Melissa Cauchi

The Future of Coral Reefs Implied by the Paleocene-Eocene Thermal Maximum

AbstractCoral reefs provide ecosystem services to coastal areas and are an integral part of fishery and tourism industries which impact major economies worldwide. Rising sea surface temperatures and ocean acidification as a result of increasing greenhouse gas emissions will threaten the health and population of the world’s reefs. This paper discusses current and future projections for coral reefs in comparison to the Paleocene-Eocene Thermal Maximum (PETM), when temperatures and CO2 levels exceeded that of present conditions. During the PETM coral reefs became dominated by foraminifera and only those that were able to migrate away from the mid-latitudes survived, narrowly avoiding extinction. RCP scenario 8.5, detailed by the 2014 IPCC report, predicts a similar and dire future for coral reefs due to the currently rapid rate of climate change. Under this scenario in which carbon emissions continue on their current trajectory, coral reefs will be put at high risk for extinction by 2050.

IntroductionAccording to the latest IPCC Report, global temperatures are projected to rise about 2°C

by the year 2100, which would offset an increase in oceanic temperatures by 1 to 4° C and cause the global sea level to rise an additional one to four feet (IPCC, 2014). Earth’s oceans act as a climate regulator by sequestering CO2 from the atmosphere. In this anthropogenic age the role of oceans in the carbon cycle has dampened the full impacts of fossil fuel emissions for the planet as a whole nit the oceans are bearing the brunt of increasing ocean acidity. Due to the rapidly acidifying oceans, warming climate, and sea level changes, coral reefs are struggling to survive and reproduce and will ultimately decline in population. This will result in decreased biodiversity and will potentially change the ocean’s ability to sequester CO2 and begin to impact human industries. In this contribution, we use the Paleocene-Eocene Thermal Maximum (PETM) as a comparison for future warming because oceans during this time period were a whole 6 to 14°C warmer. During the PETM, corals had enough time to migrate away from tropical waters that were gradually becoming thermally inhabitable. They survived extinction and were able to recuperate during the cool mid-Eocene. Unlike the PETM, modern day climate changed is occurring very rapidly and shows no sign of tapering and cooling. Coral reefs may have insufficient time to adjust to extreme warming. Coral reefs, especially those in warm tropical waters, are at high-risk of extinction by 2050 (Gattuso et al., 2015).

Background The Great Barrier Reef (GBR) and the surrounding Pacific Ocean will be highlighted in

this paper as these will be the first spots to experience rising sea surface temperatures and ocean acidification as a result of global warming. The Great Barrier Reef is considered the largest living structure on Earth as it spans 1,400 miles of coral habitats. The GBR houses about 1,800 fish species and 125 shark species, along with thousands of mollusks and microorganisms that are essential to fisheries and economies. The GBR also holds 400 different species of corals (Holland, 2011).

Although some corals are more tolerant to environmental changes than others, corals prefer shallow, clear water with sunlight to drive photosynthesis. Corals have a symbiotic

relationship with zooxanthellae, a photosynthetic algae that provides most of the corals energy and color. Under stressful situations (i.e., higher ocean temperatures), the zooxanthellae leaves the coral, which causes bleaching due to the coral’s lack of an energy source. A bleached coral is more vulnerable to disease and external threats such boat impacts and oil spills, which may ultimately kill the weakened coral (Holland, 2011).

Historically, corals were able to recover from natural fluctuations of CO2 and other greenhouse gasses primarily because they were given long stretches of time to heal. However, current and unrelenting conditions such as warming temperatures, overexposure to UV rays, and drastic changes in sea level (which either leave the corals exposed or bury them in unlit areas) have put corals at a higher risk. More acidic oceans also disturb the rate at which corals can build their calcium carbonate structures, making it harder for reefs to recover (Holland, 2011).

