Over past centuries, the crews of ships regularly measured some basic properties of the waters through which they sailed, such as temperature and salinity. 

Though their accuracy has to be questionable, these historical observations have proven to be important for climate modelers who are trying to validate their work. In recent years, the importance of the deep ocean as a sink for the extra energy trapped by anthropogenic climate change has come to the fore. 

Unfortunately, the vast majority of deep ocean observations don't start until the 1980s. From 1925 to 1927, however, the German research vessel Meteor criss-crossed the Atlantic Ocean on an expedition that collected a uniquely thorough record of oceanographic properties for the full depth of the ocean basin. The ship made 13 coast-to-coast sweeps from the Southern Ocean to the tropical North Atlantic, providing a set of observations that were largely unmatched until the 1990s. 

Working with this historical data set, Gouretski et al. identified long-term temperature and salinity trends for the entire water column of the Atlantic. They find that during the twentieth century, the upper 2,000 meters (6,500 feet) of water warmed by 0.272 degrees Celsius (0.49 degrees Fahrenheit) and became saltier by 0.030 per mil. Half of the heat content increase took place in the upper 400 meters (1,300 feet) of water, and three quarters took place in the upper 700 meters (2,300 feet). 

The water below 6,500 feet, however, became slightly cooler and fresher. The authors calculated that a reduction in density of the ocean waters due to changes in temperature and salinity would have resulted in 3.7 centimeters (1.46 inches) of sea level rise over the 80 years following the Meteor's expedition. 

The authors note that the calculated warming trend aligns with numerical models of anthropogenic warming.

Citation: Viktor Gouretski, Johann H. Jungclaus and Helmuth Haak, ' Revisiting the Meteor 1925-1927 hydrographic dataset reveals centennial full-depth changes in the Atlantic Ocean ',Geophysical Research Letters, doi:10.1002/grl.50503