MEASURING the temperature of something as stratified as the ocean has never been easy. Before the 1980s, ships automatically recorded the temperature of water flowing through their ports, but the great depth variance of these ports and the dearth of data outside major shipping routes made the figures incomplete and unreliable. Next came satellites, which were able to capture more surface-temperature data in three months than the total compiled in all the years prior to their advent. Nonetheless, they too have limitations: for example, their infrared sensors are susceptible to cloud contamination.

Continuous monitoring of sea temperatures only began in 2000, run by an international collaboration called Argo. This is a regularly replenished fleet of untethered buoys, now numbering nearly 4,000, which divide their time between the surface and the depths, drifting at the whim of the currents. Over ten-day cycles they sink slowly down to about 2,000 metres (6,560 feet) and back up, measuring temperature and salinity as they go. Although the network is still sparse—one float for every Honduras-sized patch of ocean—their data have revolutionised oceanographers’ understanding of their subject.

One of the biggest benefits of better-measured seas is the possibility of getting to grips with dramatic weather events. The top three metres of the oceans hold more heat energy than the entire atmosphere does. How much of that energy escapes into the air, and when and where it does so, drives the strength and frequency of storm systems. And more and more energy is becoming available to do that driving. During the past hundred years, the average surface temperature of the seas has risen by about 0.9°C (1.6°F), according to America’s National Oceanic and Atmospheric Administration. This means that, since the 1980s, about a billion times the heat energy of the atom bombs dropped on Hiroshima and Nagasaki has been added to the ocean—roughly an atomic explosion every few seconds.

Yet even as the amount of energy the oceans hold has risen, the details of its transfer to the atmosphere remain unknown for large swathes of water. This is particularly important when it comes to understanding a phenomenon like the South Asian monsoon. Its rains are driven by the huge size of the Bay of Bengal, and by the amount of fresh water that pours into it from the Ganges and Brahmaputra river systems. Because this buoyant fresh water cannot easily mix with the denser salty water below it, the surface gets very warm indeed, driving prodigious amounts of evaporation. Better understanding these processes would improve monsoon forecasts—and could help predict cyclones, too.

Read more in our TQ on ocean technology