The Miocene: The Future of the Past

Abstract

The Miocene epoch (23.03-5.33 Ma) was a time interval of global warmth, relative to today.Continental configurations and mountain topography transitioned toward modern conditions, and many flora and fauna evolved into the same taxa that exist today. Miocene climate was dynamic: long periods of early and late glaciation bracketed a ∼2 Myr greenhouse interval-the Miocene Climatic Optimum (MCO). Floras, faunas, ice sheets, precipitation, pCO 2 , and ocean and atmospheric circulation mostly (but not ubiquitously) covaried with these large changes in climate. With higher temperatures and moderately higher pCO 2 (∼400-600 ppm), the MCO has been suggested as a particularly appropriate analog for future climate scenarios, and for assessing the predictive accuracy of numerical climate models-the same models that are used to simulate future climate. Yet, Miocene conditions have proved difficult to reconcile with models. This implies either missing positive feedbacks in the models, a lack of knowledge of past climate forcings, or the need for re-interpretation of proxies, which might mitigate the model-data discrepancy. Our understanding of Miocene climatic, biogeochemical, and oceanic changes on broad spatial and temporal scales is still developing. New records documenting the physical, chemical, and biotic aspects of the Earth system are emerging, and together provide a more comprehensive understanding of this important time interval. Here, we review the state-of-the-art in Miocene climate, ocean circulation, biogeochemical cycling, ice sheet dynamics, and biotic adaptation research as inferred through proxy observations and modeling studies. Plain Language Summary During the Miocene time period (∼23-5 million years ago),Planet Earth looked similar to today, with some important differences: the climate was generally warmer and highly variable, while atmospheric CO 2 was not much higher. Continental-sized ice sheets were only present on Antarctica, but not in the northern hemisphere. The continents drifted to near their modernday positions, and plants and animals evolved into the many (near) modern species. Scientists study the Miocene because present-day and projected future CO 2 levels are in the same range as those reconstructed for the Miocene. Therefore, if we can understand climate changes and their biotic responses from the Miocene past, we are able to better predict current and future global changes. By comparing Miocene climate reconstructions from fossil and chemical data to climate simulations produced by computer models, scientists are able to test their understanding of the Earth system under higher CO 2 and warmer conditions than those of today. This helps in constraining future warming scenarios for the coming STEINTHORSDOTTIR ET AL.

Simulating Miocene warmth: insights from an opportunistic Multi-Model ensemble (MioMIP1)

“…The Miocene epoch encompasses much of the range of climate states between the Cenozoic endmembers of peak Eocene warmth and the modern ice-house state with extensive glaciation at both poles (Figure 1, Steinthorsdottir et al, 2020). Terrestrial and marine records show that Miocene climate BURLS ET AL.…”

“…Typical proxy estimates for the Miocene are in the 300-600 ppmv range (Figure 1, Foster et al, 2012Foster et al, , 2017Sosdian et al, 2018), although during the MCO, recent reconstructions suggest that CO 2 may have been as high as 800-1,100 ppm (Sosdian et al, 2018;Stoll et al, 2019). A full description of the Miocene CO 2 reconstructions and their uncertainties is given in Section 8 of (Steinthorsdottir et al, 2020). A prolonged carbon isotope excursion between ∼16.7 Ma and ∼13.5 Ma (the Monterey Excursion; Vincent & Berger, 1985) documents a marked perturbation of the carbon cycle at this time, modulated by orbital forcing.…”

“…Vegetation distributions resulting from a warmer global climate may itself provide a stabilizing feedback mechanism to maintain that warmth (e.g., Bradshaw et al, 2015;Knorr et al, 2011). As described in more depth in the following section, while these mechanisms have been shown to contribute to warming in models, they fall short of explaining the full extent, and the polar amplified spatial structure, of Miocene warmth (also see summary in Steinthorsdottir et al, 2020).…”

Deep Heat: Proxies, Miocene Ice, and an End in Sight for Paleoclimate Paradoxes?

“…It is important to note that Antarctic ice volume throughout the middle Miocene may have been much more variable than implied by the relatively low-resolution data of Modestou et al (2020). For example, it has been suggested that peak middle Miocene warmth was potentially associated with little ice on Antarctica (Miller et al, 2020;Stap et al, 2016; discussed in more detail in Steinthorsdottir et al, 2020). In addition, the Antarctic ice sheet response to pCO 2 depends strongly on multiple factors such as the dynamics of pCO 2 change, orbital parameters, and precipitation lapse rate (Stap et al, 2019).…”

“…The Miocene climatic optimum (MCO, ∼16.5–15 Ma) is the most recent interval with atmospheric CO 2 substantially elevated above that of the early 21 st century, with boron isotope and alkenone δ 13 C‐derived estimates (Sosdian et al., 2018; Super et al., 2018) constraining peak CO 2 to around 400–800 ppm, declining to 200–400 ppm after the Miocene climate transition (MCT, ∼14.5–13 Ma). The atmospheric CO 2 concentration of the MCO gave rise to a profoundly different world to today, with global mean surface temperature ∼3–6°C higher than preindustrial times (Hansen et al., 2013; Tierney et al., 2020) and a substantially reduced latitudinal temperature gradient (Goldner et al., 2014); also see the review paper in this issue for a more comprehensive summary of the climate and biota of the Miocene (Steinthorsdottir et al., 2020).…”