The oceans are experiencing extraordinary human-induced threats from global climate change, overfishing, pollution and an increase in invasive non-native species. Seaweeds create the largest vegetated marine habitats on the planet, which underpin global marine function but are threatened by the impacts of environmental change. Despite the importance of seaweeds, and the threats they face, they are afforded inadequate conservation measures, a major gap which urgently needs to be addressed.

The state of the world’s seaweeds has come about as a result of this need. It provides the evidence-base which will inform a ‘Seaweed Breakthrough’, a potentially powerful means of protecting seaweeds and seaweed habitats through the UNFCCC High Level Climate Champion 2030 Breakthrough Agenda. Through setting global targets to halt habitat loss, protect and restore habitats and secure sustainable investment, Breakthroughs falling under the overarching goal for Marine Conservation aim to achieve significant change to reach a resilient, zero carbon future by 2030 across every sector of the global economy.

We have compiled an up-to-date overview on the state of the world’s seaweeds. Through a series of chapters covering information on seaweed distribution, habitats, ecosystem services, as well as how they are threatened, protected and restored, we have identified knowledge gaps and drafted ambitious high-level targets that will form the basis of expert-led workshops to support the Seaweed Breakthrough.

This review demonstrates how important seaweeds and their habitats are in the functioning of marine ecosystems, global fisheries, food security, valuable materials for industrial and pharmaceutical uses and, therefore, livelihoods. It documents what is known. For example, we now have quantitative evidence for the severity of losses and degradation that kelp forests are suffering in many regions around the world. Less is known about most other seaweed habitats, as they have received far less attention. For example, rhodolith beds, habitats formed by free-living calcified red seaweeds, are extensive in many parts of the world and are still being discovered. However, they are threatened by pollution, habitat degradation, climate change, ocean acidification and trawling. Deep-water seaweed habitats have also only been studied in a tiny fraction of the oceans yet are likely to hold diversity that is still to be discovered.