Examples of Earth's outgoing longwave radiation in brightness temperatures for the far-infrared at wavenumbers of 667.6 cm-1 (top), for the stratosphere, and 621.7 cm-1 (bottom) for the troposphere. The cold features in the mid-latitudes and Tropics are clouds. ESA’s Earth Explorer 9 candidate mission, FORUM, would measure radiation emitted from Earth across the entire far-infrared part of the electromagnetic spectrum. Specifically, it would measure in the 666.67 cm–1 to 100 cm–1 range (i.e. equivalent to 15–100 micrometre wavelength), which has never been achieved before from space. The mission would improve our understanding of the greenhouse effect and, importantly, improve confidence in the accuracy of climate-change assessments that form the basis for future policy decisions.