|Week beginning on September 12, 2021:||413.09 ppm|
|Weekly value from 1 year ago:||411.65 ppm|
|Weekly value from 10 years ago:||389.38 ppm|
The graph, updated weekly, shows as individual points daily mean CO2 up to and including the week (Sunday through Saturday) previous to today. The daily means are based on hours during which CO2 was likely representative of “background” conditions, defined as times when the measurement is representative of air at mid-altitudes over the Pacific Ocean. That air has had several days time or more to mix, smoothing out most of the CO2 variability encountered elsewhere, making the measurements representative of CO2 over hundreds of km or more.
The selection process is designed to filter out any influence of nearby emissions, or removals, of CO2 such as caused by the vegetation on the island of Hawaii, and likewise emissions from the volcanic crater of Mauna Loa. We require low variability within each hour and between successive hourly averages, as well as a degree of persistence of the likely valid "background" hours between successive days. Thus, some of the hours that are initially designated as background, may still be flagged as unlikely to represent background conditions as information about the following days becomes available.
For details, see ”How we measure background CO2 levels at Mauna Loa”.
The weekly mean (red bar) is simply the average of all days in the week for which a background value could be defined. The average standard deviation of day to day variability, calculated as the difference from the appropriate weekly mean, equals 0.38 ppm for the entire record. As a visual aid, the blue lines present monthly means of background data as they are presented under Recent Monthly CO2 at Mauna Loa.
These data are still preliminary, pending recalibrations of reference gases and other quality control checks.
This figure shows the atmospheric increase of CO2 over 280 ppm in weekly averages of CO2 observed at Mauna Loa. The value of 280 ppm is chosen as representative of pre-industrial air because it is close to the average of CO2 measured and dated with high time resolution between the years 1000 and 1800 in an ice core from Law Dome, Antarctica. [Etheridge et al., 1996]. Although the time resolution of old air locked in ice cores is not enough to preserve seasonal cycles, there is no doubt that the seasonal cycle, which is mostly caused by photosynthesis and respiration of ecosystems on land, was similar to what we observe today. Therefore, for the comparison with pre-industrial times the Mauna Loa weekly data have been first deseasonalized by subtracting the observed average seasonal cycle, and then subtracting 280 ppm. The enhancement of the CO2 mole fraction in the atmosphere over pre-industrial is expressed both as ppm and as a percentage change since the year 1800.
Data are reported as a dry air mole fraction defined as the number of molecules of carbon dioxide divided by the number of all molecules in air, including CO2 itself, after water vapor has been removed. The mole fraction is expressed as parts per million (ppm). Example: 0.000400 is expressed as 400 ppm.