The Weddell Sea plays a crucial role in the global oceanic circulation system, known as the great ocean conveyor, which facilitates the movement of heat, carbon dioxide, and nutrients across Earth’s oceans. However, our recent research conducted in the Antarctic has uncovered concerning findings regarding the cooling layer of water on the sea bed. This layer of cold water, which is vital for the ocean’s ability to absorb a significant portion of atmospheric heat and greenhouse gas emissions, is experiencing both warming and shrinking due to the effects of human-induced climate change.
The impact of climate change on the Antarctic has resulted in the melting of ice shelves, thus disrupting the intricate system that governs the circulation of the ocean. However, our observations suggest that a natural cycle may have contributed in part to the changes observed over the past 30 years. It is worth noting that the ocean has absorbed over 90% of the excess heat and approximately 30% of the additional carbon dioxide generated by human activities since the industrial age began. This has had a mitigating effect on the impact of climate change on the Earth’s surface and the ecosystems we inhabit.
The exchange of heat and gases between the atmosphere and the ocean predominantly occurs in the Southern Ocean surrounding Antarctica, facilitated by the complex vertical movement of water. A major driver of this vertical movement is the formation of Antarctic bottom water, a term used by oceanographers. Near the Antarctic coastline, seawater at freezing temperatures comes into contact with frigid air, causing it to freeze into sea ice. This process expels salt and consumes freshwater, resulting in the formation of cold, dense, and salty water. The majority of this dense water is produced in specific locations near Antarctica.
In these regions, winds blowing off the freezing continent continually push newly formed sea ice away from the surrounding ice shelves, creating open water areas called polynyas. These polynyas serve as “ice factories” that generate significant amounts of cold and salty water. This water then flows down the continental slope of Antarctica like a submarine waterfall, reaching the ocean bottom. Once there, it spreads throughout the globe as Antarctic bottom water, which is the deepest and densest water mass in the world. This water mass stores atmospheric carbon for hundreds or even thousands of years, playing a crucial role in regulating global climate by driving the great ocean conveyor.
Our recent research, utilizing data from ships and satellites, has revealed a decline of more than 20% in the volume of bottom water in the Weddell Sea, which is one of the main producers of this water mass. This decline has occurred over the past three decades, resulting in the deep Weddell Sea warming at a rate four times faster than the global average. Our findings indicate that weakening offshore winds in the region are responsible for the shrinking of polynyas and the reduced production of the cold, dense, and salty water that drives the Antarctic bottom water formation and the global ocean conveyor. Consequently, this slowdown in the deep overturning circulation can have profound implications for the climate system.
Previous studies have linked the weakening of the global ocean conveyor to the reduced formation of cold and dense water in the Southern Ocean caused by the increasing influx of meltwater from ice shelves. While anthropogenic climate change certainly plays a significant role, our research suggests that natural variability in wind patterns and sea ice extent also contribute to these changes.
The weakening of offshore winds in the southern Weddell Sea over the past 30 years has limited the size of the coastal polynya, resulting in diminished sea ice production. This change in wind patterns appears to be linked to changes in surface temperatures over the tropical Pacific, forming part of a natural cycle similar to El Niño known as the Interdecadal Pacific Oscillation. Oscillations in sea surface temperatures in the tropical Pacific are powerful enough to influence local air pressure and, consequently, winds on both sides of the Antarctic peninsula. This suggests that the trend in Weddell Sea winds and subsequent effects on Antarctic bottom water formation over the past 30 years may be attributed to a longer natural cycle.
However, it is essential to note that ship-based observations have demonstrated the warming and thinning of the bottom water layer around Antarctica for several decades. In regions other than the Weddell Sea, both model predictions and observations suggest that this phenomenon can be explained by the increasing influx of freshwater from melting ice shelves, disrupting the formation of salty and dense water. A similar trend has been observed in the bottom water layer of the Weddell Sea, although the ice shelves in this region are not melting as rapidly as in other parts of Antarctica. This is primarily due to the presence of the polynya sea ice factory, which serves as a barrier to warmer Southern Ocean water. It is important to emphasize that the changes observed in the Weddell Sea are part of a broader trend across Antarctica that cannot be solely attributed to natural causes. In fact, the freshening and shrinking of bottom water align with scientific predictions regarding ice sheet melting. Satellite observations have confirmed a steady loss of ice sheet mass since 2002.
Models are valuable tools for translating our current understanding of physics, historical conditions, and present circumstances into projections of future climate. However, they often lack complete representations of critical processes, including the formation of Antarctic bottom water. This underscores the need for ongoing research to enhance our understanding of the Earth’s system and improve future climate projections.
Mounting evidence suggests that the Antarctic ice sheet is highly vulnerable to climate warming. The melting of this vast reservoir of ice poses a significant threat to the global ocean’s overturning circulation. This disruption of the climate system will contribute to accelerated sea-level rise worldwide. As scientists studying the intricate interactions between the ocean, ice sheet, and atmosphere in Antarctica, we hope that our continuous efforts to refine our understanding of the Earth’s system and future climate projections will inform decision-makers. Urgent and systematic measures are necessary to reduce greenhouse gas emissions and slow the pace of global warming.
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Jessica Irvine is a tech enthusiast specializing in gadgets. From smart home devices to cutting-edge electronics, Jessica explores the world of consumer tech, offering readers comprehensive reviews, hands-on experiences, and expert insights into the coolest and most innovative gadgets on the market.
