Healthy corals reefs are filled with life, sounds and activity as thousands of fish and invertebrates species that inhabit them, but, currently few coral reefs are this abundant in species as they were a few decades ago. Surveys from 1960s and 19970s show that coral cover on undisturbed reefs was 70% or higher, however, in recent times coral loss on individual reefs is widespread even on the isolated and managed reefs such as Florida Keys and unpublished recent surveys show pacific coral reef cover to be about 30% which could be half of what it was a few decades ago. Human induced climate change has been indicated as the main cause of coral loss. Worlds reefs destroyed by climate change was described in 1998 as 11% and another 16% damaged by the 1997-98 coral bleaching which was caused by increase in temperatures as that was the hottest year recorded in the past century (Houghton, 2004).
Corals are defined by both biological and geographical components. Coral reefs are the most biologically diverse ecosystems on earth as they provide important habitat to 25% of marine species found in shallow tropical and sub tropical waters. They are also major sources of ecosystem goods and services to small island states in the tropics (Christensen, Eakin, Hoegh-Guldberg, & Mumby, 2008). Geographically they are located in the indo-pacific and the Atlantic with the Caribbean being the most extensive coral reef biological province. They are made of limestone, which is calcium carbonate produced by corals and calcareous algae. These reefs are colonial organisms that host single celled organisms called zooxanthelleae in their body tissues. They exist in a symbiotic relationship where the corals obtain food as a result of photosynthesis by the zooxanthelleae and this in return benefit from nutrient released by the corals and the complementary effects of carbon dioxide between them.
This symbiotic relationship supports the biological communities that reside in coral reefs as they are the base for the coral food web. The coral reefs ecology is comprised of millions of animals and plant species which are otherwise known as coral communities and include corals, starfishes, shrimp, sponges, anemones, worm mounds, turtles, many fish species, sea fans, moray eel and lobsters among many other organisms (Houghton, 2004).
The earth has gone through various climate cycles but in the last 10-11000 years earth’s temperatures and climate has remained relatively stable. This is the period when civilization of men started with the agrarian revolution, growth of cities and invention of machines. This saw gradual increase in the amount of carbon dioxide and other green house gases in the atmosphere causing a warming effect, which continues up to date. Carbon dioxide current level is 385 parts per million in 2001, which is the greatest recorded so far. Temperature has also been increasing rapidly with a recorded 10F between mid nineteenth to the end of the twentieth Century. An increase of about 0.50C has also been recorded for the past 25 years with the total heating from the late 19th century to 2005 being 0.80C. This has resulted in many effects to the environment, humans and animals. In addition to this, global phosphorous and nitrogen cycles have speed up due to artificial fertilizers use and extensive land use changes, the hydrological cycle has changed due to damming of rivers and water diversion and climate change, natural ecosystems altered by fishing, forestry, and agriculture have contributed to global climate changes, which in turn have altered the coral reef ecosystems (NASA Earth Observatory, 2009).
Effects of climate change on coral reef communities
Coral bleaching happens when the symbiotic relationship between coral animal and a plant like organism, zooxanthellae, breaks due to rise in the normal ocean temperature and the coral dies. Zooxanthellae contain photosynthetic pigments which it uses to make food and oxygen for coral in return for nutrients. The greater the rise in temperature and the longer the time for the rise, the higher the mortality rate of coral which turns coral white due to loss of zooxanthellae. Global warming has led to 10 increases in ocean temperature for the last couple of decades which have resulted to severe coral loss (Houghton, 2004). Two types of bleaching happens as a result of temperature increase: algal stress bleaching which results from impairment of photosynthesis by high temperatures and light levels; physiological coral bleaching which occurs as a result of reduced tissue biomass and ability to host algae due to added demand for energy of regulating the abnormal temperatures (Buddemeir, Kleypas & Aronson, 2004).
Coral bleaching as a result of global warming in the last eight years in the Indian Ocean on the coasts of Seychelles Island and Madagascar has resulted to massive death of coral leaving the reefs as rubble and reduced diversity of fish by half in the region. This is according to a report by the study by marine biologists published in the National Academy of Sciences in May 2006 (Markey, 2006).
