# Foraminiferal isotopic evidence of abrupt mid-20th century onset of hydrographic instability in Nordic Seas inflow waters

## TL;DR

This study shows that Atlantic water inflow into the Nordic Seas became unstable around 1950, affecting Arctic climate and delaying the impact of global warming.

## Contribution

The paper presents a high-resolution isotopic record revealing a mid-20th century onset of hydrographic instability in Nordic Seas inflow waters.

## Key findings

- A previously unrecognized increase in temperature instability occurred around AD 1950 in Nordic Seas inflow waters.
- This instability impacted Greenland Ice Sheet melting, Arctic sea-ice extent, and Arctic Surface Air Temperature.
- The change in hydrographic conditions around AD 1950 was imprinted on deep waters overflowing the Nordic Seas Basin.

## Abstract

The flow of warm Atlantic waters into the Nordic Seas largely determines the transport of ocean heat to the Arctic and is a prominent feature of the Atlantic Meridional Overturning Circulation (AMOC). Here we provide a ~ 250-year-long (1750 to 1992 AD), annually- to sub-annually- resolved record of Nordic Seas inflow water characteristics inferred from changes in δ18O of planktic foraminiferal carbonate. The new record is reflective of upper ocean temperatures across the Atlantic Water Zone of the Nordic Seas and reveals a previously unrecognized increase in temperature instability ~ AD 1950 that appears to have impacted rates of Greenland Ice Sheet melting, Arctic sea-ice extent and Arctic Surface Air Temperature, delaying the regional response to Anthropogenic Global Warming by several decades. While the relationship between the sudden change in hydrographic conditions and AMOC strength and stability is not yet clear, the change in inflow characteristics ~ AD 1950 was clearly imprinted on deep waters overflowing the Nordic Seas Basin.

The online version contains supplementary material available at 10.1038/s41598-025-32210-8.