Deep Ocean Warming Solves a Persistent Mystery in Global Sea Level Rise Measurements

Climate scientists have long maintained meticulous records of the factors driving global sea level rise, carefully tracking the contributions of melting glaciers, shrinking ice sheets, and the thermal expansion of warming ocean water. Yet for years, a nagging discrepancy in these records has puzzled researchers: when all the known contributors were added together, the total fell short of the actual measured rise in sea levels. Now, a new study has identified the missing piece of the puzzle, revealing that warming in the deepest layers of the ocean, below 2,000 meters, accounts for a significant portion of the previously unexplained sea level increase. The findings represent a major advance in understanding how heat is distributed throughout the ocean and what that means for future projections of coastal flooding and erosion.

The global ocean absorbs more than 90 percent of the excess heat trapped by greenhouse gases in the atmosphere, making it the dominant heat sink in the Earth's climate system. Most ocean temperature monitoring has historically focused on the upper 2,000 meters of the water column, where the majority of ocean heat content measurements are collected by the Argo network, a global array of nearly 4,000 autonomous profiling floats. These instruments have provided invaluable data on upper ocean warming since their deployment began in the early 2000s. However, the deep ocean below 2,000 meters, which comprises roughly half of the total ocean volume, has remained far less well observed due to the extreme pressures and logistical challenges of deploying instruments at such depths.

The research team used a combination of satellite altimetry data, which precisely measures changes in sea surface height, and deep ocean temperature measurements from ship based surveys and a small number of deep Argo floats to quantify the contribution of deep ocean warming to global sea level rise. Their analysis reveals that heating below 2,000 meters has contributed approximately 0.1 to 0.15 millimeters per year to global sea level rise over the past two decades. While this may sound small, it is sufficient to close the gap between observed sea level rise and the sum of previously identified contributors. The finding confirms that the deep ocean is not the stable, unchanging environment it was once assumed to be, but is actively warming in response to the buildup of greenhouse gases in the atmosphere.

The mechanisms by which heat reaches the deep ocean are complex and involve some of the largest circulation patterns on the planet. Cold, dense water formed at high latitudes in the North Atlantic and around Antarctica sinks to the ocean floor and spreads throughout the global ocean basins in a process known as thermohaline circulation, sometimes called the ocean's conveyor belt. As surface waters warm due to climate change, this process transports increasingly warmer water into the deep ocean over timescales of decades to centuries. The study found that warming is particularly pronounced in the Southern Ocean around Antarctica, where deep water formation is most active. Changes in these circulation patterns could have far reaching consequences for ocean chemistry, marine ecosystems, and the pace of future sea level rise.

The implications of deep ocean warming extend well beyond the sea level budget. Warmer deep waters can affect the stability of marine ice sheets, particularly in West Antarctica, where warm ocean currents flowing beneath ice shelves accelerate melting from below. This submarine melting is considered one of the most significant and least predictable risks for future sea level rise, with the potential to add meters to global sea levels over the coming centuries if major ice sheets destabilize. The new findings suggest that deep ocean warming may be contributing to this process more than previously recognized, adding urgency to efforts to expand deep ocean monitoring capabilities.

The study's authors call for a significant expansion of the deep Argo program to improve coverage of the ocean below 2,000 meters. Currently, fewer than 200 deep Argo floats are in operation, compared to the nearly 4,000 standard floats monitoring the upper ocean. The researchers argue that without better observations of the deep ocean, climate models will continue to carry significant uncertainties in their projections of sea level rise and ocean heat uptake. Closing this observational gap is essential for coastal communities, urban planners, and governments around the world who depend on accurate sea level projections to plan infrastructure investments, flood defenses, and managed retreat strategies in the face of a rising ocean.