One Ocean, Many Worlds of Life - Convention on Biological Diversity

warmer temperatures prompt more rapid growth <strong>of</strong> zooplankton that eat
the algae before they have a chance to reach the bottom.121 122
Several coastal habitat types are at risk from a changing climate, for a variety
<strong>of</strong> reasons. Warming has been shown to diminish the diversity <strong>of</strong> salt marsh
communities by effectively drying out the plants and the soil in which they
grow. Kelp forests are threatened by, among other things, increased temperatures
altering the boundaries <strong>of</strong> the areas in which they can survive, by
decreases in nutrient flows, and by increases in predators such as urchins.123
And mangroves are potentially particularly susceptible to an encroaching
sea, a phenomenon that threatens coastal environments worldwide as a
consequence <strong>of</strong> a rise in global sea levels.124
As water warms, it expands, and such thermal expansion is the primary
reason why, since 1880, sea level has risen by an average <strong>of</strong> 22 centimetres;
increased warming, combined with groundwater extraction, the melting <strong>of</strong>
glaciers and some melting <strong>of</strong> the massive sheet <strong>of</strong> ice covering Greenland,
is likely to cause sea levels to rise by a metre, or possibly even significantly
more, by 2100.125 126 Clearly, such a huge increase in sea level imperils huge
areas <strong>of</strong> coast, posing immense challenges for the viability <strong>of</strong> significant
areas <strong>of</strong> coastal population.
Of particular concern is the effect <strong>of</strong> climate change on coral reefs. Coral
reefs have a relatively narrow temperature band in which they can thrive;
once the temperature exceeds the upper limit <strong>of</strong> that band—specifically,
when the sea water temperature exceeds the average summer level <strong>of</strong> a
particular location by about 2°C for more than a few weeks, the corals
become stressed and expel the symbiotic zooxanthellae that give them most
<strong>of</strong> their color, a frequently fatal phenomenon known as “coral bleaching.”
Although small-scale bleaching has long been recorded following localized
temperature increases, mass bleaching is a recent phenomenon. <strong>One</strong> such
example occurred in 1998, when elevated sea surface temperatures led to
massive bleaching episodes that extended from the western Pacific across the
Indian <strong>Ocean</strong> to east Africa, killing or severely degrading an estimated 16
percent <strong>of</strong> corals in the region.127 Another extensive outbreak took place in
2010, with bleaching observed in every ocean and major sea in which coral
occurs, from the Persian Gulf to southeast Asia, the Central Pacific to the
Caribbean. In the Caribbean, more than 80 percent <strong>of</strong> corals surveyed by
researchers had bleached, and in many places 40 percent or more had died.128
58 Marine BiOdiversity