Abstract: Coastal ecosystems in general and intertidal systems in particular, are likely to be profoundly affected by climate change. On temperate rocky shores, the responses of foundational seaweed species to climate change have the potential to trigger major changes in the entire intertidal ecosystem. We studied the impact of climate change on seaweeds meadows in the North-Atlantic rocky intertidal with focus on the brown canopy-forming macroalga Fucus serratus. More specifically, we aimed to identify the impact of climate change on 1) the geographical distribution, 2) the thermal tolerance, and 3) the genetic changes of fucoid seaweeds. Ecological Niche Model predictions suggest that suitable environments for fucoid seaweeds are opening up in the Arctic along the southern coasts of Greenland, Iceland and Spitzbergen. At the same time, our models predict that fucoid seaweeds will face extinction from rocky shores in southern Europe before year 2200. The negative impact of climate change on the southern edge populations of fucoid seaweeds is not only a prediction but is already supported by empirical findings. For example, we recorded a significant loss of allelic richness and an abundance decline of about 90 % in a Spanish southern edge population of F. serratus over the past decade. The threatened southern-edge populations of many temperate seaweeds are ancient glacial refugia and centers of genetic diversity. Thus, a remaining key question is, whether the plastic or adaptive capacities of these populations are sufficient to survive climate change or if temperate seaweeds are at risk to lose their centers of adaptive potential to future environmental change.