Satellites & Bleaching

50-km Degree Heating Week

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satellite coral bleaching Degree heating Weeks

Shown here is an example 50-km Degree Heating Week (DHW) map for the Western Hemisphere (from 2005). Click the image for more information and a larger view of the latest near-real-time global 50-km DHW data.

Earlier in this tutorial, you learned that corals are sensitive to temperatures warmer than the bleaching threshold, 1°C above the highest summertime mean SST. You also learned that Coral Reef Watch's 50-km Coral Bleaching HotSpot product provides a snapshot of where corals are stressed right now around the globe. Remember though that heat stress will build up if the temperature around the corals stays above the bleaching threshold over time. We need some way to measure not only how far the temperature is above the threshold, but how long it has stayed above that point. This is what the 50-km Degree Heating Week (DHW) product was designed to portray. As with the other 50-km products in this tutorial, the 50-km DHW is updated on the Coral Reef Watch website twice per week.

The DHW shows how much heat stress has accumulated in an area over the past 12 weeks (3 months). In other words, we add up the Coral Bleaching HotSpot values whenever the temperature exceeds the bleaching threshold. It is a running sum: as we advance one half-week in time (a timestep), a half-week timestep "falls off" the back end of the 12-week window.

Areas shown in black have not accumulated thermal stress over the previous 3 months; i.e., the temperature had not crossed the local bleaching threshold. Any colored regions indicate thermal stress to corals in those areas. The units for DHW are "degree C-weeks", combining the intensity and duration of thermal stress into one single number. Based on research at Coral Reef Watch, when the thermal stress reaches four degree C-weeks, you can expect to see significant coral bleaching, especially in more sensitive species. When thermal stress reaches eight degree C-weeks or higher, you would likely see widespread bleaching and mortality from the thermal stress. 50-km DHW animations are a good way to visualize how coral bleaching heat stress accumulates over time.

The 50-km DHW history is also shown on the time series graph for each 50-km Virtual Station site. If you follow the 50-km SST line, you can look for times when it touches or exceeds the bleaching threshold temperature. The areas highlighted in red below are added up to make 50-km DHWs:

time series graph

In the graph below, you can see that the 50-km DHWs are also plotted in a separate section along the bottom of the graph, with the scale on the right-hand axis. The four- and eight-DHW thresholds are shown as dashed, horizontal red lines. The colors that fill below the DHW line correspond to established satellite bleaching alert levels, which are discussed in the next section.

satellite sst

Try our hands-on 50-km DHW exercises to learn more about this product.

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