While it is clear that the current ENSO-driven mass bleaching event has been devastating and widespread, to put it in perspective, it’s essential to look back and compare it to similar incidents in the past. Some of these, which you may or may not remember, took place in 1982-83 and 1997-98; the latter of which resulted in NOAA declaring the first global mass bleaching event1. Analyzing data and contemporary reports from these two El Niños gives us valuable insight into the present catastrophe unfolding on our planet.
Widespread bleaching was first noticed and documented by scientists during the ENSO cycle of 1982-83. The El Niño event was the strongest that had occurred in many years, but the threat of coral bleaching was just coming to be understood. In 1997, the first bleaching events of a new ENSO cycle occurred, and researchers began analyzing data following the conclusion of the second wave of bleaching in 1998. One particularly illuminating example is a 2001 study by University of Miami researchers that sought to quantify the difference in scope and severity between the ‘82-83 and ’97-98 bleaching events2. Focusing on Panama and Ecuador, the study primarily documents the sea surface temperatures (SST) and broad-scale mortality rates of reefs in the two regions.
As is quite apparent in Figure 1, mean SST in Panama was noticeably higher in 1997 than 1982 (first years of the respective events). This distinction is not as noticeable for ’98 and ’83, and the large part of the SST data suggests that the two ENSO events were rather close in magnitude2. The data the scientists collected also reveals that 1982-83 had a much higher overall coral mortality rate (85%) in the region than ’97-98 (less than 10%)2. The study points out that seasonal upwelling, which is the rising of deep, cool water, may have contributed to this regional moderation of the 1998 El Niño2.
These conclusions demonstrate that great variation occurs between, and spatially within, mass bleaching events, even if ENSO climate effects occur in broadly similar patterns. While most coral were bleached or affected in some way, not all reefs were wiped out in 1998, as one might expect from such severe conditions, and the precedent set in 1983; natural processes seemingly prevented this from happening. A sample comparison of individual corals from the same region and species experiencing very different degrees of bleaching is shown below in Figure 2.
How does this all relate to the 2016 El Nino that just concluded? I think one essential takeaway from this study is that just because El Niños are getting stronger and longer in conjunction with climate change (as evidenced by 2015-16)3, sea temperature severity is not necessarily directly correlated with overall reef mortality in a region. Many other factors contribute to the health of reefs during an ENSO than just variation in air/sea temperature, which is usually what’s discussed in the public sphere. Considering the importance of understanding and predicting global mass bleaching events, looking at historical analyses and comparisons helps put their effects into a more accurate context.
1- “El Niño prolongs longest global coral bleaching event.” News and Features – NOAA, pub. online 23 Feb. 2016. Web. 20 Feb 2017. http://www.noaa.gov/media-release/el-ni-o-prolongs-longest-global-coral-bleaching-event
2- Glynn et al. “Coral Bleaching and Mortality in Panama and Ecuador during the 1997–1998 El Nino–Southern Oscillation Event: Spatial/Temporal Patterns and Comparisons with the 1982–1983 Event.” Bulletin of Marine Science, 69(1): 79–109, 2001. Web. 18 March 2017. http://www.ingentaconnect.com/content/umrsmas/bullmar/2001/00000069/00000001/art00007
3 – “El Nino Prolongs Longest Global Coral Bleaching Event.” American Geophysical Union, pub. online 23 February 2016. Web. 22 March 2017. https://news.agu.org/press-release/el-nino-prolongs-longest-global-coral-bleaching-event/