It has been estimated that coral reefs will become lifeless in about 50 years, with only “coral skeletons bathed in algal slime” left (Charlies Vernon of the Australian Institute of Marine Science: Holland, 2011). Many industries, including fishing and tourism, rely on the biodiversity coral reefs create. Without coral reefs, coastal areas are more susceptible to erosion and stronger tidal surges (Holland, 2011). Ultimately, the best long-term conservation method is to reduce anthropogenic greenhouse gas emissions. Other conservation methods include limiting fishing, designing reserves, breeding more heat-tolerant corals, reducing ocean acidification by adding bicarbonate or lime, building giant canopies in shallow water for shade, and artificial coral reef structures. One interesting development that will be discussed is the transplantation of steel electrical cages to aid corals in building their CaCO3 skeletons (Henderson, 2002).

This paper will also discuss the RCP 8.5 scenario compared with other RCP emission scenarios. Representative Concentration Pathways (RCPs) are greenhouse gas concentration trajectories mentioned in the latest IPCC Assessment report. They are named after the numerical range of radiative forcing values estimated for the year 2100. RCP 8.5 represents increasing greenhouse gas emissions (an estimate of 1370 ppm of CO2 by the year 2100) while RCP 2.6 for example represents a decline in greenhouse gasses (490 ppm). RCP 8.5 is the predicted “business as usual” scenario (Wayne, 2013).

Name Radiative Forcing

CO2

Equivalent(ppm)

Temp Anomaly

(°C)

Pathway SRES Temp.

Anomaly Equivalent

RCP 8.5 8.5 W/m2 1370 4.9 Rising SRES A1F1

RCP 6.0 6 W/m2 850 3.0 Stabilizing without overshoot

SRES B2

RCP 4.5 4.5 W/m2 650 2.4 Stabilization without overshoot

SRES B1

RCP 2.6(RCP 3PD)

3 W/m2 before 2100, declining to 2.6 W/m2 by 2100

490 1.5 Peak and decline None

Fig. 1 Radiative forcing, CO2 concentrations, and temperature anomalies associated with each RCP scenario. Special Report on Emissions Scenarios (SRES) are alternative scenarios that were

detailed in the 2000 IPCC Report and have been replaced by the RCP scenarios in the 2014 IPCC report (Wayne, 2013).

Materials and MethodsFor this paper, I have gathered and annotated scholarly articles regarding coral reef

behavior under current and past climate change, projected outcomes on coral reefs, and the societal impacts of these changes. I have also read further into the possible solution of using Biorock, or low-voltage steel cages, to aid in the restoration of coral reefs.

Data and Discussion Corals, Oceans, and the Carbon Cycle

Oceans have acted as a climate stabilizer and have absorbed about 93% of earth’s additional anthropogenic heat since the 1970s, which has staved off atmospheric warming but has consequently caused additional carbon to be absorbed. Uptake of excess CO2 by the ocean has decreased the pH and saturation state of calcium carbonate minerals aragonite and calcite of seawater, among other things (Gattuso et al., 2015).

CO2 enters the oceans by diffusing on the surface. The biological pump aids in transporting carbon down into the deep oceans where it is then stored. Currently, about 25% of anthropogenic CO2 enters the ocean where some of it reacts to form carbonic acid, causing ocean acidification (Hoegh-Guldberg et al., 2007). Dissolved CO2 also reacts with water to produce bicarbonate ions (HCO3

-), hydrogen ions (H+), and carbonate (CO3=). Carbonate ions can

combine with calcium ions to form calcium carbonate which is fundamental for shell-building organisms such as coral. CaCO3, a chemical component of limestone and chalk, dissolves in acid, making it difficult for the insoluble solid to be maintained under ocean acidification which is defined as an increase in the hydrogen ion concentration. The amount of dissolved CaCO3 determines the pH of the ocean through the CaCO3 pH-stat, but that takes a few thousand years to adjust the ocean’s pH (Archer, 2010).

