Since the beginning of life on Earth, oceans have influenced the fundamental processes of our planet. From their role in the global carbon cycle to providing the habitats that sustain marine biodiversity, oceans provide the living resources and services upon which humans depend. They create opportunities for economic development, for societal well-being, and for quality of life. But we are changing the World Ocean to an extent that is unprecedented in millions of years. Greenhouse gas emissions are warming the planet, affecting the global carbon cycle, and changing the chemical composition of the ocean. These fundamental changes can have serious consequences for oceanic productivity and species composition. Marine ecosystems are being disrupted by overfishing and pollution at various scales so the consequences for Earth, as we have known it, might become serious. There is general agreement that our understanding of the role of oceans in the ecology of the planet is in its infancy. While many adverse effects of climate change have been described to date, they are likely a fraction of what will become apparent in the coming years. These changes are described and debated in regional or thematic symposia and workshops, but a comprehensive view of the current state of the global ocean and comparison between regions demands an international symposium program that reaches for that scale. ICES, PICES and IOC joined forces in 2008 for the first global ocean symposium (Gijón, Spain) and it attracted 400 scientists from 48 countries. This second look at the "Effects of Climate Change on the World's Oceans" will lead us through many issues of the role of climate change on the oceans: sea level rise, changes in thermo-haline ocean circulation, acidification, oligotrophy of temperate seas, changes in species abundance, distribution and phenology, loss of biodiversity, all of which will have serious implications for marine living resources, etc. This symposium aims to bring together experts from different disciplines to exchange observations, results, models and ideas at a global scale and to discuss the opportunities to mitigate and protect the marine environment and its living resources.