High ocean temperatures have been identified as a major trigger for the massive coral bleaching event that affected the tropics, including the Great Barrier Reef, in 2015 and 2016 according to a paper published today in the journal Nature.
According to the study's lead author, Professor Terry Hughes from the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University, "The severity of the 2016 bleaching was off the chart", and more severe than the two previous major bleaching episodes on the Great Barrier Reef in 1998 and 2002.
NOAA's Coral Reef Watch program has predicted and monitored all three severe events on the Great Barrier Reef using satellite remote sensing data and was a key contributor to this study.
Since the 1980's most of the information about global SST has come from spacecraft, including NOAA’s geostationary orbiting environmental satellites (GOES) and polar-orbiting environmental satellites (POES). Since the late 1990s, Coral Reef Watch has applied these satellite data to the problem of coral bleaching. This has been critical to monitoring the third global coral bleaching event since it started in June 2014. While prior global events in 1998 and 2010 lasted less than 12 months, the current one is in its 33rd month.
Monitoring Sea Surface Temperature Via Satellite
The ocean and most other objects emit radiation in the infrared and the microwave wavelengths of light, the amplitude of which varies with the temperature of the ocean. Satellite instruments, such as the GOES I-M imager aboard the current GOES satellites and the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA/NASA Suomi NPP satellite (the successor to NOAA’s legacy POES satellites), can be used to measure these wavelengths from space.
Today in addition to satellite and shipboard measurements there are thousands of floats in the ocean measuring temperature and salinity. These are used to calibrate and validate the measurements recorded by satellite instruments in addition to sampling throughout the water column.
Among the key findings from this study are:
- 2015 and 2016 saw record temperatures that triggered a massive episode of coral bleaching across the tropics, as confirmed by both aerial and in-water surveys, including along the Great Barrier Reef.
- Coral bleaching events should no longer be thought of as individual disturbances to reefs, but as recurring events that threaten the viability of coral reefs globally.
- The Great Barrier Reef has had three major bleaching episodes, in 1998, 2002 and 2016, with the latest being the most severe and with catastrophic levels of bleaching occurring in the northern third of the Reef (a region approximately 800 km or 500 miles in length).
- The amount of bleaching on individual reefs in 2016 was tightly linked to local heat exposure.
- The cumulative, superimposed footprint of the three mass bleaching events on the Great Barrier Reef has now encompassed virtually all of the Reef.
- Better water quality or reduced fishing pressure did not significantly reduce the severity of bleaching.
- Past exposure to bleaching in 1998 and 2002 did not lessen the severity of the bleaching in 2016.