As CO2 enters the ocean the dissolved gas reacts with water to produce carbonic acid, hydrogen ions, and bicarbonate ions. Another interpretation can be drawn from the following infographic:

First, dissolved CO2 combines with water to form carbonic acid

(1): CO2 + H2O ↔ H2CO3

Then the carbonic acid dissociates in water to produce a hydrogen ion and bicarbonate ion

(2): H2CO3 ↔ H+ + HCO3-

The bicarbonate ion can dissociate further to produce an additional hydrogen ion and a carbonate ion

(3): HCO3- ↔ H+ + CO32-

Calcification of coral is significantly impacted by the carbonate concentration. Calcium ions must combine with carbonate ions to form CaCO3 skeletons for organisms such as corals. Shell-forming organisms make their skeletons indirectly by taking bicarbonate ions from the seawater and from their own metabolism

(4): Ca2+ + 2HCO3

- → CaCO3 + H2O + CO2

When carbonate concentrations drop below saturation, the lack of the CO3= restricts the ability of

corals to make their skeletons. Many corals require the ocean to over-saturated with calcium and carbonate to make their skeletons.

The EoceneSea surface temperatures during the Eocene were warmer than the predicted changes for

the upcoming century. During the Eocene, when sea surface temperatures were a whole 6°C warmer, coral reefs were mainly distributed in the mid-latitudes (Descombes et. al, 2015). As temperature dropped thereafter, the Pacific Ocean attracted a plethora of corals and became a biodiversity hotspot during the colder Quaternary period because the Pacific Ocean had the warmest sea surface temperatures. Under current carbon emissions, however, the Pacific Ocean will also be one of the first spots to experience extreme thermal spikes, putting the marine organisms that live there under stress (Descombes et al., 2015).

RCP scenario 8.5 was tested under two different climate models (EC-Earth and IPSL-CM5A-LR) and it was found that ocean suitability for corals would decrease by 46% by 2100, the most impacted area being the Pacific. Results from the Eocene simulation, with an estimated CO2 forcing of 2240 ppm, were compared to the RCP 8.5 simulation. The ideal preferences for coral reefs deteriorate after SST warm to more than 32°C shown in both the models run for the Eocene and projected emissions. Ideal conditions for coral reefs include temperatures between 23 to 29°C, well under the 32°C limit given by the models. Under RCP 8.5, the mean temperature of the Pacific Ocean may reach 34.7°C by 2100, creating an environment inhabitable for coral reefs and causing reef destruction and retraction (Descombes et al., 2015). Also during the Eocene, coral reefs mainly housed “encrusting and massive coral colonies” and very few multi-branched corals, limiting the diversity of the reef (Cacciapaglia et al., 2014). Massive and encrusting corals refer to corals that are flatter, thick, and solid with almost identical polyps in all directions (brain corals, Faviidae, are an example). Multi-branched corals grow upwards and have numerous limbs that hold their polyps (i.e., SPS corals, or Acropora).

Calcification of coral is significantly impacted by the carbonate concentration. Calcium ions must combine with carbonate ions to form CaCO3 skeletons for organisms such as corals. Shell-forming organisms make their skeletons indirectly by taking bicarbonate ions from the seawater and from their own metabolism

(4): Ca2+ + 2HCO3

- → CaCO3 + H2O + CO2

When excess CO2 dissolves in the oceans and produces carbonic acid (1), the effect on the carbonate/bicarbonate balance is for the hydrogen ions to react with carbonate to form bicarbonate, reducing the carbonate concentration

(5): CO2 + H2O + CO32- ↔ 2HCO3

-

Fig. 2 Calibration of both current (in blue shades) and Eocene (in red) run models for occurrences of coral reefs under a range of different SST. As shown and described, the

temperature suitability for coral reefs is narrow and starts to decline after ~32°C (Descombes et al., 2015).

