Published: February 25, 2020
NOAA Coral Reef Watch's (CRW) near real-time satellite products and modeled Outlook are indicating that in early 2020, it is likely the Great Barrier Reef (GBR) in Australia will experience its most widespread coral bleaching event in recent years. Current predictions suggest that this event is likely to be less severe than some of the more recent mass bleaching events (e.g., 2016 and 2017), but that the most severe bleaching is expected in the far northern portion of the GBR (which was severely impacted in 2016) and in the far southern reef areas. Additionally, peak bleaching will likely occur between February 25 and March 6, 2020 - unless weather patterns maintain heavy cloud cover over the GBR during this time.
Since early January 2020, sea surface temperatures (SSTs) along the GBR have been greater than the stress threshold (Maximum Monthly Mean, MMM). This has resulted in Coral Bleaching HotSpots greater than 1°C for the majority of the GBR (Figure 1). This means that the majority of the GBR is currently accumulating harmful heat stress that can lead to coral bleaching.
The actual DHW value at which bleaching becomes significant on a reef can vary from event to event. Previous GBR observations suggest that by a DHW of 4, significant bleaching is underway and observable. We don't definitively know why the difference exists from one event to another. However, research suggests that contributing factors include, but are not limited to, light levels, a change in species composition of reefs following previous mortality events, pre-conditioning, adaptation, and different oceanographic conditions.
Note that most of the GBR's DHW values are lower than they are in the deeper surrounding waters, due to the strong tides up and down the GBR. The tides affect the mixing of hot surface waters with cooler subsurface waters. This manifests as cooler water at the surface and is an extremely important mechanism to consider when assessing the potential for coral bleaching on the GBR.
Figures 5 and 6 are the various Outlook forecasts for the likelihood for coral bleaching on the GBR for the next five weeks. They suggest that heat stress will cause bleaching across the entire length of the GBR. More specifically, the Outlook suggests that the week with the highest probability of bleaching (Figure 6) is the period nearest the neap tides, centered on March 1, 2020. It's worth noting that the Outlook does not have tidal mixing embedded in the model, and yet we know that tidal mixing has a huge influence over heat stress on the GBR.
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