The Earth Climate Observatory (ECO) space mission concept for the monitoring of the Earth Energy Imbalance (EEI)

The Earth Energy Imbalance (EEI) is defined as the small difference between the incoming energy received by the Earth from the Sun and the outgoing energy lost by the Earth to space. Both the incoming solar and the terrestrial outgoing energy are of the order of 340 W/m² at the global annual mean level, while the EEI is of the order of 0.9 W/m². The EEI is accumulated in the Earth’s climate system, particularly in the oceans which have a high heat capacity, and results in global temperature rise. Despite its fundamental importance, the EEI is currently poorly measured from space, due to two fundamental challenges.  A new space mission concept, called the Earth Climate Observatory (ECO) - with as primary mission objective the accurate and stable monitoring of the EEI - is currently elaborated. The first fundamental challenge is that the EEI is the relatively small difference between two opposite terms with large and nearly equal amplitude. Currently, incoming solar radiation and outgoing terrestrial radiation are measured with separate instruments, which means that their calibration errors are added, and overwhelm the signal to be measured. In order to make significant progress in this challenge, a differential measurement using identically designed, intercalibrated instruments—so-called wide-field-of-view (WFOV) radiometers—to measure both the incoming solar radiation and the outgoing terrestrial radiation is needed. The second fundamental challenge is that the outgoing terrestrial radiation has a systematic diurnal cycle. An adequate sampling of the global diurnal cycle can be obtained by using an innovative constellation of two orthogonal 90° inclined orbits which provide both global coverage and a statistical sampling of the full diurnal cycle at the seasonal (3-month) time  scale. The wide-field-of-view radiometer will make accurate low-spatial-resolution measurements of the Total Outgoing Radiation (TOR) of the Earth. Auxiliary innovative multispectral visible and thermal imagers – with high information content, serving a multitude of possible secondary mission objectives - will be used to increase the spatial resolution of the radiometer observations and to separate the TOR spectrally in the Reflected Solar Radiation (RSR) and the Outgoing Longwave Radiation (OLR).