Overview of Climate Changes and Illustrative Impacts - Climate Stabilization Targets - page 43

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-
2°C above climatological maximal summer sea surface temperatures, which
corresponds to global average warming of about 1.5-3°C (Figures S.5 and
O.7). {5.8}
Rising CO 2 and 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 CO 2 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 km 2 with 0.5 m of sea
level rise; more than 90% of these losses are projected to occur in develop-
ing countries. {5.2}