They use thermometers attached to thousands of bobbing robots floating at controlled depths throughout the oceans. This system of “Argo floats” was launched in the year 2000 and there are now about 4,000 of the floating instruments.
About once every 10 days, they cycle from the surface to a depth of 6,500 feet (1,981 meters), then bob back up to the surface to transmit their data by satellite. Each year this network collects about 100,000 measurements of the three-dimensional temperature distribution of the oceans.
The Argo measurements show that about 93 percent of the global warming caused by burning carbon for fuel is felt as changes in ocean temperature, while only a very small amount of this warming occurs in the air.