Clouds are a key component of the climate system, regulating surface energy balance and temperature, and indirectly influencing soil moisture. Changes in large-scale circulation and surface forcing are altering cloud properties, particularly in rapidly warming high-latitudes. We investigate the link between changes in cloudiness and ecosystem productivity during the growing season using the ABCflux v2 database, which compiles monthly land–atmosphere CO₂ flux measurements across tundra and boreal ecosystems. Cloud properties are derived from the MODIS Level-2 Product (MOD06_L2) and aggregated to monthly averages at each flux site. Hourly records of CO₂ fluxes and cloud properties are obtained from regional flux networks and Meteorological Aerodrome Reports (METAR) for additional analysis. We find that the strongest changes in cloud fraction occur in June, with declining cloud cover over tundra and boreal ecosystems in Eurasia, contrasted by increasing cloudiness across North America. Site-level trends in cloud fraction are negatively correlated with trends in incoming shortwave and photosynthetically active radiation. At the hourly scale, light-use efficiency increases under overcast conditions, leading to enhanced gross primary production relative to clear-sky conditions. At the monthly scale, sites exhibiting increasing cloud fraction show positive trends in net ecosystem exchange, indicating a weakening of the net carbon sink with increasing cloudiness. This apparent scale dependency highlights the complex interplay between diffuse radiation enhancement and reduced total radiation input. These early results demonstrate that changes in cloud properties modify radiation budgets and may affect high-latitude carbon dynamics, with implications for predicting land–atmosphere feedbacks under climate change.