| Summary: | Comunicación presentada en los Coloquios Paco Ynduráin, celebrados en modalidad virtual el 2 de diciembre de 2020. The world´s ocean is actively involved in climate change, as modulator of the change and receptor of the process. During the last 50 years, approximately 93% of the excess heat accumulated by our planet due to anthropogenic emissions of greenhouse gases,mainly carbon dioxide (CO2), has been stored in the ocean. Moreover, since the beginning of the Industrial Revolution, the ocean has absorbed nearly 30% of the CO2 released by the humans to the atmosphere. Marine sequestration of heat and carbon is, hence, extremely beneficial at planetary scale, but in turn, entails environmental costs for the oceanic domain. Warming of the water column causes an increase in stratification, with implications for natural ocean circulation mechanisms and chemical processes (e.g. decrease in dissolved oxygen levels). On the other hand, uptake of atmospheric CO2leads to a gradual drop in the pH of seawater, the so-called ocean acidification phenomenon. The rise in acidityalters chemical speciation and biogeochemical cycles of many elements and compounds. One well-known effect is the lowering of calcium carbonate saturation states, which impacts calcifyingmarine organisms from all levels of the marine food web.In this lecture, the fundamental role of the ocean in the Earth¿s climate system will be presented and discussed in relation to the consequences alreadyexperienced by the marine ecosystems, from a global perspective to a regional scale.
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