Likewise, the Pleistocene and the Holocene also saw shifts in geographic ranges in response to warming and cooling based on temperature preferences of corals and other marine organisms. During the warmer late Pleistocene, for example, corals were 500km further south along Australia’s western coastline due to the rise in sea-level at this period, which is comparable to current sea-levels. Natural fluctuations of CO2 during glacial-interglacial periods have caused natural fluctuations and shifts in marine organism populations. Unlike the Pleistocene, current changes in coral reef populations are being propagated by anthropogenic CO2 in an ocean that is already warm and experiencing high sea-levels (Hughes et al., 2003).

The Paleocene-Eocene Thermal Maximum (PETM)The PETM is considered a major global perturbation where sea surface temperatures

were hotter than current values. It is speculated that the PETM was caused by an extremely rapid rise of greenhouse gas concentrations from the atmosphere, possibly released by the destabilization of deep sea methane hydrates or organic carbon stored within the permafrost. Oceanic life during the PETM can be divided into three stages of development. The first stage is marked by carbonate usage by coralgal reefs (I). The second stage, also called the transitional stage, showed that coralgal reefs thrived only at mid-latitudes and that foraminifera dominated the low-latitudes (II). The third stage showed the rise of the foraminifera from the low-latitudes to the mid-latitudes as they grew and produced carbonate (Scheibner et al., 2008).

Samples of coral reefs from a low-palaeolatitude setting (Egypt) and a mid-palaeolatitude setting (the Pyrenees, Spain, and France) were tested (Scheibner et al., 2008). The results indicated that temperatures during the PETM could have ranged from 31 to 33°C and may have had its peak at 34°C (Scheibner et al., 2008). Corals began to decline from stage II to stage III when foraminifera began to spread. This decline was attributed to both rising oceanic temperatures and the aggressive growth and carbonate usage by the foraminifera. It was estimated that the adaptation capacity for coral reefs during the PETM could not extend an additional rise of 5°C in low-latitudes and 8°C in mid-latitudes, as shown by the decline in reef population during this period (Scheibner et al., 2008).

Lastly, coral reefs first disappeared from the hotter tropical to sub-tropical waters of the Southern Tethys to retreat to cooler northern waters during stage II. At this same stage larger foraminifera were becoming the more dominant carbonate producers (Scheibner et al., 2008).

Current Ocean Conditions and Coral ReefsThe increase in bicarbonate and hydrogen ions reduces the availability of carbonate for

calcification used by marine organisms such as corals. A study of the Vostok Ice Core to determine that warmer inter-glacial periods had lower carbonate concentrations compared to cooler glacial periods, which had slightly higher carbonate concentrations. Current ocean conditions have warmer sea temperatures and lower pH and carbonate-ion values than any other time period in the last 420,000 years (Hoegh-Guldberg et al., 2007).

Fig. 3 Temperature and carbonate ion concentrations of past interglacial/glacial periods compared to both current (A) and projected scenarios (B for Coral Reef Scenario B and C for

CRS-C). As temperatures and atmospheric CO2 content increase, the carbonate ion concentration decreases and reef communities degrade (Hoegh-Guldberg et al., 2007).

Under a model scenario where CO2 concentrations were set to 560 ppm, coral

calcification and growth decreased by 40% because of the lack of carbonate-ions, which inhibits aragonite formation. The consequence is a decrease in linear extension rate and skeletal density, as shown in the coral Porites found on the Great Barrier Reef. Porites have experienced a 20.6% decline in growth over the last 16 years (Hoegh-Guldberg et al., 2007).

The GBR is reaching low levels of aragonite saturation quicker than other regions such as the Central Pacific. Aragonite is an essential mineral for corals that is formed alongside calcium carbonate. As atmospheric CO2 rises and is diffused into the ocean, carbonate-ion concentrations decrease. With a depletion of carbonate ions, substantial levels of aragonite cannot be formed. The minimum aragonite saturation value allowable for coral reef survival and growth (Ω aragonite > 3.25) diminishes with decreasing carbonate-ions. Hence, if atmospheric CO2

concentrations exceed 500 ppm, carbonate-ion concentrations will fall to less than 200 μmol kg-1,

making the aragonite saturation levels less than 3.3, inhabitable for healthy coral reefs (Hoegh-Guldberg et al., 2007).

