Status of Caribbean coral reefs after bleaching and hurricanes in 2005

Coral Reefs and Climate Change: Susceptibility and Consequencesgases such as CO 2. Current predictions of future coral reef bleaching events indicate that coralswill not adapt to warmer water without either a stabilization of greenhouse gas emissions oreven a decrease. If low emission technologies result in global temperatures stabilizing at 2 o Cabove the present, coral populations will initially decrease with the loss of temperature sensitivespecies, until they are replaced by more temperature resistant species. That will take decadesif not centuries. However, if greenhouse gases do not stabilize, the most likely scenario is thatcoral populations will decrease, with growing rates of extinction of corals and the thousands ofother species that depend on coral reefs. This will mean an end to the all-important ecologicalservices provided by coral reef ecosystems.2. In c r e a s i n g Co n c e n t r at i o n s o f CO 2in SeawaterThe cascading effects on ocean chemistry of rising atmospheric CO 2levels are referred toas ‘ocean acidification’. This does not generally invoke the same sense of urgency as coralbleaching, probably because:zx it is a creeping environmental problem;zx ocean acidification has only recently been accepted as a reality (partly becauseseawater carbonate chemistry is not intuitive); andzx the process is relatively invisible and does not appear to physiologically harm adultcorals.How ocean acidification will affect all life stages of organisms, reef communities, and reefstructures, however, is largely unstudied. While the nature and rate of ocean acidification iswell-known and predictable, the potential ecosystem effects of ocean acidification constitute aproblem of high uncertainty, but high risk.The uptake of atmospheric CO 2by the oceans is a double-edged sword. So far, the oceans haveabsorbed about a third of the excess CO 2released into the atmosphere from burning fossil fuelsand other human activities. Another third has been taken up by activities on the land, and theremaining third has remained in the atmosphere such that the concentration of CO 2has gonefrom 280 parts per million by volume (ppmv) before the industrial revolution to about 380 ppmvtoday. Ocean uptake of CO 2from the atmosphere reduces the severity of the greenhouse effectand climate change (and indeed the conditions that cause coral bleaching). Unfortunately, italso alters the chemistry of seawater resulting in lower pH (‘ocean acidification’) and decreasedcarbonate ion concentrations. Low pH values represent high hydrogen ion concentrationsand more acid conditions, and high pH values represent low hydrogen ion concentrationand alkaline conditions. pH is reported on a logarithmic scale; such that a 1.0 change in pHrepresents a 10-fold change in hydrogen ion concentration. The pH of tropical seawater hasremained around 8.2–8.3 for about a half million years, but will decrease to around 7.9–8.0 whenatmospheric CO 2 concentrations are double the pre-industrial levels; that represents about a30% increase in hydrogen ion concentration. This change in ocean acidity will also cause ashift in the relative proportions of the inorganic forms of carbon: dissolved CO 2; bicarbonate;and carbonate. A lowering of seawater pH also means that the carbonate ion concentration willdecrease by more than 30%. This represents a substantial change in the chemical conditionssupporting calcification, because the carbonate ion is a major skeletal building block for thecalcium carbonate (CaCO 3) skeletons of corals and other reef-building organisms.23