Impacts of Climate Change on Coral Reefs and the Marine Environment


It is estimated that 70 per cent of the Earth's surface area is made up of oceans,1 the most productive habitat, comprising 75 per cent of all known species. This unique environment, which remains generally unexplored and hidden from the world, plays an important role in regulating global temperature and is the primary producer of oxygen. Coral reefs, which comprise only about 0.5 per cent of the ocean floor, are complex three-dimensional structures built up over thousands of years as a result of the deposition of calcium carbonate skeletons of the reef building coral species. These reefs are often referred to as the "rainforest of the sea". This allegory underestimates the complexity of coral reefs, which have a greater diversity of animal and plant life than rainforests, circulate nutrients through the intricate food web and provide food at all levels of the food chain.

Historically, the sea has served as a major transportation network, a source of food and a favourite recreational area. Most major cities were developed along the coast as trading areas. The growth of these cities is manifested today in the percentage of the world's population (approximately 80 per cent) who live within 100 kilometres of the coast and depend on the sea for their livelihood (approximately 3.5 billion people).2 In fact, the survival of the world's poorest people depends on their close relationship with the sea. The economic importance of the sea is evidenced in the ecosystem services provided by way of fisheries, tourism, coastal protection, and in its role as a source of raw materials. This dependency on the sea is now threatened by environmental conditions brought on by global climate change.


The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2007)3 presented strong evidence that global warming over the last century was largely a result of human activity, such as the burning of fossil fuel, deforestation and the conversion of lands for agricultural use. Temperature records from as far back as 1850 show that the globe has on average warmed by 0.8° C, and further analysis has shown that since the 1970s each decade has been warmer. Global concentrations of carbon dioxide (CO2) have also shown increased levels from an average of 280 parts per million (ppm) in the mid-nineteenth century, at the beginning of the industrial revolution, to approximately 388 ppm at the beginning of the twenty-first century. The global warming trend is expected to continue, as IPCC estimates that the global average temperature will be 2.5-4.7° C higher in 2100 when compared to pre-industrial levels.4


In order to assess the impact of climate change on coral reefs and the marine environment, we need to examine the predicted environmental changes and evaluate the capacity of the marine organisms to adapt to these changes.5 Climate models indicate that the sea surface temperature is expected to rise by 1 to 3° C, while the sea level is expected to rise by 0.18 to 0.79 metres. Regional weather patterns are likely to change, resulting in an increase in the severity and frequency of storm events, particularly cyclones. In addition, ocean circulation patterns are expected to be modified and pH is expected to decline as a result of the absorption of CO2.6


Despite having taken millions of years to evolve, marine organisms, under today's conditions, must adapt very quickly to new conditions. Marine organisms will be affected by changes in two main aspects of their environs, namely, by changes in the natural habitat and food supply, and changes in ocean chemistry. Marine plants, mainly phytoplankton, are primary producers that form the base of the food chain. There is expected to be a gradual decrease in the quantity of these plants in warmer waters, effectively reducing the amount of nutrients available to animals further along the food chain. In addition, temperature is an important trigger in the life cycles of many marine plants and animals, and often the onset of feeding, growth and reproduction are synchronized. With these processes out of sync, organisms are likely to arrive on the scene when their food sources have long gone.

The anticipated increase in ocean temperature is predicted to stimulate the migration of marine organisms based on their temperature tolerance, with heat-tolerant species expanding their range northward and those less tolerant species retreating. This change in ocean dynamics will have a deleterious effect on species that are unable to migrate and could lead to their demise. Ocean acidification, or increased CO2 levels which result in the lowering of the pH of seawater, not only reduces the abundance of phytoplankton but also decreases calcification in certain marine animals like corals and shellfish, causing their skeletons to become weaker and growth to be impaired.

Possibly one of the greatest threats facing corals, however, is that of bleaching as a result of increased sea surface temperature. Bleaching occurs when prolonged increased sea temperatures cause a breakdown in the relationship between the corals and their symbiotic zooxanthellae (algae). The coral subsequently expel the zooxanthellae, lose their colour (bleaching) and become weak. Some corals are able to recover, often with compromised immune systems, but in many cases they die.


