Scientists tracking factors driving a global rise in sea levels in recent years say a key missing piece of the puzzle has finally been revealed.
For many years, climate scientists have worked to balance what they call the “global mean sea level (GMSL) budget,” which describes a range of contributors to global sea level rise. Among these factors are melting ice sheets, glacier loss, and the expansion of seawater as it warms.
Until around a decade ago, it was believed that all the factors that govern such processes were known. That changed in recent years, as an apparent discrepancy surfaced: the real-world sea level observations appeared to be outpacing what known factors by themselves could explain. So what could be driving the anomaly?
Now, an answer to this lingering mystery may finally be at hand, according to new research that appeared in the journal Earth’s Future, which identifies what could be the lynchpin in this mystery: warming that is occurring deep within the ocean.
Heat from Deep Below
The new research, led by Anny Cazenave of the Laboratory of Space Geophysical and Oceanographic Studies in France, and undertaken by an international team of colleagues, revealed that heat absorption in ocean layers at depths greater than 2,000 meters plays a significant role in rising sea levels.
Incorporating this deep ocean warming into current models, the previously unexplained aspects of recent global sea level rise that scientists have observed since 2016 can largely be accounted for, the researchers say.
According to new estimates, deep ocean heating likely contributes to nearly 0.4 millimeters of sea level rise that occurred annually between 2005 and 2022, which amounts to almost ten percent of the total increase recorded during that period.
That may seem like a small amount, but in the broader context of sea level changes scientists have observed, it is significant enough that the imbalance was initially deemed an anomaly, and a concerning one.
However, understanding the role of this newly recognized factor presented a challenge, since the majority of direct measurements of ocean conditions appear to arise from the global network of more than 4,000 Argo floats, which are instruments that measure factors including salinity, pressure, and temperature as they make their way through the uppermost 2000 meters of the Earth’s oceans.
Through their movement, the Argo floats capture data that is transferred to satellites, offering a game-changing ability for scientists to collect information about our oceans, including their heat content. However, due to the depths at which they operate, these devices do not collect data from some of the ocean’s deepest waters.
This is significant, as the ocean’s deepest volumes lie in the regions where the scientific capabilities of Argo floats cannot reach.
A New Approach to Gathering Deep Ocean Data
To overcome this, Cazenave and the research team employed reanalysis, a method that combines existing observations with advanced computer models to estimate conditions under circumstances that cannot be directly measured, such as those occurring within the deepest regions of the ocean.
This approach was employed with a new reanalysis system called CIGAR, which reconstructed temperature changes in the deep ocean and integrated the results with existing data compiled in various other datasets. Among these were satellite data that included information on ocean mass obtained by the GRACE satellite program, as well as glacier and ice sheet mass loss, and changes in land water storage.
With the combined data, Cazenave and her international collaborators successfully generated a more complete image of the broader forces that are currently driving sea level rise.
Deep Ocean Climate Mysteries
A significant finding from the new research involves the deep ocean climate system, the dynamics of which remain somewhat mysterious. Although upper layers of Earth’s oceans are well-known reservoirs for heat, the team’s new work reveals that the same can be said of deeper ocean waters, which also absorb large amounts of energy.
Fundamentally, this heat absorption contributes to the greater expansion of the ocean and, of key importance to the new research, also the anomalous sea level rise observed in recent years.
Along with the new light it sheds on this phenomenon, the team’s discovery also raises some new questions, including what the broader causes are that drive the warming observed in the deep ocean. One possibility is that natural climate variability could be at play, although anthropogenic factors could also be responsible, if not a combination of both.
Future research using advanced climate models will aim to better understand these processes, and ultimately, the team’s new work highlights the complexity of our planet’s climate, as well as the importance of studying the processes that often require the most challenging measurement.
The team’s study, “Evidence of Increased Deep Ocean Warming From a Sea Level Budget Approach,” was published in Earth’s Future.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.