S.K. Lauvset, N. Gruber, Long-term trends in surface ocean pH in the North Atlantic, Marine Chemistry, Volume 162, 20 May 2014
When carbon dioxide (CO2) dissolves in seawater and is hydrated, it forms carbonic acid and readily dissociates into bicarbonate and hydrogen ion, decreasing the pH of seawater. If anthropogenic CO2 emissions continue to increase at a rate of 1% per year, the average pH of the surface ocean is predicted to fall .3 to .4 by 2100. The drop in pH is equivalent to a 100 to 150% increase in the concentration of H+ ions. Ocean acidification, OA for short, is the decrease of surface ocean pH as a result of increasing atmospheric CO2. Many metabolic processes are pH-dependent; the effect of OA on the marine animals’ physiology is an active area of research today. The most commonly known effect of OA is that it inhibits shell development and formation of organisms, particularly pteropods. The effect of OA on the food chain is unknown and is one of the big questions in the OA scientific community. The outlook isn’t looking too good.
Ocean models allow us to see a glimpse into the future. By using a set of equations that define the chemistry of the marine inorganic carbon system, scientists can predict the pH of the ocean. While an ocean model can be used to see the chemistry of our ocean in the future, they can also be used to understand how we have already altered it.
Lauvest and Gruber wanted to know whether we have already altered the pH of the North Atlantic, particularly the North Atlantic Subpolar Gyre. The waters of the North Atlantic are cold. Since the ocean of the North Atlantic is cold, particularly near the Arctic, it may be the region of greatest carbon dioxide uptake. This will result in a decrease of ocean pH if atmospheric CO2 continues to increase.
One can obtain ocean pH using two different ways. The first is that pH can be measured on site using an electrode that has been calibrate to reference standards. The second method is to calculate pH. Lauvest and Gruber gathered pH data from the CARINA database, a database that contains pH data. Before they dove into data analysis they first used data another database with other marine chemistry data to calculate pH, mainly CO2 fugacity (fCO2) and alkalinity (ALK). The dataset spans from 1981 to 2007.
fCO2 is a measure of carbon dioxide and its tendency to escape from its liquid phase to gas phase in water. ALK is a measure of the water’s capacity to neutralize the effects of decreasing pH as a result of more CO2 dissolving in seawater. ALK and fCO2 came from another database, called SOCAT. They used fCO2, ALK, salinity, and temperature to determine pH. This calculated pH was then compared to the measured pH from the CARINA database. The pH compared using the two different methods were from the same location and within the same time frame; measurements of the same year and month were paired.
After comparing the two pH values, Lauvest and Gruber then determined long term trend of the North Atlantic Subpolar Gyre’s pH.
The pH calculated and pH measured nearly the same even though they were determined using separate methods.
Lauvest and Gruber also did a series of statistical tests to determine the error of their measured and calculated values. They found the error from the two methods to be insignificant. Lauvest and Gruber also wanted to know if the ocean pH is changing as a result of other parameters besides atmospheric CO2, such as the ocean’s temperature or ALK. They determined that the change of these parameters was insignificant. However, the increase of fCO2 is significant. The fCO2 of the surface North Atlantic Subpolar Gyre is increasing at a rate of 2.0±0.38 µatmyr-1.
Lauvest and Gruber found that the pH is decreasing at a rate of .0022 per year. Because temperature and ALK has not changed significantly, Lauvest and Gruber concluded that the pH of the ocean has decreased as a result of increasing fCO2. The decrease in pH is likely caused in increasing CO2 in the atmosphere.
Before we can predict the chemistry of our oceans in the future, we must understand why it has changed and by how much. The primary motivation of Lauvest and Gruber was to determine the quality of the CARINA and SOCAT dataset. By being able to confirm the pH from two different datasets, they were able to confirm these datasets are extremely valuable for future OA studies. Most importantly, they have shown that we have already decreased the pH of the North Atlantic Subpolar Gyre. OA is not in the distant future-it has already started.