Forecasting the annual atmospheric CO2 rise at Mauna Loa

For the last 9 years, the Met Office has issued forecasts of the annual increment in atmospheric carbon dioxide measured at Mauna Loa, accounting for both anthropogenic emissions and the effects of El Niño Southern Oscillation (ENSO) on natural carbon sinks and sources. The first forecast was produced when the 2015/16 El Niño was emerging, and correctly predicted the largest annual increment on record before or since. In most years, the inclusion of ENSO provides a more skilful forecast than just considering emissions alone, except for 2022-2023 when La Niña conditions in late 2022 were followed by an early emergence of El Niño conditions in the second quarter of 2023. The impacts of interannual differences in emissions on the CO2 rise are usually smaller than those of ENSO variability, except in 2020 when the emergence of an unexpected large drop in global emissions due to societal responses to the COVID-19 pandemic required the forecast to be re-issued with a new estimate of the annual profile of emissions. Our forecast methodology also provides a simple means of tracking the changes in anthropogenic contributions to the annual atmospheric CO2 rise against policy-relevant scenarios. The Met Office forecast for 2024 suggested a relatively large annual CO2 rise, but not as large as 2015/16. Even with the predicted El Niño influence removed, the human-driven rise in CO2 this year would be at the upper limit of compatibility with IPCC AR6 scenarios that limit global warming to 1.5°C with little or no overshoot.