Underwater close-up of coral polyps mixed with orange and light purple.

Satellite mapping of coral bleaching

As sea surface temperatures continue to rise above normal due to climate change, satellite data is being used to map coral bleaching events caused by increased ocean warming.

What is coral bleaching?

Coral reefs They 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. Coral reefs provide a protective environment and compounds that algae need for photosynthesis. In turn, zooxanthellae contribute to the coral’s energy needs and aid in the deposition of calcium carbonate, which is critical for the formation and maintenance of coral skeletons. It is zooxanthellae that give healthy corals their colorful appearance.

Paramoricia coral polyps. Photo: Art Howard, NOAA-OER/BOEM/USGS, public domain.

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

Global warming is raising sea temperatures, which stresses coral reefs and could disrupt the symbiotic relationship they have with algae. Coral bleaching is a condition in which stressed polyps expel the algae living in their tissues, causing the coral to lose its vibrant colors.

Rising ocean water temperatures put stress on coral reefs and cause coral bleaching.  Colonies
Rising ocean water temperatures put stress on coral reefs and cause coral bleaching. Colonies of “fire coral” that have lost their symbiotic algae, or “bleached,” on a reef off Islamorada, Florida. Photo: Kelsey Roberts, USGS. Public domain.

Climate change is also causing carbon dioxide concentrations in the atmosphere to rise. Ocean water absorbs this carbon dioxide, making it more acidic. Acidification affects the ability of corals to deposit calcium carbonate, weakening them and making them more vulnerable to stressors such as high temperatures.

Tracking coral bleaching with satellite images

As oceanic heat waves continue, heat buildup occurs. Since coral reefs will react negatively when ocean temperature conditions become too hot or too cold, scientists can use remote sensing data from satellites to map what is known as “accumulated heat stress” in the oceans that leads to coral bleaching.

Mapping heat stress accumulated in the ocean

This condition is commonly measured as “thermal warming weeks” (DHW), which determines how hot the water is compared to the average temperature and how long the high temperature will last. When accumulated heat stress reaches a certain threshold, it can have harmful effects on marine ecosystems, including causing coral bleaching, where coral loses its color and can eventually die if the stress continues.

Scientists can calculate daily changes in accumulated heat stress by combining remotely sensed data from polar-orbiting satellites, for example NASA-NOAA Suomi NPP, and geostationary satellites such as NOAA’s GOES weather satellites. This data is then entered into computer models to calculate accumulated thermal stress values.

When accumulated thermal stress values ​​reach 4, coral bleaching can result. When the accumulated thermal stress value reaches 8, these coral bleaching events will likely lead to coral death.

Extreme marine heat wave around the Florida Keys

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

This methodology was used to draw a map Extreme marine heat wave Which led to coral bleaching during the summer of 2023 in the Atlantic Ocean around the Florida Keys. During that summer, heat stress levels in the ocean waters surrounding the Florida Keys greatly exceeded the upper temperature limits at which corals could survive. Satellite-measured water temperatures in the Florida Keys reached the level where bleaching occurs on June 14, 2023. The total heat stress experienced by coral reefs in the region was nearly three times higher than the previous record.

Jacqueline de la Cour is the Director of Coral Reef Monitoring Operations Noah said, “To our knowledge, heat stress accumulating in Florida has not been this severe or widespread this early in the summer season since satellite recording began in 1985.” The last mass coral bleaching event around the Florida Keys occurred in 2014 and 2015.

In 2023, the world’s oceans experienced unprecedented warming, showing temperatures well above average surface water temperatures, such as in the tropical Pacific where El Niño occurs, the North Atlantic, the Caribbean, and the Gulf of Mexico.



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