Climate Change Impacts to Coral Reefs

Increased anthropogenic (human-produced) carbon dioxide in the atmosphere causing climate change is also increasing sea surface temperatures, causing ocean acidification, increasing tropical cyclone frequency and severity, and causing sea level rise. Many aspects of climate change can significantly affect coral reefs.

 

Increased Sea Surface Temperatures

The environmental stress to corals due to increasing sea surface temperatures (SST) disrupts the mutualistic relationship between Scleractinian corals (reef-building or hard corals) and zooxanthellae causing the expulsion of the symbiotic algae from the coral tissues, a process known as bleaching. In reef building corals, the zooxanthellae provide up to 95% of carbon necessary for growth, reproduction, and maintenance. Laboratory experiments conducted by Glynn and Croz (1989) illustrated that even slightly elevated temperatures (30 to 32 degrees Celsius) from normal (26 to 28 degrees Celsius) for several weeks lead to declines in coral health and ultimately death. Mass mortalities due to bleaching events are strongly linked to increased SSTs. However, coral bleaching has been attributed to a variety of disturbances (see Coral Bleaching lab note). Large-scale bleaching events have increased in frequency and magnitude over the past 40 years. Major bleaching events in 1997-1998, 2002, 2005 and 2010 have impacted entire reef systems. One of the largest bleaching events coincided with the distribution of abnormally high sea surface temperatures due the 1997-1998 ENSO (El Nino - Southern Oscillation). During the large-scale event, bleaching affected coral reefs in over 50 countries. If SSTs continue to increase due to climate change, bleaching events will be more frequent and severe.

Ocean Acidification

Furthermore, increasing carbon dioxide levels in the atmosphere are increasing the absorption of carbon dioxide by sea water, affecting a process known as carbon buffering (shown below). In general, the increased carbon dioxide absorption by sea water decreases pH, resulting in more acidic water. The absorption of carbon dioxide by the world's oceans reduces the carbon dioxide levels in the atmosphere. However, sea water with low pH can have severe impacts to organisms which produce calcium carbonate, such as corals.

 

Scleractinian corals have the potential to produce significant quantities of calcium carbonate, which make up the coral's skeleton, but calcification rates vary under different environmental conditions. Decreasing calcification rates have been noted due to decreasing pH. Cooper et al. (2008) noted that calcification rates have declined by 21% between 1998 and 2003 on the Great Barrier Reef. In addition to increasing ocean acidity, these significantly compromised calcification rates have also been attributed to other environmental conditions, such as temperature and nutrients. Fine and Tchernov (2007) illustrated the effects of highly acidified water on Scleractinian coral growth. The corals completely lost their skeletons and resembled anemones. While the anemone-like polyp remained healthy, the fitness of the organism becomes compromised due to skeletal loss. Spillman et al. (2011) states that rapid change in ocean chemistry is likely to outpace the potential for evolutionary adaptation to ocean acidification. If corals are unable to adapt, the functional loss of reef ecosystems will threaten the numerous important economic, social, and environmental assets provided by the coral system, including the loss of global diversity. Therefore, climate change is a great threat to coral reefs world-wide. To make matters worse, many coral reefs are already degraded due to marine pollution, overfishing, and other local-scale disturbances.