Global and Zonal-Mean Hydrological Response to Early Eocene Warmth

Earth's hydrological cycle is expected to intensify in response to global warming, with a “wet-gets-wetter, dry-gets-drier” response anticipated over the ocean. Subtropical regions (∼15°–30°N/S) are predicted to become drier, yet proxy evidence from past warm climates suggests these regions may be characterized by wetter conditions. Here we use an integrated data-modeling approach to reconstruct global and zonal-mean rainfall patterns during the early Eocene (∼56–48 million years ago). The Deep-Time Model Intercomparison Project (DeepMIP) model ensemble indicates that the mid- (30°–60°N/S) and high-latitudes (>60°N/S) are characterized by a thermodynamically dominated hydrological response to warming and overall wetter conditions. The tropical band (0°–15°N/S) is also characterized by wetter conditions, with several DeepMIP models simulating narrowing of the Inter-Tropical Convergence Zone. However, the latter is not evident from the proxy data. The subtropics are characterized by negative precipitation-evaporation anomalies (i.e., drier conditions) in the DeepMIP models, but there is surprisingly large inter-model variability in mean annual precipitation (MAP). Intriguingly, we find that models with weaker meridional temperature gradients (e.g., CESM, GFDL) are characterized by a reduction in subtropical moisture divergence, leading to an increase in MAP. These model simulations agree more closely with our new proxy-derived precipitation reconstructions and other key climate metrics and imply that the early Eocene was characterized by reduced subtropical moisture divergence. If the meridional temperature gradient was even weaker than suggested by those DeepMIP models, circulation-induced changes may have outcompeted thermodynamic changes, leading to wetter subtropics. This highlights the importance of accurately reconstructing zonal temperature gradients when reconstructing past rainfall patterns

Reconstructing southern New Zealand Miocene terrestrial climate and ecosystems from plant fossils

The New Zealand landmass occupies an area on the edge of the Southern Ocean at 35–47°S. Though situated north of the subtropical front, the New Zealand landmass experiences direct influence of the Antarctic Circumpolar Current and westerly winds. A projection of the effect of fluctuations of the Antarctic ice mass and Southern Ocean circulation on the New Zealand environment and global atmospheric circulation has to rely on the assessment of terrestrial climatic conditions during a time when the Antarctic ice sheet was reduced. The early – middle Miocene (23–12 Ma) represents a period when the extent of the Antarctic ice sheet, as well as the Southern Ocean circulation was reduced. Two terrestrial depositional systems were active in southern New Zealand during this time: the Waipiata Volcanics and Lake Manuherikia. Quantification of floral response to climate in the Waipiata Volcanic and Manuherikia Groups is used to elucidate the effect of ocean circulation change on the New Zealand terrestrial environment. The earliest Miocene environment, prior to the northward movement of the Subtropical and Subantarctic fronts, can be quantified using floras in the Waipiata Volcanic Group (Foulden and Hindon Maar). Floras are diverse and suggest subtropical to warm-temperate conditions prevailed. There is some evidence for seasonality in the precipitation regime, but temperatures appear to be relatively consistent between the two investigated floras. A seasonal mid-latitudinal light regime may have amplified seasonal contrast. During deposition of the Manuherikia Group (Grey Lake, Vinegar Hill, Kawarau River and Nevis Valley) the environment was more changeable, both seasonally and between floras. Absolute time constraints on the Manuherikia Group are poor, but all floras are between 19–15 Ma old and relative superposition of the floras is well-constrained. The trend in this period was from warm-temperate conditions and seasonal temperatures in the lowermost floras, Grey Lake and Vinegar Hill, to subtropical conditions with seasonal precipitation in the youngest floras, Kawarau River and Nevis Valley. This occurs during a period when the major front systems were shifting northwards, although still not converging with the New Zealand landmass. The seasonal precipitation regime could be caused by southern migration of the westerly wind belt in response to the presence of a subtropical high-pressure cell over mid-latitudes during the early Miocene. The middle/late Miocene boundary is represented in the Dunedin Volcanic Group (Double Hill and Kaikorai Valley), which is constituted of volcanoclastics and basalts of a multiple-vent volcanic edifice associated with the Waipiata Volcanic Field. Floras of this group are warm-temperate, but Kaikorai Valley appears to have the signature of frost-adaptation. Possibly, southern New Zealand during the middle/late Miocene boundary was subject to influence from the Southern Ocean. This period may therefore mark the northward movement of the Subantarctic Front, the interception of this front with the southernmost extent of New Zealand and initiation of the on-land climatic transition towards modern conditions. This account of the early to middle Miocene terrestrial climate of southern New Zealand is in agreement with global climatic models and Miocene climatic reconstructions from oceanic proxies. During the Miocene the New Zealand landmass became gradually more susceptible to influences from the Antarctic and therefore cooled. Additionally, under generally warmer climatic conditions in the Miocene, such as is projected for future global scenarios under increased atmospheric carbon levels, seasonality in New Zealand appears to increase

