The overarching aim of the proposed research was to bring together internationally recognised scientists currently active in Far Infrared (FIR) or related research to strengthen their individual efforts and develop a core team that can lead future international activities in this area. This particular bid explored the role of FIR radiation in shaping polar climate, building on a range of ongoing initiatives in the UK, USA and Italy. The planned work had two main science foci: (1) quantify the recently identified 'ice-emissivity' feedback using a combination of novel observations and theoretical modelling; (2) link surface based, in-situ and space-based measurements to develop a greater understanding of the controls on the longwave radiation budget in polar regions. In meeting the aims, an array of coherent, observationally based tools were developed that can be used to test and evaluate climate model performance in polar environments. Specific scientific objectives to be realised through the course of the project are: O1: the first derivation of FIR snow and ice surface emissivity from in-situ measurements over the Arctic ice sheet; O2: the delivery of a unique, spectrally-resolved surface downwelling longwave radiation (DLR) database, stratified according to meteorological/cloud regime, designed for satellite retrieval and model evaluation over Antarctica; O3: the delivery of a thoroughly validated, multi-year, ongoing satellite-based record of spectral outgoing longwave radiation covering the full infrared spectrum; O4: the application of the tools developed through WP1-3 to evaluate model performance in polar environments with initial focus on the atmospheric components of the UK's Earth System Model (UKESM) and the US Department of Energy's Community Earth System Model (CESM).

The Far infra red (FIR) is defined as the region of electromagnetic spectrum found at wavelengths greater than 15 microns. FIR radiation plays a major role in the Earth's energy balance, accounting for approximately half of the emission to space from the Earth and its atmosphere in the global mean. Fundamental physics implies that FIR radiation will play an even more important role in influencing climate variability and change in the fragile polar regions. The very cold surface temperatures found in these locations means that a greater fraction of the emitted surface energy is found at longer wavelengths. Moreover, the associated very low water vapour concentrations typically found in polar regimes effectively open up 'windows' in the FIR, making it possible to see further into the atmosphere from the ground than would normally be possible at these wavelengths. By the same argument, more of the surface energy emitted at these wavelengths is able to escape to space. Recent work has suggested that assumptions about FIR surface characteristics made in many of the most advanced models that we use to predict climate - termed Earth-system models - mean that they may be missing an important polar climate feedback process. This could lead to an additional Arctic surface warming of up to 2 K by the 2030s which would be expected to affect the rate of ice-melt and sea-level rise. Termed the 'ice-emissivity' feedback, the mechanism depends on the fact that snow and ice emit more energy at FIR wavelengths than sea-water at the same temperature. Current Earth-system models typically assume that all surfaces have the same emissivity in the FIR and so do not include this feedback process. These same models also struggle to match surface observations of the downwelling radiation emitted by the atmosphere in polar regions, a shortcoming that is believed to be principally due to inadequacies in the representation of polar clouds. However, up to now a detailed evaluation of the polar radiation budget has been hampered by a lack of dedicated observations spanning the entire infrared, including the FIR. This project seeked to address this deficiency by bringing together a team of international experts in FIR research and climate modelling to develop a suite of observationally based tools which were used to assess model performance and drive future improvements. This project derived the first ever assessment of FIR surface emissivity from in-situ airborne observations over the Greenland plateau; characterise the infrared surface radiation budget over Antarctica and assess the meteorological processes driving variability there over a range of time-scales; evaluate approaches used to derive synthetic FIR measurements from space-based observations; and begin the process of quantifying the ice-emissivity feedback in two leading Earth-system models.