The global concentration of greenhouse gases continues to increase

The rise in the global mean temperature is a central topic in the debates on climate change. There is a particular debate as to whether the 1.5 degree target agreed in the Paris Climate Agreement can still be achieved or whether humanity must prepare itself for much higher temperatures with all the associated catastrophic consequences. There is clear scientific evidence that the increasing concentration of greenhouse gases such as carbon dioxide and methane correlate directly with the global rise in temperature.

I would like to explain below what the current trend in the global concentration of CO2 and CH4 looks like. Only if these values fall or at least stagnate is there a certain probability that the impending climate catastrophe can still be averted.

The title of my article anticipates the outcome: the likelihood of stopping climate change is becoming ever smaller.

NOAA/GML calculation of global means

The NOAA GML Carbon Cycle Group computes global mean surface values using measurements of weekly air samples from the Cooperative Global Air Sampling Network [Conway et al., 1994; Dlugokencky et al., 1994; Novelli et al., 1992; Trolier et al., 1996]. Global values can be computed for nearly all trace gas species and stable isotopes routinely measured by GML and the University of Colorado INSTAAR.
The global estimate is based on measurements from a subset of network sites. Only sites where samples are predominantly of well-mixed marine boundary layer (MBL) air representative of a large volume of the atmosphere are considered. These “MBL” sites are typically at remote marine sea level locations with prevailing onshore winds. Measurements from sites at altitude (e.g., Mauna Loa) and from sites close to anthropogenic and natural sources and sinks (e.g., Park Falls, Wisconsin) are excluded from the global estimate. The use of MBL data results in a low-noise representation of the global trend and allows us to make the estimate directly from the data without the need for an atmospheric transport model.

Measurements

All measurements used to estimate surface global means are made by GML and CU/INSTAAR (for stable isotopes). Routine and ongoing comparison experiments within the Boulder labs help ensure that measurements are internally consistent with respect to calibration and methodology [WMO, 2009a]. All data used to construct the global estimates have been screened by the principal investigators. Only measurements determined to be free from sampling and analysis artifacts are considered for the calculation of the global estimate.

The World Meteorological Organization (WMO) World Data Center for Greenhouse Gases (WDCGG) also publishes global averages for CO2 and other gases. WDCGG uses curve fitting and data extension methods very similar to those developed by NOAA [WMO, 2009b], but in addition to marine boundary layer sites, WDCGG includes many continental locations strongly influenced by local biospheric sources and sinks and also by fossil fuel emissions. WDCGG also includes sites from multiple independent laboratories which raises the issue of possible artifacts due to scale or measurements differences. The WDCGG global average has a positive mean offset of ~0.35 ppm and a larger seasonal cycle amplitude compared to NOAA results. The MBL estimate is expected to be lower than a full global surface average because areas with high fossil fuel loading due to recent emissions are not represented. On the other hand, the full troposphere (up to ~8-15 km altitude) and especially the stratosphere with lower CO2 mole fraction are not represented in either approach. NOAA observe that CO2 is increasing at about the same rate everywhere it is measured. Because CO2 is a long lived gas in the atmosphere, emissions anywhere will, in about one year, contribute to higher CO2 everywhere. One cannot “hide” CO2 emissions from the MBL sites for more than about a month. Thus the MBL gives probably the bestlow-noise representation of the ongoing global increase of CO2.

Below: Global average atmospheric carbon dioxide mixing ratios (blue line) determined using measurements from the Carbon Cycle cooperative air sampling network. The red line represents the long-term trend. Bottom: Global average growth rate for carbon dioxide.

Top: Global average atmospheric carbon dioxide mixing ratios (blue line) determined using measurements from the Carbon Cycle cooperative air sampling network. The red line represents the long-term trend. Bottom: Global average growth rate for carbon dioxide.

Trends in Atmospheric Carbon Dioxide (CO2)

The table and graph show annual mean carbon dioxide growth rates based on globally averaged marine surface data. In the graph, decadal averages of the growth rate are also plotted, as horizontal lines for 1960 through 1969, 1970 through 1979, and so on.

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 estimate for the annual growth rate of the previous year is produced in April of the following year, using data through January. We finalize our estimate for the growth rate of the previous year 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.

Trends in Atmospheric Methane (CH4)

The graphs show globally-averaged, monthly mean atmospheric methane abundance determined from marine surface sites. The first graph shows monthly means for the last four years plus the current year, and the second graph shows the full NOAA time-series starting in 1983. Values for the last year are preliminary, pending recalibrations of standard gases and other quality control steps. Other impacts on the latest few months of data are described below.

Conclusion

The NOAA data show that the concentration of the two most important greenhouse gases, carbon dioxide and methane, continues to rise steadily. Despite all efforts, agreements and appeals, there is no flattening of the curves, let alone a reversal, but we are observing an unbroken rate of increase. The MDL data leaves no room for doubt: The global concentration of climate-relevant gases shows values that accelerate a further global rise in temperature and that this increases the probability of reaching tipping points that make a reversal to a tolerable climate impossible.

Lan, X., Tans, P. and K.W. Thoning: Trends in globally-averaged CO2 determined from NOAA Global Monitoring Laboratory measurements. Version Thursday, 05-Dec-2024 13:43:51 MST
Global Monitory Laboratorium, NOAA,

Bernd Riebe, DEC 2024