Pliocene and Eocene provide best analogs for near- future climates K. D. Burkea,1, J. W. Williamsb, M. A. Chandlerc,d, A. M. Haywoode, D. J. Luntf, and B. L. Otto-Bliesnerg aNelson Institute for Environmental Studies, University of Wisconsin–Madison, Madison, WI 53706; bDepartment of Geography and Center for Climatic Research, University of Wisconsin–Madison, Madison, WI 53706; cCenter for Climate Systems Research, Columbia University, New York, NY 10025; dGoddard Institute for Space Studies, National Aeronautics and Space Administration (NASA), New York, NY 10025; eSchool of Earth and Environment, University of Leeds, LS2 9JT Leeds, United Kingdom; fSchool of Geographical Sciences, University of Bristol, BS8 1SS Bristol, United Kingdom; and gClimate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO 80305 Edited by Noah S. Diffenbaugh, Stanford University, Stanford, CA, and accepted by Editorial Board Member Robert E. Dickinson November 6, 2018 (received for review June 29, 2018) As the world warms due to rising greenhouse gas concentrations, the baselines, in which the 20th and 21st century instrumental records Earth system moves toward climate states without societal precedent, are used for reference. This restriction overlooks the deep history challenging adaptation. Past Earth system states offer possible model of Earth’s climate variation and the societal, ecological, and systems for the warming world of the coming decades. These include evolutionary responses to this past variation. By considering only the climate states of the Early Eocene (ca. 50 Ma), the Mid-Pliocene shallow temporal baselines, the evolutionary adaptive capacity of (3.3–3.0 Ma), the Last Interglacial (129–116 ka), the Mid-Holocene species to future novel climates may be underestimated. Con- (6 ka), preindustrial (ca. 1850 CE), and the 20th century. Here, we quan- versely, others have drawn informal analogies between the cli- titatively assess the similarity of future projected climate states to mates of the future and those of the geological past (18, 19), but these six geohistorical benchmarks using simulations from the Hadley there has been no quantitative comparison. Here, we pursue a Centre Coupled Model Version 3 (HadCM3), the Goddard Institute for deeper baseline, formally comparing the projected climates of the Space Studies Model E2-R (GISS), and the Community Climate System coming decades with geohistorical states of the climate system Model, Versions 3 and 4 (CCSM) Earth system models. Under the from across the past 50 My. We seek to identify past states of the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario, climate system that offer the closest analogs to the climates of the ECOLOGY by 2030 CE, future climates most closely resemble Mid-Pliocene cli- coming decades, the time to emergence for various geological mates, and by 2150 CE, they most closely resemble Eocene climates. analogs, and the distribution and prevalence of “geologically Under RCP4.5, climate stabilizes at Pliocene-like conditions by 2040 CE. novel” future climates (i.e., that lack any close geological analog Pliocene-like and Eocene-like climates emerge first in continental in- among the climate states considered here). teriors and then expand outward. Geologically novel climates are uncommon in RCP4.5 (<1%) but reach 8.7% of the globe under Identifying the Closest Paleoclimatic Analogs for Near- RCP8.5, characterized by high temperatures and precipitation. Hence, Future Earth RCP4.5 is roughly equivalent to stabilizing at Pliocene-like climates, Earth’s climate system has evolved in response to external while unmitigated emission trajectories, such as RCP8.5, are similar forcings and internal feedbacks across a wide range of timescales EARTH, ATMOSPHERIC, to reversing millions of years of long-term cooling on the scale of a (Fig. 1). Since 65 Ma, global climate has cooled (20), and AND PLANETARY SCIENCES few human generations. Both the emergence of geologically novel climates and the rapid reversion to Eocene-like climates may be out- Significance side the range of evolutionary adaptive capacity. The expected departure of future climates from those experi- climate change | climate analog | no analog | paleoclimate | enced in human history challenges efforts to adapt. Possible an- planetary boundary alogs to climates from deep in Earth’s geological past have been suggested but not formally assessed. We compare climates of the y the end of this century, mean global surface temperature is coming decades with climates drawn from six geological and Bexpected to rise by 0.3 °C to 4.8 °C relative to 1986–2005 CE historical periods spanning the past 50 My. Our study suggests averages, with more warming expected for higher levels of green- that climates like those of the Pliocene will prevail as soon as house gas emissions (1) and substantial effects predicted for the 2030 CE and persist under climate stabilization scenarios. Un- cryospheric (2), hydrologic (3), biological (4, 5), and anthropogenic mitigated