Sea level rise
Sea levels are estimated to rise by about 0.1 and 0.9 meters by 2099 as a result of increase in sea water and melting glaciers according to IPCC (NASA Earth Observatory, 2009). Over several thousand years sea levels have remained stable and in response to this, reefs have developed to a point where they are sea-level-limited. This ensures that water circulation is limited and no potential for further upward growth. A small sea level increase will be beneficial to them. With increased sea level rise, reefs at the lower depth limit might be drowned due to limited light available to them but the projected increase are within the capacity of most reefs to survive. The problem with rise in sea levels could only arise as a result of sedimentation due to enhanced erosion of shore lines as they would limit light and oxygen (Buddemeir et al., 2004).
El Nino- Southern Oscallition (ENSO)
This has been associated with mass bleaching in the last two decades with most bleaching happening during the El Niños of 1982-83, 1987-88 and 1997-98. This happens when mean sea temperatures in coral regions changes. Regions of abnormal warm water occur throughout the pacific and Indian ocean for example. These correspond with seasonal maximum water temperatures which results into rise in temperatures beyond the normal levels, impairing coral functioning. El Niño occurrences have risen in rate of recurrence, severity and duration in the last few decades, though climate experts says there is no link to global warming, they agree that El Niño-like events have evolved. Severity of the bleaching in the past few El Niños have presented a “worst case scenario” especially when combined with the rise in sea surface temperatures leading to massive bleaching and mortality of coral reef communities.
Studies by the National Academy of Sciences published in 2006 showed that more than 16% of the world’s coral reefs were destroyed during the 1998 El Niño. The study also associates extinction of four species of fish, that is, butterfly ish, damselfish and two wrasse species to the effect of 1998 El Niño (Markey, 2006).
Ocean circulation changes
Circulation of waters in the ocean as a result of wind-driven upwelling (local) and thermohaline at global scale are predicted to change with increase in global warming. Most of coral reefs are found in high latitudes where boundary currents that bring warm waters from tropical regions occur through local and thermohaline circulation (THC). If these circulations are altered in strength and path, the coral ecosystems depending on them will also change. THC is threatened with shut down or slowing down due to changes in ocean temperatures and freshwater runoff, which in turn will affect ocean circulation and upwelling patterns that have the potential to impact the corals negatively. Scientists are however uncertain of how climate change is going to affect THC (Christensen et al., 2008).
Precipitation and storm patterns
Records show that precipitation over most tropical regions between 100 S-100 N have risen in the past century by around 0.2-0.3%. These have been predicted to intensify in most regions. This will lower the salinity of the ocean water and raise the sediment discharge and disposal into the river mouths, which is very armful to the coral reefs as they prevent oxygen absorption and sunlight from reaching corals resulting to massive deaths of corals. Frequency and severance of droughts have also been predicted to increase as global warming and climate change increases. This will result into vegetation cover loss in the areas affected, which will result into intensified erosion and sediment deposition in river mouths leading to coral reef ecosystem destruction (Buddemeir et al., 2004).
Global climate change has resulted in ocean warming which has led to increased incidences of infectious diseases which cause most coral deaths particularly in the tropical areas of the Caribbean. The severity of these diseases such as the white syndrome increases with rise in temperatures. White syndrome is a condition that leads to a disease on the coral reef and is caused by prevalence of relatively warm conditions over the years (Houghton, 2004).
Buddemeir, R. W., Kleypas, J. A & Aronson, R. B. Coral Reefs and Global Climate Change: Potential contributions of climate change to stresses on Coral Reef Ecosystems. A report by PEW Center on Global Climate Change, Arlington, 2004. Web.
Christensen, T., Eakin, M., Hoegh-Guldberg, O. & Mumby, P. Coral Reefs Under Rapid Climate Change. The Under sea journal online, 2008. Web.
Houghton, J. Global Warming: the complete briefing. Cambridge. Cambridge. University Press, 2004.
NASA Earth Observatory. Potential Effects of Global Warming, 2009. Web.
Markey, S. Global warming has devastating effect on coral reefs, study shows. National Geographic News Online, 2006. Web.