Climate Change May Slow Down Deep Ocean Currents, Hindering Their Ability To Hold Carbon From Atmosphere
It has long been known that oceans influence the Earth's climate in several ways. Deep ocean currents, called ocean conveyor belts, carry massive amounts of heat, oxygen, and nutrients around the world. But researchers have found, in a new study, that global warming may be affecting these redistribution patterns, and could seriously impact the world's future climate.
"Our observations are showing us that there is less formation of these deep waters near Antarctica," said Irina Marinov, lead author of the study from the University of Pennsylvania, in a statement. "This is worrisome because, if this is the case, we're likely going to see less uptake of human produced, or anthropogenic, heat and carbon dioxide by the ocean, making this a positive feedback loop for climate change."
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Recent studies have shown that the Antarctic bottom water (AABW), which surrounds Antarctica and is the densest water mass in the world ocean, has been shrinking steadily in the past few decades. These cold and salty waters are heavy, flowing up to 2,000 meters below the ocean's surface, and holding huge amounts of atmospheric heat and carbon dioxide. The southern ocean takes up approximately 60 percent of the anthropogenic heat produced on Earth and 40 to 50 percent of the anthropogenic carbon dioxide.
"The southern ocean is emerging as being very, very important for regulating climate," Marinov said. This prompted the team to determine if there was a correlation between climate change and the shrinking AABW.
They first looked to a phenomenon that occurred in the 1970s in the Weddell Sea. Satellite images taken during that period showed an area of open water, called polynyas, in the middle of large ice packs in the Weddell Sea. This area of open water is formed when warm water from the North Atlantic is pushed up toward the southern ocean's surface. While this occurs, a dense concentration of brine forms at the surface from salt being expelled during sea-ice formation. These processes result in a layering of cold and heavy salty surface water above warmer, lighter water. Eventually, the surface water sinks, and mixes with the warmer water, causing the circulation of millions of cubic meters of water in a process called open-sea convection.
Since the polynya in the Weddell Sea has not reappeared in nearly 40 years, scientists labelled it and open-sea convection a rare natural occurrence.
The current study's researchers believe that the occurrence of polynyas have been suppressed due to the effects of climate change on ocean salinity. This is because warmer climates are causing more ice packs to melt, which in turn adds fresh water to the ocean. Fresh water is not as heavy as salt water, and doesn't sink through the layers of the ocean - meaning less open-sea convection. "This is important because this process of deep convection that happens in polynyas is a big contribution to the Antarctic bottom waters, these deep currents that feed the rest of the ocean," Marinov noted.
The scientists examined a number of satellite images and data collected from 36 state-of-the-art climate change models, which showed that the southern ocean had rapidly lost its salinity in the past 60 years, and that polynyas were more common pre-industrial conditions, before anthropogenic climate change took hold. "We see that the convective process is shutting down as the water gets fresher and fresher," Marinov said in the statement.
Seven of the models suggested that increased fresh water in the southern ocean could stop the convection from occurring altogether by 2030, and most models showed strong decreases in convection during the 21st century, reducing AABW formation.
What impact does this have for current and future climate change? According to scientists, the recent absence of polynyas could mean that heat is getting trapped in the depths of the ocean, rather than being released into the atmosphere. "The slowdown of polynyas will likely be a positive feedback on warming, as the convective process is shutting down and reducing the amount of new, anthropogenic carbon and heat being taken out of the atmosphere. We are pursuing these implications in our current work."
The scientists have also examined how the ocean's natural ability to store carbon might respond as the climate warms. The ocean stores 50 times more carbon dioxide than the atmosphere, and is a crucial player in climate change regulation.
These levels of carbon are due to tiny organisms called phytoplanktons that use carbon dioxide during photosynthesis. When fish and other marine organisms that feed on phytoplanktons die and decompose, carbon is released into the ocean. If it were not for this process, atmospheric carbon dioxide levels would be about 200 parts per million (ppm) higher than the currently observed 400 ppm.
The researchers ran climate simulation software to understand how changes in wind, temperature, and salinity during the 21st century would affect the ocean's ability to store carbon. Their findings suggest that the phytoplankton-driven biological carbon pump will strengthen, leading to increased carbon storage in the ocean. But this effect may not be enough to balance the carbon dioxide expelled from a warmer ocean. "Gases are more soluble in colder liquids," Marinov said in the statement. "With climate change, we predict that the ocean will lose some of its deep, natural carbon in the future, partly because the temperature warming effect is so strong."
Marinov's future plans are to collect samples from all over the southern ocean and apply physical, biological, and chemical analyses to the climate change models. "More and more, people interested in ocean and climate sciences must also be interested in interdisciplinarity, in linking physics, biology, chemistry in the global climate context."
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