The real challenge is that modifications resulting from climate change impacts are being superimposed on a marine environment already under stress from direct and indirect anthropogenic stressors associated with overfishing and improper fishing practices, coastal development, sedimentation, land-based sources of pollution and marine pollution. This convergence of multiple stressors places the world's coral reefs under considerable pressure and it is estimated that about one third of the massive reef building coral species are facing extinction. Coral reefs all over the world are experiencing a significant decline. However, in the scientific community, it is believed that this decline predates the carrying out of detailed scientific studies.7 Due to their inaccessibility, the studies of coral reefs are fairly new when compared to other disciplines going back only about 50 years, but even over this short time span a significant decline in the status of coral reefs worldwide has been observed.8,9 During the 1980s and 1990s, the decline of coral cover was further exacerbated by the loss of the dominant algae consuming herbivores (sea urchins and herbivorous fishes), coral bleaching and coral diseases.10 Nowhere has this decline been more pronounced than in the Caribbean, which is now regarded as the poster child of coral reef demise. Data evaluated from as far back as the 1960s have conclusively shown the progressive decline in overall coral cover and the increase in abundance of fleshy algae.11,12


The options for tackling the issues related to coral reefs are twofold: adaptation and mitigation.13 Adaptation involves local research and conservation efforts in building resilience in the reef ecosystems through such activities as reef restoration, the identification of stress-tolerant species, the reduction of over-fishing and the establishment of marine protected areas (MPAs). MPAs are regarded as the best management tool for conserving coral reefs and other marine environments14 because their no-take zones provide a safe haven for populations to grow and to subsequently replenish the surrounding marine environment. Adaptation, however, is not enough; a serious global response to mitigate climate change by directly reducing emissions, improving energy efficiency, limiting deforestation and increasing carbon sinks is required. At this point, mitigation measures are only expected to prevent further warming, as the reversal of existing conditions is now considered highly unlikely.15


It is now agreed by coral reef scientists around the world that the marine environment in general and coral reefs in particular are being adversely affected by climate change. Most scientists believe that the rate of change of climatic conditions is potentially beyond the capacity of coral reefs to adapt and recover.16

The outlook for the Caribbean is less optimistic than for the Indo-Pacific. Research seems to indicate that the low sea urchin populations allow the algae to outcompete the corals for space. One possible solution is to maintain a healthy population of parrotfish to keep the algae population in check. Although research is being carried out, the ability of coral to adapt to warmer waters has not been shown for many species. However, scientists have observed that in some regions, especially the remote areas of the Pacific, where reefs are far removed from human impacts, these have shown resilience to an increase in sea surface temperature and bleaching.

With all the evidence pointing to the almost inevitable demise of coral reefs, there is now an urgent need for marine scientists to be proactive and engage the public and endangered communities to the reality of the threat. This engagement is essential if a change in attitude and behaviour is to be effected. Scientific knowledge needs to be transferred into practical solutions that will engender public support. On a wider scale, there needs to be collaboration between governments and the impacted communities in order to formulate and implement polices geared towards long-term sustainability.

Are we fighting a losing battle? Quite possibly climate change might have passed the point of no return. What is clear is that any solution to climate change is also the solution to coral reef recovery.




3 IPCC (

4 The Royal Society, "Climate change: A Summary of the Science", September 2010, p. 16. (see

5 Przeslawski, R. et al. (2008). "Beyond Corals and Fish: The Effects of Climate Change on Non-coral Benthic Invertebrates of Tropical Reefs", Global Change Biology (2008) 14, 2773-2795, DOI: 10.1111/j.1365-2486.2008.01693.x.

6 IPCC (

7 Jackson, J. 2012. "The Future of Coral and Coral Reefs in a Rapidly Changing World", International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012.

8 C. Wilkinson (ed.) (2004). "Status of Coral Reefs of the World: 2004", Vol. 1 and 2. Australian Institute of Marine Science, Townsville, Queensland, Australia.

9 Wilkinson, C., Souter, D. (eds.) (2008). "Status of Caribbean Coral Reefs after Bleaching and Hurricanes in 2005", Global Coral Reef Monitoring Network, and Reef and Rainforest Research Centre, Townsville, p. 152.

10 Sweatman, H. et al. (2011). "Assessing loss of coral cover on Australia's Great Barrier Reef over two decades, with implications for longer-term trends", Coral Reefs (2011) 30:521-531. DOI 10.1007/s00338-010-0715-1.

11 Gardner et al. (2003). "Long-term region-wide decline in Caribbean Corals", Science 301:958-960.

12 Jackson, J. (2012). "The Future of Coral and Coral Reefs in a Rapidly Changing World", International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012.

13 Hoegh-Guldberg, O. (2012). "Coral reefs and global change: where do the solutions lie?", International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012.

14 Hughes, T. et al. (2003). "Climate Change, Human Impacts, And the Resilience of Coral Reefs Science", 301, 929 (2003). DOI: 10.1126/science.1085046.

15 Lowe, J. A.; et al. (2009). "How Difficult Is It To Recover from Dangerous Levels of Global Warming?" Environmental Research Letters 4:014012. DOI:10.1088/1748-9326/4/1/014012.

16 Hoegh-Guldberg, O. (2012). "Coral Reefs and Global Change: Where do the solutions lie?", International Coral Reef Symposium, Cairns, Australia, 9-13 July 2012.