Climate change and the oceans – What does the future hold?

• Warming, acidification and deoxygenation are having multiple impacts on marine ecosystems and their functions.
• Physiological responses to climate change result in altered abundance, distribution and ecological interactions of species.
• The processes associated with climate change interact to amplify the effects of each factor.
• Temperature rise, deoxygenation and acidification, are symptoms of changes in the carbon cycle in past extinction events.
• The current CO2 rise associated with “the deadly trio” is occurring at a high rate possibly not seen for 300 my.

The ocean has been shielding the earth from the worst effects of rapid climate change by absorbing excess carbon dioxide from the atmosphere. This absorption of CO2 is driving the ocean along the pH gradient towards more acidic conditions. At the same time ocean warming is having pronounced impacts on the composition, structure and functions of marine ecosystems. Warming, freshening (in some areas) and associated stratification are driving a trend in ocean deoxygenation, which is being enhanced in parts of the coastal zone by upwelling of hypoxic deep water. The combined impact of warming, acidification and deoxygenation are already having a dramatic effect on the flora and fauna of the oceans with significant changes in distribution of populations, and decline of sensitive species. In many cases, the impacts of warming, acidification and deoxygenation are increased by the effects of other human impacts, such as pollution, eutrophication and overfishing.The interactive effects of this deadly trio mirrors similar events in the Earth’s past, which were often coupled with extinctions of major species’ groups. Here we review the observed impacts and, using past episodes in the Earth’s history, set out what the future may hold if carbon emissions and climate change are not significantly reduced with more or less immediate effect.