Today’s ocean acidification is unique in that it is human-induced and therefore occurring very rapidly. However, studies of past climate do yield valuable insights which help scientists predict the impact of ocean acidification in the future. We know that the ocean changes pH levels naturally, but these changes are very gradual. The last levels of acidification which approach today’s occurred 55 million years ago during the Paleocene-Eocene Thermal Maximum. Mass extinctions of certain marine organisms occurred during this period, and the oceans’ recovery period stretched across millions of years. Though this past acidification differs from the one we are experiencing today, it can still answer many questions about what we should expect of the future. Patterns are inferred from ancient times by the use of proxy studies. A proxy is a representative of something correlated. For example, since the presence of CO2 in the atmosphere is directly correlated with decreased oceanic pH, we can study past levels of atmospheric CO2 and extrapolate the corresponding oceanic pH to build models showing levels of ocean acidity throughout time.
There are several proxies utilizing the connection between atmospheric CO2 and ocean acidity which have been used to infer such patterns: ancient pockets of air trapped in Arctic ice, boron isotopes from carbonates, the stoma of fossilized leaves, which exhibit a higher density at lower atmospheric carbon concentrations, and rocks (Royal Society, 2005). Atmospheric carbon concentrations throughout the ages can be modeled using these proxies. The question is: do such proxy models of CO2 levels in the atmosphere have a direct correlation to the pH level of the ocean? One would assume the answer to be “yes,” because ocean pH is based on CO2 levels in the ocean, which are in turn based on CO2 levels in the atmosphere.
Sediment cores from the bottom of the ocean and fossils of certain sea creatures do show levels of oceanic acidity that correspond with oceanic CO2 content. These sea creatures, including certain types of algae and coral, are known as calcifying organisms because their skeletons and cell coverings are made from calcium carbonate. When pH decreases, the calcium carbonate is liable to dissolve and fewer such organisms are found in the ocean. Fossil records of times with fewer surviving calcifying organisms should correspond with proxies showing increased levels of atmospheric CO2 (Royal Society, 2005). Air bubbles captured in ancient ice cores told scientists that the most recent period exhibiting atmospheric CO2 levels close to ours was the Paleocene-Eocene Thermal Maximum 55 million years ago (European Science Foundation, 2009). Fossil records show massive extinctions of ocean organisms during this time, so it follows that atmospheric CO2 levels should mirror oceanic pH levels. However, this connection in proxy modeling has proven problematic.
For example, a study using boron isotopes from the most recent ice age 20,000 years ago shows that ocean pH levels decreased by .3 units, while another model based on amount of calcifying organisms present showed no ocean-wide reaction to changing pH (Royal Society, 2005). Instead of corroborating each-other, the proxy models present an apparent contradiction. A possible explanation for this is that the era’s natural changes in pH occurred slowly enough that the CO2 had time to interact with and dissolve carbonate oceanic sediments to preserve a basic natural balance. Also, natural recovery from dangerously-high CO2 levels takes on the order of several thousand years, while recovery from a mass extinction takes millions of years (European Science Foundation, 2009). This would explain why CO2 levels seemed to have no obvious effect on calcifying organisms; the timescales of natural reaction to the phenomena do not mesh (Royal Society, 205). Now that human activity has so greatly increased atmospheric CO2 concentrations, this adjustment time is even further thrown off.
Basically, studies of past climate and ocean chemistry tell us that oceanic acidity and atmospheric CO2 concentration are directly related. Increased atmospheric CO2 did impact the ocean’s pH. However, since these changes occurred naturally and slowly in the past, the disastrous effects of acidification were kept to a minimum as organisms had time to adjust to the changing nature of the ocean. Today, as the oceans’ concentration of acidic hydrogen ions has increased 30% since human industrialization, the changes are accelerated and the natural balance set dangerously awry. Ocean pH is dropping more rapidly than ever before, already decreasing .02 units in ten years. This rate of change has been unequaled for tens of millions of years. If the rise in ocean acidification continues unabated, many marine organisms will be confronted with conditions they have not faced in their whole evolutionary histories.