits at an elevation of 11.141 feet and is the world’s standard for monitoring average levels of carbon dioxide in the northern hemisphere.

Daily independent measurements by NOAA began in 1974.

The combined monitoring by Scripps at Mauna Loa and NOAA have provided a baseline for the establishment of a long-term record of the most important greenhouse gas.

“Like other greenhouse gases, CO2 acts like a blanket, trapping heat and warming the lower atmosphere. This changes weather patterns and fuels extreme events, such as heat waves, droughts and wildfires, as well as heavier precipitation and flooding. Rising CO2 levels also contribute to ocean acidification, a change in ocean chemistry that makes it more difficult for marine organisms like crustaceans, bivalves and coral to grow hard, carbonate skeletons or shells,” the press release said.

While Mauna Loa is considered the world’s standard for recording the global rise of carbon dioxide, it can’t capture all variations throughout the atmosphere. There are monitoring stations in the Southern Hemisphere with a reverse cycle that have yet to cross 430 ppm.

The measurements from Mauna Loa Observatory, along with data from other sampling stations all over the world, are incorporated into the Global Greenhouse Gas Reference Network, which serves as a basis for policymakers attempting to tackle climate change.

Carbon dioxide levels in Earth’s atmosphere are an indicator not only of how much humans are impacting the climate, but of the overall health of our planet.

“They’re telling you about your whole system health with a single-point measurement,” Keeling told NBC News. “We’re getting a holistic measurement of the atmosphere from really a kind of simple set of measurements.”

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