Mapping Coral Bleaching With Satellites

Caitlin Dempsey


As above normal sea surface temperatures continue to occur due to climate change, satellite data is being used to map out coral bleaching events that are triggered by increased ocean warming.

What is coral bleaching?

Coral reefs are made up of tiny creatures called coral polyps. These corals have a special partnership with tiny algae called zooxanthellae. Corals and zooxanthellae exist in a mutualistic relationship. Corals provide a protective environment and compounds that the algae need for photosynthesis. In return, zooxanthellae contribute to the coral’s energy needs and aid in calcium carbonate deposition, which is crucial for the formation and maintenance of coral skeletons. It’s the zooxanthellae that give healthy corals their colorful look.

A closeup underwater of coral polyps tinged with orange and light purple.
Paramuricea coral polyps. Photo: Art Howard, NOAA-OER/BOEM/USGS, public domain.

Corals are highly sensitive to temperature fluctuations — they require conditions within a narrow temperature range and become stressed when exposed to water that is either too warm or too cold. According to NOAA, the optimal ocean temperatures for corals is between 73° and 84° Fahrenheit (23°–29° Celsius). Temperatures below  64° Fahrenheit (18° Celsius) are too cold for reef-building corals. Temperatures above the optimal range can also damage and ultimately kill coral reefs — only some species can tolerate temperatures above 104° Fahrenheit (40° Celsius) for short periods.

Global warming leads to elevated sea temperatures, which stresses the corals and can disrupt the symbiotic relationship they have with algae. Coral bleaching is a condition in which stressed polyps expel the algae living in their tissues, resulting in the loss of the coral’s vibrant colors.

Warming ocean waters stress corals and cause coral bleaching. Colonies of “blade fire coral” that have lost their symbiotic algae, or “bleached,” on a reef off of Islamorada, Florida. Photo: Kelsey Roberts, USGS. Public domain.
Warming ocean waters stress corals and cause coral bleaching. Colonies of “blade fire coral” that have lost their symbiotic algae, or “bleached,” on a reef off of Islamorada, Florida. Photo: Kelsey Roberts, USGS. Public domain.

Climate change also leads to higher concentrations of carbon dioxide in the atmosphere. This CO2 is absorbed by ocean waters, making them more acidic. Acidification impacts the corals’ ability to deposit calcium carbonate, weakening them and making them more susceptible to stressors like elevated temperatures.

Tracking coral bleaching with satellite image

As oceanic heatwaves persist, a buildup of heat happens. Since corals will react adversely when ocean temperature conditions become too hot or too cold, scientists can use remotely sensed data from satellites to map what is known as “accumulated heat stress” in the oceans that leads to coral bleaching.

Mapping accumulated heat stress in the ocean

This condition is usually measured as “degree heating weeks” (DHW), which quantifies how much warmer the water is compared to the average temperature and for how long that elevated temperature has persisted. When accumulated heat stress reaches a certain threshold, it can have harmful effects on marine ecosystems, including causing coral bleaching, where corals lose their color and can eventually die if the stress continues.

A map with gradients of red showing ocean heat stress around the Florida Keys area in the Atlantic Ocean.
A map created by NASA showing accumulated heat stress values for Atlantic Ocean areas around the Florida Keys. Map: NASA, public domain.

Scientists are able to calculate the day-to-day changes in accumulate heat stress by combining remotely sensed data from polar orbiting satellites, for example NASA-NOAA Suomi NPP, and the geostationary satellites like NOAA’s GOES weather satellites. This data is then fed into computer models to calculate accumulated heat stress values.

When accumulated heat stress values reach 4, this can result in a coral bleaching event. When the accumulated heat stress value reaches 8 then these coral bleaching events are likely to lead to coral reef death.

Extreme marine heat wave around the Florida Keys

A map of the southern tip of Florida and Cuba with the ocean temperatures a range of blue for cold and deep red for hot.
This July 23, 2023 map showing areas in the Atlantic Ocean around the south tip of Florida, the Florida Keys, and Cuba reaching heat stress values above the thresholds for coral survival. Map: NOAA, public domain.

This methodology was used to map an extreme marine heat wave that triggered coral bleaching during the summer of 2023 in the Atlantic Ocean around the Florida Keys. Over that summer, levels of heat stress in the oceanic waters surrounding the Florida Keys had significantly exceeded the upper temperature limits that corals can survive in. Satellite-measured water temperatures in the Florida Keys reached the level at which bleaching occurs on June 14, 2023. The total heat stress experienced by corals in the area was almost three times higher than the previous record.

A graph showing a skyrocketing line for ocean temperatures for Florida.
Graph showing the record-high degree heating weeks readings for the Florida Keys area during the summer of 2023. Graph: NOAA, data from Coral Reef Watch, public domain.

Jacqueline De La Cour who is the Coral Reef Watch Operations Manager told NOAA, “To our knowledge, accumulated heat stress in Florida has not been this intense or widespread, this early in the summer season, since the satellite record began in 1985.” The last mass coral bleaching event around the Florida Keys happened in 2014 and 2015.

In 2023, the world’s oceans have experienced unprecedented warmth, showing much higher than average surface water temperatures such as in the tropical Pacific Ocean where El Niño is occurring, the North Atlantic, the Caribbean, and the Gulf of Mexico.


Photo of author
About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

Free weekly newsletter

Fill out your e-mail address to receive our newsletter!