Reconstructing southern New Zealand Miocene terrestrial climate and ecosystems from plant fossils

The New Zealand landmass occupies an area on the edge of the Southern Ocean at 35–47°S. Though situated north of the subtropical front, the New Zealand landmass experiences direct influence of the Antarctic Circumpolar Current and westerly winds. A projection of the effect of fluctuations of the Antarctic ice mass and Southern Ocean circulation on the New Zealand environment and global atmospheric circulation has to rely on the assessment of terrestrial climatic conditions during a time when the Antarctic ice sheet was reduced. The early – middle Miocene (23–12 Ma) represents a period when the extent of the Antarctic ice sheet, as well as the Southern Ocean circulation was reduced. Two terrestrial depositional systems were active in southern New Zealand during this time: the Waipiata Volcanics and Lake Manuherikia. Quantification of floral response to climate in the Waipiata Volcanic and Manuherikia Groups is used to elucidate the effect of ocean circulation change on the New Zealand terrestrial environment. The earliest Miocene environment, prior to the northward movement of the Subtropical and Subantarctic fronts, can be quantified using floras in the Waipiata Volcanic Group (Foulden and Hindon Maar). Floras are diverse and suggest subtropical to warm-temperate conditions prevailed. There is some evidence for seasonality in the precipitation regime, but temperatures appear to be relatively consistent between the two investigated floras. A seasonal mid-latitudinal light regime may have amplified seasonal contrast. During deposition of the Manuherikia Group (Grey Lake, Vinegar Hill, Kawarau River and Nevis Valley) the environment was more changeable, both seasonally and between floras. Absolute time constraints on the Manuherikia Group are poor, but all floras are between 19–15 Ma old and relative superposition of the floras is well-constrained. The trend in this period was from warm-temperate conditions and seasonal temperatures in the lowermost floras, Grey Lake and Vinegar Hill, to subtropical conditions with seasonal precipitation in the youngest floras, Kawarau River and Nevis Valley. This occurs during a period when the major front systems were shifting northwards, although still not converging with the New Zealand landmass. The seasonal precipitation regime could be caused by southern migration of the westerly wind belt in response to the presence of a subtropical high-pressure cell over mid-latitudes during the early Miocene. The middle/late Miocene boundary is represented in the Dunedin Volcanic Group (Double Hill and Kaikorai Valley), which is constituted of volcanoclastics and basalts of a multiple-vent volcanic edifice associated with the Waipiata Volcanic Field. Floras of this group are warm-temperate, but Kaikorai Valley appears to have the signature of frost-adaptation. Possibly, southern New Zealand during the middle/late Miocene boundary was subject to influence from the Southern Ocean. This period may therefore mark the northward movement of the Subantarctic Front, the interception of this front with the southernmost extent of New Zealand and initiation of the on-land climatic transition towards modern conditions. This account of the early to middle Miocene terrestrial climate of southern New Zealand is in agreement with global climatic models and Miocene climatic reconstructions from oceanic proxies. During the Miocene the New Zealand landmass became gradually more susceptible to influences from the Antarctic and therefore cooled. Additionally, under generally warmer climatic conditions in the Miocene, such as is projected for future global scenarios under increased atmospheric carbon levels, seasonality in New Zealand appears to increase

