Symbolic CO2 threshold crossed

Concentrations of CO₂ are now 50% higher than at the start of the industrial revolution.

The latest measurements released show that the atmospheric CO₂ concentrations at Mauna Loa Observatory, Hawaii, are now at record levels. The average for March 2021 was 417.14 parts per million (ppm).
“This is 50 per cent higher than the average for 1750–1800 which can be found with high confidence from ice core records,” write Richard Betts, Met Office Hadley Centre and University of Exeter Global Systems Institute and Ralph Keeling, Scripps Institution of Oceanography, UC San Diego.

“The human-caused 50 per cent increase in CO₂ has been a major driver of the observed global temperature rise of over 1°C. It is also symbolic because a 50 per cent increase is halfway towards ‘doubled CO₂’ which has long been an important benchmark for quantifying future global warming. For example, the long-term warming in response to a doubling of CO₂ concentrations relative to pre-industrial is defined with the standard metric of Equilibrium Climate Sensitivity (ECS), with recent research suggesting that ECS is likely to be between 2.6°C and 4.1°C, ” Betts and Kealing say.

“The build-up of CO₂ in the atmosphere has also been accelerating. It took over 200 years for levels to increase by 25 per cent, but now just over 30 years later, levels are at a 50 per cent increase. If the current trend continues, doubled CO₂ will be reached in approximately 55 years. Reversing this trend and slowing the atmospheric CO₂ rise and global warming will need global emissions to reduce. Projections by the IPCC suggest that to halt global warming at 1.5°C, global emissions will need to reach net zero by approximately 2050, possibly much sooner,” according to Betts and Kealing.

“Ongoing monitoring of the global carbon budget confirms that the atmospheric CO₂ rise is entirely caused by human activity, mainly through the burning of fossil fuels with further contributions from deforestation. Since natural carbon sinks remove CO₂ from the atmosphere much more slowly than the rate of these emissions, CO₂ levels are continuously building up. Reducing emissions slows the rate of build-up but does not stop it altogether unless the overall input of CO₂ to the atmosphere reaches zero,” conclude the two scientists.

Compiled by Reinhold Pape

Global Monitoring Laboratory, Recent Daily Average Mauna Loa CO₂, https://www.esrl.noaa.gov/gmd/ccgg/trends/monthly.html
Global Monitoring Laboratory, Monthly Average Mauna Loa CO₂, https://www.esrl.noaa.gov/gmd/ccgg/trends/
Met Office, 7 April 2021, Atmospheric carbon dioxide at record high levels despite reduced emissions in 2020,  https://www.metoffice.gov.uk/research/news/2021/record-co2-levels-despit...
Carbon Brief, 16 March 2021, Met Office: Atmospheric CO₂ now hitting 50% higher than pre-industrial levels, https://www.carbonbrief.org/met-office-atmospheric-co2-now-hitting-50-hi...
Climate Energy College, More Climate Spirals, https://www.climatecollege.unimelb.edu.au/more-climate-spirals

Figure: Atmospheric CO₂ levels from 1700 to 2021. This is based on ice core data before 1958, then the instrumental record at Mauna Loa from the Scripps CO₂ program and, finally, the 2021 CO₂ forecast from the Met Office. The apparent change in 1958 is because ice core records do not capture the seasonal cycle seen in instrumental records.