The trend of increasing CO₂ concentration in the atmosphere is well known. During recent years another, possibly as serious and wide-ranging, trend has caught the attention of scientists: decrease of dissolved oxygen concentration (deoxygenation) in the world’s oceans. In a recent review paper Ocean Deoxygenation in a Warming World. Ralph F. Keeling, Arne Körtzinger and Nicolas Gruber, Annu. Rev. Mar. Sci. 2010, 2:199–229 the authors give overview of the possible reasons, the current status and possible consequences of ocean deoxygenation. Deoxygenation is believed to occur because of an interplay of several reasons, such as lower solubility of oxygen in warmer water and stratification of the upper oxygen layers, both being consequences of global warming. There are significant zones in the world’s oceans that are termed hypoxic (meaning that marine organisms suffer from various stresses) that have emerged during just the recent decades. The largest such zone is in the northern Pacific Ocean with the area of several tens of millions of square kilometers. Current evidence indicates that more significant changes are looming, with potentially very serious impact on the marine ecosystems and on the whole world.

There is currently no full understanding of the causes of the deoxygenation processes neither their possible consequences. In order to develop models that would help to understand and predict deoxygenation and its consequences, accurate data on dissolved oxygen content in oceans are needed very much. Obtaining such data is a major measurement science challenge. The sensors (actually sensor arrays, to allow measurements at different depths) used must be automatic in order to obtain sufficient amount of data. The measurement results made in different locations and at different times must be comparable, which makes sensor stability and rigorous calibration very important. We hope to contribute to this challenge by metrological characterization of dissolved oxygen sensors, evaluation of the reliability of the data and developing new calibration approaches (see some recent results).