Fig. 4 Changes in aragonite saturation predicted to occur under different emission scenarios. Under pre-industrial emissions (280 ppm), Ω aragonite > 3.25, allowing corals to

grow. Under potential emissions such as 550 ppm, Ω aragonite < 3.25, putting coral reefs in a detrimental situation (Hoegh-Guldberg et al., 2007).

These findings are confirmed by tests of the calcification state versus the aragonite saturation state through the Hamburg Ocean Carbon Cycle (HAMOCC) model, assuming constant alkalinity and only a change in carbon concentrations. In this model, aragonite saturation drops significantly as atmospheric CO2 increases. As previously stated, increased CO2 into the ocean restricts the amount of carbonate-ions, and thus aragonite, available to corals. An increase in the concentration of atmospheric CO2 will decrease the aragonite saturation state by 30%, which means that coral calcification will also significantly decrease due to the fact that calcification depends upon the saturation state of aragonite in surface waters (Kleypas et al., 2007).

A study of 328 samples was done on colonies of oceanic and coastal Porites, a common GBR coral, from 69 reefs of the GBR to test calcification. Annual density bands from skeletal records indicate that calcification has declined by 14.2% since 1990, a decline unprecedented in

the last 400 years. The declining calcification is a response to increasing thermal stress and the decreasing saturation state of aragonite in the GBR, disabling the ability of corals in this region to deposit CaCO3 (De’ath et al., 2009).

Annual data for three growth factors of the Porites coral were tested for skeletal density, annual extension (length of polyp growth), and calcification rate from 1990 to 2005. There was also a separate control group of 139 colonies sampled prior to 1990. The results show strong declines in all three parameters with calcification rates steadily decreasing at an average of 1.44% yearly. It was also found that due to a decrease in aragonite saturation state, reef-building corals have declined 9 to 56%. Warmer SST and reduced aragonite saturation state, offset by the doubling of pre-industrial CO2, is likely responsible for the decline of potential coral growth (De’ath et al, 2009).

Fig. 5 Declines in calcification (g/cm2 yearly), linear extension of polyps (cm/yearly), and density (g/cm3 yearly) of the Porites tested from 1990 to 2005 (De’ath et al, 2009).

Similarly, 12 coral species from the Pacific Ocean processed through RCP 8.5 SST models predicted that 9 out of the 12 species would face a 24 – 50% habitat lost by 2100. Under RCP 8.5, sea surface temperature is predicted to rise to 3.1° C with the Pacific Ocean warming faster than other ocean bodies. Interestingly, the models showed that photosynthetically available radiation (PAR) dictated the distribution of the 12 coral species more so than the SST. About 80% of the 12 species reacted to PAR while 50% reacted to SST. Also the most sensitive species to anthropogenic forcing are stenotopic coral species. Stenotopic species can only survive under a narrow range of conditions and are particularly adverse to environmental changes. These corals are at higher risk of death than more thermal-tolerant corals due to their heightened inability to adapt (Cacciapaglia et al., 2015).

Both RCP scenarios 2.6 and 8.5 predict oceanic warming, sea level rise, and coral reef degradation but to drastically different extents. RCP scenario 2.6 calls for lower carbon emissions while RCP 8.5 reflects our current rising carbon emissions. Under RCP 2.6, warm-water corals will already be at high-risk by 2100, meaning that coral health would be in decline and susceptible to bleaching events, but still able to maintain some sort of reef ecosystem. Under RCP 8.5, corals will be at very high-risk by 2050, meaning that reef ecosystems will be degraded into CaCO3 skeletons and replaced by an algae-based community (Gattuso et al., 2015).

