Strengthening Atmospheric Greenhouse Effect at the Arctic North Slope of Alaska Site Evidenced by Long-Term Records of the Downwelling Longwave Radiance Spectrum

Arctic amplification, the accelerated warming of the Arctic compared to lower latitude, remains poorly understood, particularly regarding the role of clouds. In this study, we analyze 26 years of downwelling longwave radiation (DLR) measurements from an Extended-range Atmospheric Emitted Radiance Interferometer (E-AERI) at the US Department of Energy's North Slope of Alaska site. Our analysis reveals a pan-spectral increase in DLR under all sky conditions, primarily driven by an enhanced greenhouse effect from thick/low clouds and greenhouse gases. We find that the observed DLR trends are primarily driven by radiance changes within individual sky conditions, rather than changes in cloud fraction, suggesting fundamental shifts in atmospheric emissivity and/or temperature. Comparative analysis between the Southern Great Plains (mid-latitude) and North Slope of Alaska (high-latitude) sites demonstrates that clouds impact the longwave surface energy balance differently between the two locations. In the Arctic, the pan-spectral increase in DLR trend dampens out the radiative cooling of the warming surface, contributing significantly to Arctic amplification. Consistent positive trends are found in the far-infrared, a spectral region sensitive to even small changes of the dry and cold Arctic's atmosphere and of significant interest in the current warming context. Additionally, we observe a relatively weaker DLR trend in the ozone absorption band under every sky condition, indicating a tropospheric ozone radiative forcing in the Arctic climate.