The concentration of carbon dioxide in the atmosphere peaked above 430 parts per million in 2025—the highest it has been in millions of years—according to data released by the National Oceanic and Atmospheric Administration and Scripps Institution of Oceanography at the University of California, San Diego on Thursday.
The news was overshadowed by the explosive feud between U.S. President Donald Trump and his erstwhile backer Elon Musk, but climate activist Bill McKibben argued that it was ultimately more consequential.
“In the long run, this is actually going to be the important news of the day—CO2 in the atmosphere passes another grim milestone,” McKibben wrote on social media.
Carbon dioxide has been accumulating in the atmosphere due primarily to the human burning of fossil fuels, as well as by the clearing of forests and other natural carbon sinks. There, it acts as a greenhouse gas, trapping heat from the Earth, and is the primary gas responsible for the rise of global temperatures by approximately 1.1°C from the 1850 -1900 average. This warming has already had a host of dramatic impacts, from extreme weather events to sea-level rise to polar ice melt, and scientists warn these impacts will only accelerate under current energy policies, which put the world on track for around 3°C of warming by 2100.
The last time that atmospheric CO2 concentrations topped 430 ppm was most likely more than 30 million years ago, Ralph Keeling, who directs the Scripps CO2 Program, toldNBC News.
“It’s changing so fast,” he said. “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.”
“While largely symbolic, passing 430 ppm should be a wake-up call.”
Scripps and National Oceanic and Atmospheric Administration both measure carbon dioxide levels from NOAA’s Mauna Loa Observatory in Hawaii, where Charles Keeling began taking measurements in 1958. As CO2 levels rise over time, they also follow a seasonal cycle—peaking in May before falling in the Northern Hemisphere summer and rising again in the fall.
This May, Scripps Oceanography calculated an average of 430.2 ppm for 2025, which is 3.5 ppm over the average for May 2024. NOAA’s Global Monitoring Laboratory, meanwhile, calculated a monthly average of 430.5 ppm, a 3.6 ppm jump from the year before and the second-steepest yearly climb since 1958.
“Another year, another record,” Keeling said in a statement. “It’s sad.”
The news comes two months after Mauna Loa daily measurements surpassed 430 ppm for the first time in March, which Plymouth Marine Laboratory professor Helen Findlay called “extremely disappointing and worrying.”
“While largely symbolic, passing 430 ppm should be a wake-up call, especially given the accelerated response we are seeing of glaciers and ice sheets to current warming,” Dr. James Kirkham, chief scientist of the Ambition on Melting Ice coalition of governments, said at the time.
“This upward trajectory is a direct result of continued fossil fuel use, likely exacerbated by emissions from extreme wildfires last year, methane leaks from fossil fuel extraction and possibly greater permafrost emissions, alongside decreased ability of very warm oceans to absorb CO2,” Kirkham said.
The monthly record also comes a little more than a week after a United Nations report warned that there was a small chance global temperatures could surpass 2°C in at least 1 of the next 5 years, only a decade after world leaders pledged in the Paris agreement to keep global temperatures “well-below” that level.
“Carbon emissions are still rising, and the atmosphere is going to keep heating further until greenhouse gas concentrations stabilize,” Matt Kean, who chairs Australia’s Climate Change Authority, wrote in response to the Scripps and NOAA figures. “What sort of climate do we want to leave our children and those who come after them?”