The Impact of Climate Change and Coral Reef Destruction

1.1   Introduction

Carbon emissions from human activity have generated two complications for coral reefs: climate change and ocean acidification (Anthony 2016). Climate change leads to ocean warming, which greatly effects biological and ecological reef processes, causes extensive coral bleaching events, and fuels tropical storms (Hoegh-Gulberg et al. 2017). Ocean acidification decelerates reef growth, changes competitive interactions, and damages population replenishment (Noonan, Kluibenschedl, Fabricius 2018). Ocean warming and acidification represent an almost contradictory challenge by eroding reef resilience and simultaneously increasing the demand for reef resilience (Anthony et al. 2010). This literature review will address this problem in the context of challenges and potential solutions. Organization efforts can make up for reduced coral reef resilience in the face of global change, but to a limited extent and over a limited time frame (Mcleod et al. 2018). Critically, a realistic perspective on what sustainability measures can be achieved for coral reefs in the face of ocean warming and acidification is important to avoid setting unachievable goals for regional and local-scale management programs (Mcleod et al. 2014).

2.1 Background to Ocean Environment and Coral Reefs

A decade ago, it was reported that coral reefs were to face an uncertain future under climate change and ocean acidification (Hughes et al. 2003). Now, as evidence from observational and experimental research is accumulating, the report focuses on the changes we expect coral reefs to undergo (Pandolfi, Conolly, Marshall 2011), the consequences for society (Cinner et al. 2013), and potential solutions (Rau, Mcleod, Hoegh-Guldberg 2012). The expected ecological change will be a shift away from structurally diverse, species-rich seascapes toward a system lacking in numbers and variety but with more stress-resistant species (Ortriz et al. 2014). The Intergovernmental Panel on Climate Change (IPCC) supports this research along with various lines of evidence.

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Firstly, worldwide research verifies the predictions of an atmosphere progressively becoming enriched with carbon dioxide to be factual, which has not occurred to this extent for approximately 800,000 years (Ball 2018). The IPCC stated in 2015 that the warming of the climate system is unequivocal. The Australian Bureau of Meteorology at that time recognized that the year 2015 existed to be the warmest year since 1880 which was also validated by the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Information (NOAA NCEI 2016) releasing their annual analysis, stating that in 2015, the warming trend was 0.90°C warmer than the global average for the twentieth century. The Earth’s primary heat sink, based on the warming trend of the upper ocean, is highly indicated by the global warming trend (Dahlman, Lindsey 2018).

Second, after the commencement of the industrial era, the sea has absorbed nearly one-third of the carbon dioxide produced from human activity (Freely et al. 2004). This uptake has caused a decline in the pH of surface ocean waters by approximately 0.1 units, corresponding to a more than 25% increase in the concentration of hydrogen ions (Orr et al. 2005). Ocean acidification is a chemical process with less factors involved and produces a clearer cause-and-effect relationship with the atmospheric concentration of CO2 and global carbon emissions (Hughes et al. 2007). An emergent body of investigational work shows that ocean acidification from the middle to the end of this century will lead to an adverse biological and ecological responses for vital species of coral reef organisms (Kroeker et al. 2010). Areas of reef that are exposed to clean carbon dioxide provide opportunity for coral reefs under ocean acidification. The research indicates a large loss of species, ecological functions and physiological deficiency of invertebrates and fish (Gattuso, Hoegh-Guldberg, Pörtner 2014). Predictions demonstrate that if carbon emissions continue at the same rate, the ocean environment will alter conditions that are marginal for reef growth this century (Camp et al. 2018).

Coral reefs are not only exposed to the pressures of climate change and ocean acidification, but are also exposed to local and regional pressures such as overfishing, pollution in urban development, deforestation and agriculture land (Hilmi et al. 2017). These combined, lead to and increase of impacts and compromised resilience (Anthony et al. 2010).

2.2 Climate Change resulting in Ocean Warming and Acidification

Global warming trends vary geographically showing a near-linear relationship with the carbon accumulated in the atmosphere. The trend for ocean warming greatly follows the global trend. The global upper ocean’s heat content has amplified immensely from approximately 1990 showing a mean thermal difference through 2015 reaching approximately 0.15°C for the upper (0–700m) layer. Data from another layer (0-100m) show a 0.4°C difference.

