## 2 Scientific Objectives and Requirements
### Scientific Objectives
Figure 5 illustrates the flow of energy through Earth's atmosphere. Table 1 provides ERB main contributions and EEI during the 2000-2010 period. Figure 5 shows how solar radiation warms our planet, and how this energy becomes temporarily trapped as it fluxes away from Earth's surface as longwave infrared radiation. This energy trap produces the greenhouse effect, which represents the main driver of global warming [13]. EEI is the difference between the total solar irradiance (TSI) and the OSR combined with the OLR thermally emitted. At the TOA, the EEI was +0.58 \(\pm\) 0.15 Wm\({}^{-2}\) for the period 2005-2010 according to [PERSON] et al. (2011) [14]. It was estimated at +0.60 \(\pm\) 0.40 Wm\({}^{-2}\) between 2000 and 2010 by [PERSON] et al. (2012) [12]. A recently published study by [PERSON] et al. (2020) [15] shows that the EEI is currently estimated to be +0.87 \(\pm\) 0.12 Wm\({}^{-2}\) over the period 2010-2018 compared to +0.47 \(\pm\) 0.10 Wm\({}^{-2}\) over the period 1971-2018. This positive EEI describes the excess of heat in the Earth system, mainly due to human activity. Figure 6 shows the evolution of the OLR and the EEI (12-month rolling averages) over the last four decades. Changes in sea surface temperature (SST) during different phases of the El Nino Southern Oscillation (ENSO) and in cloud properties are often reflected in changes in OLR. The EEI strong variations correspond to volcanic eruptions that deposit aerosols in the stratosphere, thus cooling Earth by reflecting sunlight back to space. In 1982, El Chichon ejected million metric tons of sulfur dioxide into the stratosphere. In 1991, Mount Pinatubo represents the largest stratospheric disturbance since Krakatoa eruption in 1883, dropping global temperatures and increasing ozone depletion. Then, the global energy imbalance of the Earth is not constant over time. It depends on natural variability and the complexity of different climate forcings. The time series of measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument is compared to the ERA 5 model, which represents the fifth generation of atmospheric reanalyses of global climate from the European Centre for Medium-Range Weather Forecasts (ECMWF). These results highlight the impact of anthropogenic radiative forcing during a period where the TSI has decreased since 1980 and where solar cycle 24 (24 th cycle since 1755) is much weaker than the previous two cycles. The various studies [12, 14, 15] conclusively show the dominant role of the human-created greenhouse effect in driving global climate change. The EEI represents the most critical measure of the state of the Earth's climate. It sets expectations for future climate change (taking into account the Earth's inertia). \begin{table}
\begin{tabular}{c l l} \hline
**Contributor/Imbalance** & **Acronym** & **Flux** \\ \hline Incoming solar radiation & TSI/4 & 340.2 \(\pm\) 0.1 Wm\({}^{-2}\) \\ Outgoing shortwave radiation & OSR & 100.0 \(\pm\) 2.0 Wm\({}^{-2}\) \\ Outgoing longwave radiation & OLR & 239.7 \(\pm\) 3.3 Wm\({}^{-2}\) \\ \hline TOA Earth energy imbalance & EEI at TOA & +0.6 \(\pm\) 0.4 Wm\({}^{-2}\) \\ Surface Earth energy imbalance & EEI at surface & +0.6 \(\pm\) 17.0 Wm\({}^{-2}\) \\ \hline \end{tabular}
\end{table} 
Table 1: ERB main contributions and EEI during the period 2000–2010 from [PERSON] et al. (2012) [12]. Figure 5: Radiation budget diagram of the Earth’s atmosphere from [PERSON] et al. (2012) [12]. ERB for the period 2000–2010, where all flows are expressed in Wm\({}^{-2}\). The EEI represents the best approach to characterize global warming compared to other commonly used parameters (CO\({}_{2}\) concentration and Earth surface temperature).