The numbers that have been coming in over the past few years have challenged a comfortable assumption in a NOAA data center that processes temperature measurements from thousands of equipment placed throughout every major ocean basin. It was thought that the deep ocean, namely the water below one, two, and three kilometers, would be stable. Simple physics shields it from the rapid warming occurring at the surface, but it is not impervious to change forever. Heat travels slowly through water, and the mixing between the surface and deep levels is slow by any measure relevant to human timescales. For the time being at least, the warming was meant to be a surface issue. The assumption needs to be revised, according to the high-precision monitoring network.
This level of measurement is made possible by a variety of equipment that, when combined, provide an image of ocean temperature with a level of detail not possible a generation ago. By continually recording temperature at various depth levels, deep-sea moorings—anchored instrument packages positioned at fixed points in the water column—capture hourly fluctuations that conventional profiling surveys would completely miss.
The Argo float network, which consists of over 4,000 devices floating freely in the world’s seas, descends to a depth and profiles salinity and temperature before returning to the surface and sending the data via satellite. Overhead, radiometers on NOAA satellites measure daily anomalies in sea-surface temperature at resolutions that enable real-time tracking of heat distribution over whole ocean basins. Measurement capabilities down to thousandths of a degree are produced by the combination, much beyond what is possible with traditional instruments.
The network has discovered an unmistakable warming signal in the deep basins and abyssal plains. In deep basins that were once thought to be thermally stable, the pace is negligible in absolute terms—between 0.02 and 0.04 degrees Celsius per decade. That seems insignificant. There are two compounding reasons why it isn’t. The ocean is huge, to start. Across the Atlantic or Pacific abyssal plains, warming a deep water mass by 0.02 degrees each decade requires absorbing and holding onto a certain amount of thermal energy that, if released back into the atmosphere, would significantly increase surface warming. Second, the 0.02 figure represents a current measurement on a moving gage rather than a ceiling because the trend is directional and seems to be accelerating in some basins.
Oceanographers most frequently use the 90% figure to illustrate why ocean temperature is so important to understanding climate. The ocean has absorbed more than 90% of the extra heat that greenhouse gas buildup has added to the Earth system instead of the atmosphere. The comparatively slight increases in surface air temperature that have been observed over the past century only make up a small portion of the planet’s overall heat intake. Most have been quietly absorbed by the ocean without the kind of obvious repercussions that grab the public’s attention right away. The detection of the deep-water warming signal indicates that the heat is reaching deeper and building up in strata that were not sufficiently characterized by earlier monitoring.
The accuracy of the measurement is important because even slight increases in baseline temperature in the deep ocean can have cascade effects. The heat content of the ocean layers beneath the surface contributes to marine heatwaves, which are unusual surface temperature occurrences that bleach coral, interfere with fisheries, and push fish populations beyond of their historical limits.
El Niño occurrences use deep-ocean thermal energy in ways that become more intense as the baseline temperature of those deep layers rises. These phenomena cause changes in weather patterns across several continents and redistribute heat throughout the Pacific. Sea level rise is already being measured at tide gages worldwide due to thermal expansion, which is the physical expansion of water volume as temperature rises. Even if surface warming were to stop right away, the expansion of the deep ocean would continue to contribute to this signal for centuries.
Through decades of worldwide collaboration under initiatives like Argo and NOAA’s unwavering dedication to deep-sea instruments, the monitoring network generating these data represents a substantial infrastructure investment in ocean observation capacity. When it’s functioning properly, thousands of instruments silently measure, communicate, and contribute to a dataset that academics work with over years and decades rather than news cycles. This type of study doesn’t make headlines. Because the system is sensitive enough to pick up on things that earlier monitoring missed, the readings it is currently producing are crucial. That system is not scary. It is operational. What it discovered is concerning.
