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

The mid Miocene represents an important target for paleoclimatic study because the atmospheric CO2 concentration ranged from near modern values to ∼800 ppm, while a large, dynamic Antarctic ice sheet was likely to have been present throughout much of this interval. In this special issue, Modestou et al. (2020) (doi.org/10.1029/2020PA003927) reconstruct deep ocean warmth based on the clumped isotopic composition of benthic foraminifera, a technique that allows the ice volume and thermal components of the benthic oxygen isotope stack to be separated. These data reveal a very warm deep ocean while simultaneously suggesting that continental ice volume may, at times, have been greater than today. Here, I review these results in the context of recent developments in geochemical proxies and ice sheet modeling, and explore how the presence of a large Miocene ice sheet could be reconciled with CO2 at least as high as present. More broadly, I argue that many of the 'paradoxes' that pepper the paleoclimate literature result as much from our imperfect understanding of the proxies, as from our understanding of the climate system. Robust proxies with a well‐understood mechanistic basis, as employed by Modestou et al. (2020), as well as advances in model‐data comparability usher in a new era of palaeoclimate research; an exciting future of untangling Earth's myriad past climate states awaits.

Plain Language Summary: Reconstructing climate variation in Earth's geologic past informs us of the broad features of warm climates, which is relevant to preparing for climate change over the coming centuries. Moreover, these data can be compared to state‐of‐the‐art climate models, which provides a test of the degree to which our models can reproduce warm climate states. A paper recently published in this journal applies a new method in order to reconstruct the temperature of the deep ocean in the middle Miocene (between 17 and 12 million years ago), when the atmospheric CO2 concentration was naturally similar to or higher than it is today. Coupled with decades of previous study, these exciting results depict an unfamiliar world characterized by a warm deep ocean, and yet a large ice sheet was present on Antarctica. Both models and data agree that the Antarctic ice sheet in the Miocene was highly responsive to changes in the atmospheric CO2 concentration, a clear cause of concern in the context of ongoing anthropogenic climate change.

Key Points: The importance of a paper by Modestou et al. (2020) is explained, which reports middle Miocene benthic foraminifera clumped isotope data. Ways in which a large Antarctic ice sheet can be reconciled with a warm deep ocean and moderately high CO2 are explored. More broadly, I argue that recent advances in proxy methodology are resulting in ever increasing confidence in paleoclimate reconstructions.