The role of elevated atmospheric CO<sub>2</sub> and increased fire in Arctic amplification of temperature during the Early to mid-Pliocene

Abstract

Abstract. The mid-Pliocene is a valuable time interval for understanding the mechanisms that determine equilibrium climate at current CO2 concentrations. One intriguing, but not fully understood, feature of the early to mid-Pliocene climate is the amplified arctic temperature response. Current models underestimate the degree of warming in the Pliocene Arctic and validation of proposed feedbacks is limited by scarce terrestrial records of climate and environment, as well as discrepancies 20 in current CO2 proxy reconstructions. Here we reconstruct the CO2 and summer temperature from a re-dated 3.9 +1.5/-0.5 Ma sub-fossil fen-peat deposit on west-central Ellesmere Island, Canada, and investigate fire as a potential feedback to Arctic amplification of warming during the mid-Pliocene.Average CO2 was determined using isotope ratios of mosses to be 440 ± 50 ppm. The estimate for average mean summer temperature is 15.4±0.8°C using specific bacterial membrane lipids, i.e. branched glycerol dialkyl glycerol tetraethers. Macro-25 charcoal was present in all samples from this Pliocene section with notably higher charcoal concentration in the upper part of the sequence. This change in charcoal was synchronous with a change in vegetation that saw fire promoting taxa increase in abundance. Paleovegetation reconstructions are consistent with warm summer temperatures, relatively low summer precipitation and an incidence of fire comparable to fire adapted boreal forests of North America, or potentially central Siberia.To our knowledge, this study represents the furthest northern evidence of fire during the Pliocene and highlights the important 30 role of forest fire in the ecology and climatic processes of the Pliocene High Arctic. The results provide evidence that terrestrial fossil localities in the Pliocene High Arctic were probably formed during warm intervals that coincided with relatively high CO2 concentrations that supported productive biotic communities. This study indicates that interactions between concentrations were only ~30 ppm higher than modern.

An energy balance model for paleoclimate transitions

“…The Pliocene Research, Interpretation and Synoptic Mapping (PRISM) project of the US Geological Survey has contributed to this goal (Dowsett et al, 2011(Dowsett et al, , 2013(Dowsett et al, , 2016, as has the Pliocene Model Intercomparison Project (PlioMIP; Haywood et al, 2011Haywood et al, , 2016Zhang et al, 2013). Advances in the extraction and interpretation of proxy data have given a clearer picture of the warm Pliocene climate (Haywood et al, 2009;Salzmann et al, 2009;Ballantyne et al, 2010;Steph et al, 2010;Seki et al, 2010;Bartoli et al, 2011;De Schepper et al, 2013;Knies et al, 2014;O'Brien et al, 2014;Brierley et al, 2015;Fletcher et al, 2018). At the same time, computer models have achieved closer agreement with proxy data, see for example Haywood et al Peltier (2017, 2018), and other references therein.…”

“…Equilibrium climate sensitivity (ECS) is a useful measure of the sensitivity of a given climate model to an increase in CO 2 concentration µ. It is usually defined as the change T in the global mean temperature T , resulting from a doubling of µ, starting from the accepted pre-industrial value µ = 270 ppm (IPCC, 2013;Forster, 2016;Knutti et al, 2017;Proistosescu and Huybers, 2017). The ECS provides a first-order estimate of the amount of present-day global warming predicted by a given model as µ increases.…”

“…In recent years, significant progress has been achieved in understanding Pliocene climate, e.g. by Haywood et al (2009); Salzmann et al (2009); Ballantyne et al (2010); Steph et al (2010); von der Heydt and Dijkstra (2011); Zhang and Yan (2012); Zhang et al (2013); Sun et al (2013); De Schepper et al 2013; Tan et al (2017); Peltier (2017, 2018) and Fletcher et al (2018); see also references therein. This paper proposes that bistability and bifurcation may have played a fundamental role in determining the Pliocene climate.…”

Linking Warm Arctic Winters, Rossby Waves, and Cold Spells: An Idealized Numerical Study

“…Observations show that climate warming is not homogeneous over the globe, and Arctic Amplification (AA) refers to the accelerated rate of warming of the Arctic near the surface compared to tropics and mid-latitudes (Serreze et al 2009). AA shows a seasonality, with a maximum during late fall and winter (Lu and Cai 2009), it can be found in historical observations, in projection of future climate (Pithan and Mauritsen 2014) and even in paleoclimatic studies (Serreze and Barry 2011), with a consistent relationship with the concentration of greenhouse gases (Fletcher et al 2018).…”