Image showing water above and below the ocean surface

Scorching heat waves, flooding rains, and raging wildfires have affected large swaths of Earth’s surface in recent months, breaking records yet again and aligning with climate model projections suggesting that extreme weather will continue to become more frequent and more severe. The leading edge of the climate crisis is setting in.

Understanding what potentially lies ahead has never been more important for the long-term well-being of Earth’s people and ecosystems. However, predictions of what exactly will happen to the climate in the future are riddled with uncertainties that hinder efforts to implement—or sometimes even to consider—plans aimed at mitigating or adapting to change. One promising approach to improving our understanding of the future and reducing uncertainty is to examine the geological past.

Miocene climate archives represent an opportunity to retroactively gain insights into processes affected by climatic warming.

Warmer intervals of the Eocene (56–34 million years ago), Miocene (23–5 million years ago), and Pliocene (5–2.6 million years ago) epochs—whose climates can be reconstructed using proxies preserved in the geologic record as well as through computer simulations—serve as analogues for future warm climates. Studying these periods provides unique perspectives that can help us anticipate the patterns and impacts of future warming [Burke et al., 2018; Lear et al., 2020; Tierney et al., 2020].

Miocene climate archives, for example, represent an opportunity to retroactively gain insights into processes affected by climatic warming [Steinthorsdottir et al., 2021a]. During the Miocene, continental positions resembled their configuration today, and Earth’s systems and life forms—in the ocean and on land, from the atmosphere to the cryosphere—experienced dynamic changes.

In the early and late Miocene, widespread glaciations prevailed at high latitudes, whereas the middle Miocene was characterized by greenhouse conditions (Figure 1). By the late Miocene, many key components of the Earth system as we know it today had developed, including perennial Arctic ice, the El Niño–Southern Oscillation, strong monsoon systems, the tundra-permafrost biome, widespread grasslands, and modern forests with their associated ecosystems, as well as modern-type coral reefs. Many of these systems, however, are now viewed as vulnerable to impending climate change if conditions like those that were typical during the middle Miocene recur. It is therefore of enormous interest to study this past period as a possible future-climate analogue.

Fig. 1. Middle Miocene (~20⁠–⁠14 million years ago) sea surface temperatures (horizontal scale) and model topography (meters above sea level, vertical scale) are shown here. Sea surface temperatures are derived from the multimodel mean of all simulations forced with middle Miocene boundary conditions, as given by Burls et al. [2021, Table 2]. Topography is from the updated Herold boundary conditions as given by Burls et al. [2021]. Credit: Natalie Burls

The Miocene Climatic Optimum (MCO), from about 16.9 million to 14.7 million years ago, is a particularly appropriate analogue for assessing near-term future climate scenarios and the predictive accuracy of numerical climate models [Schellnhuber et al., 2016]. The MCO was a transient episode when carbon dioxide (CO2) concentrations were between about 400 and 650 parts per million [Steinthorsdottir et al., 2021a] (for comparison, the global average in 2019 was 410 parts per million). Average temperatures during the MCO were roughly 6°C–8°C warmer than today, on par with the upper range of future warming predictions calculated using the Intergovernmental Panel on Climate Change’s Representative Concentration Pathways [Steinthorsdottir et al., 2021a, 2021b].

Over the past decade, the paleoclimatology community has cooperated to make headway in evaluating warm climate analogues in the Pliocene and Eocene through model intercomparison projects (MIPs) such as the Pliocene MIP (PlioMIP) and the Deep-Time MIP (DeepMIP) [Haywood et al., 2016; Lunt et al., 2021]. In contrast, the Miocene has received comparatively little such attention; but that is beginning to change.

Launching Collaborative Ventures

The community recognized the opportunity to provide a collaboratively developed, updatable, and freely accessible data portal for Miocene temperature data

A necessary first step in developing a Miocene MIP, or MioMIP, is to build a comprehensive synthesis of proxy-derived observational data for comparison against numerical simulation outputs. In June 2019, a multidisciplinary group of scientists interested in all aspects of Miocene climate and biota gathered for MioMeet at the Bolin Centre for Climate Research at Stockholm University. Participants at the meeting committed to inventorying and collating all existing Miocene temperature data sets starting with sea surface temperature to create MioMIP, following the examples of PlioMIP and DeepMIP. (Additional outcomes of the meeting included a special collection of papers published in Paleoceanography and Paleoclimatology in 2020 and 2021, a comprehensive review paper on Miocene research published in the same journal, and a Miocene-focused session at AGU Fall Meeting 2020.)

