The consequences of climate change on our ocean
Climate change has garnered a tremendous amount of attention in the past several decades from scientists and governments around the world but has mostly focused on one consequence of burning fossil fuels: the warming of the atmosphere and our oceans.
However, until recently, another very important consequence of increased carbon dioxide emissions has been largely overlooked. Our oceans currently absorb approximately one-third of the roughly 79 million tons of carbon dioxide that is emitted to the atmosphere on a daily basis. As the ocean absorbs more carbon dioxide, a series of chemical changes take place that could profoundly affect marine life, especially organisms such as corals which build their skeletons out of calcium carbonate. The gradual increase in acidity of the oceans and the resulting change in seawater chemistry is known as a process called ocean acidification.
Coral reefs are incredibly important ecosystems for a number of reasons: they provide income and food to millions of people around the world, protect shorelines and small islands from devastating storms, have tremendous biomedical potential, and have been likened to the “rainforests of the sea” because of the enormous amount of biodiversity they support.
A recent economic valuation of coral reefs placed their global worth at roughly $30 billion annually; one estimate values Bermuda reefs at an average of $722 million per year. Unfortunately, warmer and more acidic oceans as a result of climate change may act as chronic stressors to coral reefs, which already face acute threats such as overfishing, pollution and sedimentation.
When carbon dioxide is absorbed by the ocean, it reacts with seawater to form a weak acid, or H2CO3. This acid quickly dissociates to form bicarbonate and carbonate ions (HCO3- and CO2-3); as a result, the overall acidity of the ocean slowly increases. Assuming carbon emissions continue at their current rate, the pH of the ocean will be approximately 7.8 by the year 2100, which is lower than it has been for millions of years.
Calcifying organisms have endured in more acidic oceans in the past, but the rate at which our oceans are acidifying is taking place 100 times faster than it ever has in geologic time, so there are fears that these organisms may not be able to adapt quickly enough. As the acidity of the oceans increases, a shift in the availability of carbonate ions takes place; carbon is found in the form of bicarbonate, rather than carbonate.
Corals and other calcifying organisms combine carbonate ions with calcium ions from the surrounding seawater to build their calcium carbonate skeletons. This effectively means these organisms will have less material available to build their skeletons, will likely have to devote more energy to grow, and might build weaker structures.
The BEACON programme at the Bermuda Institute of Ocean Sciences (BIOS) studies how coral reefs may respond to ocean acidification at multiple spatial and temporal scales. Dr Andreas Andersson and collaborators are continuously monitoring seawater chemistry at Hog and Crescent reefs while measuring growth rates of corals at each.
In tandem with these measurements, corals are also grown in aquaria with closely controlled seawater chemistry reflecting the acidity of the oceans in the years 2050 and 2100. By combining results from these studies, Dr Andersson hopes to gain insight into the environmental controls of calcification and coral reefs.
Ocean acidification is a relatively young field of research that has recently gained substantial attention from scientists and other stakeholders. Though there is very little uncertainty about the effects and magnitude of ocean acidification in the open ocean, it is difficult to predict the consequences for near-shore, shallow water ecosystems and their associated calcifiers. Interactions between organisms and their surrounding seawater add tremendous complexity to the issue, making it difficult to conclude exactly how these organisms will respond. By studying which environmental parameters control calcification in corals, the BEACON programme at BIOS hopes to contribute very important work to the field of ocean acidification research that will help us predict how corals and other marine calcifiers will grow in the future.