First published: 7 October 2016

Discussion

The atmospheric concentration of CO2 has been steadily climbing since mankind began its long-term climate experiment of liberating long-buried fossil carbon. Before we began, concentration was below 300 ppm, but since then we have sent more than 2 trillion tonnes of CO2 into the atmosphere. While some of that has been absorbed by land and ocean 'sinks', much of it remains in the atmosphere, and will stay there for many hundreds of years.

Almost all the variation from week to week is natural, probably a result of shifting wind patterns bringing different air parcels to the sampling site, such that it is highly unlikely that we can discern anthropogenic effects from week to week. The steady increase from year to year, however, is clearly driven by our global emissions.

In an article published in June 2016, Richard Betts and colleagues at the UK's Met Office Hadley Centre forecast that the monthly average concentration of CO2 at Mauna Loa would remain above 400ppm all year "and hence for our lifetimes". The figure above bears out the first part of that statement, and moreover shows that also weekly averages stayed above 400ppm throughout 2016.

In January 2020 the Met Office predicted that the annual mean CO2 concentration at Mauna Loa would be 414.2±0.6ppm. The figure below shows how their forecast is faring against monthly observations (see previous years' forecasts lower down on this page; Note that the figure presents weekly concentrations from NOAA but monthly concentrations from Scripps, and as these datasets are entirely independent they do not always line up).

The seasonal cycle is dominated by northern hemisphere forests, following the pattern of plants' photosynthesis, which stores CO2, and microbial decay, which releases it again (more info). There is about twice as much land area in the northern hemisphere as in the southern hemisphere. The figure to the right shows the global average concentration since 1959 (click to enlarge).

Here's another version of the Mauna Loa plot, with apologies to Hokusai (click to enlarge).

Data Sources

NOAA releases weekly average concentrations of CO2 measured at Mauna Loa, Hawaii here.

The forecast by the Met Office Hadley Centre is based on independent measurements of CO2 concentration at Mauna Loa made by the Scripps Institution of Oceanography, available here.

This figure was inspired by similar ones produced by Climate Central here and here.

Previous years' forecasts by the UK Met Office

In February 2017 the UK Met Office made a forecast for 2017, and the following figure indicates how well that forecast performed against measurements (click to enlarge). The success of this forecast was published in 2018.

In early 2018, Betts and colleagues repeated the forecast for 2018, and the figure below shows how they fared against observations reported by Scripps. Monthly forecast concentrations have an uncertainty of ±0.6 ppm, while the monthly observations have an uncertainty of about ±0.3 ppm.

In January 2019 the Met Office predicted that the annual mean CO2 concentration at Mauna Loa would be 411.3±0.6ppm.