As the second largest mass of ice on Earth, the Greenland Ice Sheet is a bellwether of our changing climate. It contains enough water to raise ocean levels by about 20 feet and has been shrinking with warming temperatures. Anticipating its future fate is critical for the long-term stability of our planet. To glean insights, scientists have been studying past "interglacials," or periods of mild climate between ice ages that exhibit similar conditions to today and may serve as an analogue for the future.
A recent paper published in Earth and Planetary Science Letters unveils a new method to examine the Greenland Ice Sheet's behavior over the last five interglacials, spanning ~430,000 years and providing a much longer view of past climate, one that will help unravel how fast and how much the ice sheet could melt in the future.
To get a broader picture, scientists turn to some surprisingly small clues: marine sediments, the leftovers from grinding glaciers that have been sloughed off and deposited over thousands of years. These sediments have magnetic properties that can tell researchers important information about where the sediment started its journey, how far it traveled and when the ice sheet retreated.
Traditionally, these sediments would be processed in bulk, grouping clay, silt and other materials together in a sample. Joseph Stoner, an associate professor in the College of Earth, Ocean, and Atmospheric Sciences and co-author Robert Hatfield, a post-doctoral research associate in Stoner's lab, worked with colleagues to develop a grain-specific method that provides a fine-scale view of the ice sheet's past behavior.
"The clay might have a different history than the silt, as well as the sand. They may get to this location by different processes, and they may have different histories, different transport pathways," he said.
Stoner says to think of it like an orchestra – a suite of strings, wind instruments and percussion, each adding an important element to the ensemble. The grain-specific method allows them to zero in on a particular instrument. "If you have a bunch of tuba players, you're going to get a very different sound. The whole is the sum of the parts, and if you look at the parts, you get a unique story."
The story he and colleagues have been able to tell so far is that the Greenland Ice Sheet didn't melt uniformly across all interglacials. The most dramatic deglaciation occurs when solar radiation and warming from atmospheric CO2 act in concert. However, in the Holocene, the time since the last ice age, the ice sheet has been unusually stable despite increasing CO2. Stoner explains that solar radiation has currently been decreasing, providing a kind of buffer against human-caused CO2 emissions. That might seem like good news, but he warns that it's a risky gamble.
"We're kind of running another experiment here. When is CO2 just going to be enough?" he asks.
Stoner and Hatfield's unique method applied toward several interglacials is promising progress in understanding the Greenland Ice Sheet's stability. Still unknown is precisely when the retreat begins and why it happens. Stoner is poised to apply his new methods to different sites and learn more.
"We're trying to cast a wider net to look at the signal that's there, and see how it changes. It's as if we're collecting butterflies in one location and trying to say something about the entire species. You need a bunch of people collecting butterflies to see what's really there. It's the same with marine geology and trying to understand the complexities of our climate."