Individual corals will remain healthy only when temperatures stay within a relatively narrow range. Coral polyps have a symbiotic relationship with zooxanthellae, single celled algae, that live in their tissues. The zooxanthellae provide food for the coral through photosynthesis, and get nutrients and a place to live in return. As temperatures increase, corals get stressed and may eventually expel the zooxanthellae, turning a white color. This is known as coral bleaching. The corals depend on the zooxanthellae for most of their food, and if the high temperatures continue for long enough and they stay bleached then they can die .
The usual way to measure the thermal stress on a coral is the degree heating week, or DHW. DHWs are calculated in relation to the maximum monthly mean temperature, or MMM, for the area being measured. The MMM is the mean temperature of the hottest month of the year, according to some set of historical data. One DHW is accumulated for each week and each degree Celcius spent above the MMM. For example, two DHW can be accumulated by either spending two weeks at 1°C above the MMM, or by spending one week at 2°C above the MMM. DHWs can be fractional, but are only accumulated if temperatures are at least 1°C above the MMM. Within a three month window, if corals in an area have undergone 4 DHW then there is some risk of bleaching, and if they have undergone 8 DHW then widespread bleaching and mortality is likely .
The DHW is used by NOAA's Coral Reef Watch program  to process satellite data and determine the areas that are at immediate risk of bleaching. The DHW has also been used in combination with data from climate models to make projections of future bleaching. As temperatures steadily increase, areas will begin experiencing at least 8 DHW in a three month window every single year.
Adaptation to Warming and Acidification