Global warming may be slowing deep ocean currents according to new research and observations of Antarctic ice packs. It is believed that global warming is responsible for an increase in glacier melt and precipitation, resulting in lower salt content in Antarctic waters and inhibiting the exchange between cold deep ocean water and warmer surface water. Normally, relatively warm waters at the surface of the sea are exposed to frigid air temperatures, lowering their temperatures and causing them to sink to the bottom, where they flow north bringing oxygen, carbon and nutrients to deep-sea regions before eventually cycling back to the Antarctic along the surface where the process repeats. However, the lowered salt content has reduced the density of surface waters, making them more resistant to mixing with deeper waters and stalling the current.
Areas of open water amid ice packs known as polynyas have been a common sight in Antarctic waters since their discovery via satellite in 1974, but in recent years they have become almost impossible to find. Polynyas are believed to be important players in how the relatively warm surface water is cooled enough to mix with water in colder layers. Thick caps of ice seal the water off from the cold air that encourages the temperature change that allows mixing, and without the polynyas to allow exposure, the process slows.
What is worse is that polynyas are believed to form in areas where warm water rising from below keeps the surface above freezing and clear of ice. What this means is that as the currents slow, there is less warm water, which means there are less polynyas, which means there is less area for water to be chilled, which slows the currents. The catalyst for this problem is melting sea ice, suggesting that global warming may be slowing deep ocean currents.
The reduction in salt content of the surface Antarctic waters has taken place over the last 60 years, so the researchers are not yet sure if it part of man-made changes or a naturally occurring cycle. However looking back at research strips and data gathered by floats bobbing in the Antarctic Ocean has shown that polynyas used to be a common occurrence, making their present relative rarity a cause for concern. On top of this, as the Antarctic deep ocean currents grow more sluggish, the appearance of new polynyas is likely to stagnate as well, exacerbating the problem even further.
Despite the growing amount of warm water trapped at the bottom of the Antarctic Ocean, researchers do not think the disappearing polynyas are linked to the recent global warming hiatus that has been seen around the world. It may be that a reduction in the deep water currents is a natural counterbalance to the reduction in salinity designed to allow for more ice, which is effective at reflecting heat from the sun away from the surface of the earth and reducing temperature. However if global warming may be slowing deep ocean currents, how may these changes affect us?
By Daniel O’Brien