## 2 Importance of ERB at TOA and EEI Measurements for the Next Decades  
The GCOS currently identifies 55 ECVs critical for understanding and characterizing Earth's climate, with approximately 60% relying heavily on satellite Earth observation data. Collectively, all these variables offer a comprehensive picture of the evolving global climate system and serve as fundamental indicators for informing climate mitigation and adaptation strategies, assessing climate risks, attributing climatic events to their underlying causes, and supporting climate services. Among these variables, ERB is a key component, as it directly reflects the planet's energy balance and climate dynamics. Variations in ERB indicate changes in energy accumulation within the climate system, primarily in the oceans, making it a fundamental metric for assessing global warming. Accurate ERB measurements are crucial for understanding climate processes, validating climate models, and guiding mitigation strategies. Observations of ERB at TOA are only achievable via satellite platforms. Currently, datasets from the CERES Energy Balanced and Filled (EBAF) Edition 4.2.1 [15] are available, providing monthly mean TOA shortwave, longwave, and net radiative fluxes on a 1\({}^{\circ}\)\(\times\) 1\({}^{\circ}\) spatial grid. These data offer enhanced spatial and temporal detail for analyzing the Earth's energy budget. For the period 2000-2010, [PERSON] et al. (2012) [2], using satellite data primarily from the CERES mission, estimated the incoming solar radiation at 340.2 \(\pm\) 0.1 Wm\({}^{-2}\), the reflected solar radiation at 100 \(\pm\) 2 Wm\({}^{-2}\), and the outgoing longwave radiation at 239.7 \(\pm\) 3.3 Wm\({}^{-2}\). While determining absolute values of Earth's radiative fluxes is important--as it allows for direct comparisons--it remains a challenging task due to calibration uncertainties and instrumental biases. Thus, one can account for inter-satellite biases and place greater emphasis on analyzing the variability and anomalies in these fluxes, which are particularly useful for understanding changes in the energy balance and the global climate system. Prior to the industrial era, approximately before 1850, Earth's energy balance is assessed to have been close to equilibrium, with net radiative forcing near zero. This conclusion is supported by multiple lines of evidence, including: the synthesis of pre-industrial energy balance estimates presented in Chapter 7 [16] of Working Group I of the Intergovermmental Panel on Climate Change (IPCC)'s Sixth Assessment Report (AR6); observational and model-based updates to Earth's energy budget reported by [PERSON] et al. (2012) [2]; the conceptual framework of near-equilibrium conditions prior to anthropogenic forcing articulated by [PERSON] et al. (2005) [17]; historical analyses of surface and TOA energy fluxes reviewed by [PERSON] et al. (2013) [18]; and energy budget constraints on climate response from the pre-industrial state discussed by [PERSON] et al. (2013) [19]. With the onset of industrialization in the mid- to late 19 th century, atmospheric concentrations of GHGs such as carbon dioxide (\(CO_{2}\)) and methane (\(CH_{4}\)) began to rise. However, the increase in EEI during this period remained small or nearly neutral, largely due to compensating effects from natural variability--such as volcanic eruptions--and the growing influence of anthropogenic aerosols, which exerted a cooling effect by reflecting sunlight at specific wavelengths of the solar spectrum [20,21]. Throughout the 20 th century, particularly after 1950, EEI began to increase more noticeably as GHGs emissions intensified. Nevertheless, this trend was partially masked by elevated levels of aerosol pollution, especially in the Northern Hemisphere, which continued to offset some of the warming by enhancing Earth's reflectivity. Since the early 1980s--during the satellite era--space-based observations have shown that EEI has become persistently positive, indicating that Earth is accumulating energy.