One of the great challenges of modern marine ecology is detecting trajectories of change in marine ecosystems and identifying the underlying drivers. Kelps (Laminariales) form key biogenic habitat in coastal regions of temperate and polar seas worldwide, providing ecosystem services valued in the range of billions of dollars annually. While local evidence suggests that these important ecosystems are increasingly threatened by shifts in climate stressors, no comprehensive global analysis of change in kelp abundance currently exists. Here, we build and analyze a worldwide database of kelp time series to assess global and regional changes in kelp abundances over the past half-century and then merge it with data on global long-term changes in climate drivers. We detected a small global decline in kelp, with significant variation that was characterized by both increases and decreases on a regional scale. Merging kelp abundance with environmental data (temperature and wave heights) allowed us to attribute some of the spatial variation in regional trajectories to changes in climate drivers. Specifically, kelp has been declining in regions throughout the globe with warming seawater temperatures; with wave disturbances driving losses at lower latitudes. Our results highlight the importance of examining contrasting regional signals of change. These regional contrasts can provide the variability we need to discover generalized links between synchrony in global phenomenon, such as climate change, and the future of kelp ecosystems around the globe.