Lava flow blocks road access to Mauna Loa Observatory. Photo: Hawaii EMA Civil Air Patrol

Broken Record: Atmospheric Carbon Dioxide Levels Jump Again

Average May reading 3.0 parts per million higher than in 2022

Carbon dioxide levels measured at NOAA’s Mauna Loa Atmospheric Baseline Observatory peaked at 424 parts per million (ppm) in May, continuing a steady climb further into territory not seen for millions of years, scientists from NOAA and Scripps Institution of Oceanography at UC San Diego announced today.

Measurements of carbon dioxide (CO2) obtained by NOAA’s Global Monitoring Laboratory averaged 424 parts per million in May, the month when CO2 peaks in the Northern Hemisphere. That represents an increase of 3.0 ppm over May 2022. Scientists at Scripps Oceanography, which maintains an independent record, calculated a May monthly average of 423.78 ppm. That increase is also a jump of 3.0 ppm over the May 2022 average reported by the Scripps CO2 Program.

“Sadly we’re setting a new record,” said Scripps Oceanography geoscientist Ralph Keeling, who oversees the iconic Keeling Curve record established by his father 65 years ago. “What we’d like to see is the curve plateauing and even falling because carbon dioxide as high as 420 or 425 parts per million is not good. It shows as much as we’ve done to mitigate and reduce emissions, we still have a long way to go.”

CO2 levels are now more than 50% higher than they were before the onset of the industrial era.

“Every year we see carbon dioxide levels in our atmosphere increase as a direct result of human activity,” said NOAA Administrator Rick Spinrad, Ph.D. “Every year, we see the impacts of climate change in the heat waves, droughts, flooding, wildfires and storms happening all around us. While we will have to adapt to the climate impacts we cannot avoid, we must expend every effort to slash carbon pollution and safeguard this planet and the life that calls it home.”

CO2 pollution is generated by burning fossil fuels for transportation and electrical generation, by cement manufacturing, deforestation, agriculture and many other practices. Like other greenhouse gases, CO2 traps heat radiating from the planet’s surface that would otherwise escape into space, amplifying extreme weather events, such as heat waves, drought and wildfires, as well as precipitation and flooding.

Rising CO2 levels also pose a threat to the world's ocean, which absorbs both CO2 gas and excess heat from the atmosphere. Impacts include increasing surface and subsurface ocean temperatures and the disruption of marine ecosystems, rising sea levels and ocean acidification, which changes the chemistry of seawater, leading to lower dissolved oxygen, and interferes with the growth of some marine organisms.

This year, NOAA’s measurements were obtained from a temporary sampling site atop the nearby Mauna Kea volcano, which was established after lava flows cut off access to the Mauna Loa observatory in November 2022. Scripps’s May measurements were taken at Mauna Loa, after NOAA staff successfully repowered a Scripps instrument with a solar and battery system in March.

The Mauna Loa data, together with measurements from sampling stations around the world, are incorporated by NOAA’s Global Monitoring Laboratory into the Global Greenhouse Gas Reference Network, a foundational research dataset for international climate scientists and a benchmark for policymakers attempting to address the causes and impacts of climate change.

Widely considered the premier global sampling location for monitoring atmospheric CO2, NOAA and Scripps observatory operations were abruptly suspended on Nov. 29, 2022 when lava flows from the eruption of Mauna Loa volcano buried more than a mile of access road and destroyed transmission lines delivering power to the observatory campus. After a 10-day interruption, NOAA restarted greenhouse gas observations on Dec. 8 from a temporary instrument installation on the deck of the University of Hawaii observatory, located near the summit of Mauna Kea volcano. Scripps Oceanography initiated air sampling at Mauna Kea on Dec. 14, 2022 and resumed sampling at Mauna Loa on March 9, while maintaining their Mauna Kea observations.

Continuous daily samples were obtained from both Mauna Loa and Mauna Kea by Scripps Oceanography during May, the month when CO2 levels in the Northern Hemisphere reach their maximum levels for the year. Scripps recorded a May CO2  reading from Maunakea of 423.83, which is very close to the reading of 423.78 from Mauna Loa.

The Mauna Loa observatory is situated at an elevation of 11,141 feet above sea level, while the Mauna Kea sampling location is slightly higher, at an elevation of 13,600 feet. Scientists are able to sample air undisturbed by the influence of local pollution or vegetation, and produce measurements that represent the average state of the atmosphere in the Northern Hemisphere from both locations.

Scripps Oceanography geoscientist Charles David Keeling initiated on-site measurements of CO2 at NOAA’s Mauna Loa weather station in 1958. Keeling was the first to recognize that CO2 levels in the Northern Hemisphere fell during the growing season, and rose as plants died back in the fall. He documented these CO2 fluctuations in a record that came to be known as the Keeling Curve. He was also the first to recognize that, despite the seasonal fluctuation, CO2 levels rose every year.

NOAA began measurements in 1974, and the two research institutions have made complementary, independent observations ever since. 

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

About UC San Diego

At the University of California San Diego, we embrace a culture of exploration and experimentation. Established in 1960, UC San Diego has been shaped by exceptional scholars who aren’t afraid to look deeper, challenge expectations and redefine conventional wisdom. As one of the top 15 research universities in the world, we are driving innovation and change to advance society, propel economic growth and make our world a better place. Learn more at

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