ABSTRACT
Coral reefs are a hub of biodiversity and are under threat with increasing temperatures causing mass bleaching events worldwide and increasing coral mortality. Observed historical temperatures from 1981-2010 were plotted alongside two future scenarios (2021-2050, 2071-2100) under two different RCP scenarios, RCP4.5 and RCP8.5 to predict future temperatures using the change factor method to downscale predictions from 2.5° x 2.5° to 0.5° x 0.5° to be comparable to the observed baseline data. Results found temperatures increase across the board, particularly underRCP8.5 with temperatures increasing by as much as 2.8°C which will move the climate out of the optimum temperature for coral to thrive. The reef is therefore under threat by increasing temperatures, seen with a 50% loss from two mass bleaching events in 2016/2017. Coral bleaching and biodiversity impacts are investigated and adaptative measures on a national and global scale are analysed.
1. INTRODUCTION
Climate change is responsible for numerous substantial impacts on Earth’s ecosystems. The purpose of this report is to look at the impacts of climate change on the Great Barrier Reef(GBR), the largest reef system on the planet. It stretches 1429 miles covering ~113,000 square miles, composed of almost 3000 individual reef systems(NOAA, 2020). The Great Barrier Reef is located in the Coral Sea off the East coast of Queensland, Australia (seen in Figure 2). The greatest threats to the reef are severe weather systems, rising sea temperatures, and ocean acidification –all impacts of climate change(Great Barrier Reef Foundation, 2020). As atmospheric CO2 and other greenhouse gases (GHG) rise ocean temperatures, the temperature range in which coral thrive is exceeded, leading to coral bleaching (Great Barrier Reef Marine Park Authority, 2017). The impact of the 2016 mass coral bleaching event is seen in Figure 1.This has been studied previously by Prachett et al., (2011) who presented changes in biodiversity caused by coral bleaching; however,they did not take temperature into consideration. Therefore, the aim of this report is to answer how changing climate will impact coral bleaching and ultimately, biodiversity in the area due to the deterioration of coral ecosystems (Munday et al.,2007). This will be achieved by examining how climate has changed in the past, continues to change in the present, and using two different climate scenarios (RCP 4.5 and RCP 8.5), forecasting how it may change in the future.
2. METHODOLOGY
The primary software used to investigate the impact of climate change on the GBR was Microsoft Excel. Using the co-ordinates for the GBR (18.29°S, 147.70°E), data was obtained from KNMI Climate Explorer. For baseline data, historical and observed daily temperatures were gathered using the ERA5 1979-now 0.5°which provided data from1979-2020ona scale of 0.5° x 0.5°. This resulted in a time-series output which was extracted in raw form and pasted into a prebuilt spreadsheet for Maximum Temperature (TMAX). The data was then trimmed to contain only 1981-2010 to provide a 30-year baseline period, as recommended by the International Meteorological Organisation in 1935 (now known as the World Meteorological Organisation) (IMO, 1935). The raw temperature data output was provided in Kelvin (°K) and converted to Degrees Celsius (°C) by subtracting 273.15 from the Kelvin value. For future projections, Representative Concentration Pathway (RCP) data was gathered for climate scenarios RCP4.5 and RCP8.5 using the 5th phase of the Coupled Model Intercomparison Project (CMIP5) Mean. The CMIP5 Mean provides a framework for climate change investigations whilst encouraging the use of a standard set of model simulations (PCMDI, 2020). These RCPs were chosen as RCP4.5 is the lower intermediate scenario whilst RCP8.5 was chosen as it is the highest GHG emission scenario, and the pathway that climate is currently headed towards (IPCC, 2014). Raw data was extracted and input to the spreadsheet containing values from 1861 (hindcast period) to 2100 before being cleaned to contain only from 1981-2100 to be concurrent with historical observed values.