Figure 1. a) Mean of upper oceans (0-700m) estimated global heat content for several data sources. b) mean of annual thermal anomalies for two global ocean layers. Figure from the NOAA/NESDIS/NODC Ocean Climate Laboratory.

This warming trend of the accumulated atmospheric carbon ultimately drives ocean acidification towards the expected trajectory (IPCC 2014). Forecasts are highly dependent on which carbon emissions scenario and mitigation procedures play out on a global scale (IPCC 2014).

Representative Concentration Pathways (RCPs) capture the carbon concerns of diverse societal activities across the globe, including policies, variations in vegetation land cover, technological trends, and efforts at mitigation through shifts to renewable and alternative energy sources (IPCC 2012). It is predicted by the RCP that there will be a decline in pH to less than 7.8 by the year 2100 (Hobday, Lough 2011). The carbon chemistry of the upper ocean, leading to a level below the optimum pH required for coral reef growth (Hobday, Lough 2011). Lowered pH, show potentially damaging carbon chemical conditions for coral reef builders and other marine calcifiers. A second consequence predicted is the high risk of substantial sea level rise which will melt thermal ice caps due to thermal expansion (Bennet 2018).

Figure 2. (a) Projections of mean global average surface temperatures. (b) Projections of mean global ocean surface pH for RCP. Shaded areas indicate uncertainties. Figure from the NOAA/NESDIS/NODC Ocean Climate Laboratory.

3.1 Climate Change and Coral Reefs

The euphotic zone is the layer of water, close enough to the ocean’s surface that receives sufficient light for photosynthesis to occur. Tropical coral reefs are found within the euphotic layer. The tropical reefs grow in formation of a symbiotic relationship amongst corals and unicellular algae. This relationship allows for photosynthesis to occur by the symbiotic algae producing approximately 90% of the coral’s energy that is essential for calcification, tissue growth and reproduction (Barnes, 1987). This can only take place within a range of vital environmental conditions such as the seawater temperature, light and carbon chemical condition (Barnes, 1987). 

Corals prefer high temperatures, as the rate of calcification will increase with the increase of climate and they stay within their upper thermal limits until it reaches its thermal optimum before it is required to decline (Hoegh-Gulberg 2007). However, when temperatures exceed optimum temperatures, for example in the summer, due to the narrow envelope and high sensitivity, there is risk of coral bleaching as an adaptation method (Abdo, Bellchamber, Evans 2012).

3.2 Warming Ocean: Bleaching of Coral Reefs

Coral bleaching is a standard term for a process where the coral host expels its algae (zooxanthellae) living in their tissues, causing the coral to turn a very pale color or completely white (NOAA 2018). This is due to the algae releasing free oxygen radicals, lethal to its coral host (Nielsen, Petrou, Gates 2018). This occurs when temperature and light conditions go beyond optimum range (NOAA 2018). The coral hosts’ immediate response releases all dysfunctional algae cells which creates the corals white color. Stress-caused coral bleaching, depending on severity, allows for coral recovery (Nielsen, Petrou, Gates 2018).  However, the loss of zooxanthellae, famishes the coral of energy from lack of photosynthesis which leads to death unless temperatures are normalized and algae are restored before the coral depletes its energy reserves. If the algae loss is persistent and there is constant stress, there will be extensive coral deaths and vast consequences for associated reef fauna (Anthony et al. 2010).

Figure 3. Process of coral bleaching; difference between healthy, stressed and bleached coral. Figure from NOAA 2018.

