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We describe a set of early Eocene (~ 55 Ma) climate model boundary
conditions constructed in a self-consistent reference frame and incorporating
recent data and methodologies. Given the growing need for uniform
experimental design within the Eocene climate modelling community and the
challenges faced in simulating the prominent features of Eocene c...
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... runoff in current generation climate models is impor- tant primarily for the redistribution of fresh water to the oceans and can have significant implications for deep wa- ter formation (e.g. Bice et al., 1997). Here we use the gra- dient of our Eocene topography to represent river runoff direction (Fig. 8). This data set was created using scripts made available by the National Center for Atmospheric Re- search (Rosenbloom et al., 2011). Regions where vectors do not reach the ocean (i.e. internal basins) were manually cor- rected. Topographic gradient has been used to constrain river directions in the overwhelming majority of ...
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Citations
... Earth is undergoing a rapid pace of landscape transformation, primarily from agriculture and development (Powers & Jetz, 2019). Likewise, some climate warming projections set the planet on a course to the Early Eocene (Burke et al., 2018), when northern Greenland was warm temperate forest (Herold et al., 2014;West et al., 2020). Although the concept of a native range is already fraught with ambiguities (Pereyra, 2020), this concern only escalates when organisms are no longer able to live in their historic distributions. ...
International and national conservation policies almost exclusively focus on conserving species in their historic native ranges, thus excluding species that have been introduced by people and some of those that have extended their ranges on their own accord. Given that many of such migrants are threatened in their native ranges, conservation goals that explicitly exclude these populations may overlook opportunities to prevent extinctions and respond dynamically to rapidly changing environmental and climatic conditions. Focusing on terrestrial mammals, we quantified the number of threatened mammals that have established new populations through assisted migration (i.e., introduction). We devised 4 alternative scenarios for the inclusion of assisted‐migrant populations in mainstream conservation policy with the aim of preventing global species extinctions. We then used spatial prioritization algorithms to simulate how these scenarios could change global spatial conservation priorities. We found that 22% (70 species out of 265) of all identified assisted‐migrant mammals were threatened in their native ranges, mirroring the 25% of all mammals that are threatened. Reassessing global threat statuses by combining native and migrant ranges reduced the threat status of 23 species (∼33% of threatened assisted migrants). Thus, including migrant populations in threat assessments provides a more accurate assessment of actual global extinction risk among species. Spatial prioritization simulations showed that reimagining the role of assisted‐migrant populations in preventing species extinction could increase the importance of overlooked landscapes, particularly in central Australia, Europe, and the southwestern United States. Our results indicated that these various and nonexhaustive ways to consider assisted‐migrant populations, with due consideration of potential conservation conflicts with resident taxa, may provide unprecedented opportunities to prevent species extinctions.
... 2. (Herold et al., 2014). The ocean temperature and salinity were initialized from a PETM quasi-equilibrated state, and the 18 O of seawater was initialized from a constant −1 ‰ to account for the absence of ice sheets in a hothouse climate . ...
Paleoclimate reconstructions of the Early Eocene provide important data constraints on the climate and hydrologic cycle under extreme warm conditions. Available terrestrial water isotope records have been primarily interpreted to signal an enhanced hydrologic cycle in the Early Eocene associated with large-scale warming induced by high atmospheric CO2. However, orbital-scale variations in these isotope records have been difficult to quantify and largely overlooked, even though orbitally driven changes in solar irradiance can impact temperature and the hydrologic cycle. In this study, we fill this gap using water isotope–climate simulations to investigate the orbital sensitivity of Earth's hydrologic cycle under different CO2 background states. We analyze the relative difference between climatic changes resulting from CO2 and orbital changes and find that the seasonal climate responses to orbital changes are larger than CO2-driven changes in several regions. Using terrestrial δ18O and δ2H records from the Paleocene–Eocene Thermal Maximum (PETM), we compare our modeled isotopic seasonal range to fossil evidence and find approximate agreement between empirical and simulated isotopic compositions. The limitations surrounding the equilibrated snapshot simulations of this transient event and empirical data include timing and time interval discrepancies between model and data, the preservation state of the proxy, analytical uncertainty, the relationship between δ18O or δ2H and environmental context, and vegetation uncertainties within the simulations. In spite of the limitations, this study illustrates the utility of fully coupled, isotope-enabled climate models when comparing climatic changes and interpreting proxy records in times of extreme warmth.
