How do we measure ocean acidification?

Hi, this is Aleck Wang. I am a scientist (marine chemist) from the Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution. I study the marine carbon dioxide system, also the called carbonate system or inorganic carbon system. My lab group is responsible for measuring all primary CO2 parameters during the cruise. These include seawater pH, partial pressure of carbon dioxide (pCO2), dissolved inorganic carbon (DIC), and total alkalinity (TA). I would like to talk about how we determine or measure ocean acidification. You already know from our previous blog posts what ocean acidification (OA) is. Briefly, as we (humans) release more CO2 to the atmosphere through burning of fossil fuels and other activities, more and more CO2 is dissolved in the ocean, which causes the ocean water to be more acidic (lower pH). The process is similar to when one makes a soda water: as we inject CO2 to water, it becomes more acidic (the resulting soda is acidic).

Fig. 1. Mean pCO2 and pH in the upper ocean of the Central Pacific Ocean near Hawaii between 1988-2007 (Dore J. E. et al. PNAS 2009, 106:12235-12240)

Fig. 1 shows direct observational evidence that ocean acidification is happening. The data were collected at a fixed station from the central Pacific near Hawaii between 1988 and 2007. Newer data are also available. The top panel basically shows that surface water pCO2 increased at the same pace as the atmospheric CO2 increase (~ +1.9 ppm per year) during this period. In the meantime, surface ocean pH (middle panel) was decreasing at a rate of ~ 0.02 per decade. This increase in pCO2 and decrease in pH in the surface ocean have also been observed in other ocean basins and some continental shelf regions during the past two decades. To determine if the ocean is acidifying is in fact very demanding scientific work. It took more than two decades of efforts from many scientists around the globe to measure seawater pH, pCO2, DIC, and TA precise and accurate enough to undoubtedly discern the acidification signal, such as shown in Fig. 1. Part of this is because the rate of pH decrease and pCO2 increase in the ocean is a small number on an annual basis. Our measurements have to be good and long enough for us to say for sure that ocean acidification is happening.

Fig. 2. pH profiles at 5 stations in the North Atlantic Ocean during the cruise.

Take pH measurements for example. Thanks to the analytical improvement in the 1990’s, we are able to measure seawater pH precisely to the 3rd - 4th decimal place (±0.0004 pH units) based on the colorimetric principle. This allows us to see the gradual pH decrease shown in Fig. 1. In addition, we can see many subtle features of pH from the surface to the deep ocean (Fig. 2). These will inform us what processes occur in the water column and how they may affect pH values. It takes a great deal of effort to make these measurements. Just to give you a sense; two groups of people have to work two shifts of 12 hours almost non-stop to finish all the pH measurements on board of R/V Oceanus during this current cruise.

Fig. 3. pCO2 measurements along the cruise track. Two red circles indicate we went through two boundaries (fronts) between different water bodies, where show large pCO2 changes.

There have been significant improvements on the technologies of how we measure these carbonate parameters during the recent decade. We can now measure all four carbonate parameters near-continuously and autonomously with high precision and resolution. There are even in-situ sensors to measure these parameters. These improvements have reduced the measurement cost and increased efficiency. Fig. 3 shows the pCO2 data we just collected during our cruise in the N. Atlantic Ocean using a fully automated pCO2 instrument to make continuous underway measurements of surface water pCO2 along the cruise track. Because the measurement of the instrument is quick (~ 1 minute per measurement) and precise (±1 µatm), we captured two water fronts (boundaries) where surface pCO2 underwent large changes within short distances (time). The fronts in the ocean are often places where high biological production occurs. DIC is also a very useful and important measure of the carbonate system. It is the sum of all carbonate, bicarbonate ions and dissolved CO2 in seawater. As ocean is acidifying, and more atmospheric CO2 is dissolved into the ocean, DIC will increase as well. Fig. 4. shows a few DIC profiles from the cruise. They are almost mirror images of the pH profiles. Now we can measure DIC precisely with only 0.1% error.

Fig. 4. DIC profiles at 4 stations in the North Atlantic Ocean during the cruise.

In summary, we now can measure ocean acidification precisely and accurately by measuring the key parameters related to OA, and inform the public and policy makers how fast ocean is acidifying. There should be little doubt that the ocean is acidifying. The question is how the acidification will affect all aspects, from chemistry to biology, of the ocean.