The initial severe coral bleaching event reported in a large ocean that was due to climate change and ocean warming happened in 1982-1983 due to a strong El Niño event (Eakin, Sweatman, Brainard 2019). Earlier reports of coral bleaching occurring were in smaller waters such as lakes and local nature were caused by local factors other than global warming (NOAA 2018). The worst and most severe coral bleaching event recorded was in 1998 caused by yet another strong El Niño event which was unpredicted and alerted for a closer look at what is to come for coral reefs in a warming world (Salm, Coles 2001). Coral bleaching events since then have been arising and are likely to keep increasing under both El Niño and La Niña conditions. With re-occurring bleaching events, the most damaging recorded, is an El Niño event affecting nearly 90% of Australia’s Great Barrier Reef (Eakin, Sweatman, Brainard 2019).

There is evidence for coral to undergo thermal adaptation by selective mortality and symbiont adaptation meaning the algae living within the coral, adjust to thermal resistance (Morikawa, Palumbi 2019). However, as coral taxa vary in susceptibility, bleaching risks are difficult to predict and are more variable than previously expected (NOAA 2018).  Rates of adaptation are also unlikely to be retained with the warming as recent studies indicate seasonal trajectories of the ocean summer heating under continuing climate change will inhibit corals from recovery and will possibly lower overall thermal tolerance of healthy coral which in turn, increases mortality risk  (Camp et al. 2018).                        

3.3 Changes in Storm Pattern: Destruction of Reef Structure

Storms are expected natural events in tropical environment, however research shows that the average intensity of storms shift towards heavier storms in warmer climates. Storms combined with coral bleaching overwhelm coral reefs to recover and hence make it difficult for restoration of the oceans environment, ecosystem, functions and diversity in between events (Hughes et al. 2007).  As of 2005, there have been several cyclones distressing the GBR that accounted for the 50% of coral cover decline. Storms are a primary cause of coral reef disturbance however, it was the coral bleaching due to the prolonged loss of calcification rate from increased temperatures, which in turn affected the resilience of corals from storms such as cyclones (Anthony et al. 2010). 

3.4 Ocean Acidification: Decreases Growth Rates and Structural Integrity

For over 200 years, since the industrial revolution began, the concentration of carbon dioxide (CO2) in the atmosphere has increased due to the burning of fossil fuels and land use change (Dahlman, Lindsey 2018). As stated previously, the ocean absorbs approximately 30% of the CO2 that is released in the atmosphere, and as levels of atmospheric CO2 increase, so do the levels in the ocean (Freely et al. 2004).

When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions. This increase causes the ocean to lower in pH and hence become more acidic, causing carbonate ions to be relatively less abundant (Anthony et al. 2010).

Carbonate ions are an essential building block of structures of marine biogenic calcium carbonate structures (Gattuso, Hoegh-Guldberg, Pörtner 2014). Decreases in carbonate ions can critically affect marine calcification which meaning slower coral growth. Ocean acidification indicates more susceptibility to storm damage, slower recovery rates between detrimental events and reduced reef resilience (Hughes et al. 2007).

These alterations in ocean chemistry can disturb the behavior of non-calcifying organisms as well. Certain fish’s capability to identify predators is reduced in more acidic waters. When these organisms are at risk, the entire food web may also be at risk (NOAA 2018).  Although ocean acidification is another major cause that plays part in coral reef degradation, there are unfortunately no studies indicating that coral reef organisms can adapt to it (Bennett 2018).

5.1 Summary

Climate change is the physical consequence of increasing greenhouse gasses in the atmosphere. It pushes ocean warming, fuels tropical storms, and leads to sea level rise. Ocean warming drives coral bleaching and reduces coral growth and reproduction (Anthony et al. 2010).

Globally, coral reefs shelter the largest biodiversity of any ecosystem. Although it covers less than 0.1% of oceans floor it harbors one quarter of marine species as well as providing protection and maintenance of food for other organisms. The destruction of coral reefs will lead to economic, social and health loss. Coral reefs are essential for a healthy global ecosystem and aid in warning signs to less sensitive systems if climate change is not immediately addressed. Deterioration of other systems may occur more rapidly and irreversibly.

Coral reefs provide for the livelihoods of many, and pressures to coral reefs are ultimately threats to people. Majority of literature provide evidence that climate change is affecting coral reefs globally. Measures of adaptation are being explored however these global pressures must first take place by providing awareness, participating in the global carbon emissions challenge and taking action within local management.

References

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