Coral reef bleaching and global climate change: can corals survive the next century?

C oral reef ecosystems are threatened on a worldwide basis, with overfishing, diseases, eutrophication, hurricanes, overpopulation, and global climate change all contributing to recent declines in reef-forming corals or phase shifts in community structure on time scales not observed previously (1–3). These changes are in contrast to recent periods of long-term stability in coral reef communities over geological time scales of thousands of years (4, 5). A recent meta-analysis of coral cover throughout the Caribbean has shown an 80% decline that has been both long term (e.g., decadal) in duration and region-wide (6). For the last two decades, coral reef biologists have attributed much of the increase in coral mortality to coral bleaching subsequent to elevated seawater temperatures occurring on both regional and global spatial scales (7). Coral bleaching, a stress response of reef-forming corals, results in the loss of their symbiotic algal partner that supplies a large percentage of the nutritional requirements of the coral host and causes the corals to appear white (ref. 7 and Fig. 1). Since 1979, there have been dozens of reports of coral bleaching associated with elevated sea surface temperatures (SSTs), whereas from 1876 to 1979, only three events were recorded (8). The recently released Fourth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC; www.ipcc.ch) states with 90% certainty that most of the observed warming of the planet over the last half-century has been caused by human activities from the accumulation of greenhouse gases. On the heels of the IPCC report, in this issue of PNAS, Donner et al. (9) provide a quantitative assessment of the contribution of humaninduced climate change for the most devastating coral-bleaching event on record, the Caribbean-wide coral bleaching in 2005. Donner et al. (9) use the extensive SST database of the Advanced VeryHigh-Resolution Radiometer Pathfinder satellite processed by the National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch program (http://coralreefwatch.noaa.gov). The analysis of the SST data is presented in the form of degree heating weeks (DHWs) that are equivalent to a week of SSTs higher than the local maximum in monthly climatology. When integrated over a 12-week period, values of 4.0°C-weeks indicate that coral bleaching may occur, whereas values of 8.0°C-weeks indicate that severe bleaching and mortality are likely (Fig. 2). Donner et al. use this data set to develop a degree heating month (DHM), equivalent to 1 month of SSTs higher than the maximum monthly climatology, to be more compatible with the output of their global climate model (10). The simulations were conducted by using a relatively new family of climate models created by the NOAA Geophysical Fluid Dynamics Laboratory (www.gfdl. noaa.gov). In a series of simulations, Donner et al. use the output of these coupled atmosphere–ocean general circulation models to investigate the influence of anthropogenic forcing on the increase of SSTs in the Caribbean that led to the 2005 bleaching event. In addition, those investigators simulate the trajectory of changes in Caribbean SSTs to assess the probability of events similar to 2005 occurring in the future and whether thermal acclimatization or adaptation by corals (10), specifically their algal symbionts (11, 12), can prevent bleaching events.