Future data uses a spatial resolution of 2.5° x 2.5°, a coarser grid box than the historical observed data (0.5° x 0.5°) and required downscaling through the use of Change Factors (CF) for it to be comparable locally and regionally. This was achieved by subtracting the hindcast period (1981-2010) from the future period (2021-2050 and 2071-2100) within each model, for each month, for RCP4.5 and RCP8.5 respectively. The CF works on different scales dependent on the scenario and time period used. The calculated change factor (seen in Figure 2) is added to historical observed data to produce future downscaled projections. The difference in spatial resolutions can be seen in Figure 3.
The prebuilt spreadsheet contained formulae which calculated a variety of statistics – Mean Temperatures, Q90, Summer Days, Heat Waves and Change Factors. A pivot table was created to refine an average of yearly temperatures for both observed data (1981-2010) and future predictions (2021-2100). Using these statistical calculations, two graphs and three tables were produced. Additional statistics were calculated to refine the results of the project, producing an additional two tables and one graph.
3. RESULTS
The results displayed and discussed below are a result of the methods undertaken in Section 2.
Figure 4 displays the average temperature of each year from the observed historical data (1981-2010) and predictions based under two different RCP scenarios (RCP4.5 and RCP8.5) for the 30-year baseline covered throughout. Previously known El Niño years have been highlighted (1994-1995, 1997-1998) with the impacts of El Niño predominantly seen in the temperatures the following year, such as the increase from 25.01°C in 1997 to 26.33°C in 1998 (Australian Government Bureau of Meteorology, 2020). Natural climate variation is seen throughout the observed data, with temperatures remaining mostly between 25-26.5°C. However, under the two RCP scenarios, annual average temperatures are projected to increase to 26.8°C in 2100 under RCP4.5 and 28.1°C under RCP8.5 (the current emission pathway we are on).
Figure 5(A) uses conditional formatting to create a heat map effect on the mean temperatures using the baseline value. The table shows that temperatures can increase by as much as 2.8°C to 30.5°C by 2100 under RCP8.5 in comparison to the baseline temperature of 27.7°C. The table also shows annual temperature rises, in consensus with Figure 4. Figure 5(B) shows a decrease in temperatures associated with the onset of winter in June/July, also seen in Figure 5(A), however an increase of 2.8°C is still shown during these months rising from 23.4°C to 26.2°C.
Figure 6(A) shows the table generated in the preconstructed spreadsheet. The baseline of 28.7°C for January, the peak summer month, is higher than 90% of temperatures observed in January from 1981-2010. Under different scenarios, this is only seen to increase across all months, as seen in the annual value in the final column. Figure 6(B) displays the 95thpercentile which is a more confined range of values displaying the extremes where only 5% is above these values. Under different RCP scenarios, it is seen to increase by as much as 2.7°C by 2100 under RCP8.5, however if RCP4.5 is achieved it will increase only by 1.4°C.
Figure 7 shows the amount of summer days (temperature >25°C)in comparison to baseline data. The most significant increase is conclusive with other results seen in RCP8.5 for the end of century (2071-2100) where the most notable increase is during the peak winter months of July and August. However, even under the intermediate RCP4.5, some summer days appear in comparison to the baseline(increasing by 37 days).
Figure 8(A) displays the temperature range in which coral thrive (23-29°C) with conditional formatting to show the seasonal shift. Summer months in baseline conditions contain the most days for coral to thrive, replaced by the winter months as we move into hotter climates in the next 30 years (moving from July baseline of 13 days to 22 days under RCP4.5 in 2021-2050). Annual totals for optimum temperatures decrease by 89 days by 2100.Figure 8 (B) is a visual representation which shows the magnitude of this shift, particularly under RCP8.5 from 2071-2100 with a sharp increase in the winter months of June, July and August.
4. DISCUSSION
As seen throughout the results section, temperatures are only set to increase under future climate scenarios.
