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

The IPCC Sixth Assessment Report (AR6) provided an assessment of human influence on key indicators of the state of climate grounded in data up to the year 2019 (IPCC, 2021a, Supplement Sect. S1). The next IPCC assessment report, AR7, is due towards the end of the decade. Given the speed of recent change, and the need for updated climate knowledge to inform evidence-based decision-making, the Indicators of Global Climate Change (IGCC) was initiated to provide policymakers with annual updates of the latest scientific understanding on the state of selected critical indicators of the climate system and of human influence.

This second annual update follows broadly the format of last year (Forster et al., 2023), focussing on indicators related to heating of the climate system, building from greenhouse gas emissions towards estimates of human-induced warming and the remaining carbon budget. Figure 1 presents an overview of the aspects assessed and their interlinkages from cause (emissions) through effect (changes in physical indicators) to climatic impact drivers. It also provides a visual roadmap as to the structure of remaining sections in this paper to guide the reader.

The update is based on methodologies assessed by the IPCC Sixth Assessment Report (AR6) of the physical science basis of climate change (Working Group One (WGI) report; IPCC, 2021a) as well as Chap. 2 of the WGIII report (Dhakal et al., 2022) and is aligned with the efforts initiated in AR6 to implement FAIR (Findable, Accessible, Interoperable, Reusable) principles for reproducibility and reusability (Pirani et al., 2022; Iturbide et al., 2022). IPCC reports make a much wider assessment of the science and methodologies – we do not attempt to reproduce the comprehensive nature of these IPCC assessments here. As such, we do not consider adopting fundamentally different approaches to AR6. Rather, our aim is to rigorously track both climate system change and evolving methodological improvements between IPCC report cycles, thereby achieving transparency and consistency in between successive reports.

The update is organised as follows: emissions (Sect. 2) and greenhouse gas (GHG) concentrations (Sect. 3) are used to develop updated estimates of effective radiative forcing (Sect. 4). Earth's energy imbalance (Sect. 5) and observations of global surface temperature change (Sect. 6) are key global indicators of a warming world. The contributions to global surface temperature change from human and natural influences are formally attributed in Sect. 7, which tracks the level and rate of human-induced warming. Section 8 updates the remaining carbon budget to policy-relevant temperature thresholds. Section 9 gives an example of global-scale indicators associated with climate extremes of maximum land surface temperatures. An important purpose of the exercise is to make these indicators widely available and understood. Code and data availability is given in Sect. 10, and conclusions are presented in Sect. 11. Data are available at 10.5281/zenodo.11388387 (Smith et al., 2024a).