Corals are very sensitive to water that is too warm - even temperatures that are just 1-2°C above the highest average summertime temperature. If corals bathe in water above this critical threshold for just four weeks (or at higher temperatures for even shorter durations), the accumulated heat stress can induce coral bleaching. Bleaching is a condition where coral polyps expel the algae that they rely on for nutrition through photosynthesis. While bleached coral can recover, it is susceptible to disease and can remain weakened for years, or even die.
Record-setting global temperatures in the past decade are causing more frequent episodes of coral bleaching. Currently, mass bleaching events - those that affect corals in many areas around the globe - occur about twice a decade, which still permits time for recovery. The last major episode was 2010, the hottest year on record. These events are monitored and predicted via satellite tracking by NOAA's Coral Reef Watch program. This visualization compares NOAA data from the 2010 event to conditions in 2013. Fortunately 2013 saw bleaching in only localized regions, such as the Northern Mariana Islands in the western Pacific.
Climate modeling studies predict that mass bleaching events will occur more frequently as temperatures increase. This visualization highlights one recent modeling study that projects that if greenhouse gas emissions remain on the trajectory they are, 90 percent of reefs may experience severe bleaching on an annual basis by 2055. At that rate, corals would not be likely to adequately recover.
Many factors will influence future risks to reefs. One is where the coral is located. Large-scale climate and oceanographic patterns mean that certain waters - such the southern Great Barrier Reef and the western Indian Ocean - will warm more slowly than others. Corals in these regions would experience annual bleaching perhaps 10 or 15 years later than most other reefs, allowing more time for adaptation, another influencing factor.
Evidence is mounting that some corals can increase their resilience to heat stress over time through adaptive responses. The potential for future adaptation is not factored into most current model simulations, so it's possible that more reef areas will persist for longer than they suggest. But how fast corals can adapt, and to what extent, is still unclear. The largest influencing factor is the trajectory of future emissions. Our actions to mitigate warming, not adaptation, will largely determine how reefs will fare in a climate increasingly inhospitable to their survival.
NOAA's Coral Reef Watch program documents accumulated heat stress to corals - a measure called degree heating weeks - based on sea-surface temperature measurements taken every three days from the AVHRR sensor on NOAA's polar-orbiting satellites. This visualization begins with a time series of weekly averages of degree heating weeks data for 2013. The annual images of heat stress from 2010 and 2013 are composites of the maximum values of degree heating weeks in reef areas over the given year. For more on how heat stress is measured by satellite, visit NOAA Coral Reef Watch's tutorial. Coral reef locations are from the World Resource Institute's Reefs at Risk Revisited report.
The last map in the visualization is from a 2014 study that used climate models to project when and where corals may experience severe bleaching every year, or eight or more degree heating weeks annually. The map is an average of the projections from a number of models. It represents a "business as usual" scenario for future greenhouse gas concentrations (IPCC RCP 8.5), in which only modest future progress is made to reduce energy use and emissions. [Van Hooidonk, R. J., J. A. Maynard, D. Manzello, and S. Planes. 2014. Opposite latitudinal gradients in projected ocean acidification and bleaching impacts on coral reefs. Global Change Biology: 103-112.]