Modern researchers traced the path of a 19th-century sailing ship in order to compare ocean temperatures over the past century and found that the water started getting hotter much earlier than previously expected.

A team led by researchers from the Scripps Institution of Oceanography at the University of California San Diego compared temperature data from a fleet of robots employed by the Argo ocean-monitoring program. They compared these results to temperature records taken by British scientists during the voyage of the HMS Challenger in the 1870s.

The found that the temperature of ocean waters from the surface down to 700 meters (2,300 feet) increased by an average of .33 degrees Celsius, or about a half a degree Fahrenheit, over 135 years.

The results extend the time period for which we think global ocean warming occurred, says Dean Roemmich, a professor at the Scripps Institute and the lead author of the study, which appeared in the journal Nature Climate Change on Sunday.

It's hard to tell if there are certain areas of the sea that are warming faster than others - it looks as though the Atlantic Ocean is warming faster than the Pacific Ocean, but that result is just on the edge of statistical confidence, according to Roemmich.

One of the reasons the researchers can't make more finely-tuned claims about ocean temperature warming is that the HMS Challenger took readings at only 300 temperature stations, while the Argo project has around 3,500 robots.

Another problem is that there is a lot of variance in ocean temperatures, even in the same area, so it takes a large number of measurements to average over that noise, Roemmich says.

Still the researchers' results are another drop in the bucket of evidence for climate change, and the study also highlights the significance of the Challenger expedition.

On that voyage, led by British naval officer Sir George Strong Nares, scientists fished for data with special glass thermometers built to withstand extreme temperatures at crushing depths. In addition to temperature readings, the Challenger was equipped with the tools to measure depth and to sample previously undiscovered ocean life.

The Challenger expedition was really before its time, Roemmich says. They pioneered many of the methods in physical oceanography that became widespread about 50 years later.