Anthropogenic Climate Change and Coral Reefs at the end of the 21st Century.
By: Estefania Liehr
30% of the CO2 released into the atmosphere since industrial times has already been absorbed by the oceans (Melendez et al., 2017; Van Hooidonk et al., 2013) This has caused the seawater pH to decrease by 0.1 units, from 8.2 – 8.1 (IPCC, 2007; Melendez et al., 2017; Ocean Acidification, 2014) which corresponds to a 26% increase in acidity; this phenomenon is called Ocean Acidification (OA) (Ocean Acidification, 2014). Various species are affected by OA because it causes a reduction in the saturation state of calcium carbonate minerals like Aragonite, which are essential for skeleton and shell production, hence causing slower growth rates and high mortality within calcifiers (IGBP, IOC, SCOR, 2013; Van Hooidonk et al., 2013). Additionally, when temperature levels are abnormally high, corals undergo thermal stress and are forced to eject Zooxanthellae, a photosynthetic algae with which they share a symbiotic relationship (Pandolfi et al., 2011, p.419). This leaves the corals looking white, for which this phenomenon is known as “coral bleaching” (Pandolfi et al., 2011, p. 418). If the Zooxanthellae remains away from the coral polyp for too long, it may result in the coral´s death, hence it also being associated with high mortality rates (Pandolfi et al., 2011, p.418).
Coral reefs worldwide are under vast stress due to anthropogenic climate change and both formerly mentioned problems are forecasted to intensify (IPCC, 2014). Coral reefs have already been identified as endangered ecosystems due to the many natural man-made stresses (Riegl et al., 2009, p.2). Estimates suggest that around 20% of reefs have already been lost, 24% seem to be under great threat and 26% are on the verge of irreparable damage (Riegl et al., 2009, p.2). Corals provide protection from shore erosion and storms, and they provide habitat and nurseries to countless species. Stress and mass mortality could prove devastating for marine biodiversity potentially affecting food security and economic stability in various regions, as well as compromising the ocean's ability to buffer climate change (IGBP, IOC, SCOR, 2013). This essay explores future forecasts for oceans at the end of the century. It starts by examining the impacts of OA on corals, then proceeds to analyse how temperature increases affect coral bleaching and finally studies the socio-economic impacts as well as potential policy implications.
Ocean Acidification refers to the process whereby the ocean´s uptake of CO2 causes a decrease in pH and carbonate ions in seawater (Melendez et al., 2017, p.31). The current rate of pH change in the oceans is unprecedented for the past 300 million years (IGBP, IOC, SCOR, 2013). It is a problem that will, with certainty, continue to increase with rising CO2 concentrations. In 2012 the UN declared OA to be a threat to ecosystems of great economic and ecological value, as well as to human well-being (IGBP, IOC, SCOR, 2013).
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Figure 1 - showing the projected global surface pH changes for the future. The blue line (very low emissions) represents RCP 2.6 and the red line (high emissions) represents RCP 8.5; (Ocean Acidificacion Portal, 2014)
As can be seen in Figure 1, a very low emission scenario (RCP 2.6) would result in a further decrease of 0.05 units (Ocean Acidification, 2014). In an RCP 4.5 scenario, the decrease is estimated to be of another 0.1 units by the end of the century, making the ocean 40% more acidic; for RCP 6.0 the projected decrease if of 0.2 units which equals to a 62% increase in acidity (Melendez et al., 2017, p.31). The forecasts for RCP 8.5 vary, some argue that the average pH will decrease by 0.3-0.4 units corresponding to 100-150% more acidity (Melendez et al., 2017, p.32; Hopcroft, p.3; IPCC, 2017), whereas others project the increase at about 170% (IGBP, IOC, SCOR, 2013).
As mentioned above, OA causes a decrease in carbonate ions which in turn decreases the saturation levels of calcium carbonate minerals, such as Aragonite, which are the building material of most marine skeletons (Melendez et al, 2017, p.32). The Aragonite saturation levels (Ω) have already been lowered from 4.6 to 4.0 from pre-industrial times (Riegl et al., 2009, p.10) and are projected to continue to decrease in the years to come. Low Aragonite Saturation levels compromise the capacity of many organisms to perform efficient calcification (Riegl et al., 2009, p.19) and waters with a value of Ω <1 (unsaturated) are known to be corrosive for aragonite based shells and skeletons (Ocean Acidification, 2014). In fact, the shells of some Pteropods (marine snails) already are dissolving (IGBP, IOC, SCOR, 2013, p.16).
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Figure 2 – Global projections of Aragonite saturation levels in 2100 for RCP 8.5; (Ocean Acidificacion Portal, 2014)
As seen in Figure 2, predictions for RCP 8.5 show a Ω of less than one in many polar areas and less than three around tropical areas for the year 2100, which is a daunting prospect for corals worldwide as coral growth usually occurs in levels where Ω >3 (IGBP, IOC, SCOR, 2013). If the emission trajectory remains on the present path, nearly 70% or cold-water corals will be surrounded by corrosive waters by 2100; furthermore, the erosion of coral reefs is likely to exceed reef-building before the end of the century (IGBP, IOC, SCOR, 2013). Silverman et al (2009 in Van Hooidonk et al., 2013, p.19), found that by the time atmospheric CO2 levels doubles to 560 ppm, half of the world's reefs will be net dissolving; under RCP 8.5 this will happen in the year 2050. A study done by the National Oceanic and Atmospheric Administration (NOAA, 2008) shows that OA will critically reduce coral growth by the end of the century if we continue on the current emission trajectory. Additionally, 37% of Caribbean reefs already are net eroding and many others are close to net erosion, which means that even small changes in calcification could have devastating consequences for the reefs (Van Hooidonk et al., 2013, p.19). However, some claim that current projections are overestimating the speed and intensity of the damage (Pandolfi et al., 2011, p. 418). Additionally, studies show that there are variations in calcification sensitivity and responses hence creating uncertainties in the responses of future calcifiers (Pandolfi et al., 2011, p.420-421). Likewise, light intensity, which has been shown to affect calcification rates, is often unaccounted for in various studies thus producing uncertainties (Dufault et al., 2013). Furthermore, the coral´s adaptive and evolutionary capacities are unknown to us at the moment. For example, studies show that strong natural variability can aid corals adapt better to ocean acidification (NOAA, 2008, p.1). And other studies have shown that OA and thermal stress can both be buffered by corals through an increase in feeding and lipid content (Towle et al., 2015, p.1).
In addition to low pH levels, corals are also experiencing thermal stress which occurs when corals are exposed to temperatures of 1-1.5 degrees higher than the season maximum mean temperature for sustained periods of time (Riegl et al, 2009, p.7). When this occurs, corals are forced to break their symbiotic relationship with Zooxanthellae (a phenomenon referred to as coral bleaching), which leaves them vulnerable to disease, reduces energy available for growth and reproduction and hence their capacity to build reefs, and in the long term leads to mortality (Pandolfi et al., 2011, p.419). Coral bleaching is also triggered by other stressors such as salinity levels, disease, UV-radiation and sedimentation, but recent research has mainly focused on anthropogenic temperature increases due to unprecedented mass bleaching events worldwide (McWilliams et al., 2005, p.2055). Furthermore, fish and other invertebrates that inhabit or consume the corals typically suffer after mass bleaching events (Pandolfi et al., 2011, p.419). The frequency and duration of bleaching events has risen dramatically since pre-industrial times (Riegl et al., 2009; Pandolfi et al., 2011).