By Lily Carey
Medill New Service, Dec. 23, 2024
Over 70,000 years ago, much of modern-day Europe was covered by a vast ice sheet — one that was slowly growing in size as global temperatures dropped.
This Eurasian ice sheet accumulated mass over the course of the next several thousand years, though portions of it also periodically melted, sending pulses of meltwater into the north Atlantic.
For scientists such as Gracelyn McClure, a geologist and a recent graduate of the University of Minnesota, this isn’t just a tale of an ancient ice age. These meltwater pulses, and the chemical clues they left behind, could provide crucial information about how our oceans might respond to melting glaciers now and in the future as ice loss accelerates.
“There’s going to be meltwater pulses with human-caused climate change as well,” McClure said. “It’s not necessarily going to be a consistent, gradual feed into the oceans…But we could see a pretty rapid increase of, for instance, one meter within a relatively short amount of time.”
Today, many experts are projecting that surpassing 2 degrees Celsius (3.3 degrees Fahrenheit) of global temperature increase could cause sea levels to rise anywhere from one to three feet by 2100, possibly more. But as McClure and her colleagues at UMN have found, that increase won’t come overnight. Instead, this sea level rise will likely come in the form of smaller, relatively rapid meltwater pulses — waves of melted freshwater from glaciers that spread into the ocean and sometimes further inland. We have just surpassed the 1.5-degree target set in the international Paris Agreement to cap global warming to slow down sea level rise.
Now, scientists are studying how these meltwater pulses have behaved in the past, and how they’ve changed our ocean chemistry, hoping to predict the impact of current and on-going glacial melt.
“It’s a little hard to draw comparisons between what’s happening during glacial buildup to now, when we’re concerned about the glaciers melting,” McClure said. “Somehow those abrupt pulses happened, but the glaciers didn’t become completely destabilized.”
Typically, meltwater pulses are extremely hard to track in the fossil record, as they usually disseminate into the ocean without leaving behind any major footprints. But when that meltwater pulse from Europe entered the Bay of Biscay, just off the coast of France, some of the water from the bay evaporated into rain, which fell nearby on the northern Iberian Peninsula.
On land once again, this water became runoff, traveling through the soil and collecting chemical elements. Eventually, these elements collected in underground caves as speleothems, or towers of mineral deposits, leaving behind an incredibly well-preserved record of the chemical composition of the neighboring land and ocean at the time. The layers of the deposit go back in time.
That’s where McClure and her colleagues come in. A team of researchers sampled these speleothems from what is today the northern coast of Spain, and McClure helped to analyze them, searching for a few key isotopic elements.
The freshwater contained in glaciers holds much more oxygen than salty ocean water does — but O18, a stable but heavier isotope of oxygen, is seen at much lower levels in meltwater than ocean water. So when McClure’s speleothem samples came back with lower-than-average O18 concentrations, she knew it was a sign of a freshwater meltwater pulse entering the ocean, and was able to piece together the timing of the pulse.
Seeing rapid meltwater pulses during a time when the world was getting cooler overall was intriguing to McClure. But this instance also falls in line with a historical record of small yet rapid meltwater pulses during periods of rapid global change — much like the one we’re seeing today.
“How does approaching [rapid meltwater pulses] differ from approaching a massive sea level increase further down the line?” McClure asks. “That’s a very different problem.”
Increasing levels of freshwater from glaciers and ice sheets melting in the world’s oceans can also interfere with crucial ocean currents. The Atlantic Ocean, for instance, carries water in a “conveyor belt”-type motion, as McClure described it, circulating cooler water from north to south and pushing warmer water from the south back north in a current known as the AMOC (Atlantic Meridional Overturning Circulation). It’s the reason why northern Europe is relatively warmer than it would otherwise be.
But when meltwater pulses come into play, they introduce freshwater molecules into the mix, disrupting the sinking of O18-rich saltwater into the AMOC. As the AMOC slows, the turnover of warm and cool water stagnates, leading to drastic changes in regional temperatures both in water and on land.
Modern day climate change might have a slightly different impact on the AMOC, since the meltwater pulses that McClure studied came from an ice sheet that melted in the aftermath of that last great ice age. Yet the end results could be remarkably similar.
“If Greenland were to melt a lot, it could definitely impact the North Atlantic deep water formation, which would then impact AMOC,” she said. “That is really important for the atmosphere-ocean relationship, and if there’s enough melting to completely stop the circulation, we’re going to experience a pretty rapid climate change in the opposite direction in some places.”
That impact is not yet certain. Robert Holzman, a current UMN Ph.D. student? who studied inland meltwater pulses at the end of the last Ice Age about 10,000 years ago, said that it can be hard to predict the influence of future glacial meltwater using data from different glaciers. “It’s really complex to actually work this into projections of sea level, hence the reason that we’ve got a really wide range of protections for how quickly it’ll move up,” he said.
Ultimately, any future meltwater pulses will come from Greenland or Antarctica, the two remaining landmasses covered in mammoth ice sheets. Just as the location of pulses from the Eurasian ice sheet influenced the freshwater makeup of the Bay of Biscay, “figuring out where the sources of these [future] meltwater pulses are very critical,” Holzman said.
“Any way that we can build an understanding of how ice sheets react to rapid warming can be worked into modeling to some degree, and assist in getting a better idea of how these ice sheets might behave,” he said.
Photo at top: Detail of NOAA’s graphic of the Atlantic Meridional Overturning Circulation (AMOC), the system of ocean currents circulating water within the Atlantic Ocean. The currents bring warm water north and cold water south. The AMOC is part of the global conveyor belt that circulates global ocean currents. The conveyor belt was discovered by Columbia University geochemist the late Wallace Broecker, one of the founding mentors of the Comer Foundation climate research and fellowship programs. (NOAA: Science on a Sphere)
