Her Majesty's Ship Challenger set sail in 1872. Stripped of her guns and outfitted for science, her mission was to sail around the globe sampling as she went. Among other scientific triumphs, the Challenger gathered the first global set of ocean temperature readings, more than 260 in all. The British expedition measured from the surface to a depth beyond 900 meters. In 2004, a set of drifting buoys began to make similar measurements. There are now more than 3000 of these floats bobbing in the world's seas, collecting oceanographic information. Comparing the data sets, separated by more than a century...