The instrumental record of temperature change in the oceans goes back to about 1850 and consists of thermometer measurements made on water samples taken by merchant and navy ships as they sailed the world’s oceans. The data are understandably best for parts of the oceans along major trade routes, and they are less abundant further back in time. These measurements, just like the land-based weather station data, have to be gridded to come up with a global average sea surface temperature. As might be expected, the sea surface temperature record is similar to the global temperature records, in part because the oceans make up almost 75% of Earth’s surface. But even if we separate out the land surface temperature from the global record and compare it to the ocean surface temperature, they are quite similar, as seen in the figure below.
Although the two records are quite similar, there are some differences — the SST changes over a smaller range than the land surface temperature, and the land temperature is subject to more dramatic swings. This difference is largely due to the greater heat capacity of the oceans relative to the air — it takes a long time to heat and cool the oceans, but air temperature can change quite rapidly.
Measurements from a system of hundreds of buoys stationed throughout the oceans allow us to take the temperature of the oceans over a depth range of 2000 m. These measurements go back in time to 1955 and show that not just the surface of the oceans, but the whole upper half of the oceans are slowly warming — only about 0.1 to 0.2 °C averaged over the globe during the past 50 years — but this is a vast amount of water that has been warmed.
So, while the whole ocean has absorbed a huge amount of heat, its overall temperature has changed little. Nevertheless, the very surface of the ocean has warmed almost as much as the rest of Earth’s surface.