Posted: November 12th, 2022

The Effects of Climate Change on the World’s Coral Reefs

The Effects of Climate Change on the World’s Coral Reefs

Introduction
Coral reefs are one of the most varied ecosystems on this earth. They house many marine life species and provide a plethora of advantages to natural ecosystems as well as the human population. Coral reefs bring a lot of money to coastal nations through fishing, tourism and discoveries of new medicine. In addition, they provide building materials and natural coast protection. Unfortunately, there is an immense die-out of coral reefs all over the planet. At the beginning, a majority of the population believed that the major threats to the coral reefs existence was anthropogenic effects like sedimentation and water pollution, but now it is evident that the problem is bigger in scale (Houghton & Cambridge University Press, 2012). The biggest global threat to the world’s coral reef ecosystem is climate change. Scientific evidence now plainly shows that the ocean and the globe’s atmosphere are warming, and these changes can be attributed to greenhouse gases generated by human activities. Increasing global temperatures, rising oceanic CO2 and other consequences of global warming are all affecting the health of coral reef in a negative way. Global warming will impact coral reef ecosystems, via altered ocean current patterns, rise in sea level, temperature rise and changes to the intensity and regularity of tropical storms.
The Effects of Climate Change on the World’s Coral Reefs
Climate change causes coral bleaching. A coral reef that is healthy is characterized by the symbiotic relationship that exists between the coral animals and the algae found in their tissue that nourish them. The energy of the coral, which is responsible for its bright color, takes place through photosynthesis. However, it is a fragile balance, and when the reef is affected, the relationship collapses. Therefore, the coral animal refuses the algae which fail to provide it with color and energy (Hoegh-Guldberg et al., 2007). The coral animal becomes semi-transparent once the algae have been expelled from the tissue hence the bleaching effect. The “stress” that the coral reef goes through can be largely attributed to heat. Coral reef can only thrive within a restricted thermal tolerance range; a 1 degree Celsius increase in temperature is all that it takes for bleaching to take place. Without algae to nourish it, coral cannot live past five weeks before starving to death. The rising ocean temperatures caused by global warming have been linked to coral bleaching.
Climate change negatively affects the capability of corals to make calcium carbonate. The rising levels of carbon dioxide in the ocean due to climate change have increasingly caused the ocean to acidify. Many studies have been carried out to examine the effect that rising CO2 levels have on corals, and they all conclude that high levels of CO2 make it hard for marine animals to make their calcium carbonate shells. Numerous marine organisms including corals utilize calcium and carbonate ions from ocean water to secrete calcium carbonate skeletons. Decreasing the concentration of either the ion can affect the skeletal deposition rate, but the carbonate ion is much less abundant compared to the calcium ion and seems to play an important role in the calcification of corals. Increases in CO2 in ocean water decrease the concentration of the carbonate ion in the surface water, thus decreasing the rates of calcification of a number of the most vital calcium carbonate producers. These encompass corals and calcareous algae on coral reefs.
The right formulas are as as follows:
CO2 (gas)↔CO2 (aqueous)
CO2 (aqueous)+H2O↔H2CO3
H2CO3↔ H++ HCO3-
HCO3-↔ H++ CO32-
Ca+ CO32-→CaCO3
The important compound whose quantity regulates the final production rate of calcium carbonate is the carbonate ion (CO32-).

Graph 1: a reduction in PH due to acidification leads to a fall in the carbonate ion
Source: Ocean acidification: global warming’s evil twin. (2017). Retrieved from https://skepticalscience.com/argument.php?a=243&p=2

Graph 2: the net effect of increased CO2 in water is a subsequent reduction of the carbonate ion and with it the calcification rate
Source: Ocean acidification: global warming’s evil twin. (2017). Retrieved from https://skepticalscience.com/argument.php?a=243&p=2

