Leggi in italiano

The terminus of a glacier melting and falling into the sea. Credit: Bernhard Staehli/ iStockphoto/ Getty Images.

Understanding how rapidly ice responds to changes in climate is crucial to anticipate the effects of current global warming. A study1 led by Italian researchers has now reconstructed how sea ice cover evolved from one decade to the other in the sub-polar region between Baffin Bay and the Labrador Sea 36,000 to 44,000 years ago, during the last ice age.

“The last glaciation was interrupted by moments of climate warming that occurred over 30- to 40-year periods that led to temperature increases of up to 15°C,” explains Federico Scotto, a researcher at Italy’s National Research Council (CNR) Institute of Atmospheric and Climate Sciences (ISAC) and first author of the paper, “These were then followed by cooling phases, which could last 1,000 to 2,000 years”.

Twenty-five of these heating and cooling cycles, called Dansgaard-Oeschger (D-O) events, occurred during the last glaciation. The research team studied four of them in detail to evaluate the response of sea ice, by analysing the NEEM (North Greenland Eemian Ice Drilling) ice core extracted in Northwest Greenland in 2012. The researchers evaluated the presence of bromine and sodium in the ice core, comparing them to the levels found in sea water. They then used high resolution records of oxygen isotopes to determine temperature variations. The data obtained from the NEEM core were confirmed by looking for specific biomarkers in a sediment core from the Labrador Sea. These biomarkers are chemicals produced by microorganisms that live on sea ice, and they provide evidence of water exchanges between the ice cover and the sea.

“With our study, we are unable to establish whether the melting of sea ice is the cause of atmospheric warming or vice versa,” says Andrea Spolaor, a researcher at the CNR Institute of Polar Sciences (ISP) and corresponding author. “For the first time, however, we are providing a possible explanation of what happens between stadial cold phases and interstadial warmer phases. Due to warming, the sea ice would have receded northward before being replaced by seasonal ice, which requires open sea conditions to form.”

The study achieved a temporal resolution of about three to five years per sample, a large improvement over the previous studies that only achieved temporal resolutions of around 60 to 120 years. “Previous analyses had made it possible to identify sudden changes in temperature, but not with our degree of detail,” says Spolaor. The researchers concluded that sea ice responded very rapidly to temperature changes, shifting from a thick persistent ice core to open sea and seasonal ice in the course of a decade, and vice versa. In the current global climate, sea ice thickness can reach five metres in a few years, so it is likely that the ice sheet was thicker during the ice age than today.

The study, that also involves researchers from the Ca’ Foscari University in Venice, the University of Padua, and other international institutes, was published in the Proceedings of the National Academy of Sciences.