UC San Diego News Center


Atmospheric Carbon Dioxide Levels Reach New High

Peak monthly average of 421 parts per million is 50 percent greater than pre-industrial levels

Mauna Loa Observatory. Photo: NOAA

Carbon dioxide measured at NOAA’s Mauna Loa Atmospheric Baseline Observatory peaked in May 2022 at an average of more than 420 parts per million, pushing the atmosphere further into territory not seen for millions of years, scientists from NOAA and Scripps Institution of Oceanography at the University of California San Diego announced today.

In May, Scripps Oceanography’s measurements of carbon dioxide, or CO2, at the mountaintop observatory on Hawaii’s Big Island averaged 420.78 parts per million (ppm). Scientists at NOAA calculated a monthly average of 420.99 ppm based on independent measurements at the same location.

“It’s depressing that we’ve lacked the collective willpower to slow the relentless rise in CO2,” said Scripps geochemist Ralph Keeling, who manages the iconic record of CO2 concentrations in the atmosphere named the Keeling Curve for his father, Charles David Keeling. “Fossil-fuel use may no longer be accelerating, but we are still racing at top speed towards a global catastrophe.”

“The science is irrefutable: humans are altering our climate in ways that our economy and our infrastructure must adapt to,” said NOAA Administrator Rick Spinrad. “We can see the impacts of climate change around us every day. The relentless increase of carbon dioxide measured at Mauna Loa is a stark reminder that we need to take urgent, serious steps to become a more climate-ready nation.”

CO2 pollution is generated by burning fossil fuels for transportation and electrical generation, by cement manufacturing, deforestation, agriculture and by many other practices. Along with other greenhouse gases, CO2 traps heat radiating from the planet’s surface that would otherwise escape into space, causing the planet’s atmosphere to warm steadily, which unleashes a cascade of weather impacts, including episodes of extreme heat, drought and wildfire activity, as well as heavier precipitation, flooding and tropical storm activity.

Impacts to the world’s oceans from greenhouse gas pollution include increasing sea-surface temperatures, rising sea levels, and an increased absorption of carbon, which makes sea water more acidic, leads to ocean deoxygenation, and makes it more difficult for some marine organisms to survive.

Prior to the Industrial Revolution, CO2 levels were consistently around 280 ppm for most of human civilization. Since then, humans have generated an estimated 1.5 trillion tons of CO2 pollution, much of which will continue to warm the atmosphere for thousands of years.

CO2 levels are now comparable to an ancient climate event known as the Pliocene Climatic Optimum, between 4.1 and 4.5 million years ago, when they were close to, or above 400 ppm. During that time, sea levels were between five and 25 meters (16 and 82 feet) higher than today, high enough to drown many of the world’s largest cities. Temperatures then averaged 7.7 degrees Celsius (14 degrees Fahrenheit) higher than in pre-industrial times, and studies indicate that large forests occupied today’s Arctic tundra.

NOAA’s Mauna Loa observatory, situated high on the slopes of the volcano, is the global benchmark location for monitoring atmospheric CO2. At an elevation of 3,396 meters (11,141 feet) above sea level, the observatory samples air undisturbed by the influence of local pollution or vegetation and produces measurements that represent the average state of the atmosphere in the Northern Hemisphere.

Scripps Oceanography geochemist Charles David Keeling initiated on-site measurements of CO2 at NOAA’s weather station on Mauna Loa 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. This variability is represented graphically by the Keeling Curve. He was also the first to recognize that, superimposed upon the seasonal fluctuation, CO2 levels rose every year.

NOAA began independent measurements in 1974. 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 head off the impacts of climate change.

Despite decades of negotiation, the global community has been unable to significantly slow, let alone reverse, annual increases in atmospheric CO2 levels.

“Carbon dioxide is at levels our species has never experienced before — this is not new,” said Pieter Tans, senior scientist with the Global Monitoring Laboratory. “We have known about this for half a century, and have failed to do anything meaningful about it. What’s it going to take for us to wake up?

To visualize how sea-level rise may affect your community, visit NOAA’s sea-level rise viewer at

Adapted from NOAA

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