Scientists from NOAA and the Scripps Institution of Oceanography at the University of California, San Diego recently announced that The seasonal peak in atmospheric carbon dioxide (CO2) concentration exceeded 430 parts per million (ppm) for the first time at the Mauna Loa Observatory in Hawaii.

Scripps Oceanography calculated a monthly average of 430.2 ppm for May 2025, an increase of 3.5 ppm from the May 2024 measurement of 426.7 ppm. Scientists at NOAA's Global Monitoring Laboratory reported an average of 430.5 ppm, an increase of 3.6 ppm from last year.

Ralph Keeling, director of the CO2 program at Scripps, commented: "Another year, another record. It's sad."

The Mauna Loa Observatory, located high on the slopes of the Mauna Loa volcano at an altitude of 11,141 feet above sea level, is considered global reference site for atmospheric CO2 monitoringMeasurements from this observatory represent the average state of the atmosphere in the Northern Hemisphere.

CO2 monitoring at this site began in 1958 by Scripps scientist Charles David Keeling, father of Ralph Keeling. Charles David Keeling was the first to recognize that CO2 levels in the Northern Hemisphere peak in May, decline during the growing season, and rise again as plants die in the fall. He documented these fluctuations in a record known as the Keeling curveHe was also the first to notice that, in addition to seasonal fluctuations, CO2 levels are rising every year. NOAA began daily CO2 measurements in 1974 and has maintained a complementary, independent record ever since.

Continuous daily sampling by both NOAA and Scripps at Mauna Loa for most of the past half century has provided an ideal base for generating a long-term time series of the increase in the most important human-caused greenhouse gas. The observatory was temporarily disrupted in November 2022 by an eruption of the Mauna Loa volcano, which blocked the access road. After a brief hiatus, Scripps and NOAA resumed measurements at the summit of nearby Maunakea, and NOAA later restarted measurements at Mauna Loa after installing solar power and a battery backup system.

Carbon dioxide, like other greenhouse gases, acts as a blanket, traps heat and warms the lower atmosphereThis changes weather patterns and contributes to extreme events such as heat waves, droughts and forest fires, as well as heavier rainfall and floodingRising CO2 levels also contribute to ocean acidification, which is a change in ocean chemistry that makes it harder for marine organisms such as crustaceans, bivalves, and corals to form hard carbonate skeletons or shells.

Although the Mauna Loa Observatory serves as a benchmark for capturing global CO2 increases, it does not capture the full range of CO2 variations throughout the atmosphere. For example, stations in the Southern Hemisphere have not yet exceeded 430 ppm and have an inverted seasonal cycle. NOAA’s globally distributed sampling network provides a broader picture, allowing it to not only track the overall increase but also identify locations of CO2 sources and sinks, such as boreal or tropical forests or oceans. The Scripps CO2 Program similarly operates 14 global sampling stations.

Data from Mauna Loa, along with measurements from sampling stations around the world, are incorporated into Global Greenhouse Gas Reference Network, which serves as a core research dataset for international climate scientists and as a reference point for policymakers seeking to address the causes and impacts of climate change. Spring