Zubakov and Borzenkova25 were the first to propose that the climate of their Pliocene Optimum (4.3–3.3 Ma) could be considered a past analogue for the mid-Twenty first century when atmospheric CO 2 concentrations would reach double their pre-industrial values. They pioneered the earliest efforts towards construction of regional Pliocene palaeoclimate conditions based on the palaeontology of more than 20 continental sections and available marine core sequences. From these data they reconstructed 30 Miocene and Pliocene ‘superclimathems’ defined as cycles of 100,000 to 300,000 years with amplitudes of at least 4 to 5 °C.

With the then generally accepted knowledge that the Pliocene was a time of warm, equable climate, and with growing concern over potential impacts of future global warming as a backdrop, the need for a more precise and less anecdotal assessment of Pliocene climatic conditions became clear. In 1988, the US Geological Survey endeavoured to reconstruct the palaeoenvironment during the mPWP via PRISM. Using lessons learned from earlier efforts to reconstruct the surface of the Ice-Age Earth26, PRISM and its collaborators developed a large-scale data collection project that has grown in size and scope over the past 25 years (Box 1). PRISM remains the only global-scale synoptic reconstruction of the Pliocene. Data are produced from a global distribution of localities; however, work is concentrated on a focused stratigraphic interval (currently 3.264–3.025 Ma)17 (Box 1).

Such an endeavour to understand one stratigraphic interval (time slab) is fundamentally different from efforts that have gone into developing long time series at single locations. When time series intersect the mPWP, we improve our ability to understand the dynamic development and evolution of environment and climate (see also next section). These approaches (time slab and time series) are not mutually exclusive. Rather, each benefits from the perspective only available through the other.

The tools used by the palaeoclimate community have changed over time, and researchers continue to use the most sophisticated techniques available for reconstruction (Fig. 2). Proxies have been developed to reconstruct different aspects of the environment; however, surface ocean and land temperatures are by far the primary variables reconstructed. Techniques include quantitative analysis of faunal and floral assemblages and stable isotopic composition of carbonates and biomarkers. All have strengths and limitations, but each plays an important role in our conceptual understanding of the mPWP.

Figure 2: Sample time series analyses. Time series illustrating commonly used palaeoenvironmental proxies. Vertical grey band represents position of mPWP. (a) Equatorial Atlantic SST based on alkenone unsaturation index44. (b) Caribbean Sea oxygen isotope record111 showing increasing salinity due to shoaling of the Central American Seaway. (c) Terrestrial record of the mean warmest month temperature from Lake El’gygytgyn56. (d) Equatorial Pacific SST records based on Mg/Ca palaeothermometry51. (e) Estimates of Pliocene atmospheric CO 2 with pre-industrial and present day levels (horizontal dashed lines) for comparison; dark blue dots, δ13C (ref. 3); green band, δ11B (ref. 7); pink band, alkenone5; red band, alkenone6; orange band, stomata4; yellow band, δ11B (ref. 5); blue band, Ba/Ca59; Modified from ref. 50. Full size image

Stable isotopes of oxygen have been used to estimate palaeotemperatures, and indirectly ice volume, since the pioneering work of Emiliani27. Over time they have become the multitool of palaeoceanographic inquest. Shackleton et al.28 generated the first long benthic δ18O time series spanning the Pliocene, placing limits on potential changes in the cryosphere, the stability of Antarctic ice and sea level. The development of an isotopic composite standard reference section, LR04, is possibly the most unifying development to date in pre-Pleistocene palaeoceanography29. With the LR04 composite, researchers are able to correlate remote sequences to a standardized section tied to orbital chronology. This provides the community with a high-resolution stratigraphic framework within which the temporal and spatial aspects of climate change during the mPWP can be analysed.

