Satellites & Bleaching Degree Heating Weeks

This chart shows a 2005 example of our DHW product. Click the image for more information and the latest global 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. The HotSpot chart will give you a snapshot of where corals are stressed right now. But also remember that the heat stress will build up if the temperature around the corals stays above the bleaching threshold over time. So we need some way to not only measure how far the temperature is above the threshold, but also how long it has stayed above. This is what the Degree Heating Weeks (DHW) product was designed to portray.

DHWs show how much heat stress has accumulated in an area over the past 12 weeks (3 months). In other words, we add up the 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 is eight degree C-weeks or higher, you would likely see widespread bleaching and mortality from the thermal stress. DHW animations are a good way to visualize how the stress accumulates over time.

The DHW history is also shown on the time series graph for each Virtual Station site. If you follow the 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 DHWs:

In the graph below, you can see that the Degree Heating Weeks 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 the satellite bleaching alert levels, which are discussed in the next section.

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

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