Marine science reveals climate change lessons recorded by tiny polar organisms

New research shows that cooling during the last Ice Age over parts of the North Atlantic was up to 3˚C more severe than estimated

An international research team led by University of Galway has discovered a new method to accurately measure past polar sea surface temperature changes and climate change.

In a new study published in Nature Communications, Dr Audrey Morley, lecturer in Geography and Ryan Institute and iCRAG scientist at University of Galway, reveals how polar climate history can be detected by analysing the shells of foraminifera - microorganisms no bigger than a grain of sand.

The scientists involved in the project describe the research method as invaluable, as it can be applied to new and previously published datasets worldwide to re-evaluate the magnitude and geographical extent of marine polar climate change.

Dr Morley, lead author on the research paper, said: "In the future our new method will allow us to evaluate the ability of climate models to simulate polar amplified warming and cooling, which 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. This will lay the foundation for an improved understanding of climate change."

Foraminifera are small unicellular organisms which build a miniscule shell out of calcium carbonate and other elements available in seawater. 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.

Through analysis of the magnesium and calcium (Mg/Ca) preserved in the shells, scientists can get 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 and thereby improve an understanding of future climate change. 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.

The new research method solves a long-standing problem in Arctic Climate Science.

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 and when measured together on fossil foraminifera, Mg/Ca and oxygen isotopes can be used to reveal past polar sea surface temperatures globally.

Dr Morley said: "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. 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.

"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.

"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. For example, the enhanced warming or cooling at high latitudes - also called Arctic/Polar Amplification; the disruption of heat transport by surface and deep ocean; sea ice loss; and permafrost melting, that are intrinsic to the polar regions."

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: https://www.nature.com/articles/s41467-024-53424-w

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