Primary temperature proxies commonly utilized for mPWP reconstruction include the following: Mg/Ca palaeothermometry, the alkenone unsaturation temperature proxy , the TetraEther index (TEX 86 ) and quantitative analysis of faunal and floral assemblages. These palaeotemperature proxies measure different aspects of temperature by sampling the marine environment at different times of the year as well as at different water depths. While this can provide much insight into the thermal structure of the water column, variations in the timing of production in the past can cause seasonal biases in our interpretations. Likewise, changes in preferred depth habitats would skew palaeoenvironmental reconstruction. Estimates are complicated by effects unique to different signal carriers and methods. For example, the Mg/Ca method depends on assumptions about the composition of seawater at the time calcite was precipitated and can be complicated by post-depositional processes such as dissolution. Faunal assemblage techniques require the assumption of stationarity of ecological preferences and have upper and lower limits based on calibration to present day conditions. , based on extraction of organic molecules (ketones synthesized by haptophyte algae) from sediment, is not affected by seawater chemistry but has an upper limit of ∼28 °C. TEX 86 (based on the relative distribution of membrane lipids produced by Crenarchaeota), such as , is not directly affected by seawater composition, has an upper limit close to 38 °C, but can sometimes record subsurface conditions and be complicated by terrestrial input30,31,32,33,34,35,36,37,38,39,40. The use of multiple proxies is of great benefit for gaining more robust and detailed estimates of the palaeoenvironment as long as the relative limitations of various techniques are recognized.

On the basis of these proxy techniques, past studies41 have summarized mPWP oceans as being characterized by a reduced meridional SST gradient, potentially driven by enhanced ocean heat transport42. High-latitude regions were warmer than those of today, and this warming was at least in part related to changes in sea-ice cover. SST increases in the North Atlantic appear to be particularly pronounced (Fig. 5; see also refs 43, 44, 45, 46). In the circum-Antarctic region, the Polar Front Zone was expanded but displaced towards the continent, and sea ice was greatly reduced18,47. Subtropical gyres in all oceans were displaced towards the poles.

Figure 5: Comparison of Data and Models. International Panel on Climate Change (IPCC) data model comparison. Comparison of PRISM proxy data and the PlioMIP multimodel mean (MMM) simulation, (a) circles are PRISM SST anomalies, (b) zonally averaged PlioMIP MMM SST anomalies, (c) circles are PRISM land surface air temperature (SAT) anomalies, (d) zonally averaged MMM SAT anomalies. Zonal MMM gradients (b,d) are plotted with a shaded band indicating 2σ. Site-specific temperature anomalies estimated from PRISM proxy data are calculated relative to present site temperatures and are plotted (a,b) using the same colour scale as the model data, and a circle-size scaled to estimates of data confidence9,10,11. Modified from Box 5.1, Fig. 1,83. Full size image

To highlight the benefit of using multiple proxies, we focus on efforts to understand the tropics, both in terms of data and climate modelling studies (see sections below for further discussion with regard to data/model comparison).

A clear pattern of tropical SST warming has never been evident in the PRISM reconstructions outside of upwelling regions17,18,20,35,42,48. Tropical upwelling regions are fed by intermediate depth waters that originally form and sink in higher latitudes. Since high-latitude SST reconstructions indicate substantial warming, it is plausible that warmer source waters fed tropical upwelling zones during the mPWP. Given early reconstructions of atmospheric CO 2 concentration higher than the pre-industrial3,4,49, which have been supported by more recent studies50, the lack of tropical SST warming in areas outside the upwelling zones has proven puzzling. This was one reason enhanced meridional ocean heat transport was suggested on the basis of reconstructed mPWP SST gradients42. This provided a way to explain higher concentrations of atmospheric CO 2 (compared with pre-industrial) while at the same time preserving tropical SST stability outside the upwelling zones.

Understanding the thermal stability of the western equatorial Pacific (WEP) warm pool has been a driver of Pliocene palaeoceanographic research in general for many years. While the focus of these studies has been on the early Pliocene rather than the mPWP, insights may be transferable from one to the other (Box 2).

Wara et al.51 developed Mg/Ca-based sea surface temperature (SST) records from calcareous tests of surface-dwelling planktonic foraminifers. These time series span the Pliocene to Recent at Ocean Drilling Program Sites 847 in the eastern equatorial Pacific (EEP) and 806 in the WEP (Fig. 2). The records suggest stable temperatures in the WEP and a small surface-temperature gradient across the equatorial Pacific, particularly in the early Pliocene, much similar to a modern El Niño event with warmer-than-average SST in the EEP. This led to the concept of a Permanent El Niño-like state during the Pliocene, with concomitant warming in the EEP, suggesting a weak zonal atmospheric circulation51.

