The continental shelf around Alaska is particularly vulnerable to ocean acidification due to global human emissions combined with natural local influences. The biggest natural factors that worsen ocean acidification include respiration of organic matter, melting sea ice, upwelling, and freshwater input from rivers. Additionally, a decline in annual sea ice coverage provides a positive feedback to OA, as more open water allows for greater uptake of CO2 from the atmosphere.
Researchers have paired ship-based observations with computer models to develop future projections of ocean acidification for the Bering, Chukchi, and Beaufort Seas. The studies provide predictions for when marine life is likely to feel significant affects from reduced aragonite saturation – a metric used to assess ocean acidification. The aragonite saturation state (Ωarag)describes the amount of free calcium carbonate in the water column. Many marine organisms use calcium carbonate to build their shells. In acidified conditions, less calcium carbonate is available for shell building, and the saturation state goes down. As Ωarag decreases, the shell building ability of animals decreases. An Ωarag level of 1.0 is generally understood to be the tipping point: above 1.0, shell building can continue; below 1.0, shelled organisms have trouble developing shells, or existing shells may dissolve.
Dipping below the tipping point
Projections show the annual average Ωarag in sea surface water will dip below 1.0 for all three seas within the next 50 years. The Beaufort Sea, which is currently experiencing Ωarag under 1.0 for much of the year, first crossed the threshold for mean annual Ωarag around 2001. It is expected to be followed by the Chukchi around 2033 and the Bering Sea around 2060.
The main factor affecting carbonate chemistry in the Bering Sea is the “biological pump”. This is when carbon dioxide is removed from the surface water by marine plants (like phytoplankton), which convert the CO2 to food for zooplankton and their predators. While some of the CO2 is recycled near the surface, much of it sinks into deeper waters. In the summer, the biological pump thus decreases CO2 in the surface water and increases aragonite saturation in the summer. So far, surface water with Ωarag less than 1.0 has only been measured in areas with substantial sea ice melt or river run-off, although bottom waters are consistently seasonally undersaturated and corrosive to carbonate minerals.
The Bering Sea has a naturally high degree of seasonal and interannual variability, and Ωarag changes over a very broad range. This means that many organisms are adapted to an environment with large swings in Ωarag values. This creates a natural resilience towards the degree of change that is imparted by human emissions and anthropogenic ocean acidification. Surface waters in the Bering Sea are presently supersaturated (Ωarag well above 1.0) but by 2050 these supersaturations will be weak. By 2100, it is expected to join the Chukchi and Beaufort in being undersaturated (Ωarag < 1).
In the Chukchi, carbonate chemistry is also being affected by the biological pump, but many other factors also contribute to OA vulnerability. A longer ice-free season, reduced sea ice thickness and lesser sea ice coverage all have the ability to lower Ωarag. Additionally, sea ice melt is naturally corrosive, and can acidify waters at the sea surface. At present, the Chukchi is ‘supersaturated’ with respect to aragonite, but by 2050 it is expected to be largely undersaturated.
The Beaufort Sea is a narrow basin with fewer nutrients and a much lower rate of primary production than the other water bodies. The main OA driver is upwelling – deep water coming to the surface that is naturally high in CO2 and undersaturated in aragonite. In one upwelling event observed by researchers, these waters came to the surface and moved all the way inshore along the Beaufort shelf, covering thousands of square kilometers.
In the past, upwelling slowed down each year in the fall when sea ice returned, however now that sea ice is consistently forming or arriving later, the surface is increasingly open during late autumn storms. This allows the upwelling process to continue and intensify, which exacerbates the OA problem. According to Mathis et al (2015), “In the future, the Beaufort shelf is likely to be persistently, if not continually, exposed to waters that are under-saturated with respect to aragonite as sea ice cover continues to diminish under warming conditions.”
When will organisms begin to show signs of stress?
Recently, researchers have begun to realize the 1.0 Ωarag threshold may not be the best estimate for predicting when marine life will feel the impacts of OA. This is because some species have shown a range of responses to differences in aragonite saturation – not just at the threshold. In one study, pteropod shells degraded in water where Ωarag = 1.2 while other species continued producing shells below 1.0. This makes scientists believe it may be more important to look at the natural range of variability of Ωarag in an ecosystem, and when it is expected to depart from that range.
Model results show that the Bering, Chukchi and Beaufort Seas will each pass below their respective natural ranges in the next 30 years: the Beaufort Sea by 2025, the Chukchi by 2027, and the Bering Sea in 2044. Because of the rate of acidification, it may be difficult for organisms to adapt even if Ωarag is above 1.0.
What are the challenges to forecasting OA?
Forecasting ocean acidification is challenging. First, the ocean around Alaska spans thousands of square kilometers, more than the US east coast, west coast, Gulf of Mexico, and Great Lakes combined. Additionally, there is a lot of geographic and oceanographic variability within this large area. Lastly, cold temperatures and harsh conditions create a hazardous environment for long-term sensors. These three factors make monitoring expensive and logistically difficult. Scientists work hard to target key areas to help understand ocean acidification processes and potential environmental impacts, to make monitoring and research efforts as efficient as possible.
Second, a variety of different mechanisms and feedback loops may unfold which could potentially worsen or mitigate OA in Alaska.
- Warming temperatures could increase freshwater runoff which generally makes ocean acidification worse in localized areas
- Decreases in sea ice could change the timing of primary production and alter the “biological pump” which brings CO2 into the deeper ocean.
What we do know is that ocean acidification is taking place at a pace much faster than any time in the past 50 million years, and organisms are also under pressure from other climate change factors such as warmer water temperatures, shifting circulation patterns, and sea ice change.
Information on this page came from: Mathis, J.T., and Cross, J.N., Evans, W., and Doney, S.C., 2015. Ocean acidification in the Pacific-Arctic Region. Oceanography Magazine. doi: 10.5670/oceanog.2015.36
More information on data, models, and findings for specific geographic can be found under the list of academic articles.