Described as the “rainforests of the sea” coral reefs are among the most biologically rich and productive ecosystems on Earth. Existing in a narrow band of the tropics; they cover just 0.2% of the world’s oceans and yet provide the habitat for a quarter of all marine species(1).
In the modern world, it is perhaps
unsurprising that this
ecosystem is one of the most
threatened. Recent news broadcasts of ghostly white corals provide a stark contrast to the vibrant images that come to mind when describing a coral reef. Nonetheless local threats such as overfishing, coastal development and agricultural runoff add increasing pressure to systems already being pushed to their limits by warming oceans.
A hot topic
Current projections indicate that global warming will cause a 1-4o C increase in the temperature of the world’s oceans. Like many organisms, corals are sensitive to their physical environment and have a limited temperature tolerance(2).
Warmer waters can trigger a process known as ‘bleaching’, where coral expels the symbiotic algae (zooxanthellae) living within their tissues; providing glucose, glycerol and amino acids that are vital for coral survival, as well as their characteristic colours.
Bleaching however, is not always fatal. It is possible for corals to recover providing the thermal stress is short lived and followed by a long period of stability. By contrast, prolonged and repeated thermal stress increases the probability of coral mortality(3). Once corals begin to die, reefs degrade, coral structures erode and the reef community begins to collapse.
Strong bleaching episodes coincide with periods of high sea surface temperatures(2). Current climate change projections strongly suggest that by 2060, 90% of reef locations will experience severe annual bleaching(4) thereby increasing the likelihood of mass coral death.
A global and present crisis, this year the US National Oceanic and Atmospheric Administration (NOAA) declared this to be the 3rd recorded global coral bleaching event.
The current bleaching episode coincides with a strong El Niño Southern Oscillation, which will further intensify sea surface temperatures, triggering what is predicted to be the most severe and expansive coral-bleaching event on record. It will exceed that of 1997/98 (also a strong El Niño year) where 18% of the world’s coral reefs were lost(3).
The NOAA predict that 38% of reefs will experience a degree of bleaching this year alone, and even if the majority of these regions show good recovery, it is estimated that 12,000 km2 of the worlds’ coral will die (Fig.1).
Why coral reefs matter
Aside from their biological importance as biodiversity hotspots and their role in ecosystem connectivity (mangroves, sea grasses and open oceans)(5), coral reefs provide regulating, provisioning, cultural and supporting services as described in the Millennium Ecosystem Assessment.
Approximately 275 million people live within 30km of reefs and are dependent on them for their livelihood and wellbeing.
Coral reefs provide US$30 billion annually in global goods and services; the 1998 El Niño event alone caused an annual loss of $91 million due to reef damage(1). Undoubtedly, a reduction in the extent and quality of reef systems will have extensive economic consequences in the future.
What services do coral reefs provide?
Food provisioning – 25% of the fish catch in developing countries is from coral reef associated fisheries(2). In the Philippines alone, 1 million people rely on coral reefs as a source of food. Highly productive and diverse systems, in an absence of any other protein source, 1km2 of reef can support 300 people(5).
Coastal defence – The complex three-dimensional nature of coral reefs means they reduce wave energy by 70-90%. Healthier and larger coral reef systems have a greater ability to prevent coastal erosion and buffer storm damage, thereby reducing the need (and therefore cost) for man-made coastal defences.
Tourism – This is an ever growing and incredibly valuable industry for tropical coastal communities. From beach resort to divers exploring coral reefs, reef associated tourism generates US$9.6 billion a year(1). Current predictions indicate that 60% of coral reefs could be lost by 2030; communities that are heavily reliant on reef-associated tourism will therefore suffer substantial economic and social losses. In the developing world, direct losses to household income, supporting services and infrastructure developed and maintained with reef-associated tourism would be severe.
Is it all doom and gloom?
Coral reefs are effective at resisting natural stress such as storms and short-term temperature fluctuations. However, chronic human induced pressures can diminish coral’s resilience, making them more prone to bleaching and less able to recover.
As little as 27% of global reefs are within a marine protected area (MPA). Effective MPAs successfully reduce human impacts and act as larval stores, which are vital in assisting coral recovery after a period of bleaching. Following the 1998-bleaching event, reefs inside MPAs in the Indian Ocean recovered faster than those suffering from heavy human pressures.
Local impacts such as destructive fishing will be easier to tackle than the long-term problem of climate change. As such a worldwide campaign is needed to increase the extent and effectiveness of MPAs.
The ever-looming threat of climate change proves more challenging. As temperatures increase so too will the frequency and severity of bleaching induced mortality(2).
The current El Niño event highlights the increasing pressures faced by coral reefs. We know that corals are resistant and resilient when experiencing bleaching episodes provided they are not exacerbated by intensive human activity, but warming waters will increase the frequency of periods of bleaching. As such, a global campaign to tackle local scale pressures is vital if there is to be any hope of reefs surviving climate change.
We sit at a crucial tipping point, which will determine the future of coral reefs. Without immediate international collaboration at both a local and global climate level the future of coral reefs looks fairly bleak.
- Chen P-Y, Chen C-C, Chu L, McCarl B. Evaluating the economic damage of climate change on global coral reefs. Glob Environ Chang. Elsevier Ltd; 2015;30:12–20.
- Hoegh-Guldberg O. Climate Change, coral bleaching and the future of the world’ s coral reefs. Symbiosis. 1999;48.
- Roff G, Zhao J, Mumby PJ. Decadal-scale rates of reef erosion following El Niño-related mass coral mortality. Glob Chang Biol. 2015;n/a – n/a.
- van Hooidonk R, Maynard JA, Manzello D, Planes S. Opposite latitudinal gradients in projected ocean acidification and bleaching impacts on coral reefs. Glob Chang Biol. 2014;20(1):103–12.
- Moberg F, Folke C. Ecological goods and services of coral reef ecosystems. Ecol Econ. 1999;29(2):215–33.