The IPCC (2018) concluded with high confidence that coral reefs are among the most vulnerable of marine ecosystems, with over 50% of reef’s worldwide under a high level of threat to degradation (Burke et al.,2011). Coral bleaching is the breakdown and loss of the endosymbiotic algae (zooxanthellae), the coral food source, often caused by increased temperatures of 1-2°C (IPCC, 2018; NOAA, 2020; IUCN, 2017). If bleaching occurs over a prolonged period, the coral will die(seen in Figure 9A-B), seen in the 2016 –2017 bleaching which destroyed 50% of the coral in the Great Barrier Reef (IUCN, 2017; National Geographic, 2018). The influence of El Niño/La Niñaevents (identified in Figure 4) will only be amplified by a further increase in temperatures. The IPCC (2018) have high confidence that mass coral bleaching is a direct impact of climate change (conclusive with increasing summer days displayed in Figure 7, with a range of increase from 37-128 days). A 2017 study by UNESCO concluded worldwide reef systems will perish under RCP8.5, unable to adapt to rising temperatures. This is due to the optimum temperature range for coral (23-29°C, displayed in Figure 8A-B), with only a few tolerating higher or lower temperatures for up to 6 weeks before dying. Water conditions must also be correct, with a salinity of 32-42parts per thousand (NOAA, 2020) and a stable ocean pH above 7.8 as ocean acidification prevents the growth of coral skeletons with failure to thrive below pH 7.7 displayed in Figure 9(C-D) as large coral dominate the low pH environment, with less diversity in ecology. (IPCC, 2018; Mollica et al.,2018). Ocean acidification is also found to impact biodiversity with high confidence by the IPCC (2018). Therefore, coral bleaching, ocean acidification and rising sea temperatures can impact biodiversity due to the sensitivity of coral ecosystems (Stuart-Smith et al.,2018).
However, it is important to note that climate change is not the only threat that coral reefs currently face. The IPCC reported in 2018 that it was only following the 1980’s that human-induced disturbances became more widescale. Previous to this, they remained localised as it occurred mostly through processes such as pollution, overfishing and unsustainable coastal development. A consensus among scientists places the regime shift around 1987 and global climate shift occurring due to anthropogenic warming (University of Plymouth, 2015), conclusive with the observed warming record (Figure 4). Reid et al.,(2016) discovered three of these shifts (1970s, 1980s, and 1990s) with a notable increase in temperature or sudden temporal shift in biophysical systems (supported by Beaugrand et al.,2013). Cold-stress events have also been found to cause coral bleaching such as in Florida Keys in 2010 (NOAA, 2020).
Coral reefs are composed of Calcium Carbonate (CaCO3) and occur in shallow-water ecosystems. Although they cover a small area of the ocean floor (0.1%), they are a hub of biological diversity, hosting over 600 species of hard and soft corals, 3000 species of molluscs, over 100 species of jellyfish, ~1600 species of fish, 133 species of sharks and rays, ~30 species of cetaceans and 500 species of worms (GBRMPA, 2009). Following the mass bleaching events of 2016 and 2017with a 50% loss of coral in the Great Barrier reef, biodiversity has already seen large changes in just the last few years. Several species are of conservation concern, such as seahorses and large cetaceans, whilst threatened species include marine turtles and dugongs (GBRMPA, 2014). Bleaching leads to an ecosystem restructuring –most notably, with the decline of coral-feeding fish such as finfish, molluscs, echinoderms and crustaceans (Brown et al.,2020; Prachett et al.,2011). Up to 75% of reef fishes rely on coral for shelter, settlement or food; however, not all are completely reliant on coral, and can adjust better than others, leading to short-term variations in reef species.