In their study Pandolfi et al (2011) discovered that when the concentrations of CO2 are at 560 ppm, corals will not accrue calcium carbonate and dissolve instead, leading to immense die-offs. In another research, calcifying algae were exposed to 4 different temperatures and 4 different levels of CO2; the bigger CO2 amounts caused a considerable decrease in photosynthetic efficiency and ability to accrue calcium carbonate in all corals (Sinutoh, 2011). Evidently, high amounts of CO2 in the ocean have negative impacts on coral reefs. As human activities generate large amounts of greenhouse gases, concentrations of CO2 will continue to rise in the oceans and the corals will increasingly be exposed to a chemically unhealthy ecosystem resulting in decreases in the degree and diversity of coral reefs in years to come.
Sea-level rise connected to global warming may not pose a main threat to coral reefs, as long as coral growth keeps pace with the rising sea level. Nonetheless, coral reefs can be affected in a great way if disastrous ice melting leads to major rises in the sea level. In addition, increasing sea levels have the potential of causing increased sedimentation because of shoreline erosion, which could negatively affect coral reefs. For instance, smothering of coral may occur due to the sedimentation. Smothering may in turn prevent the corals from getting enough sunlight or may cause the algal blooms to grow. The secondary impacts in the relationship between sea level and reefs are of even bigger concern. Low islands on corals have a vital connection with sea level and the regions ‘behind’ corals are normally protected by the wave-resistant structures. The structure and rise of most Indo-Pacific Islands are a consequence of a number of thousand years of stable sea level, which followed a decline from a little higher sea level some thousand years ago. Therefore, the current physical relationship between sea level and reef elevation is unlikely to be preserved by the responses of corals to increasing sea level, so that even fast growing reefs may end up becoming less efficient wave barriers (Ferrari et al., 2015).
Changes in storm patterns due to climate change results in stronger and more regular storms that can lead to the damage of coral reefs. Storms destroy corals directly via wave action and indirectly via light reduction by suspended sediment. Many studies on the effects of storms on coral reefs focus on individual events of storms; however, a meta-analysis of data dated between 1977 and 2001 revealed that the Caribbean coral cover reduced by 17 percent a year after a hurricane, and they did not recover for at least eight years after the impact (Ban, Graham & Connolly, 2014). Stronger hurricanes that followed caused more loss in the coral cover, but the 2nd of 2 consecutive hurricanes caused little extra damage, suggesting a bigger effect from increasing hurricane strength instead of increasing regularity, as a warm sea surface temperature is required for cyclogenesis (Ban, Graham & Connolly, 2014). Cyclones such as hurricanes and typhoons can be very damaging to reefs, as the waves they produce can move big reef colonies and decrease corals to rubbles. Also, storms rips loose soft corals and sponges and throws them on the shore or washes them out to the deep oceans. More so, the storms scours sediment out of lagoons leading to the formation of new beaches or sweeping of sediment into deep water hence lost from the coral (Dow & Downing, 2016). Coral reefs that have already been weakened by ocean acidification and bleaching will be at a higher risk of physical damage from tropical cyclones. Tropical storms have a tendency of generating extreme freshwater overflow with dissolved chemical and sediment releases from land. As a consequence, non-thermal coral bleaching and smothering of corals by sand and mud occur.
Climate change has led to changes in precipitation, which is also affecting corals negatively. The increasing amount of precipitation causes water in the oceans to increase further leading to sedimentation and alterations in the ocean’s salinity. For corals to grow and thrive they require water that has a certain salinity and low turbidity; unfortunately weather changes such as increasing rainfalls create an environment that is unfavorable to corals (Richey & Sachs, 2016). As patterns of weather continue to change and be more frequent, these kinds of living situations will become more ordinary for reefs. Because corals do not have the ability to adapt easily, the stressful situation brought about by immense rain threatens their survival.
Coral reefs are increasingly being affected by marine infectious illnesses. Whereas these diseases are caused by an abundance of new microbes and not by temperature as such, higher temperatures accelerate the growth of coral bacterial microbes and intensify their virulence due to increased toxin release. Therefore, climate change increases the prevalence and incidence of infectious illnesses in coral reefs. Notably, stress effects such as high temperatures are normally concurrent with worsening mortality in the affected coral organisms (Lesser & van Woesik, 2015). Alternatives for reef defense against microbial pathogens are restricted and hugely because of a protective mucus coating. Elevated temperature stimulates the generation of the mucus layer between reef tissues and bacteria. The bacterial flora and the biochemical composition on mucus can be affected by elevated temperatures, and can thus result to the substitution of advantageous species by microbes and opportunists. The mucus coating protects corals against foreign invasion and it also supports bacterial colonization; it ultimately breaks down and degrades from microbial enzyme activity (Lesser & van Woesik, 2015). When coral breaching occurs, the mucus coating is also affected; bacteria are thus able to penetrate the thin and susceptible coral tissues that do not have any further efficient antibacterial resistance. The microbes access the cell membranes of corals and the tissues accessed are damaged. Unfortunately, the attacks are not reversible and the tissue from the skeleton that lies beneath is stripped leaving necrotic remains that draw opportunistic pathogens, parasites and turf algae (Lesser & van Woesik, 2015).
Climate change has contributed to an increase in oceanic temperatures over the years. The high temperatures have caused evaporation to increase rapidly, so as the planet has become warmer in the recent years there has been a considerable increase in tropical lower atmosphere cloudiness and humidity. This has caused the salinity of ocean water to increase. Coral reefs are very sensitive to change in salinity. Reefs are restricted to a salinity ranging from 30-40 parts per thousand. Therefore, reefs adapted to the normal salinity in the ocean are not able to tolerate exposure to brackish water for long and may thus be destroyed by too much salinity (Sale, 2015). Nonetheless, a species-specific tolerance gradient exists in both directions, with a small number of reef species found in hyper-saline surroundings (for example the Arabian Gulf) and a number of species that can thrive close to river effluxes.

