Matthew McLean collaborated with nine other researchers across Europe and North America, including Malin Pinsky, to coauthor this study on community change in marine environments. Their report appears in the November 8 issue of Current Biology. Although past studies have documented extensive shifts in community temperature index (CTI), this study uniquely decomposes CTI into four underlying processes at a multi-continental scale (tropicalization—increasing abundance of warm-affinity species; deborealization—decreasing abundance of cold-affinity speciesd; borealization—increasing abundance of cold-affinity species; detropicalization—decreasing abundance of warm-affinity species). Using long-term monitoring of marine fishes across the Northern Hemisphere, McLean et al. show that one-third of increases in mean thermal affinity were primarily due to decreases in cold-affinity species. Cold-affinity decreases were stronger closer to human population centers; warm-affinity increases were stronger in warmer areas. These findings will help in anticipating future changes in biodiversity under climate change and implementing adapted management strategies.