The Earth Climate Observatory space mission concept for innovative continuity in the monitoring of the Earth Outgoing Longwave Radiation.

The Earth Energy Imbalance (EEI) is defined as the small difference between the incoming energy the Earth receives from the Sun and the outgoing energy lost by Earth to space. The EEI is accumulated in the Earth climate system and results in global temperature rise. Monitoring the EEI is of prime importance for a predictive understanding of climate change, and for estimating how well humankind is doing in implementing the Paris Climate Agreement.The current best estimates of the absolute value of the EEI, and of its long term variation are obtained from in situ observations. These observations can only be made over long time periods, typically a decade or longer. In contrast, with direct observations from space, the EEI can in principle be measured at the annual mean time scale. However, this strategy currently faces two fundamental challenges.The first challenge is that the EEI is the difference between two opposing terms of nearly equal amplitude. Currently, the Incoming Solar Radiation (ISR) and the Total Outgoing Radiation (TOR) are measured with separate instruments, which means that their calibration errors are added and overwhelm the signal to be measured. To make significant progress in this challenge, a differential measurement using identical intercalibrated radiometers to measure both the ISR and the TOR is needed.The second challenge is that the TOR has a systematic diurnal cycle. Currently, the TOR is sampled from the “morning” and “afternoon” Sun-synchronous orbits, complemented by narrowband geostationary imagers. Recently, the sampling from the morning orbit was abandoned. The sampling of the diurnal cycle can be improved, for example, by using two orthogonal 90° inclined orbits which give both global coverage, and a statistical sampling of the full diurnal cycle at seasonal time scale.For understanding the radiative forcing and climate feedback, mechanisms underlying changes in the EEI, and for climate model validation, it is necessary to separate the TOR spectrally into the Reflected Solar radiation (RSR) and Outgoing Longwave Radiation (OLR) and to map them at relatively high spatial resolution.The state-of-the-art observation of the OLR is provided by the CERES scanning 3-channel broadband radiometer aboard the Aqua, Suomi NPP and NOAA 20 satellites. We propose an innovative continuity of those measurements by replacing the radiometer by multispectral wide field of view (FOV) cameras. The wide FOV allows a full angular coverage, providing the potential for a significant reduction of the dominant angular conversion error. To realise this potential we propose to develop an innovative Deep Learning based angular conversion method. The multispectral bands of the camera should allow reconstructing the broadband OLR within the state of the art accuracy. The spatial resolution of the cameras should be sufficient to discriminate cloudy from clear-sky scenes.