Small levels of coral loss can increase biodiversity by making the habitat more suitable to invasive species such as Alexandrium minutem (toxic dinoflagellate) and Crassostrea gigas (Pacific oyster) (Cheal et al.,2008). However, rising sea surface temperatures (SST) a result of climate change (already seen with a change of 0.6°C recorded since the 1950s (Figure 10) (NCCARF, 2016) can impact the biological processes of fish,as water temperatures impact the fish body temperatures which influence growth, reproduction and swimming ability (Australian Government Bureau of Meteorology, 2020). Rising SST influences the photosynthesis rate for seagrass, a food source for marine reptiles and dugongs, whilst also providing a nursery for numerous species of fish (Descombes et al.,2015). Fish species are therefore likely to broaden their geographic range North/South. Larger reef reptiles are also affected, particularly the Crocodylus porosus (Estuarine Crocodile) whom have several vital aspects of their survival environmentally determined such as nesting periods, sex determination and swimming speed of hatchling (which can impact survival rate against predators) (IUCN, 2014).
Mammals such as seabirds, of which ~22 are present in the GBR (GBRMPA, 2009) can also be impacted as a knock-on effect of fish species variation as their food source is altered. Studied previously in El Niño/La Niña events (such as those outlined in Figure 4) with the result of fewer breeding cycles, reduced breeding success and slow chick development. Raine Island has recorded a steady decline in seabird species for the last 12 consecutive years as climate warms, with species being driven further South. (Australian Government Bureau of Meteorology, 2020).
Despite the devastating impact that climate can have on reef systems, there are some adaptative measures that can be made. One is government policies to limit the warming experienced; with policies such as the Paris Agreement already in place from 2015 to limit temperature rise globally–however, even if the efforts of the Paris Agreement are reached, it is likely corals will not recover from the damage. Further legislation from the Australian government to reduce GBR tourism and move towards sustainable coastal development and reducing land-based runoff and therefore pollutants may help the area to survive (GBRMPA, 2014). The reef became a World Heritage Area in 1981, and in 2004 became the largest marine protected area in the world. (Fidelman, 2014)however the value of the reef as a World Heritage Site will be questioned with its decreasing biodiversity (Great Barrier Reef Marine Park Authority, 2019) as the resilience of the reef decreases with rising SST. Efforts by the Australian government therefore focus on rebuilding reef resilience, alongside protecting key species and enabling the restoration of previously bleached areas such as Raine Island. Artificial reefs may also be used in the future using natural or synthetic materials (Aloysius, 2020).
Results of this report provide limitations due to the methodology, such as the change factor downscaling method. In this, the maximum temperatures and mean temperatures are shifted, maintaining the variability of observed results (seen in Figure 4). The IPCC (1992) noted 19thcentury baselines were preferable as anthropogenic effects were negligible but benefits to using late 20thcentury data were additional climate coverage and data availability. Using only two scenarios (RCP4.5, aggressive mitigation and RCP8.5, no mitigation business as usual) does not givea representative picture, as two other scenarios exist, notably RCP6.0 –a scenario between the two studied here). The ideal dataset would have been SST rather than air temperatures;however,the results provide a good approximation due to the close correlation between air and SST (Rubino et al.,2020).
5. CONCLUSION
In conclusion, climate change has already impacted biodiversity on the Great Barrier Reef with mass bleaching events becoming more frequent with increasing air and sea surface temperatures. There has already been a notable spatial redistribution in species moving Northwards/Southwards owing to increasing temperatures. Further increase in temperatures may lead to the destruction of the entire life as the optimum climatic range is exceeded. Adaptative measures therefore focus on building reef resilience and the introduction of artificial reef communities to enhance biodiversity and reduce reef mortality.
DISCLAIMER
This work is a piece of work that was originally submitted as an assignment to Queen's University Belfast by the author, all efforts have been made to erase links to the original module in order to avoid plagiarism by other students in the future.
Editors Note
I have taken the decision to remove certain tables and references to such from the above report. Sasha's original transcript contains all methods used, and provides a high degree of reliability. Although I was cautious to publish the report due to potential formatting issues, I believe Sasha's work to be accessible and that it demonstrates the very real consequences of anthropogenic climate change on the natural world. Building on from the succinct introduction on the topic by Julia Anusiak (here), I hope we can continue to understand the impact our actions have had on the natural world that we are a part of. From this, we can take practical steps to help build a better world.
-Amy Oke
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