The team also looked at subsurface temperature differences between Challenger and Argo, taking into account several sources of error in the Challenger readings. One issue with the Challenger data, Roemmich explained, is that the vessel’s scientists didn’t directly measure the depth of their thermometers; they measured only the length of the line extending the instruments into the water. Because of ocean currents, it’s nearly impossible to get a line to be completely vertical in the water, resulting in an actual depth that is a little less than the full length of the line.

“What you are then going to see is a temperature that is a little warmer than it would have been if the line has been perfectly vertical,” Roemmich said, referring to the fact that temperatures are typically warmer at shallower depths. Other Challenger errors include incorrect measurements of pressure effects on the thermometers and faulty thermometer readings, he added.

Accounting for these issues, Roemmich and his team found that, on average, global ocean temperatures increased by 0.59 degrees F (0.33 degrees C) in the upper ocean down to about 2,300 feet (700 meters). This global temperature change is twice what scientists have observed for the past 50 years, suggesting that the oceans have been warming for much longer than just a few decades.