*This is a guest post by Karuna Sehgal - student in my 'Causes & Consequences of Biodiversity' course.
The past three decades of human activity has altered the earth in more ways than one. The Earth is losing species, ecosystems and biodiversity because of warming climates, among other factors. Coral reefs, in particular, are greatly impacted by the rise of global surface temperatures.
Coral Reefs throughout tropical and sub-tropical oceans are under tremendous heat stress resulting in coral bleaching and mortality. Corals are animals that live in a symbiotic relationship with microscopic dinoflagellate algae that inhabit the coral tissues (Baker et al., 2008). Increased water temperatures result in corals expelling dinoflagellates living in their tissues, causing the coral to turn white, ending its symbiotic relationship (Heron et al., 2017). This does not necessarily mean death for the coral; however bleaching still adversely impacts corals by inhibiting growth and reproduction (Heron et al., 2017). This symbiotic relationship provides the coral with about 90% of the energy it needs to thrive, it also enables corals to construct limestone skeletons that form the three-dimensional structure of reefs, which provides habitat for over a million species (Heron et al., 2017. They are referred to as the Rainforests of the Sea because they are the most bio-diverse ecosystem in the ocean, comparable to rainforests on land. Species richness and the diversity found in these systems are phenomenal and breathtaking, and yet they are dying at an alarming rate.
|Fig. 1: Examples of a healthy and a bleached coral reef (images modified from Wikipedia pages on coral reefs and reef degradation, respectively)|
|Fig. 2: Satellite image of coral bleaching alerts from 2014–2017 (image from NOAA Coral Reef Watch)|
Bleaching and heat stress spread across tropical oceans and intensified during El Niño, and continued from La Niña and beyond (Heron et al., 2017). This period has included the three warmest years on record: 2014, 2015, and 2016 (Heron et al., 2017). Figure 2 shows that more than 70% of the global coral reef locations have experienced bleaching and most of these have experienced it twice or more, since June 2014 (Heron et al., 2017).
Baker AC, Glynn PW, Riegl B (2008) Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook. Estuarine, Coastal and Shelf Science 80:435-471.
Cofroth MA, Lasker HR, Oliver JK (1989) Coral mortality outside of the eastern Pacific during 1982-83: Relationship to El Niño. In: Global Ecological Consequences of the 1982-83 El Niño-Southern Oscillation. Glynn, PW. (ed.). Elsevier.
Heron et al. 2017. Impacts of Climate Change on World Heritage Coral Reefs : A First Global Scientific Assessment. Paris, UNESCO World Heritage Centre.
Hoegh-Guldberg O, et al. (2015) Reviving the Ocean Economy: the case for action - 2015. WWF International, Gland, Switzerland.Geneva, 60p.
O. Hoegh-Guldberg, P. J. Mumby, A. J. Hooten, R. S. Steneck. (2007). Coral Reefs Under Rapid Climate Change and Ocean Acidificaition. Science, 318, 1-7. Doi: 10.1126/science.1152509Stocker TF, et al. (2013) Climate Change 2013: The Physical Science Basis. Working Group 1 (WG1) Contribution to the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5), Cambridge University Press.