Stalagmites reveal clues to changing ocean currents, changing climate

AMOC and Climate
A depiction of the how ocean circulation normally moves warmer water north, where it gains salinity, sinks and moves colder water south, a process that moderates temperatures in northern regions. (Image courtesy of Laura Endres).

By Ilana Wolchinsky
Medill Reports

Climatologist Laura Endres began her Ph.D. a year ago at ETH Zürich in the paleoclimate field, drawing on her background in earth and climate sciences. Endres uses climate records encased in cave stalagmites to reveal past climate patterns — paleoclimate — that give clues to how fast climate change is pacing now.

But her passion for environmentalism was not always at the forefront of her career.

During the 2022 Comer Climate Conference, Endres shared how investigating past changes in the Atlantic Meridional Overturning Circulation (AMOC) can help to inform researchers about future polar ice sheet meltings and research to better predict it.

The AMOC is a large system of ocean currents that carry warm water from the tropics northward into the North Atlantic and moderate temperatures in northern latitudes.

“I was a sound engineer before transitioning to this research,” Endres said. “I was very touched by the environmental topics and wanted to use my strengths towards these issues.”

Endres’ research uses the stalagmites to better understand how changes to the AMOC has effects on climate, temperature and precipitation, based on the type of oxygen found in the calcium carbonate in the stalagmites.

She said that as we take off our paleoclimate change glasses and think about how these adaptations have worked in the past, the questions that arise are extremely important to our current situation.

Richard Alley, a geologist at Pennsylvania State University, said the history of climate results from carbon dioxide (CO2) levels. As CO2 levels rise in the atmosphere due to fossil fuel emissions, that has the biggest impact on the ocean and atmosphere. Higher levels of CO2 act as a thermostat to create a warmer environment and affects environmental conditions. The oceans have acted as a sink for CO2 and preserved the atmosphere from higher temperatures as we come nearer to the 1.5-degree Celsius rise, considered a critical limit under the international Paris Agreement.

Endres’ team found a weakening of the AMOC due to the melting of Scandinavian ice sheets. This leads to warmer subsurface temperatures in the north and unleashes a chain of events that result in melting ice sheets, releasing of icebergs and hence more fresh water into the ocean and raising global sea levels.

Ocean freshening is an event that occurs when there is a decrease in salinity levels due to an influx of freshwater. Since beginning her research, there have been two clear freshening events in the North Atlantic realm.

While freshwater leads to less saline and isotopically depleted ocean (delta-o-18, or d18O), there is more negative d18O in NISA proxy for freshening. This is oxygen-18, an isotope of oxygen with 10 neutrons instead of eight neutrons, the normal weight of oxygen (O16).

d18O is a measure of the ratio of stable isotopes. Higher values represent cold climates, and lower values indicate warm climates.

The rates of change can be successfully investigated with the NISA ship vessel stalagmites. It is important to recognize that not all freshening is accompanied by cooling events. Endres and her team are working to understand why this is and to distinguish the differences. But cooling from ice melt could have a significant temporary impact for northern countries even as the planet warms globally.

“Combining my use of paleoclimate data with modeling was a very cool project for me,” Endres said. The goal for future projections relies on the importance of a deeper understanding; there is already a strong signal of the process located at the northern border of Spain, where the stalagmites are located.

One of the most rewarding parts of Endres’ research is to be able to look at a specimen and be impressed by what they are. She said to be able to tell something of the past only based on a specimen is a humbling experience.

“The potential to talk to people not in the field, because it is a very big picture, and be able to encourage them to think,” Endres said. “It’s all about the larger questions rather than a narrow-minded approach.”

Ilana Wolchinsky is a Health, Environment and Science graduate student at Medill. You can follow her on Twitter at @IlanaWolchinsky.