Editor’s note: This article is the second in a three-part series that investigates the connections among increasing atmospheric carbon dioxide concentrations, the ocean’s role in absorbing carbon dioxide and the effect on marine ecosystems, and what happens to atmospheric oxygen levels when the base of the food chain—phytoplankton—dies off.

The first article on Fossil Fuel Emissions and Atmospheric Carbon Dioxide discussed how burning fossil fuels has increased the concentration of CO₂ in our atmosphere.  Although the rise in atmospheric CO₂ concentrations since the Industrial Revolution has been steep (a 40% increase), it would be even higher if not for the moderating effect of the ocean.

The ocean acts as a carbon sink, storing vast amounts of carbon.  It is responsible for removing a staggering 30% of anthropogenic CO₂ emissions (originating from humans) from the atmosphere each year.  This storage is the result of two processes at work within the ocean: the biological pump and the solubility pump. 

The biological pump transfers carbon from the atmosphere to the deep ocean.  Ocean plants in the surface layer of the water absorb CO₂ through photosynthesis just as plants on land do, incorporating the carbon into their bodies.  When the organisms die or excrete waste, carbon sinks to the deep ocean for long-term storage or becomes part of the sediment in the ocean bed for permanent storage.

The solubility pump dissolves CO₂ from the atmosphere into the surface layers of the ocean.  Carbon dissolves best in cooler water, which sinks into the deep ocean, sequestering the carbon for up to 500 years.   The amount of CO₂ that dissolves in ocean water depends on the concentration of carbon in the atmosphere relative to the ocean as well as the temperature of the water.  These two effects compete with each other as atmospheric CO₂ concentrations and ocean surface temperatures rise.  Greater amounts of CO₂ in the atmosphere increase the ocean’s carbon uptake, but warmer ocean water dissolves less CO₂.  Likely, the ocean will continue absorbing carbon until it reaches a saturation point, but at a slower rate due to warmer temperatures.  The ocean’s role as a carbon sink and the implications of warming for this part of the carbon cycle is a hot topic of research news lately, especially at the University of Wisconsin.   

The Ocean & Climate Change

The uptake of CO₂ by the ocean slows climate change by lowering atmospheric greenhouse gas levels, but not without consequences.  When the ocean absorbs CO₂, carbonic acid forms in the process.  Therefore, as fossil fuel emissions raise atmospheric CO₂ concentrations, the acidity of the ocean also increases (see graph above).  The global mean ocean surface pH has already decreased from 8.16 to 8.05 since the year 1850 (lower pH represents higher acidity).  Since the pH scale is logarithmic, this is a 30% increase in acidity.

Acid Test

The Intergovernmental Panel on Climate Change makes projections for future CO₂ concentrations under scenarios with varying socio-economic storylines and levels of technological and environmental development.   The projections for atmospheric CO₂ concentrations range from around 530 ppm to 940 ppm by the year 2100, depending on the emissions scenario.  Under these conditions, ocean acidity is expected to increase, with pH values dropping to between 7.7 and 8.0 by the year 2100 (see graph below).

The main reason ocean acidification is of concern is the effect it may have on the marine ecosystem.    The carbonic acid formed from dissolved CO₂ releases bicarbonate along with the hydrogen ion responsible for acidification.  Bicarbonate formation deprives calcifying organisms of the carbonate needed to form their shells or skeletal structure.  Coral reefs (which provide habitat for sea life) and phytoplankton and pteropods (which serve as the base of the food chain) are particularly at risk.  Diminishing food sources and habitat threatens larger species, such as cod and mackerel, many of which we rely on economically and nutritionally.

Another stressor on the marine ecosystem is warming sea water resulting from climate change.  Warming surface water is less dense than colder water beneath and has a lesser tendency to overturn and mix water between layers.  Mixing is necessary to transport the water from areas deeper in the ocean that are richer in nutrients to phytoplankton in surface waters that need it to survive.  Researchers have observed a decline in phytoplankton population of 40% since 1950, which they attribute to warmer ocean temperatures.

There is debate about what atmospheric CO₂ stabilization target we must meet in order to avoid the worst damage to ocean life; some argue 450 ppm while others support 350 ppm.  The global climate and the interactions between the atmosphere, ocean and living plants and animals are complex and involve uncertainty.  But clearly, we cannot continue to rely on the ocean to buffer the effects of our pollution indefinitely without consequences. Reducing our CO₂ emissions is necessary for a variety of reasons, but is crucial for saving the marine ecosystem by reducing ocean acidity and stabilizing the water temperature.        

The next article in this series will discuss how phytoplankton die-off and fossil fuel combustion affect levels of atmospheric oxygen.

Written by :

Elizabeth Klusinske