This dataset is sourced from the US Government’s Earth System Research Laboratory (ESRL), Global Monitoring Division (GMD). The study under discussion comprises of the Global Annual Mean Growth Rate series from 1959 through 2017.
The annual mean rate of growth of CO2 in a given year is the difference in concentration between the end of December and the start of January of that year. It represents the sum of all CO2 added to, and removed from, the atmosphere during the year by human activities and by natural processes. The annual mean growth during the previous year is determined by taking the average of the most recent December and January months, corrected for the average seasonal cycle, as the trend value for January 1, and then subtracting the same December-January average measured one year earlier. Our first estimate for the annual growth rate of the previous year is produced in February of the following year, using data through November of the previous year. That estimate will then be updated in March using data from December, and again in April using data through January. The estimation for the growth rate of the previous year are finalized in the fall of the following year because a few of the air samples on which the global estimate is based are received late in the following year.
The values in this dataset are subject to change depending on quality control checks of the measured data, but any revisions are expected to be small. The estimates of the global mean CO2 concentration, and thus the annual growth rate, are updated every month as new data come in. The statistics are as follows. If we estimate during a given month (“m”) the global average CO2 during the previous month (“m-1”), the result differs from the estimate made (up to almost a year later) when all the data are in, with a standard deviation of 0.57 ppm. For month m-2, the standard deviation is 0.17 ppm, and for month m-3 it is 0.10 ppm. We decided to provide the global mean estimates with a lag of two months. Thus, a December average is first calculated during the following February.
The estimated uncertainty in the global annual mean growth rate varies by year, and has been estimated by a bootstrap technique for 1980 and later. One hundred different realizations of a global network were constructed by randomly picking sites, with restitution, from our existing marine boundary layer sites in the National Oceanic and Atmospheric Administration (NOAA)/ESRL cooperative air sampling network (Conway, 1994). Each member of the ensemble of networks has the same number of sites as the real network, but some sites are missing, while others are represented more than once. An additional condition is that at least one southern high latitude site is present, one tropical and one northern high latitude site, because the researchers have always maintained broad latitude coverage in the real network. Temporal data gaps at individual sites are present in the bootstrap networks. The reported uncertainties are the 1-sigma standard deviations for each year’s growth rate of the ensemble members. Pre-1980 the annual growth rate and uncertainty have been calculated from the average of the Mauna Loa and South Pole records (before 1974 as measured by the Scripps Institution of Oceanography), as detailed in Ballantyne et al. (2012).