A new perspective on Late Eocene and Oligocene vegetation and paleoclimates of South-eastern Australia

We present a composite terrestrial pollen record of latest Eocene through Oligocene (35.5–23 Ma) vegetation and climate change from the Gippsland Basin of south-eastern Australia. Climates were overwhelmingly mesothermic through this time period, with mean annual temperature (MAT) varying between 13 and 18 °C, with an average of 16 °C. We provide evidence to support a cooling trend through the Eocene–Oligocene Transition (EOT), but also identify three subsequent warming cycles through the Oligocene, leading to more seasonal climates at the termination of the Epoch. One of the warming episodes in the Early Oligocene appears to have also occurred at two other southern hemisphere sites at the Drake Passage as well as off eastern Tasmania, based on recent research. Similarities with sea surface temperature records from modern high southern latitudes which also record similar cycles of warming and cooling, are presented and discussed. Annual precipitation varied between 1200 and 1700 mm/yr, with an average of 1470 mm/yr through the sequence. Notwithstanding the extinction of Nothofagus sg. Brassospora from Australia and some now microthermic humid restricted Podocarpaceae conifer taxa, the rainforest vegetation of lowland south-eastern Australia is reconstructed to have been similar to present day Australian Evergreen Notophyll Vine Forests existing under the sub-tropical Köppen-Geiger climate class Cfa (humid subtropical) for most of the sequence. Short periods of cooler climates, such as occurred through the EOT when MAT was ~ 13 °C, may have supported vegetation similar to modern day Evergreen Microphyll Fern Forest. Of potentially greater significance, however, was a warm period in the Early to early Late Oligocene (32–26 Ma) when MAT was 17–18 °C, accompanied by small but important increases in Araucariaceae pollen. At this time, Araucarian Notophyll/Microphyll Vine Forest likely occurred regionally. © 2022 Elsevier B.V

Climate variability, heat distribution and polar amplification in the unipolar 'doubthouse' of the Oligocene Supplement Model Data

<p>Supplementary Data for the Climate variability, heat distribution and polar amplification in the unipolar 'doubthouse' of the Oligocene paper. The SST model scripts and programs are associated with the publication "The enigma of Oligocene climate and global surface temperature evolution" by C.L. O'Brien, M. Huber, E. Thomas, M. Pagani, J.R. Super, L.E. Elder, and P. M. Hull, in Proceedings of the National Academy of Science, 2020.  https://www.pnas.org/lookup/doi/10.1073/pnas.2003914117. </p><p>The scripts and programs for the precipitation model were written by X. Liu. They were based on the scripts for Oligocene model-data temperature comparison written by M. Huber (available at https://doi.org/10.4231/SFX6-RZ18). The CESM simulations were carried out by M. Huber and collaborators (A. Goldner, N. Herold, A. Dicks). The UK model simulations were carried out by Kennedy-Asser et al. (2019) and made available through the Bridge repository (https://www.paleo.bristol.ac.uk/ummodel/scripts/papers/Kennedy-Asser_et_al_2019.html).  </p><p>The Oligocene_driver.sh script should run all the underly codes and call all the data necessary to generate the figures in the manuscript.  The climate model outputs are in NetCDF format and in the folder "NETCDF_FILES". The many code is written in NCL (http://ncl.ucar.edu) which is freely available.</p&gt

Climate variability, heat distribution and polar amplification in the unipolar 'doubthouse' of the Oligocene Supplementary Data

<p>Supplementary Data for the Climate variability, heat distribution and polar amplification in the unipolar 'doubthouse' of the Oligocene paper. Oligocene mean annual temperature (MAT) and mean annual precipitation (MAP) Data is based on a nearest living relative (NRL) analysis. </p><p>Compiled sea surface temperatures (SSTs) for the Oligocene include alkenone based Uk'37, isoprenoidal glycerol dialkyl glycerol tetraether (isoGDGT) TEX86, biogenic calcite δ18O and clumped isotope (D47) data. All SST data are associated with the publication "The enigma of Oligocene climate and global surface temperature evolution" by C.L. O'Brien, M. Huber, E. Thomas, M. Pagani, J.R. Super, L.E. Elder, and P. M. Hull, in Proceedings of the National Academy of Science, 2020.  https://www.pnas.org/lookup/doi/10.1073/pnas.2003914117 . </p&gt

