The thermal expansion of ocean depths (above 2000 metres) could contribute more to global sea level rise than previously believed (0.4 mm/year). This is what emerges from a study recently published on Earth’s Futurewhich analyzes the behavior of the oceans at depths that oceanographic buoys cannot reach. This study could solve a long-standing mystery 2016the year from which scientists are unable to explain the global balance of sea levels starting from known causes (mainly due to global warming).
What is sea level balance and why the deep oceans were not understood
Ever since satellites began measuring sea levels with millimeter precision, scientists have built that the sea level balance, that is, the sort of report in which all the known causes of the rise are added up (mainly ice melting and thermal expansion of the oceans due to global warming, given that the oceans absorb a good part of the excess thermal energy due to greenhouse gases) and it is verified that the total corresponds to what the satellites actually measure.
Since 2016, however, the data available to scientists does not completely explain the recorded increase: something was missing. Using a reprocessing of ocean data that does not include direct assimilation of satellite altimetry data, the new study estimates that the contribution of the deep oceans to sea level rise over the period 2005-2022 was approximately 0.4 millimeters per yearequal to approximately 10% of the total increase observed in the same period. If confirmed, this discovery could close the sea level balance again.
But why hadn’t the deep oceans been considered? The answer lies in the functioning of ocean monitoring, whose main instrument is the net Argona system of approx 4,000 robotic buoys scattered throughout the various oceans. These autonomously dive to a depth of 2,000 meters, collect data on water temperature and salinity, rise to the surface and transmit the information via satellite. Thanks to this network we know precisely the thermal behavior of the upper part of the oceans, but below 2000 meters depth we have no direct measurements. Even though we are talking about about half the total volume of the oceans, only 10% of historical temperature and salinity profiles extend beyond this depth.
Until now it had been estimated that the contribution of deep ocean warming was negligible, but the new study shows that – although secondary – this factor must also be taken into consideration, especially for long-term forecasts on rising oceans.
Because the ocean depths are warming
The warming of the ocean depths is a direct consequence of climate change, but it occurs at different times and in different ways compared to the surface layers. Heat trapped in the atmosphere by greenhouse gases enters the surface ocean and is then transported down to the depths via thermohaline currents — large global circulations in the deep ocean, such as the Atlantic one known as the AMOC. It is a slow but inexorable process: heat accumulates in the depths, the water expands and the sea level rises.
The fact that this contribution is increasing so rapidly is a sign that warming is reaching deeper and deeper layers, with a time lag behind surface warming.
This finding implies that future projections may have underestimated the issue of deep sea depths. This is why the network is being created Deep Argowhich aims to reach 6000 meters of depth, thus allowing us to extend our understanding of the deep dynamics that contribute to sea level rise.
