The Holocene temperature conundrum

Abstract

A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198-1201. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.global temperature | Holocene temperature | model-data inconsistency I n the latest reconstruction of the global surface temperature throughout the Holocene (1) (hereafter M13), the most striking feature is a pronounced cooling trend of ∼0.5°C following the Holocene Thermal Maximum (HTM) (∼10-6 ka) toward the late Holocene, with the Neoglacial cooling culminating in the Little Ice Age (LIA; ∼1,800 common era) (Fig. 1, blue). Numerous previous reconstructions have shown cooling trends in the Holocene, but most of these studies attribute the cooling trend to regional and/or seasonal climate changes (2-6). The distinct feature of the M13 reconstruction is that it arguably infers the cooling trend in the global mean and annual mean temperature. This inferred global annual cooling in the Holocene is puzzling: With no direct net contribution from the orbital insolation, the global annual mean radiative forcing in the Holocene should be dominated by the retreating ice sheets and rising atmospheric greenhouse gases (GHGs), with both favoring a globally averaged warming. Therefore, how can the global annual temperature exhibit a cooling trend in response to global warming forcing? This inconsistency between the reconstructed cooling and the inferred warming forced by GHGs and ice sheet poses the so-called Holocene temperature conundrum and will be the subject of this study. Here, we study the global annual temperature trend in the Holocene and its physical mechanism by comparing the temperature reconstruction with three different transient climate model simulations. Our analysis shows a robust warming trend in current climate models, opposite from the cooling in the M13 reconstruction. This model-data discrepancy suggests potentially significant biases in both the reconstructions and current climate models, and calls for a major reexamination of global climate evolution in the Holocene. Model ExperimentsWe analyzed transient climate simulations in three coupled ocean-atmosphere models [Community Climate System Model 3 (CCSM3) (7), Fast Met Office/UK Universities Simulator (FAMOUS) (8), and Loch-Vecode-Ecbilt-Clio-Agism Model (LOVECLIM) (9); Methods] that are subject to realistic climate forcings of orbitally driven insolation variations, GHGs, continental ice sheets, and the associated meltwater fluxes. The three models all simulate a robust annual mean warming (∼0.5°C) throughout ...

Intra-interglacial climate variability: model simulations of Marine Isotope Stages 1, 5, 11, 13, and 15

“…So far, the reason for these discrepancies is unsolved (cf. Liu et al, 2014), but Hessler et al (2014) pointed out that uncertainties associated with sea surface temperature reconstructions are generally larger than interglacial temperature anomalies. Thus, currently available surface temperature proxy data cannot serve as a target for benchmarking interglacial model simulations.…”

“…With increasing computer power, long-term transient simulations of interglacial climates will become more common. So far, transient CGCM simulations have been performed for the present (see, e.g., Lorenz and Lohmann, 2004;Varma et al, 2012;Liu et al, 2014) and the last interglacial (see, e.g., Bakker et al, 2013;Govin et al, 2014). More transient simulations of earlier interglacials, ideally with coupled interactive ice-sheet models, will help to develop a significantly deeper understanding of interglacial climate dynamics.…”

The PMIP4 contribution to CMIP6 – Part 2: Two interglacials, scientific objective and experimental design for Holocene and Last Interglacial simulations

“…The reliability of global temperature estimates depends on the representativeness of the data included; this point has been made abundantly clear from comparisons of records of hemispheric temperature changes during the last millennium (Fernández-Donado et al, 2013;Moberg, 2013). Currently available reconstructions of global temperature changes during the Holocene are heavily biased towards marine records, making it imperative that the reliability of these records is assessed using continental reconstructions (Davis et al, 2015;Liu et al, 2014). The lack of geographically explicit reconstructions for tropical regions and the SH would limit analysis of the Holocene transient simulations, but efforts are underway to improve this situation.…”

Warmer and wetter or warmer and dryer? Observed versus simulated covariability of Holocene temperature and rainfall in Asia

“…Secondly, most proxy reconstructions are subject to considerable uncertainty concerning their recording season [24] and their recorded climate variable [17,41]. Uncertainties with regard to the recording season in proxy reconstructions [24] are not a viable explanation for the model-data discrepancies, as the model correlation fields remain largely unchanged if annual or boreal winter season mean temperatures and precipitation are considered (Supplementary Figs.…”