Perennial vs. seasonal sea ice in the Northern Hemisphere induced by CO2 changes and tectonic activity during the Miocene

The Cenozoic history of sea ice in the Northern Hemisphere remains poorly constrained. However, several studies indicate that perennial sea ice initiated at least 18‒13 Myrs ago which emphasizes the importance of understanding climate forcings and feedbacks through the Miocene (23‒5 Myrs). In more detail, changes in CO2 might have varied between preindustrial levels (278 parts per million, ppm) and 500 ppm. Furthermore, the control of northward water mass transport by active changes of the Greenland-Scotland Ridge (GSR) sill depth might has affected the actual state of sea ice (perennial vs. seasonal) in the Arctic. Here, we investigate the sensitivity of sea ice to GSR depth and CO2 changes by means of a fully coupled ocean-atmosphere General Circulation Model (GCM) with integrated terrestrial vegetation dynamics (community of earth system models, COSMOS). The model setup comprise a global reconstruction of the mid-Miocene 20‒15 Myrs ago (continental geography, orography, bathymetry, ice-sheet geography and topography) and a change of CO2 levels in the atmosphere. Especially, we additionally integrated a high resolution bathymetric dataset for the northern North Atlantic, GSR, Nordic Seas and the Eurasian Basin. Independently, within the range of CO2 reconstructions and different ocean depth settings of the Greenland-Scotland Seaway the model is capable to reproduce perennial as well as seasonal Arctic ice cover. Based on our results, we conclude that sea ice within the Miocene climate was variable and highly sensitive to different forcings mechanisms