Pliocene and Eocene provide best analogs for near-future climates

Abstract

As the world warms due to rising greenhouse gas concentrations, the Earth system moves toward climate states without societal precedent, challenging adaptation. Past Earth system states offer possible model systems for the warming world of the coming decades. These include the climate states of the Early Eocene (ca. 50 Ma), the Mid-Pliocene (3.3-3.0 Ma), the Last Interglacial (129-116 ka), the Mid-Holocene (6 ka), preindustrial (ca. 1850 CE), and the 20th century. Here, we quantitatively assess the similarity of future projected climate states to these six geohistorical benchmarks using simulations from the Hadley Centre Coupled Model Version 3 (HadCM3), the Goddard Institute for Space Studies Model E2-R (GISS), and the Community Climate System Model, Versions 3 and 4 (CCSM) Earth system models. Under the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario, by 2030 CE, future climates most closely resemble Mid-Pliocene climates, and by 2150 CE, they most closely resemble Eocene climates. Under RCP4.5, climate stabilizes at Pliocene-like conditions by 2040 CE. Pliocene-like and Eocene-like climates emerge first in continental interiors and then expand outward. Geologically novel climates are uncommon in RCP4.5 (<1%) but reach 8.7% of the globe under RCP8.5, characterized by high temperatures and precipitation. Hence, RCP4.5 is roughly equivalent to stabilizing at Pliocene-like climates, while unmitigated emission trajectories, such as RCP8.5, are similar to reversing millions of years of long-term cooling on the scale of a few human generations. Both the emergence of geologically novel climates and the rapid reversion to Eocene-like climates may be outside the range of evolutionary adaptive capacity.

Keywords: climate analog; climate change; no analog; paleoclimate; planetary boundary.

Fig. 1.

Temperature trends for the past 65 Ma and potential geohistorical analogs for future climates. Six geohistorical states (red arrows) of the climate system are analyzed as potential analogs for future climates. For context, they are situated next to a multi-timescale time series of global mean annual temperatures for the last 65 Ma. Major patterns include a long-term cooling trend, periodic fluctuations driven by changes in the Earth’s orbit at periods of 10⁴–10⁵ y, and recent and projected warming trends. Temperature anomalies are relative to 1961–1990 global means and are composited from five proxy-based reconstructions, modern observations, and future temperature projections for four emissions pathways (Materials and Methods). Pal, Paleocene; Mio, Miocene; Oli, Oligocene.

Fig. 2.

Time series of the closest geohistorical climatic analogs for projected climates, 2020–2280 CE (MD). Colored lines indicate the proportion of terrestrial grid cells for each future decade with the closest climatic match to climates from six potential geohistorical climate analogs: Early Eocene, Mid-Pliocene, LIG, Mid-Holocene, preindustrial, and historical for RCP8.5 (A) and RCP4.5 (B). No LIG simulation from GISS was available at the time of analysis.

Fig. 3.

Projected geographic distribution of future climate analogs (RCP8.5). Future climate analogs for 2020, 2050, 2100, and 2200 CE according to the ensemble median. Geohistorical periods are rank ordered according to global mean annual temperature as follows: preindustrial, historical, Mid-Holocene, LIG, Pliocene, and Eocene, with no analog placed at the end due to the prevalence of no-analog climates in the warmest and wettest portion of climate space (Fig. 4). Hence, a projected future location matched to Pliocene, Eocene, and no analog in the three ESMs would be identified as Eocene in the ensemble median.

Fig. 4.

Projected future climate space by closest analog (RCP 8.5). (Upper) DJF vs. JJA temperature space. (Lower) DJF vs. JJA precipitation space. Each point represents a terrestrial grid location from the model ensemble for the specified decade in the RCP8.5 projection. Points are color coded according to the geohistorical climate from which their closest analog sources. Box-and-whisker plots show the data range, median, and first and third quartiles for two time periods: the specified decade (black) and 2020 CE for reference (gray).