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

Changes in climate and weather extremes are among the most visible effects of human-induced climate change. Within AR6 WGI, a full chapter was dedicated to the assessment of past and projected changes in extremes on continents (Seneviratne et al., 2021), and the chapter on ocean, cryosphere and sea level changes also provided assessments on changes in marine heatwaves (Fox-Kemper et al., 2021). Global indicators related to climate extremes include averaged changes in climate extremes, for example, the mean increase in annual minimum and maximum temperatures on land (AR6 WGI Chap. 11, Fig. 11.2, Seneviratne et al., 2021) or the area affected by certain types of extremes (AR6 WGI Chap. 11, Box 11.1, Fig. 1, Seneviratne et al., 2021; Sippel et al., 2015). In contrast to global surface temperature, extreme indicators are less established.

The climate indicator of changes in temperature extremes consists of land average annual maximum temperatures (TXx) (excluding Antarctica). As part of this update, we provide an upgraded version of Fig. 6 from Forster et al. (2023), which in turn is based on Fig. 11.2 from Seneviratne et al. (2021) (Fig. 9). As last year, three datasets are analysed: HadEX3 (Dunn et al., 2020), Berkeley Earth Surface Temperature (building off Rohde et al., 2013) and the fifth-generation ECMWF atmospheric reanalysis of the global climate (ERA5; Hersbach et al., 2020). HadEX3 is currently static and is not being updated. Berkeley Earth has been updated, resulting in TXx differences for most years (less than 0.1 °C), and now includes data for 2022. Of the three datasets, only ERA5 covers the whole of 2023 at the present time. TXx is calculated by averaging the annual maximum temperature over all available land grid points (excluding Antarctica) and then converted to anomalies with respect to a base period of 1961–1990. To express the TXx as anomalies with respect to 1850–1900, we add an offset of 0.52 °C to all three datasets. See Sect. S9 for details on the data selection, averaging and offset computation. We note that Berkeley Earth has slightly smaller trends than ERA5 and that this might warrant further investigation.

Our climate has warmed rapidly in the last few decades (Sect. 6), which also manifests in changes in the occurrencepage2646 and intensity of climate and weather extremes. From about 1980 onwards, all employed datasets point to a strong TXx increase, which coincides with the transition from global dimming, associated with aerosol increases, to brightening, associated with aerosol decreases (Wild et al., 2005, Sect. 3). The ERA5-based TXx warming estimate w.r.t. 1850–1900 for 2023 is at 2.3 °C, which is an increase of more than 0.5 °C compared to 2022 and which shatters the previous record by more than 0.3 °C. On longer timescales, land average annual maximum temperatures have warmed by more than 0.6 °C in the past 10 years (1.81 °C with respect to pre-industrial conditions) compared to the first decade of the millennium (1.21 °C; Table 9). Since the offset relative to our pre-industrial baseline period is calculated over the 1961–1990, temperature anomalies align by construction over this period but can diverge afterwards. In an extensive comparison of climate extreme indices across several reanalyses and observational products, Dunn et al. (2022) point to an overall strong correspondence between temperature extreme indices across reanalysis and observational products, with ERA5 exhibiting especially high correlations to HadEX3 among all regularly updated datasets.