Mauna Loa Observatory
The Mauna Loa Observatory. Photo by Mary Miller for NOAA

Atmospheric carbon dioxide measured at NOAA’s Mauna Loa Observatory peaked for 2021 in May at a monthly average of 419 parts per million, the highest level since accurate measurements began 63 years ago, Scripps Institution of Oceanography in San Diego reported Monday.

Scripps scientist Charles David Keeling initiated on-site measurements of carbon dioxide atop the volcano on the Big Island of Hawaii in 1958. NOAA began measurements in 1974, and the two research institutions have made complementary, independent observations ever since.

The average of the two scientific groups’ measurements a year ago in May 2020 was 417 ppm.

Twice so far in 2021, daily levels recorded by Scripps Oceanography have exceeded 420 parts per million. These levels represent the highest concentrations in the atmosphere ever experienced by humans. 

“The ultimate control knob on atmospheric CO2 is fossil-fuel emissions,” said Scripps geochemist Ralph Keeling, who took over the measurement series named the Keeling Curve after his father’s death in 2005, “but we still have a long way to go to halt the rise, as each year more CO2 piles up in the atmosphere. We ultimately need cuts that are much larger and sustained longer than the COVID-related shutdowns of 2020.”

Pieter Tans, a senior scientist with NOAA’s Global Monitoring Laboratory, noted that CO2 is by far the most abundant human-caused greenhouse gas, and persists in the atmosphere and oceans for thousands of years after it is emitted.

“We are adding roughly 40 billion metric tons of CO2 pollution to the atmosphere per year,” said Tans. “That is a mountain of carbon that we dig up out of the earth, burn, and release into the atmosphere as CO2 — year after year. If we want to avoid catastrophic climate change, the highest priority must be to reduce COpollution to zero at the earliest possible date.”

CO2 pollution is generated by emissions from carbon-based fossil fuels used for transportation and electrical generation, by cement manufacturing, deforestation, agriculture, and many other practices. Along with other greenhouse gases, CO2 traps outgoing heat from the planet’s surface that would otherwise escape into space, causing the planet’s atmosphere to warm steadily.

The highest monthly mean CO2 value of the year typically occurs in May, just before plants in the northern hemisphere start to remove large amounts of CO2 from the atmosphere during the growing season. In the northern fall, winter, and early spring, plants and soils give off CO2, causing levels to rise through May. 

In February, the United States officially rejoined the Paris Agreement on climate change, an international treaty signed by 196 countries that have committed to limiting global warming and avoiding its potentially destabilizing impacts.

Chris Jennewein is Editor & Publisher of Times of San Diego.