Cooling over parts of the north Atlantic during the last Ice Age was up to 3˚C more severe than estimated, new research has found.
The finding has been made by an international research team led by University of Galway scientists, which says it has discovered a new method to accurately measure past polar sea surface temperature changes and climate change.
The new method involves analysing shells of foraminifera, which are micro-organisms as tiny as a grain of sand.
Living foraminifera marine organism as it floats in the water. Photo: Dr Audrey Morley
Foraminifera are small unicellular organisms which build a miniscule shell out of calcium carbonate and other elements available in seawater, the scientists explain.
“In doing so, they record the chemistry and climate of seawater in their shell. At the end of their life, the empty shells sink to the seafloor and are deposited in sediment, like a marine archive year after year, millennia after millennia,”they say.
L-R: Dr Audrey Morley (Chief Scientist and lead author, University of Galway), Avery Fenton (Research Assistant, University of Galway), Dr Alison Jacobel (Middlebury College, USA) and Adele Westgard (PhD student Tromso University). Photo: Alan Burns
In the research, just published in scientific journal Nature Communications, University of Galway geography lecturer Dr Audrey Morley and colleagues involved in the project describe the research method as invaluable, as it can be applied to new and previously published datasets worldwide.
This will allow for a re-evaluation of the magnitude and geographical extent of marine polar climate change, they suggest.
Dr Morley, lead author on the research paper and Ryan Institute and iCRAG scientist, says that the new method “will allow us to evaluate the ability of climate models to simulate polar amplified warming and cooling”.
She says this is “especially important, as climate model simulations targeting warmer than present climates have historically not captured the full extent of polar amplified warming”.
“This information will enable a major leap forward in our ability to assess the sensitivity of Arctic climate and its role and variability within the global climate system,”she says.
“This will lay the foundation for an improved understanding of climate change.”
Analysis of the magnesium and calcium (Mg/Ca) preserved in the foraminifera shells allows scientists to calculate an indirect measure or ‘proxy’ of sea surface temperatures.
These “climate proxies” allow scientists to reveal earth climate history from a few hundred years to billions of years ago, improving understanding of future climate change, the research team says.
“However, in cold polar waters this method doesn’t work because it is compromised by the carbonate chemistry of seawater, leaving us without a tool to measure past marine polar climates,”they say.
The new research method solves a long-standing problem in Arctic climate science, they say.
The team set out on several oceanographic cruises, including the Marine Institute’s RV Celtic Explorer in 2020, to collect living polar foraminifera together with the seawater that they lived in.
This allowed the researchers to identify exactly how the carbonate chemistry of seawater impacts the temperature signal recorded in the magnesium and calcium Mg/Ca values of the tiny organism.
“The research showed that for polar foraminifera, the oxygen isotopes preserved in the shells can be used as a proxy for the carbonate chemistry of seawater,”the scientists say.
“When measured together on fossil foraminifera, Mg/Ca and oxygen isotopes can be used to reveal past polar sea surface temperatures globally,”they say.
Dr Morley explains that “for example, when applied to the last ice age, this method shows that current estimates of cooling over North Atlantic mid-latitudes have been underestimated by up to 3˚C”.
“Direct observations of sea surface temperatures in the Arctic are short and at best 150 years long,”she says.
“These short records leave us with a gap in our understanding and large uncertainties when predicting how future climate change will respond to rising greenhouse gas emissions,”she says.
“To improve our understanding and reduce uncertainties we look to the past using climate proxies – such as the foraminifera. Yet, most proxies of essential climate variables, such as sea surface temperatures, suffer from limitations when applied to cold temperatures that characterise Arctic environments,”she says.
“These limitations prevent us from constraining uncertainties for some of the most sensitive climate tipping points that can trigger rapid and dramatic global climate change,”she says.
The research was funded by MSCA-IF Project ARCTICO funded by the European Research Council, the Marine Institute of Ireland Research Programme 2014-2020, Science Foundation Ireland Frontiers for the Future Project, and grant in aid funding from the Marine Institute for research expedition CE20009 on the RV Celtic Explorer.
Read the full study in Nature Communications here
