The far-infrared: A frontier in remote sensing of earth's climate and energy balance

The radiative balance of the troposphere, and hence climate, is influenced strongly by radiative cooling associated with emission of infrared radiation by water vapor, particularly at far-infrared (far-IR) wavelengths greater than 15 μm and extending out beyond 50 μm. Water vapor absorption and emission is principally due to the pure rotation band, which includes both line and continuum absorption. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are well-recognized as major uncertainties in understanding and predicting future climate. Up to half of the outgoing longwave radiation (OLR) from the Earth occurs beyond 15.4 μm (650 cm^-1) depending on atmospheric and surface conditions. Cirrus clouds also modulate the outgoing longwave radiation in the far-IR. However, despite this fundamental importance, far-IR emission (spectra or band-integrated) has rarely been directly measured from space, airborne, or ground-based platforms. Current and planned operational and research satellites typically observe the mid-infrared only to about 15.4 μm. In this talk we will review the role of the far-IR radiation in climate and will discuss the scientific and technical requirements for far-IR measurements of the Earth's atmosphere.