Hauri, Claudine, Tobias Friedrich, and Axel Timmermann. “Abrupt onset and prolongation of aragonite undersaturation events in the Southern Ocean.” Nature Climate Change (2015).doi:10.1038/nclimate2844
Marine ecosystems in the Southern Ocean are important for the global cycling of nutrients and carbon and support economically valuable fisheries and tourism. Many marine animals form shells or skeletons from calcium carbonate (CaCO3), including corals, oysters, pteropods and clams. These animals use carbonate and calcium dissolved in seawater as the building blocks for their shells or skeletons. When excess carbon dioxide from the atmosphere dissolves in the ocean, it upsets the equilibrium state of seawater, making it more acidic. This sets off a chain of chemical reactions which act to reduce this excess carbon dioxide and return the system to equilibrium. In the process, the concentration of carbonate (CO3) decreases by combining with extra hydrogen ions to form bicarbonate (HCO3). When carbonate is removed from the ocean, the ability of organisms to form shells and skeletons is reduced. If the amount of carbonate in the water becomes extremely low, solid shells could actually dissolve to replace the carbonate removed from the seawater. In the Southern Ocean, pteropods, small sea snails which float in the surface ocean (Fig. 1), are particularly sensitive to this effect.
The threshold at which calcium carbonate shells could begin to dissolve is known as the saturation state. If the saturation state drops below 1, we can expect calcium carbonate to dissolve. When more carbon dioxide is added to the ocean and carbonate is taken away, shallower waters become undersaturated, exposing more organisms to undersaturated conditions. The response of organisms to undersaturated conditions is complex, with some experiencing negative effects at concentrations above the saturation state, and others surviving below the saturation state. The survival of organisms depends not just on the saturation state, but the length of time they spend in undersaturated conditions. If the exposure is short, they may be able to wait it out for a month or two but if they remain in undersaturated conditions continually, the likelihood of survival is much lower. A second important factor is how quickly conditions switch from saturated to undersaturated. Animals can adapt or evolve to slow changes in their environment to compensate for the changes but if the switch to undersaturation happen very quickly, then there is no time to adapt.
Focusing on how rapidly changes in saturation occur, and how long undersaturated conditions last, researchers from the University of Hawaii collected the projections of saturation state from ten Earth System models. These models are the most recent, state-of-the-art climate models used for predicting how the atmosphere, ocean and land will respond to increases in carbon dioxide in the atmosphere. However, all of the models have substantial errors and different biases so to reduce this the authors of the study average the results from all of the models. Before looking at the model’s predictions for the future, the authors compared the model’s predictions for the 1990’s to observed ocean conditions during that time and found the models did well representing the spatial patterns and variations in the surface ocean.
The model analysis suggests that by 2045 much of the surface of the Southern Ocean will be exposed to undersaturated conditions at the surface at some times of the year (Fig. 2). By 2095, they predict that the entire Southern Ocean surface will be undersaturated during the year, and in many locations it will be undersaturated year round at 100 m depth. The long duration of undersaturation shown in the models may have a major impact on survival of pteropods and other organisms. However, because organisms could potentially adapt to the new conditions, the authors also looked at how quickly the ocean transitions from saturated to undersaturated conditions (Fig. 3). The transition time is less than 20 years in most locations, which may be too fast for organisms to adapt as the changes will occur within just a few generations. However, the ability of organisms to adapt to changes in saturation state is poorly understood and scientists are continuing to work to predict how these changes will affect different species.
It is important to remember that these models make assumptions about future global carbon dioxide emissions, and this study focused on a single scenario which predicts continued emissions of carbon dioxide at a similar rate to today. However, even in the scenario that carbon dioxide emissions are decreased rapidly in the near future, the surface of the Southern Ocean is still predicted to become undersaturated toward the end of the century. There are other factors that the models do not take into account which may also have a big impact on calcium carbonate saturation state, such as changes in ocean circulation and changes in biological production. However, despite these limitations, the results still give a useful indication for the timescales and regions where we expect undersaturation of calcium carbonate to potentially impact marine ecosystems in the Southern Ocean.
This study emphasizes the need for more observations of the carbon cycle and ecosystems in the Southern Ocean, particularly in regions which are vulnerable to undersaturation in the near future. It also underscores the need for studies to understand the response of organisms to changes in saturation state. In particular, pteropods are very vulnerable to undersaturated conditions and are a very important component of the marine food web, as they are an important food source for krill, whales and other larger marine animals. It is important to understand how the entire ecosystem will respond to these changes, so that fisheries and other industries can incorporate this information into future planning and conservation efforts.