Thursday, May 2, 2013

Greenhouse gases carbonate the ocean, but I’m not feeling bubbly

A blog of Bridge Environment, updated most Thursdays

The gases we are producing by burning fossil fuels and cutting/burning natural reservoirs of carbon, such as forests, are making the ocean carbonated. San Pellegrino and Perrier need not worry, though. This ocean cocktail is bitter to swallow. This chemical process creates what I believe is the most imminent danger from global warming, ocean acidification. Most other climate change dangers relate to increased temperatures. While these are very real threats, we don’t have a good understanding of exactly how temperatures will change with increased greenhouse gas concentrations, and there’s at least the possibility that the outcome will be mild. When it comes to the threat from ocean acidification, temperature is secondary. The concentration of greenhouse gases, specifically carbon dioxide, is the direct threat. These concentrations are unanimously recognized to have increased and predicted to continue to grow. In the next couple of paragraphs, I will go through the relevant chemistry. If that’s not of interest, feel free to skip past the chemical equations for a summary and discussion of implications.

Carbon dioxide (CO2) and water (H2O) have an interesting equilibrium:

CO2 + H2O  H+ + HCO3-

In this way, when more carbon dioxide is added to water, more bicarbonate (HCO3-) is produced along with free hydrogen ions. In solution, this combination is familiar to us as carbonated water. When you pop open a can or bottle of soda, the carbon dioxide built up in the headspace dissipates and more emerges from the liquid as bubbles, the fizz in soda pop. While carbon dioxide is still in solution, it continues to support free hydrogen ions, the acidity that gives soda its invigorating quality. Through this reaction, our production of carbon dioxide makes the ocean more like soda pop.

At first glance, this chemical relationship appears to be a good thing for marine organisms that produce calcium carbonate shells because there’s a second equilibrium:

H+ + HCO3-  2H+ + CO32-

and the fundamental chemical reaction for shell (CaCO3) production is:

Ca2+ + CO32-  CaCO3

For the non-chemistry-inclined, these relationships suggest that more carbon dioxide would lead to more carbonate ions (CO32-) for shell production (CaCO3), in combination with calcium. The problem lies with the fact that the equilibrium for shell production is sensitive to the concentration of free hydrogen ions in the solution, also known as acidity and measured as pH. As pH changes, the relative concentrations of carbon dioxide, bicarbonate ions, and carbonate ions change as well. Here’s a useful figure showing that relationship, from Wikipedia. The blue band indicates the increased acidity (lower pH) already observed in the ocean, and the arrow shows the direction of change with increasing build up of cabon dioxide. These changes in pH are associated with decreased concentrations of carbonate ions. As a result, marine organisms that produce calcium carbonate shells are facing an increasingly hostile environment due to increasing carbon dioxide concentrations.

This is no small change. Calcium carbonate shells are incredibly important in marine ecosystems and, in fact, the history of life. Life existed for literally billions of years in forms that were soft and mostly single-celled. That all changed about 500 million years ago, when many major groups of life forms first appeared during an event called the Cambrian explosion. One of the most distinguishing characteristics of the new life forms was their use of calcium carbonate shells. In modern-day ecosystems, shells are essential for the survival of many marine organisms, from some plankton (the little things, both plant and animal, that float in the water column and form the base of most marine food webs) on up. Corals are of particular concern because their calcium carbonate shells not only protect them, they also create the foundation of an entire and quite spectacular ecosystem. By making shell production more difficult, we are adding strain to these already stressed ecosystems.

The news is not all bad. Though some forms of plankton are at risk as a result of the difficulties in shell production, the plant varieties (phytoplankton) may actually thrive due to increased concentrations of carbon dioxide, which they use to capture the sun’s energy. Also, shell production in corals may be more complex, at least for some species, than the version presented here. There is some hope that at least a few species may be able to rely on bicarbonate-based pathways for producing shells. These findings prove once again that nature is resilient. They should not give us false hope, though. We are looking at dramatic changes to ocean ecosystems as a result of ocean acidification, even if certain life forms may find ways to adapt or even thrive.

In the past, my advice about climate change has been to refocus the debate on the uncertainties surrounding predictions of warming trends, and on identifying alternative policy options, their immediate costs, and the degree to which they would reduce the risk of bad outcomes from warming. I stick with that advice, but we should keep in mind that marine ecosystems face pretty certain dramatic changes, including the likely loss of coral reefs, if we do not start the process of reducing carbon emissions very soon.


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