Deep ocean current may slow due to climate change

Far beneath the surface of the ocean, deep currents act as conveyer belts, channeling heat, carbon, oxygen and nutrients around the globe.A new study by the University of Pennsylvania’s Irina Marinov and Raffaele Bernardello and colleagues from McGill University has found that recent climate change may be acting to slow down one of these conveyer belts, with potentially serious consequences for the future of the planet’s climate.”Our observations are showing us that there is less formation of these deep waters near Antarctica,” Marinov said. “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.”Marinov is an assistant professor in Penn’s School of Arts and Sciences’ Department of Earth and Environmental Science, while Bernardello was a postdoctoral investigator in the same department and has just moved to the National Oceanography Centre in the United Kingdom. They collaborated with Casimir de Lavergne, Jaime B. Palter and Eric D. Galbraith of McGill University on the study, which was published in Nature Climate Change.Oceanographers have noticed that Antarctic Bottom Waters, a massive current of cold, salty and dense water that flows 2,000 meters under the ocean’s surface from near the Antarctic coast toward the equator has been shrinking in recent decades. This is cause for concern, as the current is believed to “hide” heat and carbon from the atmosphere. 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.Along with colleagues, Marinov used models to discern whether the shrinking of the Antarctic Bottom Waters could be attributed to anthropogenic climate change.They looked to an unusual phenomenon that had been observed from satellite images taken between 1974 and 1976. The images revealed a large ice-free area within the Weddell Sea. Called a polynya, this opening in the sea ice forms when warm water of North Atlantic origin is pushed up toward the Southern Ocean’s surface. In a separate process, brine released during the sea-ice formation process produces a reservoir of cold, salty waters at the surface of the Weddell Sea. …

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Dust in the wind drove iron fertilization during ice age

Researchers from Princeton University and the Swiss Federal Institute of Technology in Zurich have confirmed that during the last ice age iron fertilization caused plankton to thrive in a region of the Southern Ocean.The study published in Science confirms a longstanding hypothesis that wind-borne dust carried iron to the region of the globe north of Antarctica, driving plankton growth and eventually leading to the removal of carbon dioxide from the atmosphere.Plankton remove the greenhouse gas carbon dioxide (CO2) from the atmosphere during growth and transfer it to the deep ocean when their remains sink to the bottom. Iron fertilization has previously been suggested as a possible cause of the lower CO2 levels that occur during ice ages. These decreases in atmospheric CO2 are believed to have “amplified” the ice ages, making them much colder, with some scientists believing that there would have been no ice ages at all without the CO2 depletion.Iron fertilization has also been suggested as one way to draw down the rising levels of CO2 associated with the burning of fossil fuels. Improved understanding of the drivers of ocean carbon storage could lead to better predictions of how the rise in manmade carbon dioxide will affect climate in the coming years.The role of iron in storing carbon dioxide during ice ages was first proposed in 1990 by the late John Martin, an oceanographer at Moss Landing Marine Laboratories in California who made the landmark discovery that iron limits plankton growth in large regions of the modern ocean.Based on evidence that there was more dust in the atmosphere during the ice ages, Martin hypothesized that this increased dust supply to the Southern Ocean allowed plankton to grow more rapidly, sending more of their biomass into the deep ocean and removing CO2 from the atmosphere. Martin focused on the Southern Ocean because its surface waters contain the nutrients nitrogen and phosphorus in abundance, allowing plankton to be fertilized by iron without running low on these necessary nutrients.The research confirms Martin’s hypothesis, said Daniel Sigman, Princeton’s Dusenbury Professor of Geological and Geophysical Sciences, and a co-leader of the study. “I was an undergraduate when Martin published his ‘ice age iron hypothesis,'” he said. “I remember being captivated by it, as was everyone else at the time. But I also remember thinking that Martin would have to be the luckiest person in the world to pose such a simple, beautiful explanation for the ice age CO2 paradox and then turn out to be right about it.”Previous efforts to test Martin’s hypothesis established a strong correlation of cold climate, high dust and productivity in the Subantarctic region, a band of ocean encircling the globe between roughly 40 and 50 degrees south latitude that lies in the path of the winds that blow off South America, South Africa and Australia. However, it was not clear whether the productivity was due to iron fertilization or the northward shift of a zone of naturally occurring productivity that today lies to the south of the Subantarctic. This uncertainty was made more acute by the finding that ice age productivity was lower in the Antarctic Ocean, which lies south of the Subantarctic region.To settle the matter, the research groups of Sigman at Princeton and Gerald Haug and Tim Eglinton at ETH Zurich teamed up to use a new method developed at Princeton. …

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