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March 2015: 400.83 ppm March 2014: 398.10 ppm

Last updated: May 5, 2015

The graph shows recent monthly mean carbon dioxide globally averaged over marine surface sites. The Global Monitoring Division of NOAA/Earth System Research Laboratory has measured carbon dioxide and other greenhouse gases for several decades at a globally distributed network of air sampling sites [Conway, 1994]. A global average is constructed by first fitting a smoothed curve as a function of time to each site, and then the smoothed value for each site is plotted as a function of latitude for 48 equal time steps per year. A global average is calculated from the latitude plot at each time step [Masarie, 1995]. Go here for more details on how global means are calculated.

The last four complete years plus the current year are shown here. The last year of data are still preliminary, pending recalibrations of reference gases and other quality control checks.

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.

The dashed red line with diamond symbols represents the monthly mean values, centered on the middle of each month. The black line with the square symbols represents the same, after correction for the average seasonal cycle.

vía ESRL Global Monitoring Division – Global Greenhouse Gas Reference Network.

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