Global warming and its implications for coral reefs

At current projected levels of temperature increase it has been suggested that tropical coral reefs could be lost altogether as soon as 2050.

Coral reefs are marine structures of limestone built from the skeletons of coral animals, and it has taken millions of years to build the reefs we see today. Coral reefs occur in at least two types. Barrier reefs extend along coasts. The best-known example is the Great Barrier Reef off the east coast of Australia, which is the world’s largest coral reef (see more below). Coastal reefs form an addition to an island or a coast. Corals prefer such shallow areas where they eventually reach the water surface and create a kind of platform.

Coral reefs are also divided into two groups according to the water temperature and depth of occurrence. The most well-known and studied ones are the shallow-water coral reefs in tropical and subtropical waters, while the other type occurs in northern temperate areas in much colder water and at great depth. Most of this overview is focused on the tropical reefs, because they are already heavily impacted by global warming.

The projected water temperature increase will greatly affect the most biodiverse ecosystems of the oceans – the tropical coral reefs – which harbour some 25 per cent of all marine species over an area covering less than 0.1 per cent of the total ocean surface (Spalding et al. 2001). Another side effect of global warming is the increased content of carbon dioxide in the atmosphere. The liming processes in the corals are disturbed by increasing carbon dioxide.

Tropical coral reefs are found in more than 100 countries. This figure refers to the warm-water coral reefs, which are by far the most well known, because they occur in very shallow water and even at the present sea water level.

Much less known are the stone corals of temperate waters, which may occur at depths from 40 to 3,000 metres and at temperatures ranging from 4 to 13 degrees (Corcoran et al., 2006). These reefs have so far been found in 41 countries and their total area is estimated to be as great or even greater than the tropical coral reefs (Corcoran et al., 2006). In the North Atlantic these cold-water reefs are mainly built up of eyes corals (Lophelia pertusa), which occur off the Swedish West Coast for example (www.marbipp.se – 2016). The largest yet discovered cold-water coral reef made up of eyes corals is found off the Lofoten archipelago in Norway, and appears to cover an area of close to 100 km² (Fosså et al., 2003). The high density of reefs shown in the North Atlantic most probably reflects the intensity of research in this region and further discoveries are expected mainly in the deeper waters of tropical and subtropical regions.

The global oceans regulate our climate and weather conditions, like rainfall and floods. The oceans also absorb roughly one third of all carbon dioxide emitted through human activities, and have also absorbed some 90 per cent of the extra heat trapped by the rising concentrations of greenhouse gases (Gattuso et al., 2015). The temperature rises so far experienced mean that three-quarters of the world’s coral reefs are currently under threat (Burke et al., 2011).

As mentioned above, tropical coral reefs are probably among the least resilient marine ecosystems, as well as the first victims of global warming. These reefs occur in shallow water, where the temperature is rarely less than 20 degrees and salinity is around 3.5 per cent. I have emphasised (see below) that the number of fish species has declined significantly, but the most destructive effect on the well-being of a coral reef ecosystem is the effect on the reef itself. This process is termed coral bleaching. Corals are adapted to live within a narrow temperature regime. A temperature increase above the corals’ threshold level by as little as 1–2 degrees (C) for 5–10 weeks can lead to coral bleaching (Lang, 2002–2014).

Coral bleaching means that the corals lose their colouration and the underlying white calcium skeleton becomes visible. What happens is that the corals’ endosymbionts, called zooxanthels, lose their photosynthesising pigments. When coral bleaching occurs, some 60–90 per cent of the zooxanthels are lost and in addition individual zooxanthels can lose as much as 50–80 per cent of their photosynthesising pigments (Lang, 2002–2014). Some corals, however, may also have additional fluorescent pigments that are not associated with the zooxanthels and these corals may not lose all their colour. Bleached corals grow much more slowly than healthy ones, but under extreme conditions such as prolonged temperature rise above what is normally tolerated by the corals, mass mortality can occur, which may require decades to recover. Thus, mass coral bleaching events are projected to increase in frequency and intensity, and even preserving more than 10 per cent of present coral reefs would require warming to be limited to below 1.5 C (Frieler et al., 2013).

In the period 1997–98 warm-water coral reefs all over the world suffered from massive bleaching. The major impact was noted in the coral reefs of the Central Indian Ocean, where some 90 per cent of all corals in an area covering several thousand km² died, including most corals in the Maldives, the Chagos Archipelago and the Seychelles (Lang, 2002–2014). Other coral reefs, e.g. off the coasts of Thailand and Vietnam, were also badly damaged but they included more robust coral species that allowed the reefs to recover. Even parts of the Great Barrier Reef, Indonesia, the Philippines and the Caribbean area were also damaged but with a lower mortality of some 20–50 per cent (Lang, 2002–2014). These events were attributed to a combination of two weather scenarios, ENSO (El Nino-Southern Oscillation) and La Nina. The combination of those two weather phenomena probably caused the warmest water temperatures that “modern” corals have been subjected to, and it seems likely that some of the most severely damaged reefs may require years or even decades to recover. Even if most coral reefs will slowly recover there is a chance that local extinction of coral species may occur, which will lead to reduced biodiversity and, at worst, to less resilience to future accidents. This worst-case scenario may in future occur annually as a consequence of continued temperature increase, i.e. cause local or even regional destruction of entire coral reef ecosystems. At current projected levels of temperature increase it has been suggested that tropical coral reefs could be lost altogether as early as 2050 (Hoegh-Guldberg et al., 2015) and, in fact, fluctuating and rising ocean temperatures caused by global warming present the largest primary threat to coral reefs.

Lennart Nyman

The next issue of Acid News will include an overview of the impact of global warming on regional coral reef ecosystems.