In order to achieve this, they compared a historic meteorological data pool with oceanographic measurements. Oceanographers mostly do research on the upper 400 meters of the ocean which is technically very complex to do. Meteorologists, however, are more interested in the ocean surface due to ocean-atmosphere interactions. It is a lot simpler to collect data from the ocean surface, so it is not surprising that meteorologists have access to an amount of data about 40 times larger than the amount of data collected by oceanographers.
Even though different methods and data were used, both time series yield the same result: The ocean surface has warmed since 1900. Although in a few areas, the surface temperatures have declined, overall the temperature in the first 20 meters of the ocean has risen about 0.8 °C. Especially the North Atlantic is striking: The temperature of its upper layer has increased by almost 1.5 °C. In addition, a temperature rise of about 0.5°C has been noted at depths of 400 meters. This might not seem like a lot at first glance. However, the large volume has to be considered and consequently, the heat energy of the North Atlantic has risen considerably.
To evaluate the data properly, the researchers had to close data gaps and estimate errors. Because of the weak spatial coverage of data before the 1950s, the data of the temperature curves scatter quite a lot. Furthermore, the researchers had to take into account certain measurement errors. E.g. before World War II, water samples were taken with a bucket which was then heaved on deck. This is only one of the errors corrected by the researchers with a numerical model. Thus, by comparing historical databases, they were able to improve the quality of the data.
(Gouretski et al., 2012, Geophysical Research Letters, in Press: Consistent near-surface ocean warming since 1900 in two largely independent observing networks)