# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 2 Scientific Rationale of the UVSQ-SAT Mission
### Earth's Energy Imbalance
The main goal of the future satellite constellation is to obtain constant flow of direct measurements from space by using miniaturized instruments (volume, mass, power, telemetry) with narrow and broadband sensors to derive EEI at small spatiotemporal scales with an uncertainty at 1\(\sigma\) of \(\pm\)1 Wm\({}^{-2}\) for a 1-10 km resolution. For longer timescales, EEI direct measurements are also very challenging with a required measurement uncertainty at 1\(\sigma\) of \(\pm\)0.1 Wm\({}^{-2}\) during a decade. Today, the best estimates of the EEI long term timescales are currently derived from temporal changes in ocean heat content. The global average EEI estimated for the past decades ranges from around +0.4 to +1.0 Wm\({}^{-2}\)[7; 9; 10; 11; 12], largely consistent with the radiative forcing caused by anthropogenic greenhouse gases. [PERSON] et al. [9] inferred a planetary energy imbalance of +0.58 \(\pm\) 0.15 Wm\({}^{-2}\) (Earth is absorbing more energy from the Sun than it is radiating to space as heat) during the 6 y period 2005-2010 using ocean heat content. Recently, [PERSON] et al. [13] estimated EEI at +0.71 \(\pm\) 0.10 Wm\({}^{-2}\) for the period May 2005-May 2015 from ocean heat content changes measured by Argo's automated floats. Satellites' measurements from the former generation of Earth Radiation Budget Experiment (ERBE) sensors, along with the current generation of Clouds and the Earth's Radiant Energy System (CERES) sensors are the basis of an ERB multi-decadal record at the top of atmosphere. Currently, CERES sensors provide the most reliable and stable TOA flux measurements of the ERB components. However, uncertainties in CERES absolute calibration and in the algorithms used to determine ERB from satellite measurements are too large to enable Earth's energy imbalance to be quantified accurately [14]. The CERES data products are more useful for providing the spatial and temporal variability of EEI. Actually, there is a risk of a gap in the ERB data since all current CERES missions are close to the end of their lifetime after 2026 when only tropical missions such as the Franco-Indian Megha-Tropiques or short duration missions such as ESA-JAXA's EarthCARE (Earth Clouds, Aerosols and Radiation Explorer) carry ERB instruments. Indeed, a project intended to complete and replace the CERES instruments by the Radiation Budget Instrument (RBI) mission in 2021, in 2026, and in 2031 was canceled by NASA. Since then, the possibility of a constellation of small satellites in orbit before 2026 is being explored. It represents a major challenge and could meet most of the RBI measurement required for continuity of the climate data records. To conclude, a measure of the energy imbalance at the top of the atmosphere is crucial, but extremely difficult. It is a key step in the chain linking climate warming to the increase in greenhouse gases. This would be an additional element in the scientific basis for climate change mitigation, notably the magnitude of reduction in GHG emissions required to limit global warming (e.g., 2 \({}^{\circ}\)C). Ideally, accurate long term direct measurements of EEI would confirm the extent of climate warming. Short term measurements of EEI at high spatiotemporal resolution would allow constraining better poorly known radiative forcings associated with aerosols, aerosol-cloud interactions, surface albedo, UV solar irradiance, etc. The accurate measurements of solar and terrestrial radiative fluxes at TOA over a wide range of surfaces and conditions (e.g., clear-sky, with/without specific clouds or aerosols) would enable a better evaluation of the overall radiative effects of clouds and aerosols and their representations in climate models. Indeed, aerosol and cloud feedbacks arguably remain the dominant source of uncertainties in climate modeling and of its more societally relevant aspects (e.g., changes in precipitation, etc. ), as explained in [15; 16]. The incoming and outgoing shortwave flux measurements required to calculate EEI also can be used to derive the albedo.