The simple, cheap instruments measuring global warming in the oceans

Earth is warming due to the release of heat-trapping greenhouse gases. Scientists are working hard to measure how fast the planet is warming, how much warming has occurred over the past few decades, and how this is affecting coastal areas, ecosystems, and fisheries. By understanding these factors, scientists can better project future climate impacts.

A large component of Earth’s warming involves the oceans, which absorb excess heat. The difficulty of gathering measurements in the oceans is that they are vast, deep, and often hard to reach. It’s also costly. Think about it: if you wanted to take the ocean’s temperature, how would you do it?

Centuries ago, ocean measurements were made with buckets dropped from the sides of ships. Over time, measurements have become more numerous and more accurate, partly thanks to technology advances. Today, a global array of floats that take continuous profiles of the upper ocean monitors ocean temperatures at more than 3000 locations to depths of 2000 meters. 

However, this array was put in place in 2005. Prior to that, the backbone of ocean measurements was a device or probe called the expendable bathythermograph (XBT for short). These small, torpedo-like probes, deployed from ships, gather temperature data to depths of 300 to 2000 meters as they descend through the water.

XBTs were designed as a simple, inexpensive way to obtain temperature measurements from virtually any ship. These XBTs were originally used by navies to determine the depth of the sound channel, where sound waves can travel thousands of miles. They were first introduced in 1967 and immediately adopted by scientists worldwide. Since their debut, several million have been deployed, with some 20,000 launched annually in all ocean basins. 

A very important and critical component of their success has been the excellent relationship established by the scientific community with commercial shipping companies. Commercial vessels aid scientists by voluntarily deploying XBTs along routes that are continuously repeated, often in remote regions not sampled by other types of oceanographic equipment.

With XBT use dating back to the 1960s, these measurements offer a unique historical perspective on temperature change in the oceans, which is often associated with global warming or even varying location and the intensity of ocean currents. XBT records, together with those of other observational tools later put in place, are crucial for determining how fast the ocean is warming — an essential factor for quantifying our effect on climate. XBT data are also used to measure how ocean currents change and how heat is transported across ocean basins, both of which are linked to extreme weather events worldwide. 

Background information and photos of XBT devices are provided at several sites, such as through the US National Oceanic and Atmospheric Administration (NOAA), Scripps Oceanographic Institution, and Woods Hole Oceanographic Institution. 

My own research has been partially focused on how to make XBT measurements more accurate. As part of this research, I recently attended an international meeting on XBTs hosted by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) in Tokyo. At the meeting, I met with colleagues from around the world to discuss how to refine and improve XBT data from the various models of this probe. 

Dr. Gustavo Goni, an oceanographer at NOAA and co-organizer of this meeting, reminded me that despite their basic technology, XBT probes are still relevant for scientific research—they are simple to use, cost-effective, and provide unique data that other, more technologically-advanced measurement systems still cannot retrieve. I asked Dr. Goni to summarize the role of XBTs in measuring the ocean.