Coral reefs are the metropolises of the underwater world, housing more than a quarter of all oceanic species while covering less than a thousandth of the ocean floor.1 Sponges, sharks, clown fish, sea stars, and all the intrinsic biodiversity rely on the health of these reefs for both shelter and hunting grounds.
Unfortunately, as anthropogenic global warming raises sea surface temperatures, coral reefs are showing signs of sickness that threaten the stability of these rich ecosystems. Algae tucked away in the coral’s tissue provide nutrition as well as the characteristic rainbow of colors. Increased sea temperatures are dangerous to these symbiotic algae and, as they die in the warmer waters, corals turn white, a process known as bleaching. If such conditions last longer than a couple months, entire coral colonies die off from increased vulnerability to disease and lack of necessary nutrition.2
Such mass mortality has already occurred once in recent history. In 1998, an anomalous temperature increase, likely due to El Nino, bleached corals in the Pacific and Indian Oceans. The latter was particularly hit hard, with more than 70 percent of reefs dying near Maldives and the Lakshadweep Islands in the following years. Increased sea temperatures correlated with the amount of bleaching in Pacific reefs3, a finding that resonated with the scientific community. This led to many models using the temperature-bleaching connection to predict that rising temperatures would have dire consequences for reef survival around the world.
But is temperature really the only factor affecting coral survival? And are increased bleaching observations always the death knell for reefs? Data from the Indian Ocean revealed a less clear connection between warming and coral bleaching4, suggesting a heterogeneity in reef responses and resilience of particular reefs that likely depends on other factors.
Read more about what other factors are involved at the Eyes on Environment blog!
- NOAA. “Where are corals found?” Accessed June 2, 2015.
- Union of Concerned Scientists. “Early warning signs of global warming: coral reef bleaching.” Accessed June 2, 2015.
- McClanahan TR et al. “Effects of climate and seawater temperature variation on coral bleaching and mortality.” Ecological Monographs, 77, 503-525, 2007.
- Wilkinson C et al. “Ecological and socioeconomic impacts of 1998 coral mortality in the Indian Ocean: an ENSO impact and a warning of future change? Ambio, 28, 188-96, 1999.
McClanahan, T., Maina, J., & Ateweberhan, M. (2015). Regional coral responses to climate disturbances and warming is predicted by multivariate stress model and not temperature threshold metrics Climatic Change DOI: 10.1007/s10584-015-1399-x