Recent climatic changes are reshaping biological communities by shifting species distributions and favouring warm-affiliated over cold-affiliated species, a process referred to as thermophilisation. However, how this balance between the increase in warm-affiliated species versus decline in cold-affiliated species affects total species richness at multiple spatial scales remains poorly understood. Here, we resurveyed 54 temperate forest plots in Belgium (western Europe) after 25 years to assess how climate change altered carabid beetle communities over time and how this affects species richness at both local and regional spatial scales. Community composition changed consistently across plots, largely due to a significant decline in the relative abundance of cold-affiliated species, resulting in thermophilisation in 832scale analyses of species richness revealed a significant decline in overall carabid species richness across spatial scales, which was primarily driven by declines in the abundance of cold-affiliated species, with no compensatory increase in warm-affiliated species. Additionally, rarefied richness of cold-affiliated species declined markedly, indicating a reduction of this species pool, potentially due to local population extinctions. This was further supported by the absence of several previously abundant populations of cold-affiliated species with reduced dispersal capacities. Together, these patterns point towards recent climatic changes as a key driver of arthropod diversity loss in temperate forests, with pervasive effects across spatial scales.