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

The Earth energy imbalance (EEI), assessed in Chap. 7 of AR6 WGI (Forster et al., 2021), provides a measure of accumulated surplus energy (heating) in the climate system and is hence an essential indicator to monitor the current and future status of global warming. It represents the difference between the radiative forcing acting to warm the climate and Earth's radiative response, which acts to oppose this warming. On annual and longer timescales, the global Earth heat inventory changes associated with EEI are dominated by the changes in global ocean heat content (OHC), which accounts for about 90% of global heating since the 1970s (Forster et al., 2021). This planetary heating results in changes in all components of the Earth system, such as sea level rise, ocean warming, ice loss, rise in temperature and water vapour in the atmosphere, and in changes in ocean and atmospheric circulation, ice loss and permafrost thawing (e.g. Cheng et al., 2022; von Schuckmann et al., 2023a), with adverse impacts for ecosystems and human systems (Douville et al., 2021; IPCC, 2022).

On decadal timescales, changes in global surface temperatures (Sect. 5) can become decoupled from EEI by ocean heat rearrangement processes (e.g. Palmer and McNeall, 2014; Allison et al., 2020). Therefore, the increase in the Earth heat inventory provides a robust indicator of the rate of global change on interannual-to-decadal timescales (Cheng et al., 2019; Forster et al., 2021; von Schuckmann et al., 2023a). AR6 WGI found increased confidence in the assessment of change in the Earth heat inventory compared to previous IPCC reports due to observational advances and closure of the energy and global sea level budgets (Forster et al., 2021; Fox-Kemper et al., 2021).

AR6 estimated that EEI increased from 0.50 [0.32–0.69] W m-2 during the period 1971–2006 to 0.79 [0.52–1.06] W m-2 during the period 2006–2018 (Forster et al., 2021). The contributions to increases in the Earth heat inventory throughout 1971–2018 remained stable: 91% for the full-depth ocean, 5% for the land, 3% for the cryosphere and about 1% for the atmosphere (Forster et al., 2021). Two recent studies demonstrated independently and consistently that since 1960, the warming of the world ocean has accelerated at a relatively consistent pace of 0.15±0.05 W m-2 per decade (Minière et al., 2023; Storto and Yang, 2024), while the land, cryosphere and atmosphere have been warming at a pace of 0.013±0.003 W m-2 per decade (Minière et al., 2023). The increase in EEI over the most recent quarter of a decade (Fig. 4) has also been reported by Cheng et al. (2019), von Schuckmann et al. (2020, 2023a), Loeb et al. (2021), Hakuba et al. (2021), Kramer et al. (2021), Raghuraman et al. (2021) and Minère et al. (2023). Drivers for the observed increase over the most recent period (i.e. past 2 decades) are discussed to be linked to rising concentrations of well-mixed greenhouse gases and recent reductions in aerosol emissions (Raghuraman et al., 2021; Kramer et al., 2021; Hansen et al., 2023) and to an increase in absorbed solar radiation associated with decreased reflection by clouds and sea ice and a decrease in outgoing longwave radiation (OLR) due to increases in trace gases and water vapour (Loeb et al., 2021). The degree of contribution from the different drivers is uncertain and still under active investigation.

We carry out an update to the AR6 estimate of changes in the Earth heat inventory based on updated observational time series for the period 1971–2020 (Table 4 and Fig. 4). Time series of heating associated with loss of ice and warming of the atmosphere and continental land surface are obtained from the recent Global Climate Observing System (GCOS) initiative (von Schuckmann et al., 2023b; Cuesta-Valero et al., 2023; Vanderkelen and Thiery, 2022; Nitzbon et al., 2022; Kirchengast et al., 2022). We use the original AR6 time series ensemble OHC time series for the period 1971–2018 and then an updated five-member ensemble for the period 2019–2023. We “splice” the two sets of time series by adding an offset as needed to ensure that the 2018 values are identical. The AR6 heating rates and uncertainties for the ocean below 2000 m are assumed to be constant throughout the period. The time evolution of the Earth heat inventory is determined as a simple summation of time series of atmospheric heating; continental land heating; heating of the cryosphere; and heating of the ocean over three depth layers – 0–700, 700–2000 and greater than 2000 m (Fig. 4a). While von Schuckmann et al. (2023a) have also quantified heating of permafrost and inland lakes and reservoirs, these additional terms are very small and are omitted here for consistency with AR6 (Forster et al., 2021).

In our updated analysis, we find successive increases in EEI for each 20-year period since 1974, with an estimated value of 0.42 [0.02 to 0.81] W m-2 during 1974–1993 that more than doubled to 0.87 [0.65 to 1.08] W m-2 during 2004–2023 (Fig. 4b). In addition, there is some evidence that the warming signal is propagating into the deeper ocean over time, as seen by a robust increase in deep (700–2000 m) ocean warming since the 1990s (von Schuckmann et al., 2020, 2023a; Cheng et al., 2019, 2022). The model simulations qualitatively agree with the observational evidence (e.g. Gleckler et al., 2016; Cheng et al., 2019), further suggesting that more than half of the OHC increase since the late 1800s occurs after the 1990s.

The update of the AR6 assessment periods to end in 2023 results in systematic increases in EEI: 0.65 W m-2 during 1976–2023 compared to 0.57 W m-2 during 1971–2018 and 0.96 W m-2 during 2011–2023 compared to 0.79 W m-2 during 2006–2018 (Table 4). The trend and interannual variability of EEI can largely be explained by a combination of surface temperature changes and radiative forcing (Hodnebrog et al., 2024), although there was a jump in 2023, which is still being investigated (Hansen et al., 2023).