Forecasting the annual 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 2015-2016 El Niño was emerging, and correctly predicted the largest annual CO₂ increment on record at the time. 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 2023-2024 predicted a relatively large annual CO₂ rise, but the observed rise was even larger, with exceptional wildfires in the Americas a likely contributor to the additional increase. Even without the effects of El Niño and other climatic influences on carbon sinks, the human-driven rise in CO₂ in 2023-2024 would have been too fast to remain compatible with IPCC AR6 scenarios that limit global warming to 1.5°C with little or no overshoot. While the 2024-2025 rise is predicted to be smaller than 2023-2024, it will still be above these 1.5°C scenarios.