Published in Nature Communications, the work solves a paradox of Milanković’s theory and would allow future climate to be predicted more precisely
CIM researcher Gianluca Marino, from the Marine Research Centre of the University of Vigo, is part of the international team of scientists that discovered that a prolonged warm period in the North Atlantic caused ice melt in Greenland, raising sea levels by more than ten meters above the current level around 400,000 years ago. The study, published in the journal Nature Communications, resolves one of the mysteries of the ice age cycles of the past million years.
As early as the first half of the 20th century, Serbian mathematician Milutin Milanković proposed a theory to explain these cycles. He argued that they stem from long-term changes in the radiation received from the Sun in the summer. Stronger summer solar radiation melted the large ice sheets that formed in North America and Eurasia during an ice age, causing warm climates similar to the present, in which ice was only found in Greenland and Antarctica. But by 400,000 years ago, most of the Greenland ice had disappeared despite the fact that summer solar radiation was weak, which puzzled the scientific community, seemingly disproving Milanković’s theory.
In this regard, the CIM researcher highlighted that “understanding the causes of the warm interval that occurred 400,000 years ago, known as marine isotope stage 11, is key to understanding the cycles of the ice ages”. “During marine isotope stage 11, the Milanković theory, in its strictest formulation, fails to explain why the climate warmed so much and the Greenland ice sheet almost completely disappeared, when solar radiation was so weak”, he added.
International collaboration
The scientific team of which Gianluca Marino was a member included world-class academic institutions in Asia, Europe and Australia and was led by Dr. Hsun-Ming Hu, a postdoctoral researcher in Professor River Shen’s group at National Taiwan University, where they master an analytical technique that allows dating of paleoclimate records with unparalleled precision up to 600,000 years ago. They applied this technique to cave carbonates, called speleothems, from northwestern Italy, and found that the mid-latitudes warmed, albeit moderately, well before 400,000 years ago and carried heat towards Greenland for 15,000 years through a system of ocean currents known as the “Atlantic Southern Overturning Circulation”. This system of currents includes a flow of warm water that transfers heat to high latitudes in the North Atlantic and operates today keeping western Iberia and Galicia warmer than expected for their latitudes.
Gianluca Marino commented that “in this study we found that the transport of heat to the high latitudes of the North Atlantic by ocean currents maintained warm temperatures, slowly destabilizing the Greenland ice sheet until it almost disappeared. This mechanism had been overlooked until now.”
The scientist explained that “it all started when the lead author, Dr. Hsun-Ming Hu, then a predoctoral student in the Department of Earth Sciences at the National Taiwan University under the direction of Professor Chuan-Chou Shen, contacted me because he knew my work and wanted to use some statistical and interpretative methods that are used in the Palaeoclimatology Lab of the CIM in his project focused on speleothems from the Bàsura cave in Liguria, northern Italy”.
Specifically, Gianluca Marino contributed his knowledge on the role of the ocean in (paleo)climate change and working in collaboration with Hsun-Ming Hu to decipher the sequence of events that melted the Greenland ice sheet 400,000 years ago. The CIM researcher clarified that “I mainly contributed with a quantitative analysis of the data and shared the knowledge acquired on the ice age cycles during previous projects. The quantitative analysis of the data, together with cutting-edge analytical techniques, are key aspects of the work”.
A breakthrough in predicting future climate
“The climate approximately 400,000 years ago was quite similar to our pre-industrial climate. Therefore, the almost complete melting of the Greenland ice sheet that occurred then could definitely be repeated,” said Gianluca Marino. In this regard, the scientist indicated that “this interval is crucial for the climate community for that reason. However, there is one aspect that I consider even more relevant and that should interest the climate modeling community. If they manage to simulate the complete melting of Greenland, including all the conditions and processes that we highlighted in our study, then we can have greater confidence in how those same models predict future climate.”
Gianluca Marino wants to continue the work done. “The idea is to closely examine these outstanding episodes of climate change with the same level of detail, or even better, than that achieved in this study. This will allow us to decipher the processes that control the ice age cycles, as well as the role of solar radiation, ocean circulation and the carbon cycle.”
The scientist stressed that “we hope that the modeling community will begin to incorporate the mechanism we documented in our study into their climate models to investigate the causes of ice ages.”
The research activity of the CIM is supported by the Xunta de Galicia and the European Union through their co-financing under the Galicia ERDF Operational Programme 2014–2020.