RCP Scenario 2.6: Low Emissions RCP Scenario 8.5: High Emissions

CO2 emissions continue as usual until 2020, decline, and become negative in 2100 (decline to ~ 490 ppm by 2100)

Triple current CO2 emissions (~ 1370 ppm) by 2100 and increases in methane emissions

Global mean temperature rise of 0.3 – 1.7°C by 2100

Global mean temperature rise of 2.6 - 4.8°C by 2100

SST rise of ~ 0.71°C SST rise of ~ 2.73°C

Sea level rise of 0.36 – 0.82 m by 2100 Sea level rise of 0.81 – 1.65 m by 2100

Corals will be at high-risk by 2100 Corals will be at very high-risk by 2050

World population of 9 billion by 2100 World population of 12 billion by 2100

Fig. 6 A comparison of RCP Scenarios 2.6 and 8.5 (Wayne, 2013 and Gattuso et al., 2015).

RCP scenario 2.6 would limit global temperature to staying below a 2°C whereas RCP 8.5 may raise temperatures to an additional 3.2°C. Under RCP 8.5, 69% of the surface ocean will warm by 1.5°C and acidify by more than -0.2 units pH compared to preindustrial levels. This will in turn decrease net calcification for calcifying organisms such as corals and also impact many other marine organisms. Avoiding a high-risk scenario such as RCP 8.5 involves limiting the increase of global surface temperature to below 2°C and the SST to below 1.2°C, as RCP 2.6 calls for. Ideally under RCP 2.6, carbon emissions would be limited to 490ppm. Warming and ocean acidification is predicted to be less severe under RCP 2.6 and is predicted to impact a significantly smaller percentage of global oceans. RCP 2.6 creates more of a hopeful future for coral reefs, mangroves, seagrass beds, and reliant fish communities (Gattuso et al., 2015).

Fig. 7 Severity of impacts of marine organisms under RCP scenario 2.6 and RCP scenario 8.5 (Gattuso et al., 2015).

Fig. 8 Projected frequency of thermal stress for 2030 and current reef locations (World Resources Institute, 2011).

Fig. 9 Projected frequency of thermal stress for 2050 and current reef locations (World Resources Institute, 2011).

The above maps show the estimated frequency of thermal stress for the years 2030 and 2050 with data collected from the World Resources Institute. The data represents temperature changes and reef locations. Using multiple datasets extracted from the World Resources Institute, I plotted the data in ARCGIS. The black dots represent reef locations and density while the ranges of blue, orange, and red represent temperature. The blue hues show less frequency while the orange to red hues show higher frequencies. Under current or “business as usual” emissions, the oceans of 2030 will see times of thermal stress, especially around the low-latitudes and in tropical waters. The oceans of 2050 will see even more frequent warmings. These oceans will not be thermally suitable for coral reefs. Under these conditions by 2030, 88 to 92% of global corals reefs will be threatened. By 2050, the number will increase to 99 to 100% (World Resources Institute, 2011).

Coupling with thermal stress, ocean acidification is another major problem. Shown from Figure 10 (below) and as previously stated, ocean acidification of the earth’s oceans won’t change evenly. Again, the saturation state of aragonite, the crystalline mineral of calcite, is an indicator of ocean acidification (Hoegh-Guldberg et al., 2007). As CO2 increases the aragonite saturation state of warm Pacific waters will diminish first and faster than the cooler waters of the Atlantic (World Resources Institute, 2011).

Fig. 10 as CO2 rises as predicted for the years 2030 and 2050, aragonite saturation state decreases, becoming unsuitable for reefs in the area. Warmer tropical waters of the Pacific see a diminishing state faster than the cooler waters of the Atlantic (World Resources Institute, 2011).

