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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.

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. 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.]

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