EarthLabs > Corals > Lab 6: Using Data to Identify Hot Spots and Predict Bleaching Events > 6C: Degree Heating Weeks

Using Data to Identify Hot Spots and Predict Bleaching Events

Part C: Degree Heating Weeks

Degree heating week map for North America on April 17, 2008. Image source: NOAA.

Heat stress on corals will accumulate if the SST stays above the bleaching threshold for an extended period of time. So, in addition to measuring how far above the bleaching threshold the SST is, scientists also measure how long the SST stays above the threshold. These measurements are know as degree heating weeks (DHWs). DHWs tell us how much thermal stress has built up in a given area over the last 12 weeks.

  1. Go to NOAA's Coral Reef Watch home page. Click on the Degree Heating Weeks icon in the left-hand navigation bar to access the most up-to-date DHW data. The Coral Reef Watch DHW maps highlight those areas around the world where corals have been under thermal stress for extended periods of time.
  2. Find the cell in the table with the most current data. Click on the Global link for that date to open up a global map of current DHW data.

  3. Examine the map to familiarize yourself with how the data are reported.
    • black areas have not accumulated thermal stress over the previous 12 weeks (the temperature did not crossed the local bleaching threshold)
    • colored regions indicate thermal stress to corals in those areas
    • the units for DHW are "degree C-weeks", which combine the intensity and duration of thermal stress into one single number
    • when the thermal stress reaches 4 degree C-weeks, you can expect to see significant coral bleaching, especially in more sensitive species
    • when thermal stress is 8 degree C-weeks or higher, you would likely see widespread bleaching and mortality from the thermal stress

    Checking In

    Answer the following question about the DHW map.

    • Using the dotted line grid as a way of dividing the map into smaller regions, how many regions were experiencing thermal stress?


    Stop and Think

    1: Did you identify any areas where you would expect to see significant or widespread bleaching? Explain.


  4. Use your browser's back button to return to the main DHW map page. Scroll down to the bottom of the page and click on the DHW Animations link.
    How to find DHW animation link


  5. Watch the animation at least once all the way through, noting which months appear to have more or less accumulated thermal stress than is seen in the most current data.

    Checking In

    Answer the following question about accumulated thermal stress over the last 6 months.

    • How do the current levels of accumulated thermal stress compare to those over the last 6 months?
    • Do certain regions appear to experience prolonged thermal stress more often than others? (i.e., Do certain regions appear more at risk than others?)

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