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Greenhouse gases top 400 ppm for three months in a row at Mauna Loa
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Greenhouse gases top 400 ppm for three months in a row at Mauna Loa

Record high levels of carbon dioxide sustained for longest period at key testing site

For the first time since carbon dioxide in the atmosphere has been measured, the levels of this greenhouse gas at Mauna Loa, Hawaii, have been above 400 parts per million every single day for three straight months.

“We’ve reached another benchmark, reminding us that carbon dioxide concentrations continue to increase every year as carbon dioxide emissions continue,” said Pieter Tans, who leads NOAA’s measurement program. “Humans have caused carbon dioxide concentrations to rise 120 parts per million since pre-industrial times, with half of that since the early 1980s. Half of all emissions of CO2 from fossil fuel burning have taken place since 1986.”

In 2013, carbon dioxide levels at Mauna Loa, the oldest continuous measurement station operating since the 1950s, reached 400 ppm for several days for the first time during May, but did not stay at this level for an entire month. 

Peggy's Data

Peggy's Data

NOAA's M2 moored buoy--nicknamed "Peggy"--records water temperatures and other physical and biological data from the surface to the floor of the coastal shelf, giving scientists an important glimpse into the status of the Bering Sea's summer cold pool. This graph shows Peggy's observations of water temperatures near the sea floor (depths of 60 meters, or 197 feet) from 2010 to mid-May 2021. In the early part of the 2010 decade, sub-zero winter temperatures were common. Recent winters have been much warmer. NOAA image, based on NOAA buoy data from EcoFOCI/S. Bell. Background image from NOAA/NASA Suomi NPP satellite on February 27, 2021, via Worldview.
This year, the 400 ppm mark was reached two months earlier in March and the average surpassed 400 ppm for the months of April, May and June. You can track greenhouse gas concentrations online at NOAA’s website. 

The global average has not yet reached 400 ppm. The global average for May, according to the most recent data, was 398.83 ppm. The average for June is also not expected to reach 400 ppm. 

Carbon dioxide concentrations at Mauna Loa begin to decline in June every year as seasonal plant growth drives the uptake of carbon dioxide from the atmosphere. This trend is expected to continue until the end of summer in late September as part of the natural seasonal swing.

Other measurement sites

Arctic sites all reached 400 ppm in May of 2012, about a year before Mauna Loa.  Southern hemispheric sites are expected to follow with the South Pole expected to reach 400 ppm in late 2016. 

“To reverse this trend of rising greenhouse gases, nations would need to quickly eliminate about half of fossil fuel emissions globally, and gradually continue further reductions until zero net emissions have been reached,” Tans said.

For more information, please contact Monica Allen, director of public affairs at NOAA Research at 301-734-1123 or by email at

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Oceanic and Atmospheric Research (OAR) - or "NOAA Research" - provides the research foundation for understanding the complex systems that support our planet. Working in partnership with other organizational units of the NOAA, a bureau of the Department of Commerce, NOAA Research enables better forecasts, earlier warnings for natural disasters, and a greater understanding of the Earth. Our role is to provide unbiased science to better manage the environment, nationally, and globally.


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