The general scientific consensus agrees that coral reef degradation is one of the many consequences of climate change (Gattuso et al., 215 and Cacciapaglia et al., 2014). Therefore, it is safe to say that part of coral reef conservation includes a global reduction of carbon emissions, ideally following RCP scenario 2.6. This requires multi-national effort and a conscientious change in lifestyle and politics. Until then, different solutions have been proposed to aid in coral reef degradation. Developed by Thomas Goreau, a marine biologist, and Wolf Hilbertz, a structural engineer, electric steel cages were built and then transplanted into coral reef zones of the ocean. Goreau calls these steel cages “Biorock electrolytic technology” or just “Biorock.”

The idea behind Biorock is to help corals grow limestone structures through the passing of low-voltage electrical currents. The electric currents on the steel frame initiate the electrolysis of water and hydrogen as electrons bind to the hydrogen ions in the water. Due to the loss of ions, carbonic acid in the surrounding seawater breaks down, releasing hydrogen and carbonate ions. When these carbonate ions reach a certain concentration, limestone and aragonite is deposited on the steel structure. Since the Biorock reduces the need for corals to expend their own energy in making their CaCO3 skeletons, the coral polyps are better equipped to withstand stress and can focus on growth and reproduction (Goreau, 2014).

In his recent publication on Biorock, Goreau tests corals on Biorock structures versus corals on natural formations as the control group. Coral recruitment is when tiny coral larvae attach to structures and become individuals of the community. The coral recruitment on Biorock is 1 to 4 magnitudes higher than on natural habitats. Also, corals grown on the Biorock grew 2 – 10 times faster and have a higher survival rate than the control group. In the major 1998 Maldives bleaching event, less than 5% of the corals on the natural reef survived whereas 80% of the corals implanted on the Biorock not only survived but flourished thereafter. During the 2010 Thailand bleaching event, corals tended to bleach less and recover faster on the Biorock as compared to the surrounding natural reef (Goreau, 2014).

Fig. 11 Different parameters tested on Biorock as compared to natural structures. Coral recruitment, growth rate, and survival of corals all have higher values on the Biorock than the

control (Goreau, 2014).

Corals tested under electrical stimulation also had higher densities of the symbiotic algae. Biorock does not cause significant disturbance to already healthy corals in natural reefs and Biorock is ideal for both stony and soft corals. Biorock is also relatively cheap to construct and the electricity is low (less than 300 watts/100 m2) and can be generated with energy from solar cells or tidal turbines. Biorock has only been tested on a small scale in Indonesia, Panama, Mexico, and Thailand so more experimentation, large-scale and globally, is required (Goreau, 2014).

ConclusionEarth’s geologic history has experienced fluctuations of climate in relation to natural

fluctuations of carbon and other greenhouse gasses. There have been many near-extinction events that have threatened corals but coral reefs have had sufficient time in between warming events to recuperate. The late Paleocene to early Eocene saw temperatures estimated to have been 6° C higher than current. During the PETM, coral reefs suffered heavily and only corals living or migrating to cooler Northern waters (away from the mid-latitudes) survived. Generally, the PETM warming led to a steep decline in coral reefs and a disappearance of corals from lethally hot tropical waters. Areas with a lack of corals became easily and quickly dominated by foraminifera. Following the PETM came the significantly cooler mid-Eocene during which corals were able to shift back towards the warmer tropical waters that now house the Great Barrier Reef.

Current warming trends and those projected for the next century are beginning to mimic the conditions of the PETM but with no sign of cooling relief. In addition, the rising levels of atmospheric CO2 are causing both the warming and the decreasing pH of the oceans. Corals will respond directly to these changes by either migrating, bleaching, or attempting to adapt if the corals are more heat-tolerant. Coral communities will most likely switch to foraminifera-dominated communities.

The disappearance of coral reefs will have societal and economic impacts. Coral reefs protect coastal areas by reducing wave energy and limiting coastal erosion, reducing the cost of damage after natural catastrophic events. Fishing and tourism industries will also decline due to habitat loss. Degradation of the Great Barrier Reef alone due to ocean warming and acidification puts up to $5.7 billion USD and 69,000 jobs in Australia at risk (Gattuso et al., 2015). Without healthy coral-zooxanthellae relationships, reefs will become bleached caverns inviting the rapid growth and distribution of photosynthetic algae.