One study (ref. 48) used quantitative faunal assemblage techniques to estimate SST and found no compelling evidence that the WEP was different from that of present day. Since faunal techniques (calibrated to present day) have a maximum limit of ∼30 °C, and index becomes fully saturated at 28 °C, neither can document warming above modern warm pool conditions. Some late-Pliocene equatorial assemblages do show small non-analogue increases in thermophilic taxa that could be explained by brief periods with SST in excess of 30 °C (ref. 48).

O’Brien et al.52 and Zhang et al.53 addressed the issue of stability of the tropical warm pool during the early Pliocene by applying Mg/Ca, and TEX 86 techniques. They found close agreement between and TEX 86 where they could both be applied, giving confidence to the application of TEX 86 to the warm pool regions. The TEX 86 -based estimates of WEP SST were higher than previous studies based on Mg/Ca or faunal assemblages. They concluded that previous WEP Mg/Ca temperatures were underestimated because of changes in the Mg/Ca of Pliocene seawater (see next section for discussion of the Pliocene tropical Pacific and El Niño Southern Oscillation (ENSO)). It is important to note that WEP temperature reconstructions remain highly debated and no clear consensus has yet emerged within the palaeoceanographic community (see comments and counter comments on Zhang et al.53). This is perhaps unsurprising. Elevating atmospheric CO 2 concentration to 400 p.p.m.v. (a 120-p.p.m. increase over pre-industrial) would create an additional radiative forcing capable of increasing tropical SSTs by 1–3 °C at most. Hence, the signal of change is small relative to the inherent uncertainties in SST reconstruction resulting in an unfavourable signal to noise ratio. Such scenarios of claim and counterclaim regarding tropical temperatures have been the subjects of significant discussion in other communities studying other time intervals. Overall, this highlights the need for further study, while at the same time anticipating divergent views, given the aforementioned signal to uncertainty ratio.

Besides documenting palaeoceanographic conditions during the mPWP, there have been many studies that focus on the terrestrial environment10,16,19. Work in the terrestrial realm is dependent on the heterogeneous distribution of localities where palaeoclimate signal carriers are preserved. Those outcrops and cores with suitable Pliocene chronology, or that contain continuous sequences, are few in comparison with the quantity of cores containing mPWP marine sediments retrieved by the International Ocean Discovery Program and its predecessors. Terrestrial proxies from near-shore marine cores allow for high-resolution continental–marine correlation of Pliocene palaeoclimate records (for example, refs 54, 55). Salzmann et al.16 summarized vegetation-based estimates of Pliocene (Piacenzian Stage) climate as being generally warmer and moister. Evergreen taiga, temperate forest and grasslands shifted northwards, which resulted in a reduction in tundra vegetation. Warm temperate forests spread in middle and Eastern Europe and tropical savannahs and woodland expanded in Africa and Australia, replacing deserts.

Arguably, the longest and most complete record of late-Pliocene high-latitude continental climate, including the mPWP, comes from Lake El’gygytgyn56, located in Northeast Arctic Russia in a basin formed ∼3.6 Ma by a meteorite impact. The many environmental proxies from the Lake El’gygytgyn core can be correlated to the LR04 stack and thus placed within the same chronostratigraphic framework as marine sequences (Fig. 2). The ability to correlate time series of terrestrial data with time series of marine data is rare in a Pliocene context because of the difficulties in assigning precise chronologies to most available terrestrial data. Lake El’gygytgyn records document polar amplification such as is seen in marine records57 and summer temperatures 8 °C warmer than present day, which persisted until ∼2.2 Ma. This supports other estimates of strong Arctic warming during parts of the late Pliocene, including the mPWP58.

