The impacts of climate change on ecosystems are highly uncertain but potentially profound. One such impact may be the emergence of extensive mats of seaweed (macroalgae), due to the extraordinary proliferation of pelagic sargassum species, in the tropical Atlantic since 2011. Sargassum blooms are now an annual event and reached record levels across what is now known as the ‘Great Atlantic Sargassum Belt’ (GASB) in summer 2022. Monitoring across scales, from satellite surveillance to in-situ beach surveys, is bringing step changes in process-level understanding of sargassum. Physical and biogeochemical drivers of sargassum act at basin scale to shape the GASB, highly variable from year to year. In the local environment, sargassum is sensitive to ambient conditions, with new findings confirming that growth rates are temperature dependent. Multidecadal ocean warming may therefore be detrimental to sargassum, although projected changes in other drivers are uncertain. Emerging options for climate change action around sargassum include valorisation and carbon sequestration, although uncertainties are again considerable. In conclusion, the emergence of sargassum across the tropical Atlantic highlights interconnected systems that embrace physical, biogeochemical, and socioeconomic dimensions, with considerable scope for improved monitoring, process-level understanding and prediction.