prehensively across many marine taxa and geographic regions, but a recent study suggests the potential for significant changes in community structure in the Arctic and Southern Ocean biodiversity due to invasion of warm water species and high local extinction rates in the tropics and subpolar domains. {5.8}

Coral bleaching events will likely increase in frequency and severity under a warmer climate. Over the past several decades, warmer sea surface temperatures have led to widespread tropical coral bleaching events and increased coral mortality, and warming and more local human impacts are associated with declines in the health of coral reef ecosystems worldwide. Bleaching can occur for sea surface temperature changes as small as +1-2ºC above climatological maximal summer sea surface temperatures, which corresponds to global average warming of about 1.5-3ºC (Figures S.5 and O.7). {5.8}

Rising CO2and ocean acidification will likely reduce shell and skeleton growth by marine calcifying species such as corals and mollusks. Some studies suggest a threshold of 500-550 ppm CO2 whereby coral reefs would begin to erode rather than grow, negatively impacting the diverse reef-dependent taxa (see Figure O.7). Polar ecosystems also may be particularly susceptible when surface waters become undersaturated for aragonite, the mineral form used by many mollusks. Indirect impacts of ocean acidification on non-calcifying organisms and marine ecosystems as a whole are possible but more difficult to characterize from present understanding. {5.8}

Impacts of 21st Century Sea Level Rise

Depending on socioeconomic development, population growth, and intensity of adaptation, it has been projected that 0.5 m of sea level rise would increase the number of people at risk from coastal flooding each year by between 5 and 200 million; as many as 4 million of these people could be permanently displaced as a result. More than 300 million people currently live in coastal mega-deltas and mega-cities located in coastal zones. The corresponding projections for 1.0 m of sea level rise suggest that the number of people at risk of flooding each year would increase by 10 to 300 million. {5.2}

Coastal erosion is expected to occur as sea level rises with warming temperatures. Global aggregate estimates suggest that wetland and dry-land worldwide losses would sum to more than 250,000 km2 with 0.5 m of sea level rise; more than 90% of these losses are projected to occur in developing countries. {5.2}



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