Present day climate is characterized by the presence of two ice sheets, one in each hemisphere, which is rare in the climate history of the Earth. This feature is strongly associated with the fact that during quaternary, the amount of GHG, especially atmospheric CO2 content in the atmosphere, was low compared to cenozoic's one. At the scale of centuries, the warming of high latitudes will be pivotal for humankind, but are we really able to diagnose such climate changes?
The warm climates of Cenozoic and Mesozoic offer the unique opportunity to investigate the climate behaviour in a rich GHG world. For many decades, a large number of scientists, from climate modeling groups to data reconstruction communities, have addressed several issues concerning the comparison of temperature simulations and proxies reconstructions for many warm periods at mid and high latitudes, but also between the surface and the bottom of paleo ocean. These efforts pointed out a large mismatch for mid and high latitude surface temperatures. Models largely under estimating temperature reconstructions derived from many terrestrial and oceanic proxies. A consequence was that the thermal gradient form equator to pole, which was very low by the reconstructions, has remained over estimated by the models, with a weak polar amplification. These issues have been exacerbated by Model Intercomparison Projects (MIP), which clearly pointed out that these mismatches were shared by most models (Pliomip for mid Pliocene and Deepmip for Eocene), and therefore, this mismatch can be considered as a robust feature.
This long-lasting paradox is associated with our ability to simulate earth's climate with a very low equator to pole temperature gradient compared to quaternary glacial inter-glacial cycles has been arising since the 1970s'. Therefore, it could be interesting to revisit this issue together with modelers of past warm climates and data people and discuss the plausible causes of this mismatch: lack of processes (cloud physics, GHG atmospheric content, aerosols...), but also uncertainties on data reconstructions. Moreover, this issue has important consequences for our ability to correctly understand and simulate the future climates, especially at high latitudes, and the interactions with ice sheets at scales of decades to centuries.
The ongoing climate change is already prominent. Its evolution during this century is the major topic tackled by the IPCC, even though some projections are investigated up to 2300. The different scenarios used by the IPCC, from SSP1-1.9 to SSP5-8.5 depict pCO2 emissions that may lead to a drastic increase, reaching 125 Gt/year at the end of the century. Such high values correspond to warm climates of the Cenozoic. It is therefore pivotal to know how the climate models currently used for projections are able to describe this warm and Pco2 rich climates.
In a first step, thanks to mips (model intercomparison projects) similar to those used for projections (Cmip), we will depict the robust features and weaknesses of modelling mid-Pliocene, mid-Miocene, and Eocene climates when comparing model results and data reconstructions. For all these climates, there is a large mismatch between model simulation and reconstruction from different proxies at mid and high latitudes. Most of the models largely underestimate the temperature increase for these latitudes. This long-lasting paradox is also puzzling for future long-term projections.
In a second step, we will investigate the uncertainties, limitations, but also the important advantages due to the possibility of comparing model results to real data.
Last but not least, in a final step, we will investigate the possible explanations to partly solve this paradox, pointing out how these studies may help to better constrain long-term future climates.
16h15-17h: Part I. How current MIPs deal with comparison with temperature reconstructions: a review from Cenozoic warm climates Mid/late Pliocene, Mid Miocene Climate Optimum and deeper warm climates
· 16h45-17h: Alan Haywood: Pliocene climate and the high latitudes: a data/model perspective
· 16h30-16h45: Natalie Burls: Simulating Miocene warmth: insights from an opportunistic MultiModel ensemble (MioMIP1) and efforts towards a coordinated intercomparison (MioMIP2)
· 16h15-16h30: Dan Lunt: DeepMIP-Eocene: A window to a super-warm world, 50 million years ago, through an model-model-proxy-proxy intercomparison approach
17h-17h30: Part II. Forcing factors and validation
· 17h-17h10: James Rae: Cenozoic CO2: from the deep ocean to the atmosphere
· 17h10-17h20: Erin McClymont: Pliocene climate variability on glacial-interglacial timescales (PlioVAR): lessons learned from multi-proxy reconstructions of seasurface temperatures and data-model comparisons
· 17h20-17h30: Aisling Dolan: Efforts towards reconstructing ice sheets during the Pliocene
17h30-18h???Part III: Some hints to better understand this long-standing paradox
· 17h30-17h40: Ran Feng: Revisiting the low-gradient problem with weather-resolving atmosphere-ocean coupled simulations
· 17h40-17h50: Gerrit Lohmann: Effects of CO2 and Ocean Mixing on Miocene and Pliocene Temperature Gradients
· 17h50-18h: Peter Hopcroft: Potential role of methane and other non-CO2 trace gases in past warm climates