Deep ocean currents slowing sooner than anticipated based on observations, as the world's most magnificent waterfall unfolds beneath the ocean's surface while Antarctica serves as the backdrop for an extraordinary spectacle. This mesmerizing event occurs when trillions of tons of frigid, dense, and oxygen-rich water cascade from the continental shelf, plunging to immense depths. Known as Antarctic "bottom water," this formidable torrent then undertakes a remarkable journey, traversing the ocean floor through deep-sea currents and gradually ascending thousands of kilometers away.
In this grand performance, Antarctica commands a global tapestry of oceanic currents known as the "overturning circulation," orchestrating the redistribution of heat, carbon, and vital nutrients across the planet. The ceaseless motion of this circulation not only plays a pivotal role in maintaining Earth's climate stability but also serves as the primary conduit for oxygen to reach the profound depths of the ocean.
Antarctica's extraordinary waterfall thus assumes a vital role in shaping the delicate balance of our planet's interconnected systems, contributing to the harmonious equilibrium of the world's oceans and the well-being of life itself.
There are indications that the circulation is decelerating, occurring much earlier than previously anticipated. This decrease in speed carries the potential to disturb the link between the Antarctic coastlines and the vast depths of the ocean, resulting in significant ramifications for the Earth's climate, sea levels, and marine ecosystems.
In a recently published study in the journal Nature Climate Change, new research based on actual observations has shed light on the transformation of the deep ocean surrounding Antarctica over the past three decades. The data reveals a significant slowdown, approximately 30%, in the overturning circulation, alongside a decline in oxygen levels within the deep ocean. Remarkably, these changes are transpiring even earlier than what climate models had initially predicted.
The study highlights that the melting of Antarctic ice is a key factor contributing to the disruption of Antarctic bottom water formation. As the ice melts, the resulting influx of freshwater renders the surface waters of Antarctica less salty and less dense, impeding their tendency to sink. Consequently, this process acts as a brake on the overturning circulation, further exacerbating the slowdown.
An ocean with a seagull flying over it
With the deceleration of bottom water flow, the deep ocean experiences a decline in its oxygen supply. As the oxygen-rich layer of bottom water diminishes, it is gradually replaced by warmer waters that contain lower levels of oxygen, exacerbating the overall decrease.
Creatures inhabiting the ocean, regardless of their size, are sensitive to even minor fluctuations in oxygen levels. While deep-sea organisms have adapted to survive in low oxygen conditions, they still rely on available oxygen for respiration.
Consequently, the diminishing oxygen levels may compel these organisms to seek refuge in other regions or modify their behavior to cope with the changing conditions. Predictive models indicate that we are headed towards a significant reduction of up to 25% in the "viable" environment accessible to these marine animals.
The deceleration of the overturning circulation has additional implications for global warming. This circulation plays a crucial role in transporting carbon dioxide and heat to the deep ocean, serving as a storage mechanism that effectively conceals them from the atmosphere.
However, as the capacity of the ocean to store carbon dioxide and heat diminishes due to the slowdown, a larger portion of these elements remains in the atmosphere. This feedback loop amplifies the pace of global warming, exacerbating its effects.
Moreover, the reduction in the volume of Antarctic bottom water reaching the ocean floor leads to an increase in sea levels. The replacement of this bottom water with warmer waters results in thermal expansion, causing the occupying water mass to take up more space. Consequently, this expansion contributes to the rising sea levels observed as a consequence of the slowdown in Antarctic bottom water formation.
The task of observing bottom water in the Southern Ocean poses significant challenges. This region is not only remote but also characterized by the most robust winds and largest waves on Earth. Access to this area is further constrained by the presence of sea ice during the winter months, precisely when the formation of bottom water occurs.
The inhospitable nature of the Southern Ocean, coupled with its extreme conditions, makes it difficult for researchers to collect reliable and comprehensive data on bottom water properties. The remote location, harsh weather, and seasonal ice coverage impose limitations on scientific expeditions and restrict our ability to closely monitor and study the processes associated with bottom water formation and its subsequent effects on the ocean system.
