Graphical banner with image of bleached stag horn corals and title of page, namely, Climate Impacts: Coral Bleaching


What is Coral Bleaching?

Graphical depiction of the components of a coral sitting on a calcium carbonate skeleton, including the body (corallite), polyps, tentacles, and mouth. An inset, zoomed-in circle from the edge of a tentacle shows colored zooxanthellae as dots in oblong cells.Coral bleaching is one of the most widely recognized impacts of rising ocean temperatures due to climate change. Corals are animals that live in colonies, but each individual coral, or polyp, has a symbiotic relationship with microscopic algae, zooxanthellae (zo-oh-zan-thel-ee), that live within the coral’s soft tissues. When water temperatures are elevated for prolonged periods, or repeatedly with only brief pauses, the stress on the corals causes the zooxanthellae to leave or be expelled.

While corals do feed directly from their surrounding waters through filter-feeding tentacles, most of their energy comes from the zooxanthellae. The relationship is mutual: the algae are protected and absorb carbon dioxide from the coral to use in photosynthesis, and in exchange, the coral receives oxygen and other nutrients, as well as their color. So when the zooxanthellae are expelled, the corals appear white, as their skeletons becomes visible, and they begin to starve.

Bleaching does not mean automatic death for corals. Given a chance to recover, some corals can bounce back, incorporating new zooxanthellae. However, with prolonged or repeated heating events, corals do not have a chance to recover, and with a lack of algae for too long, most corals will die.

Why Study Coral Bleaching?

Beyond their beauty and draw for tourism, coral reefs play an important role in the health of coastal ecosystems, supporting immense biodiversity, and providing important protection for coastlines from storm surge. With increasing frequency of marine heatwaves, including those predicted for this summer triggered by the most recent El Niño event, understanding coral bleaching and exploring how to mitigate its effects becomes critically important to the preservation of these ecologically, culturally, and economically valuable organisms.

Stories of Coral Bleaching

A scuba diver is observing a bed of staghorn corals, which are varying in shades of white and dark yellow

Pacific Islands brace for coral bleaching event

Coral reefs around the world are age-old providers and defenders of our livelihood. From cultivating food and medicine to protecting our shores from powerful storms, these ecosystems have proven to be vital resources on a global scale. Indigenous Pacific Island communities also find value in coral reefs, and their resources are often integrated into their cultures and traditions.

But a lingering threat to the vitality of these precious ecosystems remains: coral bleaching.

PI-CASC Projects Related to Coral Bleaching

Coral scape with a few small yellow fish

Ecological and socio-cultural responses to transplanting corals to enhance reef resilience near Oʻahu
PI: Crawford Drury; FY 2021
The increasing frequency and severity of coral bleaching events means that human intervention is needed to support the adaptive capacity of reefs. This project proposes to identify the sources of resilience in two important reef-building species known to be more resilient to climate stressors, by temporarily transplanting specimen between six reefs in two bays of windward Oʻahu.


Underwater image of colorful varieties of coral

Identifying locations for coral reef climate resilience
PI: Monica Moritsch; FY2020
Coral bleaching events jeopardize coral reefs’ ability to continue to provide important ecosystem services and cultural value. This project aims to identify locations where resilience-based management strategies for reefs could have the greatest chance of success in Guam and American Samoa.


Corals spread across an outcrop, some beautifully purple, others deadly white

Using cutting-edge technology to assess coral reef bleaching events and recovery rates in Guam and the Commonwealth of the Northern Mariana Islands
PI: Romina King; FY2020
Multiple coral bleaching events between 2013 and 2017 impacted coral reefs in Guam and CNMI, resulting in the loss of more than a third of shallow living coral cover, with some species experiencing more than 90% mortality. This project assess new NASA remote sensing technology, MiDAR, to conduct rapid assessments of coral bleaching over a wide geographic area.


Underwater picture of a large rice coral outcrop

Mitigating climate change impacts: What drives thermal resilience in Hawaiʻiʻs coral reefs?
PI: Ruth Gates/Shayle Matsuda; FY2016

Corals, the bedrock of marine ecosystems, are particularly susceptible to the stressors of climate change. But species’ reactions vary. This study identifies biological characteristics that enhance coral thermal tolerance, for improved future management.


Underwater view of spreading and branching corals

Coral adaptation and acclimatization to global change: Resilience to hotter, more acidic oceans
PI: Robert Toonen; FY2015

Excess CO2in the environment causes ocean acidification with potentially severe repercussions for coral reefs and coastal societies. This work clarifies coral resilience mechanisms and evaluates potential for adaptation to future conditions.

Other Coral Bleaching Resources

Coral Gallery

Corals and coral landscapes capture the imagination, even when bleached and ghostly. These images were all taken by Dave Burdick in the waters around the island of Guam. Click on an image to see a larger version.