Penn prof explores interplay between climate change and Southern Ocean

To investigate the interplay between climate change and the ocean, Irina Marinov, an assistant professor in the Department of Earth and Environmental Science, has found it necessary to straddle disciplines.

“I started out just loving physics and math and wanting to save the world,” she says. “Only later did I stumble into climate change. More and more, people interested in ocean and climate sciences must also be interested in interdisciplinarity, in linking physics, biology, chemistry, computer science, and math in the global climate context.”

Marinov, who studied physics as an undergraduate before earning her doctorate in atmosphere and ocean sciences, exemplifies this interdisciplinary approach. Her research into the Southern Ocean, the waters that encircle Antarctica, integrates analyses of the physics and chemistry of oceans and atmosphere with studies of the biological components that play a role in the global carbon cycle.

Though sometimes overlooked by scientists, the Southern Ocean below the 30th parallel takes up more than 60 percent of the anthropogenic heat produced on Earth and 40 to 50 percent of the anthropogenic carbon dioxide penetrating into the oceans.

“The Southern Ocean is emerging as being very, very important for regulating climate,” Marinov says.

In a recent line of work, Marinov and colleagues focused on one of the ocean’s deepest currents, called the Antarctic Bottom Waters. Acting as a conveyer belt [around 6,500 feet] below the surface, these bottom waters channel heat, carbon, oxygen, and nutrients from the Southern Ocean to oceans around the globe.

Writing in Nature Climate Change earlier this year, Marinov and colleagues attempted to explain why the massive current has been shrinking in recent decades. Because the current “hides” heat and carbon from the atmosphere, climate scientists have feared that its slow-down could have repercussions for global warming.

The researchers used 36 finely tuned models that simulate climate change patterns to see if a connection exists between the shrinking current and human-caused climate change. Incorporating 20,000 data points, they found that the surface of the Southern Ocean has become less salty over the last 60 years. And whereas the ocean has always been somewhat stratified, with deeper waters being saltier and denser, they found that these gradients had become more extreme over time.

These changes have occurred, Marinov explains, because increased precipitation around Antarctica—a consequence of climate change—has made the ocean surface waters fresher, and thus less dense. These lighter waters are less prone to move down through the water column and mix with deeper waters.

“We see that the convective process is shutting down as the surface of the ocean gets fresher and fresher,” Marinov said.

The researchers’ models also point to some concerning implications for the future. A handful of the models indicate that growing levels of Southern Ocean fresh water could stop convection from occurring at all by 2030, and most of the models suggest convection will slow, reducing formation of the Antarctic Bottom Waters.

“This is worrisome,” Marinov says, “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.”

Ocean science is not only complex, but logistically difficult, Marinov says. But technological advancements have made it easier in recent years. Marinov is involved with a new endeavor that will hopefully facilitate data collection from the farthest reaches of the Southern Ocean, allowing for more informed climate projections.

In this project, researchers will add special sensors to small, remotely controlled floats that dive deep into the ocean and resurface, transmitting information about the ocean’s conditions to scientists working thousands of miles away via satellite.

“Right now these little floats record measurements of things like salinity and temperature, but we’re hoping to collect some Southern Ocean biogeochemistry measurements from them for the first time,” Marinov says.

Bottom Waters story

The origin of the Antarctic Bottom Waters is near the West Antarctic Peninsula.

Another effort will enable Marinov and colleagues to better understand how phytoplankton—tiny organisms that live at the ocean’s surface—figure into the ocean-climate picture.

“They are all microscopic so we don’t see them, but they are mighty,” Marinov says. “They account for 50 percent of the photosynthesis that occurs on the planet, and they might significantly change with global warming.” A grant from NASA to Marinov’s group supports the use of climate models and satellite images to monitor the concentration of ocean phytoplankton from space.

“These new techniques will allow us to get essentially a global map of ocean biology and help us answer some important questions about how phytoplankton responds and contributes to fluctuations in climate,” she says. “Right now our models are way ahead of our ability to collect data, but we’re starting to catch up.”

Originally published on May 8, 2014