The data from ocean temperature and salinity profiles are fundamental in our understanding of Earth’s climate system and particularly ocean heat storage‐, a key contributor to sea level rise. The main source of these profiles is now Argo floats, which in the twenty‐first century have become a central element in climate change monitoring and forecasting.

The meeting From HMS Challenger to Argo and beyond , which took place on 21 November 2018 at the National Oceanography Centre (NOC) in Southampton, highlighted the key developments in our ability to collect ocean profiles over the past 140 years, assessed the present situation, and looked to the future. The meeting was sponsored by the History Groups of the Royal Meteorological Society and the Challenger Society for Marine Science.

Chris Folland (Met Office, Retired) introduced the meeting, highlighting in particular the role of Argo float data in climate change studies and forecasting.

John Gould (NOC, Emeritus) summarised the technological developments that shaped the collection of ocean profiles, starting in the late nineteenth century. He pointed out that throughout the twentieth century large areas of the ocean remained unobserved, particularly at depth, and then went on to show how Argo floats had their roots in the World Ocean Circulation Experiment of the 1990s. The number of profiles, after Argo prevalence, increased around four times compared to the total of profiles from 1900 to 1999. (Figure 1). John ended his talk by comparing temperature and salinity data from the late nineteenth century with present values.

Brian King (NOC) explained how the Argo project developed, highlighting the importance of the system of freely shared data. Further, he focused on construction and operational principles of Argo floats. Currently there are 4000 active floats, deployed by 30 countries and collecting high‐resolution profiles from the upper 2000m. These profiles provide continuous and almost global ocean data coverage that was not possible before. The challenges of this project are to maintain the present array, to increase the profiling depth and to equip Argo with additional biogeochemical sensors.

To verify their quality, Viktor Gouretski (Chinese Academy of Sciences) examined the various sources of error in historical profiles. He pointed out the bias in temperature from bathythermographs between the 1970s and 1990s due to uncertainties in the fall rate and the way in which correction methods allowed the use of the data for climate research. Future improvements in the ocean profile database will come with the International Quality Controlled Ocean Database project (IQuOD) creating intelligent metadata and defining realistic uncertainties.

Rachel Killick (Met Office) gave an overview of the procedures used in creating the EN4 dataset from the ocean profile data. She described the automatic quality control checks and different bias corrections applied to individual profile types. Next, Rachel showed quality controlled temperature fields from the EN4 objective analysis, along with their uncertainty estimates. Her talk ended by showing how the dataset is used in seasonal forecasting, decadal prediction and ocean monitoring.

Matthew Palmer (Met Office) concentrated on the use of ocean profiles in researching climate change resulting from Earth’s Energy Imbalance (EEI). He started by showing that surface temperature is not sensitive to EEI variability, whereas ocean heat storage is. He highlighted the role of ocean profiles in assessing climate models and, finally, focused on how heat storage affects sea level rise and the use of Argo data in understanding the hydrological cycle and heat and freshwater transport.

Leon Hermanson (Met Office) discussed the application of ocean profile data to seasonal‐to‐decadal predictions, which are central to the provision of climate services. Studies have shown that predictability for an instantaneous state remains low, but that average conditions over a season as a whole are partially predictable – like rolling loaded dice. Leon highlighted that the primary source of predictability is the ocean and further described the principles of seasonal forecasting. He also showed that Argo’s data used in predictions lead to improvements in decadal ocean heat content trends and reduce the uncertainty in the estimation of ocean dynamics.

Katy Hill (World Meteorological Organisa­tion, WMO) started by describing the structure and complexity of the current observing system and highlighted the regional and global oversight mechanisms of international and intergovernmental co‐ordination. These systems are important in shaping the future observing system, to ensure the integrity of the system by avoiding duplication and ensuring that key observations are maintained. She ended by showing how the sequence of OceanObs conferences in 1999, 2009 and 2019 shaped the observing system, and looked forward to the opportunities in the UN Decade of Ocean Science for Sustainable Development (2021–2030).

Karen Heywood (University of East Anglia) presented some novel ocean observing systems. She started by describing how sensors worn by seals collect very valuable data from ice‐covered areas that are inaccessible to Argo, before moving on to ocean gliders. These work as Argo floats but are equipped with wings and rudders to steer them. Gliders are remotely controlled, allowing researchers to collect profiles in regions inaccessible to Argo, such as over the shelf break. Karen closed by showing a wave‐powered vehicle, AutoNaut, which can transport other gliders to the start point of their mission.

The meeting was closed by considering the role that the two history special interest groups might play to ensure that the technology and science developments that have occurred in the 20th century, are adequately documented and preserved.