Efforts to collate records highlighted an extensive existing suite of marine temperature data sets from the Miocene [Burls et al., 2021]. Rather than concluding its work with a static compilation of these accumulated data, however, the community recognized the opportunity to build on this foundation to provide a resource that would continue to be useful—a collaboratively developed, updatable, and freely accessible data portal for Miocene temperature data.

A Living Metadata Inventory

The inventory of ocean temperature records initiated at MioMeet revealed more than 40 published ocean temperature records from individual stations around the world, but more than a dozen more such records were in development or in the publication pipeline. Most of these records span millions of years of Earth history and typically represent hundreds of individual temperature estimates collected over a span of years. The prospect of the existing inventory of temperature records soon growing by 25% or more made clear that the Miocene research community needed a platform to support data access and discovery for researchers, including modelers and scientists from outside the discipline.

Fig. 2. Version 1.0 of the Miocene temperature portal provides access to 45 Miocene sea surface temperature records from more than 35 sites around the world. This modern global map of sea surface temperature [Kershaw, 2008] is overlain by site labels showing the spatial distribution of the records and the paleotemperature proxy type. The tabular data can be sorted using various filtering options, and it can also be downloaded. Paleotemperature proxy types are biomarker-based proxies (TEX86 and UK’37) and calcareous fossil-based proxies (magnesium/calcium ratios (Mg/Ca) and clumped isotopes (Δ47)). Ages are Oligocene–Miocene (O/M) boundary, Miocene Climatic Optimum (MCO), and Miocene–Pliocene (M/P) boundary. Click image for larger version.

An enormous amount of effort goes into obtaining these records—each spot on the map in Figure 2 arises from a dedicated ocean or land-based geological sampling expedition (most often deep-sea coring). Each record also requires countless hours of subsequent lab work to isolate signal-carrying materials (e.g., microscopic calcareous fossils or organic remains that preserve climate information; see Figure 3), establish precise sample ages, and perform geochemical analyses. In addition, raw geochemical data must be transformed into robust temperature estimates using state-of-the-art calibration equations that relate the abundance of certain chemical species in the samples (e.g., trace amounts of magnesium in the calcium carbonate shells of plankton) to the environmental temperature at the time the material grew.

Fig. 3. Eighteen-million-year-old planktonic foraminifera fossils from the Atlantic Ocean, seen here under a light microscope, are one type of geological material used for producing Miocene sea surface temperature estimates. Using these types of calcium carbonate shells, scientists can measure three different geochemical temperature proxies: clumped isotopes (Δ47), traditional oxygen stable isotopes (δ18O), and magnesium/calcium ratios (Mg/Ca). Records using all of these approaches appear in the Miocene temperature portal. Other temperature proxy types (TEX86 and UK’37) are measured on fossilized organic molecules. Credit: Helen Coxall

The Miocene temperature portal was born of collaboration between this community and experts from the Bolin Centre Database Team, who together produced a supported contribution in the established climate database framework. The portal itself is not a data repository, because paleoclimate data are not actually archived there. Rather, it is designed as an up-to-date and easy-to-use routing center to help scientists find published Miocene temperature records at the repositories where they are archived.

The portal offers researchers a way to identify gaps in the Miocene temperature record and develop much-needed new records, and to explore potential interpretations of their data sets.

The portal’s user interface provides map visualizations of sites for which existing data are available and the ability to sort or filter entries to identify data on the basis of proxy type, ocean basin, and age. It also offers one-click links to the source publications and to data repository pages where the archived data can be accessed. The organization of the portal offers researchers—including emerging researchers new to the study of Miocene climate—a way to identify spatial or temporal gaps in the Miocene temperature record and develop much-needed new records, to explore potential interpretations of their data sets, or to test hypotheses by comparing new data sets to previously published records.

