Decadal trends in global ocean pH: From data and methods to analysis and understanding.

One of the major goals of DECpH was to improve our scientific understanding of how ocean pH has changed over the last few decades in response to the increasing CO2 levels in both the atmosphere and the ocean. Such understanding is important in order to advance our understanding of ocean acidification in the future and where the possible consequences of ocean acidification will be the greatest. Therefore, to achieve this goal DECApH has both quantified the global trend in ocean pH over the past 20-30 years, and the spatial variabilities in this trend. The results are that the average global ocean pH trend has been -0.0018 units per year between 1991 and 2011. Unfortunately, there are not enough data in the 1980s for a trend estimate in the period 1981-2011, but the Pacific Ocean trend in the 30-year period (-0.0020 per year) is roughly consistent with the global 20-year trend. This signifies that there has not been any significant change and/or acceleration in ocean acidification the past few years. Even though these trends may seem small they represent a perturbation of the delicate natural balance in the ocean. Such a perturbation may have large consequences for life in the ocean, and especially phytoplankton – which are the bottom of the food chain – are vulnerable. Because of the large-scale ocean circulation and how this transports carbon in the ocean, it is expected that ocean pH changes are spatially very variable. At least on the short time scales used in DECApH. However, the results in DECApH show that ocean pH changes have been very spatially homogeneous, but that there are large uncertainties due to the high levels of noise in the data and the relatively short time frame in which we have observations. A comparison with pH changes in the Norwegian Earth system model NorESM shows that we need to improve the data coverage both in time and in space in order to further understand the underlying mechanisms that drive the pH changes and their spatial variability. It is likely that changes in the ocean buffer capacity is a main reason for spatial variability in the pH changes, but it is yet not fully understood how and how much the buffer capacity has changed. Overall the observed pH changes found in DECApH are consistent with the ocean being in thermal equilibrium with the atmosphere. Such that the increasing CO2 in the atmosphere drives increased CO2 concentrations in the ocean which again drives the pH change. As expected, no results show that changes in salinity or alkalinity are important drivers of pH changes. Somewhat more surprising, the results in DECApH also show that changes in temperature has only a minor effect on the pH changes, at least on a global scale.

DECApH is a basic science research project seeking to increase our understanding of the decadal trends in ocean pH and in this regard DECApH aims to: 1) Provide a quantification of the magnitude and uncertainty of decadal pH change on a global scale. 2) D escribe the spatial variability in these decadal trends in the surface and deep ocean. 3) Gain an improved understanding of how global ocean pH has changed in the past decades in response to increasing CO2 levels in both the atmosphere and ocean. One o f the challenges in studying global pH trends is that there is not a good enough global coverage of pH measurements to determine decadal trends, so pH has to be calculated using other measurements and approximations. To accomplish this DECApH will first contribute to developing GLODAP2 from the existing CARINA and GLODAP data sets. Surface ocean pH will be calculated using data in the SOCAT data set. A significant part of the work in DECApH will be to quantify the errors and uncertainties associated wi th the calculation method, and verify the pH calculations using the GLODAP2 data set. The calculated pH will be used to determine and analyze decadal trends in pH for both the surface ocean and the deep ocean. Ocean acidification and pH changes are key research topics and are of great interest to many different scientific communities, to the fisheries industries since ocean acidification has potentially severe implications for this industry, and also the general public. The results in DECApH will be im portant in identifying and understanding this new peril, and this knowledge will be important in determining societal actions. The project has a strong international profile and includes close collaboration with a large international project and a resear ch stay at ETH Zürich in Switzerland. DECApH will further the scientific qualifications of a young woman, and act to improve the gender balance in an existing research group.