For coral reefs, climate change due to enhanced greenhouse gasses is not some distant threat that might happen in the future. Recent climate has been affecting coral reefs since the mid 1980s, and many locations have by now already experienced multiple bouts of coral bleaching in the past 25 years or so, following periods of unusually high water temperatures (Fig. 9.4). Coral bleaching occurs when corals become physiologically stressed and lose most of their symbiotic zooxan-thellae. Localised bleaching has been described in the older coral reef literature following extreme weather and floods. However, regional scale bleaching such as the 1998 El Niño event (see Chapter 10) is a new phenomenon driven by global warming. Mortality of bleached corals is often high over very large areas, and
the frequency and intensity of warm water events is set to rise in coming decades. Unprecedented large scale coral bleaching occurred on the GBR in 1998 and 2002 (Chapter 10). On the GBR, researchers have documented a steady increase in the incidence of coral disease over recent years, which may relate to rising sea-surface temperatures. In the longer term, ocean acidification is also likely to affect the growth rate and skeletal composition of corals.
Coral bleaching, like most forms of disturbance, affects some species more than others. For example, branching and tabular Acropora species are usually much more susceptible to thermal stress than many faviids and Porites (Figs 9.5, 9.6). There are two important unknowns concerning the responses of corals and their symbiotic zooxanthellae to global warming: whether they can adapt quickly enough to rising temperatures, and the extent to which warm-adapted genotypes may be able to move. Corals typically have very large geographic ranges that usually straddle the equator and extend to 25-30° north and south. Consequently, local populations experience substantially different temperature regimes. Importantly, bleaching does not occur at the same absolute temperature everywhere. Rather, it happens when
temperatures have risen by about 2°C for several weeks above the ambient local temperature regime. Clearly, local populations have adapted to their local thermal environment (see Chapter 10). However, nobody knows how long this local adaptation has taken to evolve, or the extent to which warm-adapted strains can migrate via larval dispersal to higher latitudes.
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