By Hannah Magnuson, Dec. 20, 2018 –
Tiny bubbles of gas trapped in glacial ice are giving scientists clues about Earth’s sea level 125,000 years ago.
The gas bubbles serve as bite-sized samples of ancient atmosphere for researchers from the Scripps Institution of Oceanography at the University of California San Diego. They traveled to the ice cores of the Taylor Glacier blue ice area in East Antarctica to collect them.
The atmospheric samples give the researchers clues to understanding ocean temperature, circulation and sea level from eras long ago—clues that can help predict where today’s accelerated climate change may be taking us.
Sarah Shackleton, a sixth year Ph.D. student in the Geosciences Research Division at Scripps, explained that her research team measured the ratio of noble gas atoms present in the ice samples as an indirect way to reconstruct mean ocean temperature from the Last Interglacial, a period of warmer than average global temperature, which occurred 116,000 to 129,000 years ago.
Atmospheric noble gases like xenon and krypton are valuable to researchers because they don’t participate in biological or chemical reactions, just physics-based processes, Shackleton explained. And measuring their ratio in atmospheric samples gives the scientists predictable clues about the ratio that must have been in the ocean.
“What happens is, if the ocean cools, [xenon] becomes more soluble in ocean water so more of these gases can essentially fit into ocean water,” Shackleton explained. “And so if the ocean cools by a significant amount, that means that more of these xenon atoms can reside in the ocean, which means there are less in the atmosphere.”
Shackleton, who is mentored by Scripps geosciences professor Jeff Severinghaus, explained that oceans are a major focus of climate research because they absorb most of the excess energy that greenhouse gases trap in Earth’s atmosphere, buffering the air from more dramatic global warming.
Oceans have a very high heat capacity which allows them to take in a lot of energy without warming significantly. But the excess heat does cause ocean water to expand, raising sea levels.
The goal of Shackleton’s research is to determine how much of elevated sea level rise during the Last Interglacial period was due to ocean warming and how much was due to ice sheet loss. Though still waiting for peer review, Shackleton presented her unpublished research to fellow climate scientists in fall at the Comer Climate Conference in Wisconsin.
Shackleton’s research found that global ocean temperatures were 1-degree Celsius warmer than pre-industrial levels during the Last Interglacial period due to an ocean circulation phenomenon dubbed the “bipolar seesaw,” which refers to periods when the Northern and Southern Hemisphere temperature change is out of sync, with Antarctica warming as Greenland cools, and vice versa.
A 1-degree temperature rise should equate to about 0.7 meters (2.3 feet) of sea level rise, yet the sea levels during the Last Interglacial were a full 6 to 9 meters (19.7-29.5 feet) higher than pre-industrial levels, according to outside scientific research, mainly on fossilized coral reefs.
The rest of this elevated sea level rise must have come from Antarctica or Greenland losing significant ice mass, Shackleton explained. If researchers can determine precisely when and where this ice loss occurred, they can gain insight on where to expect ice loss in the future, as the planet continues to warm.
“When we talk about this period of time, it really comes down to the fact that sea level was higher, and we want to know—was it Greenland that lost [ice] mass? Was it West Antarctica? East Antarctica?” Shackleton said. “Because that gives us some indication of which one might be considered more unstable, or more prone to really lose mass and contribute to sea level rise.”
The Scripps research team speculates that, at the beginning of the Last Interglacial, a mass of warm water called Circumpolar Deep Water melted Antarctic ice sheets from below. Most scientists believe this process is causing the accelerating mass loss from West Antarctica today, so it’s possible it occurred during past warm periods, too, Shackleton said. To do so, the water mass, which circulates around Antarctica, would have been driven onto Antarctica’s continental shelf–the portion of the continent submerged in shallow water. Scientists think that changes in wind-driven ocean circulation could be responsible for driving the Circumpolar Deep Water onto the continental shelf.
Shackleton emphasized that her team doesn’t know for certain whether the Circumpolar Deep Water intruded onto the continental shelf during the Last Interglacial period, “But if it did, then it would be a good way to cause a lot of mass loss from Antarctica and get the sea level rise that we’re talking about from that period of time,” she said.
Her team’s data can help validate ice sheet models, which are predicting current and future sea level rise by projecting when—and how quickly—certain ice sheets will melt and how much mass and sea level rise they will add to the ocean.
By inputting sea level and climate data from the past, researchers can test the models to make sure they are realistic. Shackleton explained that her climate science colleagues are researching all aspects of past climate conditions, to inform models with a wide array of data and make future predictions more accurate.
“We’re a very small cog in a bigger wheel of trying to figure out what’s going on,” she said.
Photo at Top: Sarah Shackleton, left, and her colleague collect ice samples at Antarctica’s Taylor Glacier. (Photo by Bernhard Bereiter.)