Abstract:
The Earth’s energy imbalance is the net radiative flux at the top-of-atmosphere. Climate model simulations suggest that the observed positive imbalance trend in the previous two decades is inconsistent with internal variability alone and caused by anthropogenic forcing and the resulting climate system response. Here, we investigate anthropogenic contributions to the imbalance trend using climate models forced with observed sea-surface temperatures. We find that the effective radiative forcing due to anthropogenic aerosol emission reductions has led to a 0.2 ± 0.1 W m−2 decade−1 strengthening of the 2001–2019 imbalance trend. The multi-model ensemble reproduces the observed imbalance trend of 0.47 ± 0.17 W m−2 decade−1 but with 10-40% underestimation. With most future scenarios showing further rapid reductions of aerosol emissions due to air quality legislation, such emission reductions may continue to strengthen Earth’s energy imbalance, on top of the greenhouse gas contribution. Consequently, we may expect an accelerated surface temperature warming in this decade.

The Earth’s Energy Imbalance (EEI) is the difference in the net downward shortwave (SW) radiative flux and outgoing longwave (LW) radiative flux at the top-of-atmosphere (TOA). It causes changes in the heat content of the oceans, ice, land, and atmosphere, and is principally estimated by looking at in situ observations of the heat content of the oceans, which absorb approximately 90% of the excess heat due to EEI1. Recent estimates for the 2010–2022 period give a value of 0.89 ± 0.26 W m−2 (ref. 2), increasing from the value of 0.79 ± 0.27 W m−2 for 2006–2018 reported in the Intergovernmental Panel on Climate Change (IPCC) Assessment Report 6 (AR6)3.

The mean EEI from the Clouds and the Earth’s Radiant Energy System (CERES) is constrained by adjusting SW and LW TOA fluxes within their ranges of uncertainty such that the mean EEI from CERES for 07/2005-06/2015 is consistent with EEI from in situ observations for the same period4,5. Variations in EEI from CERES are independent of in situ data and have been demonstrated to be especially useful for examining trends in EEI6. Two decades of satellite observations are now available from CERES, and show a positive 2005–2019 trend in the EEI of 0.50 ± 0.47 W m−2 decade−1 (ref. 6). The CERES trend is in agreement with ocean-derived trends1,2. Single climate model simulations indicate that the recent trend is only explained when anthropogenic forcing and response are included7.

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