The deep ocean is assumed to be slow in the majority of climate debates. Photographs and headlines can depict how the surface churns, hurricanes form, and sea levels rise. However, things are meant to move on geological timescales, not human ones, at depths of 2,000 and 4,000 meters. We’re changing that presumption. Additionally, there are some unsettling ramifications to the modification.
Ice is the link between deep-ocean current dynamics and surface warming. The meltwater entering the ocean is fresh, lighter, and less dense than the cold, salty water that propels thermohaline circulation, as the Greenland and Antarctic ice sheets lose mass at rates that have significantly increased during the past 20 years. Density differences power the globe conveyor belt that transports water from the surface to the depths and back. Oceanographers have been charting the system’s circulation patterns for decades: warm water rises and cold, salty water sinks. The density gradient that the system depends on is altered when significant amounts of fresh water are added to the surface in polar regions.
Before believing that a reduced circulation simply indicates calmer deep waters, it is important to comprehend the contradiction in the piece’s title. AMOC, the Atlantic Meridional Overturning Circulation, has been shown to diminish under increased freshwater intake, suggesting that thermohaline circulation as a worldwide system may slow. However, localized deep-current behavior may actually speed up in response to shifting temperature gradients. Faster movement along the boundary can be caused by the pressure differential between two water masses of differing temperatures that meet at depth. In some circumstances, this means that even as the wider circulation slows, deep-sea currents will be quicker and more energetic than those generated by the pre-warming baseline.
Oceanographers have reported benthic storms, which are the undersea equivalent of severe weather events at the seafloor. However, their frequency and the processes that lead to them are not well understood. These are periods of fast-moving bottom currents that scour the seafloor, shifting sediment in ways that alter topography, destroy delicate benthic habitat structures, and sometimes seem to be correlated with surface storm events that are conveyed throughout the water column. In physical oceanography, there is still much to learn about how the ocean’s shifting density and temperature structure under accelerated warming may affect the frequency and intensity of these events. This is because it is challenging to keep sensors at depth long enough to record rare, high-energy events.
More frequent or intense deep-current occurrences have ramifications for infrastructure that should be given more consideration, but they don’t always get it. The vast bulk of the world’s internet traffic is carried by an international network of underwater data cables that travel along seafloor routes selected using data on sediment stability and historical current. Deepwater oil and gas production infrastructure is built in accordance with guidelines established from the same historical foundation. The margins included in those criteria begin to appear less pleasant if the benthic environment becomes more energetically variable under circulation changes led by warming. A data cable can be snapped by a sediment turbidity current with enough speed. This can occur solely from current-driven sediment movement and has already occurred, most notably in relation to earthquakes causing undersea landslides.
An additional layer that relates to the larger picture of climate feedback is added by the carbon dimension. Seafloor sediments contain a significant amount of organic carbon that has collected over geological time. This material is stable as long as the sediment is not disturbed. When the silt is scoured and moved by high-velocity current events, it may be exposed to oxygenated water and microbial breakdown, which releases carbon dioxide into the deep water column. That CO2 remains in the ocean for extended periods of time at depth before rising to the top. However, it is now back in the active carbon cycle and will eventually need to be processed by the ocean’s buffering chemistry rather than being stored in sediment.
The extent to which faster deep-sea current dynamics could contribute to the climate feedback scenario in the upcoming decades is yet genuinely unknown. There is currently no monitoring infrastructure in place to describe these occurrences on an oceanic scale. It is becoming increasingly evident that the deep ocean is more than just a passive reservoir shielded from surface changes in a warming environment. It is a system that reacts to everything that occurs above it using its own methods and timelines.
