Researching heat waves in the ocean: Divers collect temperature data

According to the EU climate service Copernicus, 2024 was the warmest year since weather records began, and the summer season was especially noteworthy. The changing climate associated with global warming is manifesting itself across Europe in the form of extreme weather events, including forest fires, droughts, flooding after heavy rainfall, and marine heat waves.

The BlueDOT citizen science project, coordinated at MARUM—Center for Marine Environmental Sciences at the University of Bremen, is working to better understand these changes by collecting temperature data in the upper 40 meters of the Mediterranean Sea. The goal is to more accurately track marine temperatures and monitor marine heat waves, helping to improve our understanding of how climate change is affecting the ocean.

Divers at the heart of the project

Scuba divers are at the heart of the BlueDOT project. By uploading the profiles from their dive computers onto the Divelogs portal and consenting to share these to the BlueDOT scientific database, divers make a valuable contribution to marine research.

The project team uses this citizen-sourced data, in compliance with data-protection regulations, to monitor and record temperature changes across different parts of the Mediterranean Sea and from coastal regions worldwide. Dr. Christophe Galerne and Prof. Achim Kopf, both from MARUM at the University of Bremen, Dr. Rebecca Zitoun from GEOMAR Helmholtz Center for Ocean Research Kiel, and Arne Schwab from Schwab Research Technology are leading the project.

Data extends from Spain to Heligoland

To improve the quality of the collected data and ensure that temperature readings from the different dive computers are comparable, BlueDOT has installed permanent high-precision temperature sensors at selected reference dive sites on the Costa Brava, Heligoland, and the Maltese island of Gozo. These permanently deployed sensors record the temperature at various depths, allowing scientists to calibrate the data collected from dive computers against consistent, high-resolution measurements. To support this effort, BlueDOT is collaborating with two diving centers in Spain and Malta. These centers play a key role in engaging the diving community, raising awareness about the project, and helping to test and maintain the sensors.

According to Christophe Galerne, the use of the sensors increases the accuracy of the database, "which creates a more reliable basis for research and helps to develop an optimal approach for the global expansion of the project in the long term." The diver-sourced data is an important complement to existing ocean-monitoring platforms such as satellite observations, Argo floats, and hydrographic surveys.

"With an estimated six million active scuba divers worldwide, this citizen science initiative represents huge potential for enhancing climate research through widespread, community-driven observations of ocean temperature."

Initial findings: Ocean temperatures are rising

The team has already evaluated diving data from the Mediterranean Sea. As Galerne expected, these indicate that average ocean temperatures are rising steadily. The water masses of the oceans act as heat reservoirs that interact with the atmosphere and thus influence the climate.

If this system becomes unstable with the continued warming of surface water, it could lead to intensified evaporation and, ultimately, regionally limited extreme precipitation events in the surrounding areas. Galerne explains that the associated rain belt has continuously shifted farther northward over the past 20 years, leading to sporadic droughts as well as heavy rainfall and flooding.

"The constant warming and increasing frequency of marine heat waves also have significant implications for biodiversity and the ecosystem services our oceans provide, making these phenomena a critical factor to consider in both research and management.

"There presently exists what is known as sampling bias in the data. This is exhibited by a clear predominance of data obtained during the warmer months and the holiday seasons. In order to be able to establish an average value, we would like to encourage divers to enter their data—including older data—into our portal and also to record and upload data from cooler seasons," says Galerne.

By filling these seasonal gaps, divers can play a crucial role in building a more complete and accurate picture of how ocean temperatures are changing throughout the year.