... To show the evolution of the East Asian monsoon climate in the Cenozoic era, we chose four representative geological periods, including the early Eocene (hereafter referred to as Eocene 55 Ma;Herold et al., 2014;Kiehl et al., 2021), mid-Miocene (hereafter referred to as Miocene 15 Ma;Frigola et al., 2018), mid-Pliocene (hereafter referred to as Pliocene 3 Ma;Feng et al., 2020;Haywood et al., 2016) and present day (i.e., pre-industrial period; Eyring et al., 2016). These periods cover the decrease in atmospheric CO 2 concentration and the uplift of Tibetan Plateau, which are seen as main drivers for the Asian climate evolution (Farnsworth et al., 2019;Wu et al., 2022). ...
The modern East Asian summer monsoon (EASM) features an extension from tropical to subtropical areas. However, the fundamental process that determines the northward extension of EASM in the geological history remains unclear. Here, we showed evidence from proxy data, climate modeling, and theoretical solutions that the northward extension of EASM to today's boundary emerged no later than the Miocene. The extension was driven by the monsoon seasonal march which features stepwise northward rainfall stages. The seasonal progression of monsoon was determined by Rossby wave responses from early summer to late summer and caused by the weakening of westerly jet colliding with the Tibetan Plateau (TP). The Rossby wave responses further led to a northward migration of the western Pacific subtropical high and thereby monsoon precipitation. Our findings propose a novel physical linkage between the geological evolution of EASM and the TP uplift in the context of monsoon seasonal march.
... Previous studies have suggested that paleogeographic uncertainty is a major cause of the disparities in different Eocene simulations (Li et al., 2018;Zhang et al., 2018). In recent years, several Eocene paleogeographic maps have been developed, such as those of Sewall et al. (2000), Bice and Marotzke (2001), Markwick (2007), Scotese (2014), Herold et al. (2014), Baatsen et al. (2016), He et al. (2019), and Straume et al. (2022). These maps differ in their plate tectonic models, which result in substantial variations in the position of tectonic plates and land-sea distributions. ...
... Here, with the fast version of the Norwegian Earth System Model (NorESM1-F, Guo et al. (2019a)), we further investigate differences in simulations with the two reference frames focusing on atmospheric circulations. We use three early Eocene paleogeographic maps presented by Herold et al. (2014), He et al. (2019), and Lunt et al. (2017) (see Fig. 3b in this reference) to carry out experiments. The first two maps (called HEROLD and HE in this study) use a hotspot reference frame (one type of mantle reference frame), and the other map (called PMAG in this study) uses a palaeomagnetic reference frame. ...
... With NorESM1-F, we employed the paleogeographic maps reconstructed by Herold et al. (2014), Lunt et al. (2017), andHe et al. (2019) to simulate the early Eocene climate (Table 1, Fig. 1). The HEROLD map (Fig. 1, top) in Herold et al. (2014) was used in DeepMIP standard experiments and many other early Eocene simulations (Lunt et al., 2017). ...
... This resulted in a palaeolocation (102.5° N, 39.5° E) that fits well in relation to palaeotopographic features such as the Hangay Dome and Qinling belt that are observed on the palaeogeographic map used in the DeepMIP simulations75 . ...
The dry continental interior of Asia has remained arid throughout most of its geological history, yet the future of this unique ecosystem remains unclear. Here we use palynological and isotopic records to track vegetation and moisture throughout the warm early Eocene (57 to 44 million years ago) as an analogue for extreme atmospheric CO2 scenarios. We show that rainfall temporarily doubled and replaced the regional steppe by forested ecosystems. By reconstructing the season of pedogenic carbonate growth, we constrain the soil hydrologic regime and show that most of this rainfall occurred during the summer season. This humid event is therefore attributed to an inland expansion of monsoonal moisture following the massive greenhouse gas release of the Palaeocene–Eocene Thermal Maximum as identified by a negative carbon isotope excursion. The resulting abrupt greening of the Central Asian steppe-desert would have enabled mammal dispersal and could have played a role in carbon cycle feedbacks by enhancing soil organic carbon burial and silicate weathering. These extreme Eocene proto-monsoons, albeit different from the topography-driven Asian monsoon today, highlight the potential for abrupt shifts in Central Asian rainfall and ecosystems under future global warming.
