Advancing climate change research requires accurate and traceable measurement of the atmospheric longwave irradiance. Current measurement capabilities are limited to an estimated uncertainty of larger than ± 6 W/ m2 using the interim World Infrared Standard Group (WISG). Two independently designed and calibrated absolute radiometers measuring downwelling longwave irradiance were compared during five outdoor comparisons in 2013, 2015, and 2017 at the Physikalisch Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC) and the USA-DOE Atmospheric Radiation Measurement program (ARM) in the Southern Great Planes (SGP). Two Absolute Cavity Pyrgeometers (ACPs) developed by the National Renewable Energy Laboratory (NREL) and four Integrating Sphere Infrared Radiometers (IRISs) developed by PMOD/WRC took part in these intercomparisons. From the five comparisons, the difference between the irradiance measured by the ACPs and IRISs varied from 0.2 W/ m2 to 2.5 W/ m2 based on the atmospheric conditions, which is within the combined stated uncertainties of ±3 W/ m2. The irradiance measured by the WISG is lower than the average irradiance measured by ACPs and IRISs, magnitude of the difference varied from 0.2 W/m2 to 6.6 W/ m2 depending on the integrated water vapor. A concerted effort at the World Meteorological Organization (WMO) Commission for Instruments and Methods of Observation Commission (CIMO) Task Team on Radiation References (TT) started during a meeting from November 15 to 17, 2017 in the National Physical Laboratory (NPL), Teddington, United Kingdom to establish a world reference for measuring the atmospheric longwave irradiance with lower uncertainty and with traceability to SI units by using the ACPs and IRISs as the reference.