The Holocene temperature conundrum PNAS PLUS Zhengyu Liua,b,1, Jiang Zhua, Yair Rosenthalc, Xu Zhangd, Bette L. Otto-Bliesnere, Axel Timmermannf, Robin S. Smithg, Gerrit Lohmannd, Weipeng Zhengh, and Oliver Elison Timmi aNelson Center for Climatic Research and Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, WI 53706; bLaboratory of Climate, Ocean, and Atmosphere Studies, Peking University, Beijing 100871, China; cInstitute of Marine and Coastal Sciences and Department of Earth and Planetary Sciences, Rutgers University, New Brunswick, NJ 08901-8521; dAlfred Wegener Institute for Polar and Marine Research, D-27570 Bremerhaven, Germany; eClimate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80307-3000; fInternational Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii, HI 96822; gNational Centre for Atmospheric Science-Climate, Department of Meteorology, University of Reading, Reading RG6 6BB, United Kingdom; hState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; and iDepartment of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, NY 12222 Edited by Mark A. Cane, Lamont Doherty Earth Observatory of Columbia University, Palisades, NY, and approved July 15, 2014 (received for review April 21, 2014) A recent temperature reconstruction of global annual tempera- nental ice sheets, and the associated meltwater fluxes. The ture shows Early Holocene warmth followed by a cooling trend three models all simulate a robust annual mean warming (∼0.5 °C) through the Middle to Late Holocene [Marcott SA, et al., 2013, throughout the Holocene (Fig. 1, black and yellow), leaving a Science 339(6124):1198–1201]. This global cooling is puzzling be- model-data inconsistency in global annual temperature of ∼1°C. cause it is opposite from the expected and simulated global warm- This inconsistency affects model-data comparison of the tem- ing trend due to the retreating ice sheets and rising atmospheric perature trend mainly for the Holocene, rather than the last greenhouse gases. Our critical reexamination of this contradiction millennium (after ∼1 ka) or the early deglaciation (before ∼10 ka). between the reconstructed cooling and the simulated warming In the last millennium, climate models indeed generate a global points to potentially significant biases in both the seasonality of cooling toward the LIA, after the imposition of realistic vol- the proxy reconstruction and the climate sensitivity of current canic aerosols and solar variability (10) (Fig. 1, Inset, gray line), climate models. consistent with the M13 reconstruction. During the early de- glaciation and mainly in response to the large increase of atmo- SCIENCES global temperature | Holocene temperature | model-data inconsistency spheric GHGs, both the data and model show a large deglacial ENVIRONMENTAL warming (3–4 °C) that is much larger than the data-model in- n the latest reconstruction of the global surface temperature consistency (11) (Fig. 1). Ithroughout the Holocene (1) (hereafter M13), the most strik- The forcing mechanisms of the simulated global warming are ing feature is a pronounced cooling trend of ∼0.5 °C following further assessed with transient sensitivity experiments that are the Holocene Thermal Maximum (HTM) (∼10–6 ka) toward the forced by the prescribed variations in GHGs, orbital (ORB) in- late Holocene, with the Neoglacial cooling culminating in the solation, ice sheet (ICE), and meltwater flux (MWF) in- Little Ice Age (LIA; ∼1,800 common era) (Fig. 1, blue). Nu- dividually, with other model forcings/settings fixed at the Last merous previous reconstructions have shown cooling trends in Glacial Maximum (∼21 ka). These forcings can be considered the Holocene, but most of these studies attribute the cooling independent external forcings from the perspective of our cou- trend to regional and/or seasonal climate changes (2–6). The pled physical ocean–atmosphere system here, although they are distinct feature of the M13 reconstruction is that it arguably all ultimately caused by the orbital forcing from the compre- infers the cooling trend in the global mean and annual mean hensive perspective of a coupled physical-biogeochemical earth temperature. This inferred global annual cooling in the Holo- system. It should also be pointed out that no volcanic forcing and cene is puzzling: With no direct net contribution from the orbital solar variability are imposed throughout the Holocene in our insolation, the global annual mean radiative forcing in the Ho- CCSM3 transient simulations here, a point to be returned to locene should be dominated by the retreating ice sheets and later. In all three models (Fig. 2 A–C and SI Text 1), each forcing rising atmospheric greenhouse gases (GHGs), with both favoring a globally averaged warming. Therefore, how can the global Significance annual temperature exhibit a cooling trend in response to global warming forcing? This inconsistency between the reconstructed Marine and terrestrial