Indicators of Global Climate Change 2023: annual update of key indicators of the state of the climate system and human influence

The second year of the Global Climate Change (IGCC) initiative has built on last year's effort and the AR6 report cycle to provide a comprehensive update of the climate change indicators required to estimate the human-induced warming and the remaining carbon budget. Table 10 and Fig. 11 present a summary of the headline indicators from each section compared to those given in the AR6 assessment and also summarise methodological updates. The main substantive dataset change since AR6 is that land-use CO2 emissions have been revised down by around 2 Gt CO2 (Table 10). However, as CO2 ERF and human-induced warming estimates depend on concentrations, not emissions, this does not affect most of the other findings. Note it does slightly increase the remaining carbon budget, but this is only by 5 Gt CO2, less than the 50 Gt CO2 rounding precision.

Last year witnessed a large increase in GMST (Sect. 6), approaching 1.5 °C above 1850–1900 levels, which has widely been reported in the press. The 2022–2023 increase was the third-largest annual increase in the instrumental record after 1876–1877 and 1976–1977, two other periods with a strong transition from La Niña to El Niño conditions. The reasons for the change, especially regarding the potential role of external forcings such as shipping emission reductions compared to internal variability, are currently being investigated (e.g. Schmidt, 2024; Gettelman et al. 2024). Our work looks at long-term changes and does not directly investigate the reasons for the jump in GMST levels, yet we note that our best estimate of human-induced warming in 2023 is 1.31 (1.1 to 1.7) °C (Table 6), below the observed GMST estimate of 1.43 [1.32 to 1.53] °C in 2023 (Sect. 6). This indicates a potentially large role for El Niño and other wind-driven ocean changes.

Methane and biomass emissions had a strong component of change related to climate feedbacks (Sects. 2 and 3). Such changes will become increasingly important over this century, even if the direct human influence declines. These changes need to be properly accounted for to explain atmospheric concentration and energy budget changes. The approach to methane taken in this paper (where changes to natural sources are excluded) is inconsistent with that taken for aerosol emissions (where wildfire changes are included). In future years and in the next IPCC report, a consistent approach to attribution of atmospheric emissions, concentration change and radiative forcing should be developed. Similarly, we follow the underlying literature in treating wildfire-related CO2 emissions and removals as natural only (Friedlingstein et al., 2023), even though their intensity and frequency are shifting under anthropogenic climate change.

It is hoped that this update can support the science community in its collection and provision of reliable and timely global climate data. In future years we are particularly interested in improving SLCF updating methods to get a morepage2650 accurate estimate of short-term ERF changes. The work also highlights the importance of high-quality metadata to document changes in methodological approaches over time. In future years we hope to improve the robustness of the indicators presented here but also extend the breadth of indicators reported through coordinated research activities. For example, we could begin to make use of new satellite and ground-based data for better greenhouse monitoring (e.g. via the WMO Global Greenhouse Gas Watch initiative). Parallel efforts could explore how we might update indicators of regional climate extremes and their attribution, which are particularly relevant for supporting actions on adaptation and loss and damage.

Generally, scientists and scientific organisations have an important role as “watchdogs” to critically inform evidence-based decision-making. This annual update traced to IPCC methods can provide a reliable, timely source of trustworthy information. As well as helping inform decisions, we can use the update to track changes in datasets between their use in one IPCC report and the next. We can also provide information and testing to motivate updates in methods that future IPCC reports might choose to employ.

This is a critical decade: human-induced global warming rates are at their highest historical level, and 1.5 °C global warming might be expected to be reached or exceeded within the next 10 years in the absence of cooling from major volcanic eruptions (Lee et al., 2021). Yet this is also the decade that global greenhouse gas emissions could be expected to peak and begin to substantially decline. The indicators of global climate change presented here show that the Earth's energy imbalance has increased to around 0.9 W m-2, averaged over the last 12 years. This also has implications for the committed response of slow components in the climate system (glaciers, deep ocean and ice sheets) and committed long-term sea level rise, but this is not part of the update here. However, rapid and stringent GHG emission decreases such as those committed to at COP28 could halve warming rates over the next 20 years (McKenna et al., 2021). Table 1 shows that global GHG emissions are at a long-term high, yet there are signs that their rate of increase has slowed. Depending on the societal choices made in this critical decade, a continued series of these annual updates could track an improving trend for some of the indicators discussed herein.