Assessing Mechanisms and Uncertainty in Modeled Climatic Change at the Eocene‐Oligocene Transition

Abstract

The Earth system changed dramatically across the Eocene‐Oligocene Transition (EOT) on a variety of spatial and temporal scales. Understanding the many complex and interacting factors affecting the Earth's atmosphere and oceans at the EOT requires the combination of both data and modeling approaches and an understanding of the uncertainty in both of these elements. Here uncertainty in the Earth system response to various imposed forcings typical of changes at the EOT is assessed. By using an ensemble of simulations from the fully coupled general circulation model, HadCM3L, the uncertainty due to differences in the boundary conditions and insufficient model spin‐up is quantified. The surface temperature response in high‐latitude ocean regions, particularly where deep water formation occurs, is found to be highly sensitive to differences in boundary conditions (i.e., have the greatest magnitude of uncertainty), while low‐latitude oceans are the most insensitive to differences in boundary conditions (i.e., have the lowest magnitude of uncertainty). The length of spin‐up (or how far the model is from equilibrium) can have a significant effect on the response to some forcings and on the magnitude of uncertainty due to differences in boundary conditions. These findings are important to consider for future modeling work and for interpreting previous published simulations.

Changes in the high-latitude Southern Hemisphere through the Eocene–Oligocene transition: a model–data comparison

“…A second option that could partly explain the model-data discrepancy is that local-to regional-scale warming signals in response to Antarctic glaciation due to enhanced circulation, deepwater formation and sea ice feedbacks (as identified in models by Goldner et al, 2014;Knorr and Lohmann, 2014;Kennedy et al, 2015;; and some of the FOAM simulations used here from Ladant et al, 2014; figure not shown) could be compensating for some of the cooling. When this warming is combined with a pCO 2 decline, the models do suggest that some very localized areas (i.e.…”

“…Global cooling and the significant expansion of glacial ice over Antarctica at the Eocene-Oligocene transition (EOT) ∼ 33.7 million years ago (Myr; Zachos et al, 2001;Coxall et al, 2005) would have potentially resulted in large but uncertain changes in the Southern Ocean and the climate of the high-latitude Southern Hemisphere (Bohaty et al, 2012;Passchier et al, 2013). Numerous palaeoclimate modelling studies have shown that changes in Antarctic Ice Sheet extent, atmospheric pCO 2 levels and palaeogeographic reconstruction around this period of the Earth's history can all impact the modelled global and/or regional climate (Goldner et al, 2014;Knorr and Lohmann, 2014;Kennedy et al, 2015). Interestingly, all of these studies show some areas of warming in the Southern Ocean in response to the imposition of an Antarctic Ice Sheet in their models, but the different models find the warming to occur in different regions.…”

Quantifying the Effect of the Drake Passage Opening on the Eocene Ocean

“…Conversely, DP opening in the low-resolution FOAM general circulation model produces a smaller impact in terms of temperature and ocean circulation in an Eocene configuration with high CO 2 levels compared to a modern one (Zhang et al, 2010(Zhang et al, ). et al, 2015Kennedy-Asser et al, 2019;Vahlenkamp et al, 2018). These studies describe an ACC with a moderate intensity during the Eocene/Oligocene (around 4 to 46.2 Sv; Kennedy et al, 2015), which strengthens as a result of pCO 2 decrease, Antarctic Ice Sheet formation, and opening of the Southern Ocean (up to 89 Sv; Hill et al, 2013;Kennedy et al, 2015;Ladant et al, 2014a;Lefebvre et al, 2012;Zhang et al, 2010).…”

“…These studies describe an ACC with a moderate intensity during the Eocene/Oligocene (around 4 to 46.2 Sv; Kennedy et al, 2015), which strengthens as a result of pCO 2 decrease, Antarctic Ice Sheet formation, and opening of the Southern Ocean (up to 89 Sv; Hill et al, 2013;Kennedy et al, 2015;Ladant et al, 2014a;Lefebvre et al, 2012;Zhang et al, 2010). Among these studies, the impact of DP opening on temperatures is variable with either a regional cooling of the Atlantic sector of the Southern Ocean (up to 6°C; Kennedy et al, 2015;Kennedy-Asser et al, 2019) or quasi-insignificant temperature changes (<1°C) (e.g., Goldner et al, 2014;Inglis et al, 2015;Zhang et al, 2010). Goldner et al (2014) have illustrated the particularly weak contribution of the opening of Southern gateways to EOT ocean temperature changes, in comparison to pCO 2 decrease or Antarctic Ice Sheet build-up.…”

Supplementary material to &quot;Changes in the high latitude Southern Hemisphere through the Eocene-Oligocene Transition: a model-data comparison&quot;

“…Value from Francis et al, 2009 (p.333 The model simulations used here have been previously described in Kennedy-Asser et al (2019) and Ladant et al (2014). 60…”