Calculating Thermal Stress and Predicting Coral Bleaching:
A Lesson in NOAA Coral Reef Watch Methodology
Mass coral beaching (loss of symbiotic algae) is primarily caused by unusually high water temperatures. If the bleaching is severe or prolonged then mass mortality of corals can result as occurred in 1998 when over 90% of shallow water branching corals were killed over much of the Indian Ocean. The temperature of the sea surface is easily measured from satellite sensors such as the Advanced Very High Resolution Radiometer (AVHRR) on the polar orbiting environmental satellites of the US National Oceanographic and Atmospheric Administration (NOAA). Because of the importance of coral reefs to hundreds of millions of people whose livelihoods are dependent on them, it is useful to be able to predict warm water stress and when and where coral bleaching is likely to occur.
NOAA Coral Reef Watch (CRW) data products (http://coralreefwatch.noaa.gov/) measure the temperature stress that can lead to coral bleaching. In this lesson, you will learn how to use NOAA CRW data to predict bleaching and gain an in-depth knowledge of how NOAA CRW data are produced.
The lesson begins with sea surface temperature (SST) data files derived from AVHRR imagery, and leads you through the steps to calculate a long-term average temperature from these. This average, or climatology, is subtracted from a temperature image to produce an SST anomaly image, which shows deviations from the long-term average. You will then calculate a specialized anomaly, called a HotSpot, that shows areas that are above the average temperature for the warmest month of the year. It is these areas where corals are under stress and likely to be bleaching. Experience shows that corals become stressed when the SST is greater than 1°C above the highest monthly average temperature. Finally, you will look for regions under HotSpot stress for a prolonged period, calculating a metric called the Degree Heating Week. The number of DHWs show how much thermal stress has accumulated in an area over the preceding 12 weeks. In an area with 8 DHWs, you would expect widespread bleaching and coral mortality.
To demonstrate how these satellite data are used in the real world, the lesson ends with an activity based on a bleaching event in the Caribbean Sea in 2005. You will interpret satellite data to predict coral bleaching, then compare your predictions with real-life bleaching data.
See Brown et al., 2019: Long-term impacts of rising sea temperature and sea level on shallow water coral communities over a ~40 year period,
Nature Scientific Reports, 9, 8826, https://doi.org/10.1038/s41598-019-45188-x for an example application.
How to download the lesson
The lesson can also be downloaded from the NOAA Coral Reef Watch website at http://coralreefwatch.noaa.gov/satellite/education/bilko_lesson.html.
Image examples from the lesson
NOAA Coral Reef Watch sea surface temperature (SST) image of the world's oceans for 17 September 2005. Yellow to red colours show SSTs of 28-34°C.
SST anomaly image for the world for 17 September 2005. Positive anomalies (up to +5°C) are shown in yellow to red colours and negative anomalies (down to -5°C) are shown in cyan to purple colours. Note the pronounced warming anomaly in the eastern Pacific.
A CRW Coral Bleaching HotSpot image for 17 September 2005. Areas shown in lavender/purple have a HotSpot value between 0 and 1, i.e. are above the maximum monthly mean (MMM) but below the bleaching threshold. Regions shown in orange to red are more than 1°C above the MMM (the bleaching threshold) and experiencing thermal stress.
A Degree Heating Weeks (DHW) image for 17 September 2005. The different colours show DHWs from 1-16 (pale blue to magenta). Note that the eastern Caribbean is suffering from 4-12 DHW at this time. 2005 was the worst year on record for coral bleaching and mortality for the Caribbean.
Development of this Bilko training lesson was supported by the Remote Sensing Working Group of the Coral Reef Targeted Research (CRTR) Program. The CRTR Program is a partnership between the Global Environment Facility, the World Bank, The University of Queensland (Australia), the US National Oceanic and Atmospheric Administration (NOAA), and approximately 40 research institutes and other third parties around the world.
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