Many solutions have been proposed to mitigate coral reef bleaching. Evident from the models run, coral reefs may be able to survive and adapt under RCP scenario 2.6. Scientific consensus urges an avoidance to RCP scenario 8.6. If future carbon concentrations can be maintained to less than 490 ppm, coral reefs have a greater chance of survival than if we continue with “business as usual” emissions. For now coral health mitigation strategies, such as the implementation of Biorock, may be effective means to ensure coral reef survival.

Resources1. Archer, David. The Global Carbon Cycle. Princeton: Princeton UP, 2010. Chapter 4. Print.2. Burke, L., K. Reytar, M. Spalding, and A. Perry. Reefs at Risk Revisited. (Washington, DC,

USA: World Resources Institute, 2011). Online linkage: www.wri.org/reefs.

3. Cacciapaglia, Chris, and Robert Van Woesik. "Reef-coral Refugia in a Rapidly Changing Ocean." Global Change Biology, 2015, 2272-2282.

4. De'ath, G., J. M. Lough, and K. E. Fabricius. "Declining Coral Calcification on the Great Barrier Reef." Science 323 (2009): 116-119.

5. Descombes, Patrice, Mary S. Wisz, Fabien Leprieur, Valerianio Parravicini, Christian Heine, Steffen M. Olsen, Didier Swingedouw, Michel Kulbicki, David Mouillot, and Loïc Pellissier. "Forecasted Coral Reef Decline in Marine Biodiversity Hotspots under Climate Change." Global Change Biology, 2015, 2479-2487.

6. Gattuso, J-P., A. Magnan, R. Billé, W. W. L. Cheung, E. L. Howes, F. Joos, D. Allemand et al. "Contrasting futures for ocean and society from different anthropogenic CO2 emissions scenarios." Science 349, no. 6243 (2015): aac4722.

7. Goreau, Thomas J. "Electrical Stimulation Greatly Increases Settlement, Growth, Survival, and Stress Resistance of Marine Organisms." NR Natural Resources, 2014, 527-537.

8. Hoegh-Guldberg, O., P. J. Mumby, A. J. Hooten, R. S. Steneck, P. Greenfield, E. Gomez, C. D. Harvell, P. F. Sale, A. J. Edwards, K. Caldeira, N. Knowlton, C. M. Eakin, R. Iglesias-Prieto, N. Muthiga, R. H. Bradbury, A. Dubi, and M. E. Hatziolos. "Coral Reefs under Rapid Climate Change and Ocean Acidification." Science, 2007, 1737-1742.

9. Holland, Jennifer. "A Fragile Empire." National Geographic, May 1, 2011, 1-4. 10. Hughes, T. P. "Climate Change, Human Impacts, and the Resilience of Coral Reefs." Science

301 (2003): 929-33. Accessed October 6, 2015. 11. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II

and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

12. Kleypas, J. A. "Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs." Science, 1999, 118-120.

13. Osborne, Kate, Andrew M. Dolman, Scott C. Burgess, Kerryn A. Johns, and Brian Gratwicke. "Disturbance and the Dynamics of Coral Cover on the Great Barrier Reef (1995–2009)." PLoS ONE 6, no. 3 (2011): E17516.

14. Scheibner, C., and R. P. Speijer. "Decline of coral reefs during late Paleocene to early Eocene global warming." eEarth Discussions 2, no. 3 (2007): 133-150.

15. Vince, Gaia. "How Can We Save Our Dying Coral Reefs?" BBC Future, September 6, 2012.16. Wayne, Graham G.P. "The Beginner's Guide to Representative Concentration Pathways."

Skeptical Science. Skeptical Science, Aug. 2013. Web. 19 Dec. 2015.