Coral Bleaching

NOAA operates two polar-orbiting satellites, each with an infrared sensor that detects the temperature of the ocean’s surface.  Because the satellites constantly orbit the earth, they measure the water temperature around the entire globe each day.

Widespread coral bleaching affected these Siderastrea Radians at Buck Island, St. Croix, US Virgin Islands, in October 2005. Learn more...

Scientists use NOAA’s Coral Reef Watch to monitor coral reefs around the world.  Corals start to feel stressed when the sea surface temperature is more than 1°C above the average we expect to see in the hottest month.  Using this technique NOAA maps “HotSpots,” areas that are higher than the expected maximum.  Even more important for corals is build-up of warm-water stress over time.  We also map this cumulative stress by adding up the HotSpots over a 3-month period.  These are called Degree Heating Weeks (DHWs), which pinpoint areas where corals are at risk for bleaching.  The status is updated twice per week, and the data are posted to the Coral Reef Watch website for the public to access. 

In October 2005, heat stress values in the eastern Caribbean were associated with a massive coral bleaching event. Learn more...

The Satellite Bleaching Alert (SBA) system is an automated alert system that monitors thermal stress conducive to coral bleaching via the HotSpot products. The SBA was developed as a tool for coral reef managers, scientists, and other interested people. Currently, the alerts are available for 24 coral reefs around the world.

Status levels are defined in terms of the HotSpot and Degree Heating Weeks (DHW) products:

Status Level	Definition	Description
No Stress	HotSpot ≤ 0	corals are not currently experiencing any thermal stress
Bleaching Watch	0 < HotSpot < 1	temperatures are above normal summer maximum, but corals are not yet stressed
Bleaching Warning	1 ≤ HotSpot and 0 < DHW < 4	corals are experiencing a low-level buildup of thermal stress
Bleaching Alert Level 1	1 ≤ HotSpot and 4 ≤ DHW < 8	corals are currently stressed, accumulating to a level where bleaching is expected
Bleaching Alert Level 2	1 ≤ HotSpot and DHW ≥ 8	corals are currently stressed, accumulating to a level where widespread bleaching and some coral mortality is expected

This figure shows the locations monitored by the SBA system.  The number of sites will expand in the coming years, to improve global coverage.  Courtesy ReefBase Learn more...

E-mail alerts are available for all of the reef sites.  An automated e-mail will be sent to a subscriber when the thermal stress level changes at the reef site. For more information visit our subscription page.

For more information on all of NOAA's coral activities, visit the Coral Reef Conservation Program: http://www.coralreef.noaa.gov