Due to the challenges associated with observing the deep Southern Ocean, our understanding of its dynamics remains limited. However, intermittent full-depth measurements obtained during ship voyages have offered valuable insights into the ongoing transformations occurring in the deep ocean. These observations indicate that the bottom water layer is undergoing warming, becoming less dense, and experiencing thinning.
Satellite data has also provided evidence of the shrinking Antarctic ice sheet. Furthermore, measurements taken in ocean regions downstream of areas with rapid ice melt demonstrate that the influx of meltwater is diminishing the salinity and density of coastal waters.
Although these indications are concerning, it is important to note that direct observations of the deep overturning circulation are still lacking. This knowledge gap highlights the ongoing challenges in comprehensively monitoring and understanding the intricate processes occurring in the deep Southern Ocean.
A red ship moving towards several icebergs in an ocean
A new approach was adopted, incorporating diverse observations by leveraging their individual strengths. By incorporating the comprehensive depth measurements obtained from ships, valuable snapshots of ocean density were acquired.
However, these ship-based measurements are typically conducted only once every ten years. On the contrary, moored instruments offer continuous density and speed measurements, albeit limited to specific locations and for a finite period of time.
In this new methodology, these distinct observation techniques were merged to maximize their benefits and overcome their limitations. The result was a combined dataset that provided a more holistic understanding of ocean dynamics by capturing both comprehensive depth measurements from ships and continuous density and speed data from moored instruments.
A new method was devised to integrate ship data, mooring records, and a high-resolution numerical simulation, enabling the estimation of the Antarctic bottom water flow strength and its associated oxygen transport to the deep ocean.
The primary objective of the study was to examine a deep basin located south of Australia, which serves as a recipient of bottom water from multiple origins. Notably, these sources are situated downstream of substantial meltwater inputs, making this particular region a potential early indicator of climate-induced alterations in the deep ocean.
By combining the aforementioned datasets and computational modeling, researchers were able to gain valuable insights into the intensity of the Antarctic bottom water flow and the extent of oxygen transfer it facilitates towards the deep ocean. This innovative approach contributes to our understanding of the dynamics of this crucial oceanic process and its implications for climate change monitoring.
The results are remarkable. Over a span of thirty years, from 1992 to 2017, there has been a significant slowdown of the overturning circulation in a particular region, with a decrease of nearly one-third (30%). This deceleration has led to a reduced supply of oxygen reaching the deep areas. The primary cause of this slowdown has been identified as the freshening of waters near Antarctica.
The process of freshening has resulted in a decrease in both the density and volume of the Antarctic bottom water formation, as well as a reduction in its flow velocity. The impact of this freshening has been substantial, and the observed decline would have been even more pronounced if not for a temporary climate event that occurred. This event triggered a partial and short-lived recovery of bottom water formation, driven by an increase in salinity. This recovery highlights the vulnerability of bottom water formation to changes in salinity along the Antarctic continental shelf.
These findings raise concerns as they indicate that the anticipated changes projected to happen by 2050 are already unfolding. The implications of these early developments are alarming, emphasizing the need for proactive measures to address the ongoing shifts in oceanic processes.
Antarctica's ice loss is predicted to persist and potentially intensify as the Earth's temperature rises. It is highly likely that we will surpass the critical 1.5℃ global warming threshold by 2027. As more ice melts, there will be an increase in freshening, which in turn will lead to a continuation of the slowdown in ocean circulation and profound oxygen depletion.
The repercussions of this slowdown will extend far beyond Antarctica. The global ocean's overturning circulation spans the entire planet and significantly influences the rate of climate change and the rise of sea levels.
Furthermore, marine life will face disruption and harm as a result. This research serves as yet another compelling incentive to redouble our efforts and accelerate the reduction of greenhouse gas emissions.