Preface to special issue (Impacts of surface ocean acidification in polar seas and globally: A field-based approach)

Abstract

IntroductionBoth ocean acidification and global warming are consequences of the rise in atmospheric CO 2 . Ocean acidification is not itself a consequence of global warming, but rather of the invasion of atmospheric CO 2 into the ocean. Time-series of carbonate chemistry measurements in different locations around the world all document the continuous and ongoing increase in the amount of CO 2 in the ocean, and the consequential accompanying decrease in surface ocean seawater pH at all sites over the last years (Bates et al., 2014).The 40% rise in atmospheric CO 2 levels over and above preindustrial values has led to a decrease in surface seawater pH of about 0.1 units, and if emissions continue to rise according to worst-case scenarios, as they have done until now, then at the end of this century (the year 2100) surface seawater pH may be about 0.4 units below preindustrial levels. The fundamental seawater chemistry is well understood and leads to predictions that surface seawater pH should decline in all oceans, with the single exception of regions where upwelling of deep waters occurs, where the recentlyupwelled surface seawater has not yet had time to exchange CO 2 with the higher concentrations now in Earth's atmosphere. This expectation is borne out by the records at the time-series sites which confirm that ocean acidification is a geographically global phenomenon (Bates et al., 2014). Nevertheless, the chemical impacts of ocean acidification are not equal in all locations, since seawater carbonate chemistry is changed from a different initial baseline in different locations.For calcifying organisms, that build their shells or skeletons out of calcium carbonate (CaCO 3 ), the saturation state of surface seawater with respect to calcium carbonate (Ω, primarily controlled by the carbonate ion concentration) has been a major concern because it may affect their ability to build shells and skeletons and subsequently to maintain them against dissolution. For instance, both the PIC:POC (Particulate Inorganic Carbon : Particulate Organic Carbon) ratio of coccolithophores (Meyer & Riebesell, 2015) and the dissolution of pteropod shells (Bednaršek et al., 2012) appear to be affected by the value of Ω. As a result of high gas solubility associated with cold temperatures towards the poles,, there are naturally higher dissolved carbon dioxide gas concentrations and, associated with them, naturally lower carbonate ion concentrations in polar waters. These regions are, as a result, intrinsically more susceptible to becoming undersaturated with respect to calcium carbonate (Ω < 1). For this reason polar waters may suffer more severe consequences of ocean acidification. The pH of polar waters, on the other hand, is not especially low compared to other latitudes.The study of ocean acidification is about 15 years old: the first papers were published around about the year 2000. Much of the work into ocean acidification so far (Gattuso & Hansson, 2011) has taken the form of laboratory-based studies. Although this method of inq...