Acid oceans and the next 800,000 years

Have you heard the term “ocean acidification” being thrown around in the popular media recently? If you have heard a bit about this phenomenon but you’re not a climate scientist, it’s likely that you’ve been left with the impression that ocean acidification is yet another in a long list of the potentially nasty consequences of climate change. Probably something for the scientists to be concerned about, but not nearly as pressing as the melting of polar ice caps or the possibility of a twenty foot rise in sea level. However, given the invaluable ecosystem services and role in climate regulation that our oceans provide, I find it shocking that so little attention has been paid to ocean acidification, a process that we understand much more precisely than the elusive changes in global climate associated with rising CO2 levels.

Ocean water is slightly basic, just around pH 8 (the water we drink is generally neutral or slightly acidic, around pH 7). When CO2 from the atmosphere dissolves at the ocean surface, it reacts with water to form a balance of different “carbonate species”. The most abundant form that this dissolved CO2 converts into is bicarbonate, HCO−3. The prevalence of bicarbonate in oceans is due to their basicity. However, as more CO2 dissolves in water, the balance of carbonate species shifts. Bicarbonate begins losing a hydrogen ion and becoming carbonate, CO-3. This results in a release of free hydrogen ions (H+) into the water, which increase its acidity.

If you’re not a chemist, why would you be remotely interested in the balance of carbonate and hydrogen in the water? The reason ocean pH is so important is that all organisms and biological processes that occur in the ocean are finely attuned to changes in water chemistry, much as we can feel the slightest changes in atmospheric chemistry  that cause the difference between a hot sticky day and a hot dry day. However, the acidification of oceans isn’t just a comfort problem for marine organisms- a slight attenuation of ocean pH can represent a lethal environmental change.

Many marine organisms produce exoskeletons made of calcium carbonate. Calcium carbonate, molecular formula CaCO3,  forms when a calcium atom and a carbonate ion bond together and precipitate out of solution. This is called calcification. It is essential to the survival of corals, a variety of shelled invertebrates, and single celled planktonic organisms that play an essential role in ocean photosynthesis and nutrient cycling. However, an increased abundance of H+ ions in the water interferes with the formation of calcium carbonate. In fact, calcium carbonate formation is so sensitive to pH that a fraction of a pH unit can make the difference between a habitable and a lethal environment.

So, a summary of why we should be worried about CO2 in the oceans: Increased CO2 alters the balance of carbonate in ocean water and causes the release of protons, which by definition increases acidity. This in turn prevents the formation of calcium carbonate, which is essential to the survival of a variety of marine organisms.

What kind of damage are we looking at if calcium carbonate stops forming? One of the largest challenge climate scientists face is giving accurate predictions of the ramifications of climate change (the other is communicating these predictions and their significance to the public, a task which has thus far been a resounding failure). The most useful resource we have in making these predictions is the past. Climates have been highly variable over geologic history, and the earth has experienced periods of much higher CO2 levels and correspondingly warmer climates. With regard to oceans, we know ocean CO2 levels increased dramatically about 251 million years ago, at the end of an era known as the Permian. The end of the Permian also marks the largest extinction event in Earth’s history, with up to 96% of all marine species disappearing. There is still much debate as to the cause of the Permian extinction, but there is growing evidence that rapid acidification may have been a primary driver in the oceans. But how fast is ocean pH dropping today compared to past acidification events?  William Howard of the Antarctic Climate and Ecosystems Cooperative Research Center in Hobart, Tasmania stated this past July that  “the current rate of ocean acidification is about a hundred times faster than the most rapid events” in the geologic past.

Maybe this is a little dramatic. But whether or not we believe that acidification caused a huge extinction event in the past, the fact remains that the functioning of many marine ecosystems today is entirely dependent on the ability of a select group of organisms to precipitate calcium carbonate. Coral reefs provide the a habitat for thousands of other species, but without their calcareous exoskeletons, their soft tissues would not be able to survive. Calcareous phytoplankton nourish huge regions of open ocean due to their ability to produce sugars from sunlight. Many human populations rely on calcifying shellfish as a valuable food and economic resource.

Globally, ocean pH has already dropped about 0.1 unit. A drop of another 0.3 to 0.5 pH units is predicted by 2100. In most surface oceans, this level of pH change will make the precipitation of solid calcium carbonate energetically impossible. In short, we are on a trajectory for change that we may only have a small window of opportunity to alter, if our chance has not already past. If we are worried about economic costs associated with dramatically reducing our CO2 emissions today, it might be valuable in our cost-benefit analysis to consider the reduction in quality of life our children will face in a world whose oceans are biologically impoverished.


2 thoughts on “Acid oceans and the next 800,000 years”

  1. I agree with most of your points, however some need to be discussed further, I will hold a small conversation with my buddies and maybe I will ask you some advice shortly.

    – Henry

    1. I’d love to talk about this further if you’re interested, Henry. I don’t pretend to be any expert on the subject but I have read quite a bit of current research on ocean acidification, and I agree that it’s an incredibly complex problem that my brief description only begins to tap the surface of. There are a couple recent Nature papers that delve pretty thoroughly into the science behind ocean acidification.

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