The portal also provides a simple interface by which community members can add new entries to the inventory. All entries provide source metadata, including URL addresses, so other researchers can readily access newly published temperature records and associated peer-reviewed publications. Paleotemperature experts screen new metadata entries prior to their addition to the portal to ensure quality control. This process entails vetting metadata accuracy but makes no judgment about the temperature proxy data themselves or about how the data were calibrated (considerations that would have been addressed previously in the peer review process).

The ability for researchers to contribute information about new records provides a mechanism for the portal to remain a useful community resource in perpetuity. Whereas the current version 1.0 contains only ocean surface temperature records, the soon-to-be-released version 2.0 will also include Miocene bottom water temperatures, and a potential future version could include terrestrial temperature records.

The Miocene temperature portal offers a robust starting point as the Miocene data community undertakes the development of an exhaustive data synthesis in support of a MioMIP effort. This effort entails not only collating but also reviewing and recalibrating all existing temperature records. We are confident that the portal will also stimulate other collaborative research projects among Miocene researchers.

For example, the portal should be especially useful for assembling disparate data from various climate proxies and locations for studies comparing model outputs with cataloged observations. These studies are the gold standard for using paleodata to inform our understanding of modern climate change. In addition, researchers specializing in reconstructing atmospheric CO2 in the Miocene could benefit from the portal because it provides an easy means of accessing ocean temperature records that will enable them to assess climate sensitivity (the climate system’s response to changes in atmospheric CO2). By providing a centralized overview of available records from different proxies, the portal may aid in proxy comparison studies evaluating the robustness of existing paleothermometers and assessing whether estimates from different proxies can be compiled into a comprehensive and reliable Miocene climate synthesis.

A Model for Other Communities

The portal contributes to the growing number of volunteer-driven, discipline-wide efforts to create publicly available, continuously updated databases that support data discovery and access.

The Miocene temperature portal contributes to the growing number of volunteer-driven, discipline-wide efforts to create publicly available, continuously updated databases (e.g., SISAL (Speleothem Isotopes Synthesis and Analysis), Paleo-CO2) that support data discovery and access. Similar recent efforts, such as in the speleothem research community, provide additional examples of the approach we describe and the benefits we anticipate with this portal.

The collaborative effort to develop this temperature platform has helped galvanize the Miocene data community to plan a Past Global Changes (PAGES) working group. This working group will coordinate efforts to synthesize existing and new paleodata to determine the drivers and mechanisms of Miocene climate change, to support the initiation of a MioMIP, and to gain insights about consequences that future warming may hold for Earth.

The portal should also broaden participation in Miocene climate research by helping avoid repetition of effort in the data synthesis stage. It should also reduce barriers to participation for students or for experienced researchers who have studied other time periods and are seeking to understand data coverage and locate key data-based works from the Miocene.

In the future, funding agencies should prioritize grants for collaborative meetings and data portals to ensure that sustainable data platforms are developed and maintained to provide gateways for sharing, accessing, and visualizing published scientific data within given research disciplines. We hope the Miocene temperature portal inspires other research communities to undertake similar efforts in support of access to and advancement of scientific inquiry in their own fields.


The authors thank Anders Moberg and Rezwan Mohammad from the Bolin Centre Database Team for their technical expertise and collaborative spirit. The authors also thank the Miocene research community, most especially Natalie Burls, Matthew Huber, Sevasti Modestou, Francesca Sangiorgi, Timothy Herbert, Carrie Lear, and Ann Pearson, for their contributions to the Miocene temperature portal.


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Author Information

Kira T. Lawrence (, Department of Geology and Environmental Geosciences, Lafayette College, Easton, Pa.; Helen K. Coxall, Department of Geological Sciences, Stockholm University, Stockholm; also at Bolin Centre for Climate Research, Stockholm; Sindia Sosdian, School of Earth and Environmental Sciences, Cardiff University, Cardiff, U.K.; and Margret Steinthorsdottir, Department of Paleobiology, Swedish Museum of Natural History, Stockholm; also at Bolin Centre for Climate Research, Stockholm

Citation: Lawrence, K. T., H. K. Coxall, S. Sosdian, and M. Steinthorsdottir (2021), Navigating Miocene ocean temperatures for insights into the future, Eos, 102, Published on 5 October 2021.
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