The unexpected role of certain glaciers in global warming processes

Knowing precisely the speed of the melting process of the great masses of polar ice is one of the great scientific challenges in the face of current climate change. The study of the deglaciations of the past —despite not being analogous to the current situation— provides a useful experimental scenario to be able to analyze the response speed of these ice masses.

To study the melting processes on the planet, until now only solid chronologies were available during the last deglaciation, a climatic period that extended over about 9,000 years.

A new study, partially carried out at the Scientific and Technological Centers of the University of Barcelona (CCiTUB), now presents the first record of the melting of the penultimate deglaciation with a robust and contrasted chronology, and reveals that this melting was concentrated along some 5,000 years —from 135,000 to 130,000 years before the present— and introduces significant changes in the chronologies hitherto accepted.

The study was based on a research project co-directed by Isabel Cacho, professor at the Department of Earth and Ocean Dynamics at the Faculty of Earth Sciences at the University of Barcelona (UB) and member of the Consolidated Research Group in Marine Geosciences of the UB, and Heather M. Stoll, professor at the Federal Polytechnic School of Zurich in Switzerland.

The penultimate deglaciation is a period of difficult dating through marine records, always based on indirect techniques that are very imprecise to analyze changes in the climate system on a time scale of decades, centuries or even millennia. The new work is based on the analysis of stalagmites in the caves of the Cantabrian slope of the Iberian Peninsula, climatic files that reveal changes in salinity in the North Atlantic derived from the melting of large polar ice caps and, in addition, provide information on the evolution of atmospheric temperatures of the region in the past.

“Until now, this penultimate deglaciation was only well dated in cave records from tropical areas (Asia and South America) but in no case were they able to capture the signal of melting over the North Atlantic,” explains Isabel Cacho, ICREA Academy of Sciences researcher. the UB.

The use of stalagmites as climate sensors first allows establishing high-precision chronologies that are scientifically sound. But, in addition, the chemistry of the carbonate that forms the stalagmites collects climatic variables that are decisive for reconstructing the climate. In the case of the caves in this study, the rain that recharges in the North Atlantic transfers the melting signal to the carbonate, while the biological activity of the terrain fixes the air temperature signal to the chemistry of the waters that percolate in Cave.

The integration of these three elements —solid chronologies, melting and temperature indicators— provide the published records with a unique character of extraordinary value in order to understand the atmosphere-ocean interaction processes during the phases of planetary global warming. These results have made it possible to reformulate hypotheses accepted until now and outline a new chronological framework that has been transferred to existing marine records, providing a new perspective on the speed of the processes that acted during the penultimate deglaciation.

“Our work establishes a point of anchorage to the chronology of the beginning and end of the melting, confirming the long-accepted hypothesis that the changes in insolation controlled by the Earth’s orbital movements are the triggers of this great climatic change”, details Isabel Cacho. “But it allows us to establish -continues the researcher- for the first time a robust chronology in the oceanic and atmospheric feedback processes triggered by this change in initial insolation, a change that by itself was very modest in terms of the Earth’s energy balance”.

“Thus, the intensity of the warming of the last deglaciation was controlled not so much by the triggering insolation change, but rather by climate feedback processes between the ocean, the atmosphere, and the cryosphere or ice mass,” he adds.

The fragility of marine-based glaciers

Marine-based glaciers were instrumental in accelerating the warming of the penultimate deglaciation. “Sea currents contribute to the melting of the base of these glaciers, and as these structures become more fluid and brittle, the speed of glacier progression accelerates, and the ice is discharged directly into the sea at a rate that does not allow regeneration of the glacier”, explains Professor Judit Torner, member of the Consolidated Research Group in Marine Geosciences of the UB and co-author of the work.

However, the direct discharge of ice into the ocean has a direct impact on marine currents and caused an abrupt halt in the marine circulation in the North Atlantic. “In the past, this has happened repeatedly, but our work indicates that this process was particularly intense, fast and prolonged during the penultimate deglaciation,” adds Torner.

This change in circulation was decisive in climatic evolution since it directly affected the oceanic carbon cycle, with an increase in atmospheric carbon dioxide (CO2) levels, and therefore, the greenhouse effect of the atmosphere. This caused a huge amplification of the warming process during this penultimate deglaciation, the researchers say.

Currently, many of the glaciers in Greenland and Antarctica have a marine base that shows signs of melting and destabilization. (Photo: Rafel Simó / ICM / CSIC)

Glaciers of the past, lessons of the present

Currently, many of the glaciers in Greenland and Antarctica have a marine base that shows signs of melting and destabilization. Another reason for concern is that the oceanic and atmospheric processes that reacted to the melting described in the new study are not different from those described in other deglaciations, «but the penultimate deglaciation -indicates Isabel Cacho- has a singular character because it gave way to a period interglacial warmer than today (on the order of 0.5-1.5 degrees Celsius higher than pre-industrial temperatures)’. These conditions lasted for centuries and caused a higher melting in Greenland and Antarctica, raising the sea level some 5 or 6 meters above the current level. “This suggests that not only the feedback processes themselves, but also the speed with which they react, are capable of shaping the intensity of climate change,” adds Isabel Cacho.

“This fact is very worrying, since today we are experiencing the fastest climate change in the history of the planet. The observations we make about the climate of the past confirm the climate projections available, which urges us to establish measures to contain global warming below 1.5 degrees Celsius and thus halt a series of changes that will imply a high cost for us. and for the ecosystems that sustain us. But this containment in the face of climate change requires immediate action at all levels”, the researchers conclude.

The study is titled “Rapid northern hemisphere ice sheet melting during the penultimate deglaciation.” And it has been published in the academic journal Nature Communications. (Source: UB)