proxy records suggest global cooling cooling and the inferred warming forced by GHGs and ice sheet during the Late Holocene, following the peak warming of the poses the so-called Holocene temperature conundrum and will Holocene Thermal Maximum (∼10 to 6 ka) until the rapid be the subject of this study. Here, we study the global annual warming induced by increasing anthropogenic greenhouses temperature trend in the Holocene and its physical mechanism gases. However, the physical mechanism responsible for this by comparing the temperature reconstruction with three differ- global cooling has remained elusive. Here, we show that cli- ent transient climate model simulations. Our analysis shows a mate models simulate a robust global annual mean warming in robust warming trend in current climate models, opposite from the Holocene, mainly in response to rising CO2 and the retreat the cooling in the M13 reconstruction. This model-data dis- of ice sheets. This model-data inconsistency demands a critical crepancy suggests potentially significant biases in both the re- reexamination of both proxy data and models. constructions and current climate models, and calls for a major reexamination of global climate evolution in the Holocene. Author contributions: Z.L. designed research; J.Z., X.Z., B.L.O.-B., and A.T. performed re- search; Y.R., B.L.O.-B., A.T., R.S.S., G.L., W.Z., and O.E.T. contributed new reagents/analytic Model Experiments tools; J.Z. analyzed data; and Z.L. wrote the paper. We analyzed transient climate simulations in three coupled The authors declare no conflict of interest. ocean–atmosphere models [Community Climate System Model 3 This article is a PNAS Direct Submission. (CCSM3) (7), Fast Met Office/UK Universities Simulator Freely available online through the PNAS open access option. (FAMOUS) (8), and Loch-Vecode-Ecbilt-Clio-Agism Model 1To whom correspondence should be addressed. Email: [email protected]. Methods (LOVECLIM) (9); ] that are subject to realistic climate This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. forcings of orbitally driven insolation variations, GHGs, conti- 1073/pnas.1407229111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1407229111 PNAS Early Edition | 1of5 1.0 available records from the Southern Hemisphere, both proxies suggest SST cooling in the Early Holocene (15, 16). These discrepancies are likely related to the different ecological (e.g., depth habitat) and seasonal (e.g., production season) biases of 0.0 the proxy carriers, as well as the regional distribution of the different proxy records (12, 13, 17). Therefore, the true annual SST trends in these regions remain unclear. To test the potential −1.0 0.5 impact of this seasonal bias on the cooling trend, we construct an artificial seasonally biased global temperature stack in our models. This biased stack samples the surface air temperatures 0.0 from the models in annual mean and the seasons suggested by −2.0 the original authors, except over the Northern Hemisphere ocean sites, where the SST takes the boreal summer season (Fig. SI Text 2 −0.5 S1 and ). It is interesting that the biased stack reverses −3.0 −2 −1 0 the global warming to a cooling (Fig. 1, red) and bears some resemblance to the M13 reconstruction. A further examination Temperature anomaly (degC) model, model−grid, ann shows that this global cooling trend in the biased global stack is model, site−stack, ann associated mainly with the summer bias in the Northern Hemi- −4.0 Marcott et al. Shakun et al. sphere. Fig. 3 shows the temperature evolution in the Holocene model, site−stack, biased averaged over the globe, Northern Hemisphere (30–90° N), CCSM4, model−grid, ann tropics (30° S–30° N), and Southern Hemisphere (90–30° S). The −5.0 global cooling trend (Fig. 3A) is contributed overwhelmingly by −22 −20 −18 −16 −14 −12 −10 −8 −6 −4 −2 0 a large cooling trend (1 °C) in the Northern Hemisphere in both Time (ka BP) M13 and the biased model stack (Fig. 3B). Physically, in the Northern Hemisphere, the seasonal insolation changes by over Fig. 1. Evolution of the global surface temperature of the last 22 ka: the ∼10% throughout the Holocene and is the dominant forcing for reconstructions of M13 (1) (blue) after 11.3 ka and by Shakun et al. (11) (cyan) before 6.5 ka, the model annual mean temperature averaged over the the trend of seasonal and regional temperature; a similar strong global grid points (black), and the model seasonally biased temperature cooling trend in summer temperature has been produced in averaged over the proxy sites (red). The models are the CCSM3 (7), FAMOUS climate models over large areas of the Northern Hemisphere (8), and LOVECLIM (9), with the ensemble mean shown by heavy solid lines (3, 5). We also tested other biased schemes in the model and and individual members shown by light thin lines [the LOVECLIM (○) and found that the cooling trend in the biased stack is caused mainly FAMOUS (□) are marked].