... The iCESM1.2 simulations were performed following the Deep-time Model Intercomparison Project protocols (Lunt et al., 2017) with early Eocene paleogeography and vegetation (56.0-47.8 Ma) (Herold et al., 2014) and atmospheric CO 2 levels of 1×, 3×, 6×, and 9× preindustrial values (284.7 ppmv). Seawater δ 18 O in the simulations was initialized from a constant value of −1.0‰ to account for the absence of ice sheets in a hothouse climate (Hollis et al., 2019;Shackleton & Kennett, 1975). ...
Plain Language Summary
Over the past 541 million years, the Earth has oscillated between warm (greenhouse) and cold (icehouse) climates. The most recent transition between a greenhouse and icehouse climate state occurred during the Eocene (56–34 million years ago). This transition shows a gradual cooling, previously suggested to be driven by a decline in atmospheric carbon dioxide (CO2). However, we know little about this transition in the North Atlantic Ocean. Previous studies show limited cooling of surface waters in this region. This suggests that changes in North Atlantic temperatures are not driven by CO2. To understand how sea surface temperature changes in the western North Atlantic, we analyzed the chemistry of microscopic marine fossils in sediments. Our results show a 4°C decline in temperature from the early (∼53 Ma) to the middle Eocene (∼42 Ma). This matches computer simulations of Eocene climate and confirms CO2 was responsible for the transition. The lack of cooling observed in previous work is probably due to the development of an ancient water mass known as Northern Component Water (observed today as North Atlantic Deep Water) and changes in how the Eocene ocean transported heat.
... In the early Eocene, the Arctic was ice free with warmer average surface temperatures and a weaker seasonality than today [1][2][3][4] . Warm-temperate and temperate forests were abundant in the Northern Hemisphere high latitudes 5,6 . While the concentration of carbon dioxide (CO 2 ) was much higher (~1,500 parts per million by volume) than today 7 , there are no proxies to constrain methane (CH 4 ) concentrations. ...
... Our baseline 'Eocene' simulation is conducted with the early Eocene greenhouse gas (GHG) concentrations and surface boundary conditions 5 , including the prescribed SSTs from a coupled simulation of the early Eocene 29 . A set of sensitivity experiments is then conducted by successively doubling the CH 4 concentration and changing the magnitude of polar amplification of SSTs (Extended Data Fig. 2). ...
... Extended Data Table 1 describes the experiments. forest 4 , an enhanced hydrological cycle 12 and expansive shallow shelf seas 5,13 , in particular in the southern Tethys and Arctic oceans. Earlier studies also suggested a nonlinear increase in the lifetime of CH 4 due to weaker sinks when concentrations of CH 4 are increased from modern values 14,15 . ...
Proxy data suggest that the early Eocene (∼56–47.8 million years ago) was characterized by a much weaker equator-to-pole temperature gradient than today. However, general circulation models consistently underestimate high-latitude temperatures indicated by proxy records, suggesting that they may miss important processes. Previous studies hypothesized that wintertime polar stratospheric clouds may have played an important role in Arctic warming through greenhouse forcing, but these studies did not consider the effects of atmospheric chemistry or the early Eocene topography. Here we examine these factors using a high-top atmospheric model with interactive chemistry. The lower orography in the low- to mid-latitude Northern Hemisphere early Eocene weakens the stratospheric circulation which, in combination with sufficiently high methane concentrations, leads to a substantial increase in polar stratospheric clouds in the Arctic winter. Furthermore, an increase in early Eocene polar stratospheric clouds due to a 16- to 64-fold higher than pre-industrial methane concentration results in a radiative forcing larger than the direct greenhouse effect from the methane itself. This polar stratospheric cloud-induced radiative forcing could cause up to 7.4 K of Arctic surface warming. These results point to the potential for nonlinear interactions between individual forcings.