scenarios of greenhouse gas emissions produce cli- (6) components of the Earth system. Understanding and preparing mates like those of the Eocene, which suggests that we are for climate change are challenged in part by the emergence of effectively rewinding the climate clock by approximately 50 My, Earth system states far outside our individual, societal, and species’ reversing a multimillion year cooling trend in less than two experience. Traditional systems for designing infrastructure, miti- centuries. gating natural hazard risk, and conserving biodiversity are often based on implicit assumptions about climate stationarity and recent Author contributions: K.D.B. and J.W.W. designed research; K.D.B. performed research; K.D.B., J.W.W., M.A.C., A.M.H., D.J.L., and B.L.O.-B. analyzed data; M.A.C., A.M.H., D.J.L., historical baselines (7), which fail to encompass expected trends and B.L.O.-B. contributed climate model simulation data; and K.D.B., J.W.W., M.A.C., and recent extreme events (8, 9). Calls to keep the Earth within a A.M.H., D.J.L., and B.L.O.-B. wrote the paper. “safe operating space” seek to keep Earth’sclimatesintherangeof The authors declare no conflict of interest. those experienced during the Holocene, which encompasses the This article is a PNAS Direct Submission. N.S.D. is a guest editor invited by the Editorial Board. time of development of agriculture and the emergence of the Published under the PNAS license. complexly linked global economy (10, 11). Societally novel climates Data deposition: The Matlab code used to identify closest analogs as well as the output are expected to emerge first in low-latitude and low-elevation re- files, which contain the geographic information, climatic distances, and analog matches, gions (12–14), while locally novel climates (future climates that have been deposited in the Dryad Digital Repository (doi.org/10.5061/dryad.0j18k00). have exceeded a baseline of local historical variability) are expected 1To whom correspondence should be addressed. Email: [email protected]. to begin to emerge by the mid- to late 21st century (15–17). This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. However, all prior efforts to quantify the pattern and timing of 1073/pnas.1809600115/-/DCSupplemental. novel climate emergence have been narrowly restricted to shallow www.pnas.org/cgi/doi/10.1073/pnas.1809600115 PNAS Latest Articles | 1of6 Downloaded by guest on September 29, 2021 atmospheric CO2 concentrations have declined (21). Several variable vector of mean summer and winter temperatures and warm periods offer possible geological analogs for the future: the precipitation (Materials and Methods). The climate for each ter- Early Eocene (ca. 50 Ma; hereafter the Eocene), the Mid- restrial grid location for a given future decade is compared with all Pliocene Warm Period (3.3–3.0 Ma; hereafter the Mid- points in a reference baseline dataset that comprises the climates Pliocene), the Last Interglacial (LIG; 129–116 ka), and the of all global terrestrial grid locations from all six geohistorical Mid-Holocene (6 ka). During the Eocene, the warmest sustained periods (SI Appendix, Figs. S1 and S2). For each location, we state of the Cenozoic, global mean annual surface temperatures identify for each future climate its closest geohistorical climatic were 13 °C ± 2.6 °C warmer than late 20th century temperatures analog (i.e., the past time period and location with the most (22), there was no permanent ice, and atmospheric CO2 was ap- similar climate). We apply this global similarity assessment to each proximately 1,400 parts per million volume (ppmv) (23). The Mid- future decade from 2020 to 2280 CE. Future climates that exceed Pliocene is the most recent period with atmospheric CO2 com- an MD threshold are classified as “no analog” (Materials and parable with the present (ca. 400 ppmv) (24), with mean annual Methods), indicating that they lack any close analog in the suite of surface temperatures approximately 1.8 °C to 3.6 °C warmer than geological and historical climates considered here. preindustrial temperatures, reduced ice sheet extents, and increased sea levels (25). During the LIG, global mean annual temperatures Results were approximately 0.8 °C (maximum 1.3 °C) warmer than pre- Historical climates and preindustrial climates quickly disappear as industrial temperatures (26), and amplified seasonality characterized best analogs for 21st century climates for both RCP scenarios (Fig. the northern latitudes (27). During the Mid-Holocene, tempera- 2). By 2040 CE, they are replaced by the Mid-Pliocene, which tures were 0.7 °C warmer than preindustrial temperatures (28), becomes the most common source of best analogs in the three- with enhanced temperature seasonality and strengthened Northern model ensemble and remains the best climate analog thereafter Hemisphere (NH) monsoons (27). (Fig. 2). Hence, RCP4.5 is most akin to a Pliocene commit- Recent historical intervals also provide potential analogs for near- future climates (Fig.