# Evaluation of the Landsat-8 Albedo Product across the Circumpolar Domain
## 1 Introduction  
Land surface albedo, defined as the ratio of upwelling to downwelling radiative flux, is an essential climate variable (ECV) required to accurately model the global surface energy budget [1]. Albedo describes how incoming solar energy interacts with the Earth's surface, which is key to monitoring and modeling the surface energy and the hydrologic and biogeochemical functioning of land and coastal ecosystems over the globe. Albedo is a particularly critical factor in surface radiative warming and cooling in the northern latitudes, in large part to the presence or absence of seasonal snow cover [2, 3]. Albedo is influenced by both biotic and abiotic factors including land cover, vegetation structure and distribution, and landscape topography. As such, it is locally, regionally and temporally variable. Higher spatial resolution albedo products allow for both the discrimination of the drivers of albedo change within a landscape and the impact of these changes on the overall energy balance of a system. Climate warming is occurring more strongly in the northern latitudes, with some regions of the Arctic experiencing rates two to three times the global average, leading to widespread and measurable shifts in vegetation and snow cover [4]. Correspondingly, satellite albedo measures from sensors such as AVHRR and MODIS have shown decreasing spring and summer albedo across the northern hemisphere since the 1980s [5]. This decrease in albedo has been linked to changes in snow extent and duration [5, 6] as well as decreasing Arctic sea ice extent [7, 8]. Surfacewater has also been shown to be a contributor to decreasing albedo across the Arctic-broreal domain, accounting for up to a quarter of the spatial variation in monthly albedo from 2000 to 2019 [9]. Local changes in growing season length, soil hydrology, soil nutrition, and disturbance regimes alter vegetation productivity, composition, and structure [10] and in turn affect the landscape albedo signatures. The importance of albedo in the surface energy budget is also highly dependent on the seasonality of solar insolation which can compound the impact of changes in landcover at the landscape scale. The complexity of the drivers of change coupled with the small-scale heterogeneity in vegetation, topography and seasonal snow patterns of high latitude systems highlight the need for a higher resolution satellite albedo product. Variations in small scale patterns are also associated with snow accumulation and retention across the northern hemisphere. Changes in seasonal snow and ice coverage in the biomes have particularly important implications as the high albedo of snow and ice increases the proportion of solar radiation reflected from the Earth's surface and can produce a measurable cooling effect. The Snow Albedo Feedback (SAF) is a leading cause of shifts in atmospheric circulation and climate in the Northern Hemisphere and is especially influential in the spring when higher incoming solar radiation and thinning snow cover correlate to earlier snowmelt [11] and enhanced soil thawing [12], resulting in variable growing seasons. The magnitude of cooling provided by higher snow albedo is related to several factors including snow depth, cloud cover, solar zenith angle, day length, solar insolation and vegetation type and structure, and as such, is highly variable over heterogeneous land surfaces. The largest variations in surface shortwave forcing are seen over shrulands, grasslands, and sparsely vegetated terrain. In addition to changes in water, snow and ice regimes, the pan-arctic region is also seeing changes in vegetation structure and distribution with shrubs and trees expanding in both range and stature [13; 14; 15; 16; 17]. These shifts in vegetation structure and extent have measurable effects on the surface energy balance and have been observed through greening and browning trends identified with satellite-based vegetation indices [10; 18; 19; 20; 21].