# UVsq-Sat NG, a New CubeSat Pathfinder for Monitoring Earth Outgoing Energy and Greenhouse Gases
## 1 Introduction  
Climate change is a critical challenge of the 21 st century, marked by the long-term alteration of global weather patterns. This transformation encompasses various phenomena, including temperature rise, extreme weather events, sea level rise, and changes in precipitation in many areas. The primary driver of these changes is the unprecedented increase in anthropogenic greenhouse gas (GHG) emissions since the mid-20 th century, primarily carbon dioxide (CO\({}_{2}\)), methane (CH\({}_{4}\)), nitrous oxide (N\({}_{2}\)O), and fluorinated gases [1; 2]. Changes in atmospheric composition, driven by anthropogenic emissions of GHGs, ozone precursors, aerosols, and their precursors, have a significant impact on climate by perturbing the Earth's Radiation Budget (ERB). Simultaneously, alterations in land use also influence climate dynamics. The ERB, representing the balance of energy between incoming and outgoing radiation at the top of the atmosphere (TOA), plays a pivotal role in this complex interplay. The increase in GHG concentrations results in a rise in temperature at the surface and in the lower atmosphere by limiting the escape of emitted terrestrial infrared (IR) radiation. This phenomenon leads to global warming, which is necessary for re-establishing the Earth's radiation balance. Monitoring GHG emissions from space is of utmost importance. It allows for the accurate tracking of current GHG levels, trends, and budgets (sources and sinks) [3], deepening our understanding of the processes that drive GHG levels. Accurately monitoring the ERB [4] and its various components, notably the IR one, from space is also crucial for understanding radiative changes and to assess the Earth's Energy Imbalance (EEI). Currently, two key parameters are closely monitored for understanding climate change: the concentration of CO\({}_{2}\) and the Earth's surface temperature. However, the strong link between increases in surface temperatures and GHG levels is complicated by other climate forcings and several feedback processes, in particular linked to the hydrological cycle, that are not yet well understood. Also, surface temperature trends can be unreliable indicators of climate change over shorter time scales due to the ocean's ability to absorb, store, and redistribute most of this excess heat vertically on longer time scales. To gain a comprehensive understanding of climate change, it is important to be able to monitor the EEI, i.e., the difference between the incoming solar radiation absorbed by the Earth and the Outgoing Longwave Radiation (OLR) emitted by the Earth (IR radiation). The EEI represents the excess of heat accumulated in the Earth's system from the increased longwave trapping by GHGs and hence is a direct driver of global warming. The global energy imbalance is not constant in time; it is influenced by natural climate forcings (e.g., volcanic, solar) and by modes of climate variability. For instance, the El Nino-Southern Oscillation (ENSO) recurring climate pattern causes fluctuations in heat storage in the ocean, leading to temporary global warming during El Nino phases (recent major events: 1982-1983, 1997-1998, and 2014-2016) when the ocean surface warms up in the central and eastern tropical Pacific Ocean, removing heat by radiating it back to space. Since May 2023, negative OLR anomaly values indicate the return of El Nino to the tropical Pacific after seven years. Observing the EEI and OLR in conjunction with GHGs and temperatures allows us to assess different steps in the global warming mechanism and, consequently, gain a better understanding of climate change. While the levels of GHGs, such as CO\({}_{2}\) and CH\({}_{4}\), are an indicator of anthropogenic global warming, the EEI is a better and broader indicator of actual climate change because it is a measure of the existing overall energy imbalance in the Earth's system.