# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 2 Scientific Rationale of the UVSQ-SAT Mission  
### Earth's Energy Imbalance  
Currently, Earth's surface temperatures have been rising by about 0.2 K per decade since 1981 [4] (considering deseasonalized monthly surface temperature anomalies from HadCRUTv4.5). Thus, climate change and global warming pose a severe threat to humanity. Climate processes are controlled by energy exchanges within and among the different components of the Earth system. Monitoring the Earth's influx and outflux of both longwave and shortwave radiation from all sources is essential to advance our understanding of climate variability and change and for developing more accurate and reliable climate models and forecasting. Human activities have led to rising levels of heat trapping Greenhouse Gases (GHG) in the atmosphere with less terrestrial radiation being able to escape. This unequivocal anthropogenic radiative forcing of the climate system creates an imbalance in the Earth's energy budget, which causes surface and lower atmospheric warming in order to reestablish a balance in the energy budget [5]. For this reason, EEI represents a measure of the excess of energy that is being stored in the climate system as a response to anthropogenic forcing. As such, it has been identified as a fundamental diagnostic for analyzing climate variability and anticipating future climate changes. Direct measurements of variations in the energy entering and leaving the Earth system are of primary importance for determining the rate of climate change at regional and global scales [6]. Actually, the most accurate measurement strategy to determine EEI is to monitor the temporal evolution of the ocean heat content since more than 90% of the excess energy that is gained by the Earth in response to the positive EEI accumulates in the ocean in the form of heat [7]. This can be combined with satellite radiation measurements to derive the high frequency variability in EEI. Indeed, the absolute value of EEI can be best estimated from changes in ocean heat content on long timescales, whereas the high spatiotemporal variations in EEI can be provided by satellite observations of net radiation flux variability at the Top Of Atmosphere (TOA). The information on EEI at high spatial and temporal resolution is crucial for advancing our understanding of climate change because the Earth's radiative balance is partly driven by the radiative impacts of aerosols and clouds, which are highly variable spatially and temporally and are still relatively poorly quantified (IPCC, 2014). Satellites' remote sensing provides a practical and efficient method for mapping Earth's Radiative Balance (ERB) components spatially and temporally at different scales. A large satellite constellation would allow a high frequency and sampling in measurements and consequently a more accurate determination of the Earth's global energy imbalance along with the diurnal and multi-directional sampling needed to capture spatiotemporal scales relevant to aerosol and clouds (e.g., every three hours and ideally a few km resolution). Advances in small satellite technology now enable the cost effective global solution of monitoring Earth's environment with a minimum constellation of 15 small satellites [1]. Cloud data sharing is a cost effective solution for collecting the constellation data and providing high quality science data in near real time. UVSQ-SAT is one of the first in orbit demonstration CubeSats that is intended to demonstrate the ability to build a low cost satellite with good precision measurements (relative EEI uncertainty at 1\(\sigma\) of \(\pm\)5 Wm\({}^{-2}\) during the mission). Recently, the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) 3U CubeSat [8] demonstrated technologies for high accuracy measurement of Earth's radiation budget. The new UVSQ-SAT concept is designed to explore whether it is possible to achieve the EEI required accuracies using broadband small Earth Radiative Sensors (ERS) onboard multiple satellites (constellation).