Rainforests are to the land, as what coral reefs are to the ocean. They provide a high range of biodiversity, and wide range of free “ecosystem services”: food for up to a billion people around the world, habitat for economically valuable fish species, sand for beaches, and protection from storms and waves.
From global warming to pollution and overfishing, coral reefs face a collection of serious threats. These were an unprecedented three-year-long global ocean heat wave that stressed corals, causing them to bleach and die off on tropical reefs worldwide. See below:
Image from https://oceanservice.noaa.gov/facts/coral_bleach.html
Many coral reefs experienced mass bleaching back-to-back in 2015 (top) and in 2016 (bottom). This also happened in Australia in 2017.
Coral bleaching depends on how high the temperatures are above the annual monthly maximum and how long the unusual heat persists. Severe coral bleaching was reported in areas circled in white. Please note that these are NOAA Climate.gov images adapted from State of the Climate in 2017, using NOAA Coral ReefWatch data (version 3.1).
Heat stress and bleaching
A brief bleaching event doesn't necessarily kill coral, but prolonged, severe bleaching can lead to disease and starvation.
As mentioned previously, corals get their colour from pigments in their symbiotic algae, which provide food to corals through photosynthesis. This partnership breaks down during periods of extreme heat stress, and the corals expel the zooxanthellae.
Historically, global-scale coral bleaching has been associated with ENSO (El nino southern oscillation, El Niño and La Nina) events, which generally (El Niño) raise global temperatures.
The first mass coral bleaching was observed during the El Niño in 1983, and the first global event coincided with the El Niño of 1998.
The world’s tropical reefs were stressed again during a moderate-strength 2010 El Niño. The coral-bleaching event of 2014–2017 was unusual not just for its long duration,
Though an El Niño was anticipated in 2014, it didn't really materialize until 2015, yet bleaching-level stress was already well underway by that time. A strong El Niño arrived in 2016, and heat stress occurred at 51 percent of the world's coral reefs into early 2017, when a La Niña was in place.
The 36-month heatwave and global bleaching event were exceptional and many reefs globally lost viability as there where consecutive events. More than 75 percent of Earth’s tropical reefs experienced bleaching-level stress between 2014 and 2017 and reached mortality level.
Bleaching occurring at an average rate of once every 25–30 years in the 1980s, mass bleaching now returns about every 4 years and is expected to further accelerate.
Severe bleaching is now occurring more quickly than reefs can recover, with severe consequences to ecosystems and people.
Can we turn this around?
References
Eakin, C.M., Liu, G., Gomez, A.M., De la Couri, J.L., Heron, S.F., Skirving, W.J., Geiger, E.F., Marsh, B.L., Tirak, K.V., Strong, A.E. (2018). Unprecedented three years of global coral bleaching 2014–17. Sidebar 3.1. [in State of the Climate in 2017]. Bulletin of the American Meteorological Society, 99(8), S74–S75
