There are a variety of human induced ocean changes that have degraded coral reef systems; these changes have greatly intensified in the last two centuries driven by the human population explosion. Proximate drivers of these changes include ocean warming, marine pollution, overfishing and mechanical abrasion of reefs. Although ocean warming may contribute only a fraction of the total stress to coral reef systems, the combined impact of the totality of all these environmental stressors is significant.
Colonies of corals, marine invertebrate animals a few millimeters in diameter, develop a symbiotic relationship with coralline algae in the shallow, nutrient-poor waters of the tropics. Algae supply the coral invertebrates with carbohydrates, and their photosynthetic pigments impart color to the corals. In return, coral invertebrates provide the algae with a stable environment, carbon dioxide, and nutrients. Together, they secrete calcium carbonate (CaCO3) that forms elaborate reefs, which provide habitat for a diverse array of animal species. The world’s largest coral reef system is the Great Barrier Reef, which extends for 2000 kilometres along the northeastern coast of Australia, and covers approximately 37,000 sq km.
Phenomenology of warming
Many corals live at close to their upper thermal tolerances.  Warming of the seas by as little as 1°C or 2°C above historical averages tends to stress corals, such that they expel algae. Corals without algae turn white, a process appropriately called coral bleaching. If bleaching persists for several months, corals typically die. Global warming has generally exacerbated the incidence of coral bleaching. Coral bleaching in conjunction with other abiotic factors, including the decrease in seawater pH, may cause irreparable damage to 40% of the reefs during the next few decades. 
It has long been known that corals are under attack from a number of environmental stressors that are induced by activities of humans. Chief among these threats are overfishing and water pollution (including nutrient discharge, herbicide and pesticide pollution and sewage discharge).There is an ongoing negative synergistic effect cumulating from the suite of these stressors, which is systematically degrading coral reefs worldwide. Thus the effect of any ocean warming is magnified by the backdrop of these other stressors.
With or without future changes in ocean temperature, the outlook for global coral reef systems is not favourable. The ongoing growth in worldwide human population threatens continuing assault upon coral reefs through demand for food (evinced by overfishing) and through water pollution discharge (sewage, excess fertilizers, herbicides) and sediment runoff. The large quantites of sediment runoff entering oceans arises from intensive farming pratises, overgrazing and urbanisation. Locations such as Madagascar and other Indian Ocean reefs are in severe threat, given the regional population growth pressure and lack of strong environmental protection standards. The Australian reefs are an exception, since the Australian government has been proactive in ocean water quality protection and reef conservation. Many Caribbean locales are also under severe threat where some countries' national standards are not adequate to provide protection for coral reefs.
 Harley, C. D. G., A. R. Hughes, K. M. Hultgren, B. G. Miner, C. J. B. Sorte, C. S. Thornber, L. F. Rodriguez, L. Tomanek, and S. L. Williams (2006) The impacts of climate change in coastal marine systems. Ecology Letters 9:228-241.
 Knowlton, N. (2001) The future of coral reefs. Proceedings of the National Academy of Sciences of the United States of America 98:5419-5425.
 Madeleine J.H.Van Oppen and J.M.Lough (2009) Coral bleaching: patterns, processes, causes and consequences. 178 pages.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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