Atmospheric concentration of CO 2 during the Pliocene remains only partially constrained. A number of techniques exist to estimate CO 2 (alkenones, B/Ca, δ11B, δ13C and leaf stomatal density), but the variability of estimates is high (Fig. 2). The majority of estimates indicate that CO 2 concentration during the mPWP was higher than the pre-industrial; however, the increase above pre-industrial levels reconstructed from certain records is small and presents a challenge to attribute mPWP warmth to CO 2 forcing alone3,5,6,7,50,59,60.

It is increasingly recognized that CO 2 is just one agent of radiative forcing and that other greenhouse gases such as methane (CH 4 ) are also important; however, these cannot be reconstructed at the current time for the Pliocene. Changes in continental features (including but not limited to orography and land cover) are also hypothesized to have increased long-term warmth. Furthermore, Unger and Yue61 have demonstrated the potential importance of atmospheric chemistry–climate feedbacks, as well as aerosols, in augmenting surface-temperature warming derived from a given increase in atmospheric CO 2 .

While palaeoenvironmental reconstructions have contributed greatly to understand the nature of the mPWP, the information available is insufficient to fully explain all aspects of the Earth’s climate during this time.

Box 1: Pliocene Research Interpretation and Synoptic Mapping The initial PRISM reconstruction was confined to the North Atlantic region. Palaeotemperature estimates were based on quantitative analysis of foraminifer and ostracode faunas from deep-sea cores and continental sections. Even this early and relatively primitive analysis documented a level of polar amplification of surface temperatures unseen today42. With the addition of marine data from the North Pacific and for the first time vegetation or land surface cover, sea level and ice volume/distribution estimates, PRISM was extended to a Northern Hemisphere Reconstruction112. Since that time each successive phase of the project (PRISM1 through 3) has added a new aspect of the palaeoenvironment or refined existing data sets16,17,18,19,20,21. As the magnitude of the Pliocene temperature anomaly became better defined, new methods of standardization (for example, calibration of various palaeothermometres to the same modern temperature data set) and refinement of the chronologic limits of the PRISM time slab using newly developed oxygen isotope stratigraphy29, PRISM consistently improved the overall confidence of the reconstruction20,21. The PRISM3D global reconstruction consists of six discrete data sets representing different aspects of the Pliocene world: sea surface temperatures, deep-ocean temperatures, land vegetation, ice distribution, topography and sea level17. It differs from its predecessors in that it includes a deep-ocean temperature reconstruction114, integrated geochemical sea surface temperature proxies to supplement faunal and floral-based estimates and uses numerical models for the first time to augment palaeobotanical data in the creation of a biome-based land cover scheme16. The chronostratigraphic framework within which a palaeoenvironmental reconstruction is developed is fundamental to its uncertainty. When the PRISM project commenced, confident correlation of sequences from one ocean basin to another was limited by the resolution of the palaeomagnetic timescale and temporally calibrated fossil evolutionary first and last appearance events. This limitation and initial observations from the North Atlantic reconstruction defined a 300,000-year time slab, the PRISM interval, currently known as the mPWP, within which long-distance correlations were possible, and the warm phases of climate could be averaged. This, however, created a situation where non-coeval conditions were smoothed and compared, and a super-equilibrium climate was created that may never have existed. This has significant implications for the validity of data/model comparisons110,115 (see Data Model Comparison section for discussion on the importance of climate variability during the mPWP). The PRISM4 reconstruction has new topography and ocean bathymetry, ice sheets, sea level, soils and lake data sets. SST and BIOME land cover remain the same as in PRISM3. The PRISM4 reconstruction represents the most complete and internally consistent global-scale conceptual model of the Pliocene. New high-resolution time series are being developed with millennial-scale resolution to provide information on regional variability. These time series use multiple factors (for example, T, S, productivity, precipitation, diversity, nutrients and so on), allowing for a more holistic and nuanced description of the palaeoenvironmental setting and, where possible, will have a strong marine–terrestrial correlation. These elements of the PRISM4 reconstruction will be best suited for evaluation of new climate model simulations that will be carried out as part of the second phase of PlioMIP (PlioMIP2). PRISM4 data can be found here http://geology.er.usgs.gov/egpsc/prism/index.html.