Marine Climate Change
The biggest threat of our lifetime
Climate change is becoming a huge problem, rapidly affecting our oceans, and the life within them. Climate change or global warming as it is sometimes called is defined as the change to the usual or normal weather patterns or conditions experienced in any one location and is driven by anthropogenic emissions of greenhouse gases such as carbon dioxide. Carbon dioxide is produced in great quantities during the burning of fossil fuels for energy production, and when released into the atmosphere, creates a layer of insulating gases around the earth, trapping light in and causing a heating effect. This not only heats the planet but also causes shifts and changes in normal weather patterns, increasing phenomena such as hurricanes and El Niño.
Marine climate change is a largely invisible, unquantified and multifaceted threat to oceans themselves and coastal communities that is characterised by increased sea surface temperatures (SST), acidification, changes in salinity, as well as shifts in upwelling and currents (Hoegh-Guldberg and Bruno, 2010), with potential impacts on the incidence and prevalence of marine diseases (Ward and Lafferty, 2004). The effects of climate change on disease incidence can be mediated through effects on the physiology (Timmis et al., 2019), distribution, phenology, and/or demography of the host species, or indirectly through changes in interspecific interactions within communities, for instance, when the abundance of one species increases to the detriment of another or when new species come into contact (Poloczanska et al., 2013).
We have already seen some of the devastating effects of climate change here on Koh Tao, when in 2010, an increase in water temperature sparked a mass bleaching event across the island, forcing coral communities to expel their photosynthetic friends, zooxanthellae, due to the temperature shock. However, we believe the effects of climate change around the island may be more complex and widespread.
It’s not just coral that’s at risk. Prolonged temperature increases attributed to anthropogenic climate change has been strongly linked to ulcerative skin diseases in reef fish in the Galapagos Islands (Lamb et al 2018), and we really need to keep a close eye on fish community health here to make sure we are able to report this phenomenon if it repeats itself.
The acidification of the oceans is often overlooked or misunderstood, when in reality, it is a dangerous symptom of climate change in oceans that has the potential to greatly affect some of the less charismatic than some but still incredibly economically and ecologically important, marine arthropod molluscs.
Ocean acidification is the process in which the pH of the ocean is decreased (acidified) as atmospheric carbon dioxide gas dissolves in the water. This is how it works: as water (H2O) combines with carbon dioxide (CO2) an acidic compound called carbonic acid (H2CO3) is produced.
The pH scale is a scale used for measuring the acidity or alkalinity of substances and runs from 0 – 14, 7 being neutral, below 7 is acidic and above 7 is alkaline. The pH scale is an inverse scale of hydrogen ion concentration therefore higher hydrogen ions equates to more acidity and lower pH. Seawater is naturally slightly basic and before the industrial revolution had a pH of approximately 8.2, it has since fallen to 8.1.
Now, this might not seem like a drastic decline, however the pH scale, just like the Richter scale (use for measuring earthquakes) is logarithmic. Let’s put that into numbers: for example, a pH of 3 is ten times more acidic than a pH of 4, and therefore 100 times more acidic than a pH of 5. By the end of the century, it is predicted that the pH of the ocean will drop by another 0.3 pH units, at this current acidification rate the ocean will be 120% more acidic by the end of the century, which is more acidic than this planet has experienced in over 20 million years.
So why is acidification of the ocean so disastrous for marine life? The hydrogen ions (H+) released by acids in solution bonds very readily to carbonate (CO3-2), which is a key component of the calcium carbonate (CaCO3) exoskeletons of a vast array of marine species such as bivalves, corals and crustaceans. Hydrogen ions are highly polar, which means they bond more readily to carbonate than the calcium ions that these animals require to build calcium carbonate skeletons. Animals that require carbonate ions free in ocean water to build their homes are now much less able to extract carbonate ions as they are already bonded with hydrogen. Even when organisms are able to extract carbonate ions from water to build their exoskeletons, they may need to expend more energy doing so, using vital resources required for other processes like reproduction, feeding or immune responses.
As we emit more carbon dioxide into the atmosphere and the heating greenhouse effect of the gases trapped underneath the ozone intensifies, so does the heating of our planet. As atmospheric temperatures increases, vast amounts of ice in polar regions melts and a symptom of this is sea-level rise. Simply put, this is the raising of sea levels due to the input of water usually trapped as solid forms such as glaciers and ice caps.
It is thought that ice caps in Greenland and the Antarctic region are the main contributors to sea-level rise, and it has been calculated that ice mass in these regions is decreasing by as much as 3% per year. This doesn’t sound like a lot, but the latest Intergovernmental Panel on Climate Change (IPCC) projections on global sea‐level rise (SLR) range from 0.47 to 0.63 m by the year 2100.
This sea-level rise, when aggravated by increased storm activity has the potential to destroy important coastal habitats for a variety of marine creatures such as marine turtles. Marine turtles nest on beaches all over the world, and in key rookeries in the Mediterranean have been documented to nest on average 1.32m above sea level in the case of the loggerhead turtle (Caretta caretta) and green turtles (Chelonia mydas) at 1.8m. Taking the SLR projections of the IPCC into consideration, an SLR scenario of only 1.2m would result in a loss of an estimated 59.1% of green turtle nests and 67.3% of loggerhead nests in the Mediterranean alone.
Marine turtles are already at risk of considerable threats such as overexploitation, bycatch and plastic ingestion and entanglement. Losing this percentage of nests to SLR would be disastrous for an already very threatened group of species. Not only this, but humans would also feel these negative effects with coastal communities losing their homes and being pushed further inland.
Coral is one of nature’s great examples of how animals and plants can live in symbioses together, each benefiting from the relationship in their own way. Coral polyps are able to draw calcium and carbon from the ocean to form secure skeletons to live within, creating a stable habitat. Polyps have stinging cells called nematocysts which they can use to capture food of their own from the water column, however warm water corals typically also have photosynthetic symbiotes, known as zooxanthellae, which live within these stable skeletons, taking energy from the sun through photosynthesis, offering some of this to their polyp symbiotes in exchange for nutrients and all-important CO2 required for photosynthesis.
Coral bleaching is a phenomenon in which acute or prolonged stress causes corals to expel their photosynthetic symbiotes, eventually causing mortality of corals if the process isn’t reversed and the corals re-recruit symbiotes. The link between coral bleaching and elevated sea temperatures due to climate change has been well documented for years now, however, its not only the high temperature alone that causes bleaching – in some instances it is more accurate to say that the stress caused by increased temperatures immunosuppresses the coral and gives the pathogen a competitive advantage to launch an infection. Ocean acidification driven by climate change can also act as a stressor to corals, as in these conditions their calcium carbonate skeleton becomes eroded, providing entry points for infection as well as weakening the polyp itself.
At Big Blue Conservation we are keeping a close eye out for signs of climate change-induced stress at every one of our dive sites through our reef monitoring protocols; read more about how we survey the reefs here.