Understanding the scale and mechanisms behind extreme high latitude warmth during Greenhouse climates of the Cenozoic

The Earth is in a period of global warming, manifested in a net loss of land-based ice masses and an overall trend of reduced Arctic sea-ice extent. Predicting the future warming trend as well as its consequences for the climate system is a major challeng e in climate research. High latitude climate and the cryosphere are particularly sensitive to change, and might act as wild cards in the climate system with amplified trends and possible tipping points triggering irreversible changes. Understanding the dy namics and interactions of the ocean, atmosphere, sea ice, ice sheets and glaciers at high latitudes is therefore crucial. The last known time in Earths history with levels of atmospheric greenhouse gas levels at the present high level was during the Pli ocene (~3 million years ago), and one has to look even further back in time to find a period with greenhouse gas levels as high as what is expected in the future. In this collaborative project between the Bjerknes Centre and LSCE we will endeavour to bett er understand these past warm greenhouse climates with a focus on the high latitudes. In particular, we will test the ability of climate models to reproduce the climate during these periods with high levels of greenhouse gases, with a focus on the mechani sms of poleward heat and moisture transports by the atmosphere and ocean. A key question is how the observed extreme high latitude warmth during these past periods were maintained. This question is exacerbated by the fact that state-of-the art climate mod els underestimate the recent observed polar amplification and changes in Arctic sea ice.