Source: Landschützer, P.; Gruber, N.; Bakker, D. C. E.; Stemmler, I.; Six, K. D. Strengthening seasonal marine CO2 variations due to increasing atmospheric CO2, Nature Climate Change 2018, 8 (2), 146-150.
The amount of dissolved CO2 in the seawater fluctuates naturally depending on the time of year, but what happens to this seasonal pattern when human-emitted CO2 is added to the ocean? We expect there to be some impact on the seasonal variations, but this hasn’t yet been backed up with observations. More CO2 in the seawater means more acidic seawater, and the potential of stronger seasonal swings in dissolved CO2 bodes ill for marine organisms sensitive to acidification, which would be subject to even higher levels of acidity than usual. In this recent study, Landschützer and his other biogeochemical modeler colleagues gathered 34 years of data on seawater CO2 levels and set out to determine whether anthropogenic CO2 was having any effect on the contrast between winter and summertime dissolved CO2 levels.
Any discussion on CO2 and ocean acidification first requires an explanation of how it occurs, which brings us to the chemistry of seawater. Seawater is host to a variety of different dissolved chemical compounds. Of particular interest to many ocean scientists are those compounds containing carbon. When carbon dioxide dissolves into seawater, it breaks into several dissolved species (aka ions), which comprise the seawater carbonate chemistry system. These include carbonic acid, bicarbonate, carbonate, and hydrogen ions (Figure 1). The reason these ions are given so much attention in the oceanographic community is twofold: first, because their interchange are the reactions that make the uptake of atmospheric CO2 possible, and second because when CO2 is dissolved, the amount of hydrogen ions in the water increases. When hydrogen ion levels go up, water becomes more acidic, resulting in what is referred to as ocean acidification.
The ocean’s pH level over the past 300 million years has been, on average, 8.2. But depending on the time of year, the pH of a certain region of the ocean will rise and fall. The two main factors that influence seawater CO2 concentrations, and hence pH, are the seawater temperature and biological activity. CO2 is less soluble in warmer water, and phytoplankton photosynthesis takes CO2 out of the seawater. As a result, CO2 (and pH) levels will vary depending on photosynthetic activity and water temperatures. As ocean pH creeps downward due to the extra CO2 in the atmosphere, scientists expected there to be some sort of impact on the seasonal differences in pH, but haven’t yet been able to show it in observations.
Because CO2 measurement records are much more plentiful and robust than seawater pH measurements (and can directly inform us about pH levels), Landschützer and the scientists focus on seasonal variations in dissolved CO2 levels to infer changes in pH levels. With over 34 years of ship-based and moored measurements of surface ocean dissolved CO2 in hand, Landschützer and his colleagues investigated whether or not they could detect any recent changes in the seasonal dissolved CO2 (or pCO2) contrast. From the data, they calculated differences between the average January-March pH (representing winter) and the average July-September pH (representing summer).
Though there was a lot of variability from year to year in pCO2, the trend was clear. Over the years, the difference between winter and summertime pCO2 levels has been increasing (Figure 2). When they compared their results to a similar analysis of CO2 records taken at two stations at which dissolved CO2 measurements are regularly taken, the Hawaii Ocean Time Series (roughly 60 miles north of Oahu) and the Bermuda Atlantic Time Series (roughly 70 miles southeast of Bermuda), the results were indistinguishable.
The authors wanted to dig a little further into which oceanic processes were being impacted by human CO2 and were driving these changes. Dissolved CO2 levels are mainly controlled by two factors: 1) seawater temperature, and 2) amount of biological activity (remember that photosynthesis removes CO2 from the seawater). These two variables can be referred to as the thermal and non-thermal components of CO2 dissolution into seawater, respectively.
The researchers separated out the seasonal fluctuations of surface water CO2 levels into the part driven by each of the components. However, it is important to note that the thermal and non-thermal components oppose each other; that is, when the thermal component is causing dissolved CO2 to be higher, the non-thermal component goes in the opposite direction of lowering dissolved CO2 concentrations (Figure 3). Thus the question that follows is whether the increase in the thermal component outweighs the increase in the non-thermal component, or vice versa.
Landschützer and his colleagues measured the seasonal contrast as the winter pCO2 minus summer pCO2 difference. In high latitudes, the winter – summer difference is positive, meaning that seawater pCO2 levels peak during the winter. Conversely, pCO2 peaks during the summer months in the low latitudes. From the observations, the scientists found that in high latitudes (between 10˚ and 40˚ North, or roughly the band between Caracas, Venezuela and New York City) the non-thermal component dominates, whereas in low latitudes, the thermal component drives the seasonal contrast. And as it turns out, in high latitudes, the change in the non-thermal component dominated the overall shift in pCO2 levels, and in the low latitudes, the change in the thermal component dominated the change in pCO2. What this means is this: due to the addition of human-emitted CO2, the peaks in pCO2 become more extreme in both the high and low latitudes (Figure 4), implying that the swings in seawater pH will also become more extreme. In both high and low latitudes, the seasonal amplitude of the dominant factor has increased, even though the dominant driver differs between the locations.
Landschützer and his colleagues have shown that human-emitted CO2 has already impacted the seawater carbonate chemistry system to the extent that it is detectable in seawater CO2 measurements. This work is one of many that clearly demonstrates the value of having long-term ocean monitoring schemes; the 34 years of data proved sufficient for detecting human-caused changes in surface ocean pCO2, and by extension, pH. But their findings also are ominous for of the fate of marine organisms that are particularly sensitive to pH levels. These organisms are not only being stressed by the decreasing average pH, but in addition, the seasonal variation of pH will expose them to more intense acidity than they were before.
Julia is a PhD student at Scripps Institution of Oceanography in La Jolla, California. Her focus is on biogeochemistry, which, as the name suggests, centers on the combined effects of biological, geological and chemical processes on the earth system. She is advised by Dr. Ralph Keeling and is modeling the global carbon cycle to better understand how much carbon dioxide ends up in the atmosphere. When not at her computer writing code, Julia can usually be found reading and/or thinking about food.