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Equilibrium climate sensitivity (ECS) quantifies the amount of warming resulting from a doubling of the atmospheric CO2 forcing. Despite recent advancements in climate simulation capabilities and global observations, there remains large uncertainty on the degree of future warming. To help alleviate this uncertainty, past climates provide a valuable insight into how the Earth will respond to elevated atmospheric CO2. However, there is evidence to suggest that ECS is dependent on background climate warmth, which may interfere with the direct utilization of paleo-ECS to understand present-day ECS. Thus, it is important that a range of different climate states are considered to better understand the factors modulating the relationship between CO2 and temperature. In this study, we focus on three time intervals: the mid-Pliocene Warm Period (3.3 – 3.0 Ma), the mid-Miocene (16.75 – 14.5 Ma), and the early Eocene (~50 Ma), in order to sample ECS from Cenozoic coolhouse to hothouse climates. Here, we combine the Bayesian framework of constraining the ECS and its uncertainty with several published methods to estimate the global mean surface temperature (GMST) from sparse proxy records. This framework utilizes an emergent constraint between the simulated GMST changes and climate sensitivities across the model ensemble. For each time interval, we employ a combination of parametric and non-parametric functions, coupled with a probabilistic approach to derive a refined estimate. Preliminary results for the Pliocene indicate a GMST reconstruction of approximately 19.3°C, which is higher than previous estimates that were derived using only marine records. Using this estimate, we calculate an ECS that is also higher than previously published values, especially due to the inclusion of high-latitude terrestrial temperature records into our estimates. Intriguingly, using the consistent methodology, our calculated ECS for the early Eocene is lower than that of the mid-Pliocene. This result does not support an amplified ECS in hothouse climate, and points to a potentially important role of ice albedo feedback in amplifying the ECS in coolhouse climate. Ongoing work will apply the same methodology to the mid-Miocene and further investigate the source for the estimated ECS state dependency between these climate intervals. 

This short summary presents selected results of an ongoing investigation into the feedbacks that contribute to amplified Arctic warming. The consequences of warming for Arctic biodiversity and landscape response to global warmth are currently being interpreted. Arctic North American records of large-scale landscape and paleoenvironmental change during the Pliocene are exquisitely preserved and locked in permafrost, providing an opportunity for paleoenvironmental and faunal reconstruction with unprecedented quality and resolution. During a period of mean global temperatures only ~2.5°C above modern, the Pliocene molecular, isotopic, tree-ring, paleofaunal, and paleofloral records indicate that the high Arctic mean annual temperature was 11°C–19°C above modern values, pointing to a much shallower latitudinal temperature gradient than exists today. It appears that the intense Neogene warming caused thawing and weathering to liberate sediment and create a continuous and thick (>2.5 km in places) clastic wedge from at least Banks Island to Meighen Island to form a coastal plain that provided a highway for camels and other mammals to migrate and evolve in the high Arctic. In this summary we highlight the opportunities that exist for research on these and related topics with the PoLAR-FIT community.RÉSUMÉCe bref résumé présente les résultats choisis d'une enquête en cours sur les déclencheurs qui contribuent à l’amplification du réchauffement de l'Arctique. Les conséquences du réchauffement sur la biodiversité arctique et de la réponse du paysage au réchauffement climatique sont en cours d’être interprété. Des dossiers nord-américains de paysage à grande échelle et le changement paléoenvironnementales durant le Pliocène sont exceptionnellement préservés et scellées dans un état de congélation qui fournissant une occasion pour la reconstruction paléoenvironnementale et faunistique avec une qualité et une résolution sans précédent. Pendent une période de réchauffement global seulement ~2,5°C au-dessus de moderne les dossiers, moléculaire, isotopique, annaux de croissance, paléofaunistique et paléovégétation indiquent que l'Arctique a connu une augmentation de la température annuelle moyenne de 11°C–19°C au-dessus de moderne, en montrant un inferieur gradient de température latitudinal qu'aujourd'hui. Il semble que le réchauffement intense pendent le Néogène a provoqué la décongélation et erosion pour libérer les sédiments et créer une plaine côtière continuel et épaisse (> 2,5 km dans lieux) qui a fourni une route pour les chameaux et autres mammifères pour migrer et évoluer dans l’Haut-Arctique. Dans ce résumé, nous soulignons les opportunités qui existent pour la recherche sur ces sujets et les sujets connexes avec la communauté PoLAR-FIT.