Chart 1: oceanic temperature has been increasing
Source: Levitus 2009
Climate change also affects coral reproduction. The temperature rises associated with global warming reduces coral reproductive output since thermal stress and increased respiration decreases the energy available for the development of the gonad (reproductive organs responsible for making sperm and eggs) even before bleaching takes place. Coral reefs that have gone through bleaching do not reproduce at all. Immature sperm and eggs are released prematurely because of thermal bleaching, in effect resulting to mass abortion (Cinner et al., 2016). For that reason, climate change will greatly decrease sexual reproduction of coral reefs. High temperatures also decrease the survival rate of the larvae of the coral and the competence of the larvae as well (the capability of changing into juveniles and settling to generate a skeleton), so climate change will also hugely decrease coral recruitment and dispersion.
Climate change is responsible for altering oceanic currents. The ocean often moves huge amounts of heat around the earth via worldwide ocean currents (e.g. via downwelling, upwelling and thermohaline circulation). Ocean currents will be impacted by changes in rainfall, wind, salinity and temperature because of global warming. Alterations of ocean currents can impact the transport or preservation of pollutants, larvae movement, and temperature regimes that may negatively affect species such as corals that are sensitive to thermal changes i.e. can trigger and worsen coral bleaching (Kleypas et al., 2016). Notably, the altered can currents can contribute to lack of food and obstructs coral larvae dispersal. This is so because the strength and timing of currents that bring nutrients to the corals or carry their larvae have gone through some changes attributable to climate change. It is important to note that where the speeds of currents increase, the water will become more green and the ranges of corals decreased because of reduced light regardless of higher food levels.
Conclusion
The discussions above demonstrate that the impacts of global warming on reefs are extensive enough to raise concern all over the world. These impacts are significant to the point that corals are now under threat. Unless appropriate actions are taken to minimize the effects of ocean acidification and climate change, then more corals will die-off. Despite the fact that there are different factors that should be tested to determine their relationship with ocean warming and acidification, it is now more evident that global warming is one of the earliest contributing elements. Whatever the reasons for the changes are, they are adversely influencing corals and at disturbing rates.

References
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Richey, J. N., & Sachs, J. P., 2016, Precipitation changes in the western tropical Pacific over
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