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Regional and Global Climate for the Mid-Pliocene Using the University of Toronto Version of CCSM4 and Pliomip2 Boundary Conditions

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Regional and Global Climate for the Mid-Pliocene Using the University of Toronto Version of CCSM4 and Pliomip2 Boundary Conditions Clim. Past, 13, 919–942, 2017 https://doi.org/10.5194/cp-13-919-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Regional and global climate for the mid-Pliocene using the University of Toronto version of CCSM4 and PlioMIP2 boundary conditions Deepak Chandan and W. Richard Peltier Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario, M5S 1A7, Canada Correspondence to: Deepak Chandan ([email protected]) Received: 20 February 2017 – Discussion started: 24 February 2017 Revised: 6 June 2017 – Accepted: 19 June 2017 – Published: 19 July 2017 Abstract. The Pliocene Model Intercomparison Project 1 Introduction Phase 2 (PlioMIP2) is an international collaboration to simu- late the climate of the mid-Pliocene interglacial, correspond- The mid-Pliocene warm period, 3.3–3 million years ago, ing to marine isotope stage KM5c (3.205 Mya), using a wide was the most recent time period during which the global selection of climate models with the objective of understand- temperature was higher than present for an interval of time ing the nature of the warming that is known to have occurred longer than any of the Pleistocene interglacials. The preva- during the broader mid-Pliocene warm period. PlioMIP2 lence of widespread warming during this time has been in- builds on the successes of PlioMIP by shifting the focus to a ferred from proxy-based sea surface temperature (SST) re- specific interglacial and using a revised set of geographic and constructions from a number of widely distributed deep- orbital boundary conditions. In this paper, we present the de- sea sedimentary cores (Robinson et al., 2008; Lawrence tails of the mid-Pliocene simulations that we have performed et al., 2009; Dowsett et al., 2010; Fedorov et al., 2012). with a slightly modified version of the Community Climate The PRISM (Pliocene Research, Interpretation and Synop- System Model version 4 (CCSM4) and the “enhanced” vari- tic Mapping) version 3 (PRISM3; Dowsett et al., 2010) ant of the PlioMIP2 boundary conditions. We discuss the compilation of mid-Pliocene SSTs contains data from 100 simulated climatology through comparisons to our control sites. In contrast, surface air temperature (SAT) estimates simulations and to proxy reconstructions of the mid-Pliocene are more difficult to reconstruct owing to the limited avail- climate. With the new boundary conditions, the University ability of land-based proxies that are useful for palaeother- of Toronto version of the CCSM4 model simulates a mid- mometry. However, some records available from high lati- Pliocene that is more than twice as warm as that with the tudes in the Northern Hemisphere provide an important per- boundary conditions used for PlioMIP Phase 1. The warm- spective on the magnitude of warming in that region. Ry- ing is more enhanced near the high latitudes, which is where bczynski et al.(2013) found mid-Pliocene plant fossils in most of the changes to the PlioMIP2 boundary conditions peat deposits on Ellesmere Island (north of the Arctic Circle have been made. The elevated warming in the high latitudes in the Queen Elizabeth archipelago), which led them to esti- leads to a better match between the simulated climatology mate that the local mean annual SAT during the mid-Pliocene and proxy-based reconstructions than possible with the pre- was 18:3 ± 4:1 ◦C warmer than present, while summer tem- vious version of the boundary conditions. perature hovered around 14 ◦C. Brigham-Grette et al.(2013) studied lacustrine records from the arctic lake El’gygytgyn in NE arctic Russia on the basis of which they inferred that the summer temperature there was ∼ 8 ◦C warmer than today. The warming during the mid-Pliocene is expected to have resulted in a much higher eustatic sea level (ESL) owing to the melting of large amounts of polar ice over the extended Published by Copernicus Publications on behalf of the European Geosciences Union. 