... Many other Fur Formation localities are 1.5 times farther to the west, suggesting that they were much less accessible to insects dispersing from the east. Examination of fossil plants (Herold et al., 2014) as well as similarity of the upper microthermal western American Okanagan fauna with Mo-clay (e.g., Archibald & Cannings, 2022) support an upper microthermal paleoclimate for the latter. Extant Cicadellini are most diverse in the tropics but also include some groups, particularly the cosmopolitan Cicadella generic group that occur in the temperate zone. ...
The Fur Formation, located in the western Limfjord region of northern Jutland, Denmark, is famous for its numerous and often exceptionally well-preserved marine and terrestrial fossils (e.g., Bonde et al., 2008). The geological background of the Konservat-Lagerstätte was described by Pedersen & Surlyk (1983). The formation preserves a rich record of earliest Ypresian (Eocene, ca. 55 Ma) fossil insects (e.g., Madsen & Rasmussen, 2021). The fauna includes many hemipterans mostly representing Heteroptera and Sternorrhyncha but the diverse modern family Cicadellidae (leafhoppers) is represented by a single specimen among the >20,000 available insect fossils discovered so far.
... Based on CLAMP data, Wolfe (1994Wolfe ( , 1995 suggested generally high MAP from 1000 to 2000 mm for the Eocene of Pacific North America and thus proposed conditions that are comparable to our East Asian Pacific data. For the early Eocene climate of northern part of North America, warm and wet conditions are suggested, with MAT (mean annual temperare) ~20 °C and MAP at 1000-1500 mm (Wing and Greenwood 1993;Wing 1998;Shellito and Sloan 2006;Smith et al. 2012;Breedlovestrout et al. 2013;Herold et al. 2014;Greenwood et al. 2016;West et al. 2020). For the early Eocene floras from the London Clay, UK, MAP was determined as more than 2000 mm (Van Beuskom 1971). ...
Early Paleogene latitudinal precipitation gradients and patterns along the Pacific coast of Eurasia are studied in time and space using the Coexistence Approach, for the first time applied on an extensive regional palaeobotanical record. The palaeobotanical data used in this reconstruction are compiled from literature resources on 110 reasonably well-dated floras, including terrestrial deposits of 73 sites located in the Far East of Russia, Eastern Siberia, China, and Japan, and covering the early Palaeocene to early Eocene. Our reconstructions of precipitation for the Pacific side of Eurasia in the early Paleogene demonstrate a clear division (especially pronounced in the early Eocene) into two zones at ca. 50° N palaeolatitude on all precipitation parameters. Our results reveal very weak latitudinal precipitation gradients during the early and late Palaeocene. In the early Eocene, the gradient became more clearly pronounced, and a larger “arid” zone can be distinguished in the mid-latitudes. Our data suggest that in the early Paleogene, the global atmospheric circulation consisted of two well-defined cells, Hadley and Ferrell, while the polar cell was either absent or located over the Arctic Ocean and was very weak. Based on our reconstructions, the records could not be interpreted in terms of a monsoonal type of climate. The regional distribution of hygrophilous and xerophilous taxa in our early Eocene record largely coincides with the reconstructed precipitation pattern and generally corresponds to the distribution of coals and/or oil shales and red beds and/or evaporites, respectively.
... The paleogeography used here, represents the Ypresian stage of the Eocene, where the most notable differences to today's geography are the lack of the Himalaya, the lack of the enclosed Mediterranean basin, the proto-Paratethys and the Siberian Sea and a narrower Atlantic basin. The digital reconstruction from Herold et al. (2014) is used for the paleogeography, vegetation, and river routing. The orbital configuration is set to the modern values, because it represents a forcing close to the long-term average (Lunt et al., 2017). ...
The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP). These simulations target the early Eocene climatic optimum, a geological time period with high CO2 concentrations, analog to the upper range of end‐of‐century CO2 projections. Preindustrial and early Eocene simulations, at a range of CO2 levels are used to quantify the MHT changes in response to both CO2 and non‐CO2 related forcings. We found that atmospheric poleward heat transport increases with CO2, while oceanic poleward heat transport decreases. The non‐CO2 boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO2 concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. Our results show that the changes in the monsoon systems' latent heat transport is a robust feature of CO2 warming, which is in line with the currently observed precipitation increase of present day monsoon systems.
