Global warming has been going on for a long time. What were the temperatures like a hundred years ago? Terrestrial records go back that far and farther, but what about ocean temperatures? In the late 1800’s the HMS Challenger conducted extensive measurements of ocean temperatures across the globe.

Researchers from the University of Tasmania, Sandy Bay, Australia; and NASA’s Jet Propulsion Laboratory, Pasadena, Calif., combined the ship’s measurements of ocean temperatures with modern observations from the international Argo array of ocean profiling floats. They used both as inputs to state-of-the-art climate models, to get a picture of how the world’s oceans have changed since the Challenger’s voyage.

The Challenger expedition, from 1872 to 1876, was the world’s first global scientific survey of life beneath the ocean surface. Along the way, scientists measured ocean temperatures, lowering thermometers hundreds of meters deep on ropes.