920 D. Chandan and W. R. Peltier: PlioMIP2 and CCSM4 period of time during which warmer conditions prevailed. quently, mid-Pliocene ESL estimates above 12 m imply sub- One of the earliest estimates for the mid-Pliocene ESL was stantial amounts of additional melting in the most vulnerable provided by Dowsett and Cronin(1990), who derived a lo- sectors of the EAIS. cal estimate based on measurements of present-day elevation It would not be particularly surprising if higher warming above sea level for mid-Pliocene marine deposits along the and the accompanying melting of polar ice took place in Orangeburg Scarp, near the border between South Carolina a world with much higher radiative forcing from the pres- and North Carolina. After correcting the observed elevation ence of atmospheric greenhouse gases in higher concentra- for the regional long-term tectonic uplift rate, they arrived at tions. However, proxy reconstructions for the mid-Pliocene a C35±18 m estimate of the mid-Pliocene ESL (the plus sign atmosphere do not show any significant increase in green- denotes a sea level higher than present). This value was used house gas concentrations compared to the present day. Var- to perform the first global reconstruction of the mid-Pliocene ious reconstructions for the atmospheric CO2 concentration, climate: PRISM1 (Dowsett et al., 1994). such as those based on alkenone data (Pagani et al., 2010; Based on an analysis of palaeoclimatic data from high- Seki et al., 2010; Badger et al., 2013; Zhang et al., 2013a), latitude deep-sea sediments, Kennett and Hodell(1993, δ13C measurements in marine organic matter (Raymo et al., 1995) argued for a mid-Pliocene sea level of no more than 1996), δ11B measurements (Seki et al., 2010; Bartoli et al., C25 m and “probably significantly less than this for most 2011; Martínez-Botí et al., 2015), foraminiferal B = Ca ra- of this interval”. Their analysis led to a downward revision tios (Tripati et al., 2009), and stomatal density in fossilized of the sea level estimate for PRISM2 (Dowsett et al., 1999) leaves (Kürschner et al., 1996), show that during the mid- to C25 m. Subsequent versions of the PRISM reconstruc- Pliocene CO2 varied between ∼ 200 and ∼ 450 ppmv with tion, namely PRISM3 (Dowsett et al., 2010) and PRISM4 most measurements reported in the range of 300–400 ppmv. (Dowsett et al., 2016), also include a C25 m mid-Pliocene A value of 400 ppmv is often used as a boundary con- ESL. A number of other estimates for the ESL all fall in dition in atmosphere-only and coupled climate models of the range of C10 to C30 m (Wardlaw and Quinn, 1991; the mid-Pliocene. Even if CO2 concentrations during the Krantz, 1991; Dwyer and Chandler, 2009; Naish and Wil- mid-Pliocene were only equal to those characteristic of the son, 2009; Rowley et al., 2013). Miller et al.(2012) per- present day, the significant melting of polar ice might still formed a multi-proxy analysis using backstripped records be reasonably expected due to the extended period of time from Virginia, New Zealand, and the Enewetak Atoll benthic during which this trace gas concentration was maintained. foraminiferal δ18O constraints and Mg=Ca−δ18O estimates Very little is known about the concentration of the very to argue that the mid-Pliocene sea level was 22±10 m higher potent greenhouse gas methane, although it is expected to be than present within a margin of 2 standard deviations. More at least as high as the modern concentration, if not higher, recently, Winnick and Caves(2015) argued for a reexami- mainly due to its release from the thawing Arctic permafrost nation of the transfer function used to convert mid-Pliocene and the breakdown of methane clathrates, both of which are benthic δ18O measurements into sea level in order to correct expected consequences of a warm Arctic. In the absence for the changing δ18O content of Antarctic ice. On the basis of proxy estimates for methane concentration in the mid- of the revised transfer function, they estimate that the mid- Pliocene atmosphere, the modern-day concentration value is Pliocene ESL was only 9–13.5 m higher than present. usually used (for example in the context of the PlioMIP pro- Although estimates for the mid-Pliocene ESL show con- gram) in climate models. The uncertainty in the methane con- siderable spread, all of them strongly suggest a substantial centration is partly compensated for in climate models by or total loss of the most vulnerable ice sheets: the Greenland choosing a higher concentration of CO2. Ice Sheet (GIS) and the West Antarctic Ice Sheet (WAIS). The inference of warm temperatures and reduced land ice The GIS contains ∼ 7 m of ESL equivalent ice (Alley et al., during a time period when the atmosphere was characterized 2005) and is known to have been greatly diminished un- by greenhouse gas concentrations not much different than der the influence of mid-Pliocene warmth (Lunt et al., 2008; the present, and significantly lower than projections for the Contoux et al., 2015). The WAIS, which presently contains future, is the primary reason that the mid-Pliocene has re- ∼ 5 m of ESL equivalent ice (Fretwell et al., 2012), is mostly ceived considerable interest in recent decades. Since the fu- grounded below sea level and is therefore dynamically unsta- ture of our climate will be “Pliocene-like”, it is of vital im- ble and vulnerable to a runaway collapse (Weertman, 1974). portance that we understand the magnitude and distribution The combined collapse of these two ice sheets could con- of the mid-Pliocene warming, the mechanisms for the warm- tribute up to 12 m to the mid-Pliocene ESL. The largest of the ing, and the consequences of the warming.
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