Late Pliocene to early Pleistocene climate dynamics in western North America based on a new pollen record from paleo-Lake Idaho

Marked by the expansion of ice sheets in the high latitudes, the intensification of Northern Hemisphere glaciation across the Plio/Pleistocene transition at ~ 2.7 Ma represents a critical interval of late Neogene climate evolution. To date, the characteristics of climate change in North America during that time and its imprint on vegetation has remained poorly constrained because of the lack of continuous, highly resolved terrestrial records. We here assess the vegetation dynamics in northwestern North America during the late Pliocene and early Pleistocene (c. 2.8–2.4 Ma) based on a pollen record from a lacustrine sequence from paleo-Lake Idaho, western Snake River Plain (USA) that has been retrieved within the framework of an International Continental Drilling Program (ICDP) coring campaign. Our data indicate a sensitive response of forest ecosystems to glacial/interglacial variability paced by orbital obliquity across the study interval, and also highlight a distinct expansion of steppic elements that likely occurs during the first strong glacial of the Pleistocene, i.e. Marine Isotope Stage 100. The pollen data document a major forest biome change at ~ 2.6 Ma that is marked by the replacement of conifer-dominated forests by open mixed forests. Quantitative pollen-based climate estimates suggest that this forest reorganisation was associated with an increase in precipitation from the late Pliocene to the early Pleistocene. We attribute this shift to an enhanced moisture transport from the subarctic Pacific Ocean to North America, confirming the hypothesis that ocean-circulation changes were instrumental in the intensification of Northern Hemisphere glaciation.Deutsche Forschungsgemeinschaft (DE)Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

The Paleogene to Neogene climate evolution and driving factors on the Qinghai-Tibetan Plateau

The growth of the Qinghai-Tibetan Plateau (QTP) during the Cenozoic drove dramatic climate and environmental change in this region. However, there has been limited comprehensive research into evolution of climate during this interval. Here we present a quantitative reconstruction using Bioclimatic Analysis (BA) and Joint Probability Density Functions (JPDFs) based on data available for 48 fossil floras, including macrofossils and palynological fossils collected in the QTP area from the Paleogene to Neogene (66–2.58 Ma). Both methods indicate that there was an overall decline in temperature and precipitation. Paleoclimatic simulations using Hadley Centre Coupled Model version3 (HadCM3) show that the most prominent climate change was very likely driven by QTP orographic evolution from the late Eocene, which was accompanied by a shift in temperature from a latitudinal distribution to a topographically controlled pattern. In addition, with the growth of the QTP, temperature and precipitation decreased gradually in the northeastern part of the plateau. Different sources of evidence, including plant fossil records, climate simulations and other proxies, indicate that the topographic evolution of the QTP and other geological events, in conjunction with global cooling, may have been the main factors driving climate change in this region. This research can provide insights into Cenozoic environmental change and ecosystem evolution.</p

Deriving temperature estimates from Southern Hemisphere leaves

The percentage of woody dicots with entire-margined leaves in a flora is known to be positively correlated with mean annual temperature (Leaf Margin Analysis - LMA) but this relationship is not globally uniform. In particular the floras of Australia and New Zealand have been regarded as displaying a different physiognomic relationship to climate than floras seen in the Northern Hemisphere. This difference is more marked in New Zealand where the LMA relationship appears entirely absent. Here we amass data for both Northern and Southern hemispheres using standard protocols and show that regional variations in the leaf margin-mean annual temperature relationship are real but become less significant when other characters are included. Even New Zealand falls into line and most of the mean annual temperature signal in New Zealand floras is encoded in non-margin features. We introduce a new CLAMP (Climate Leaf Analysis Multivariate Program) calibration dataset for the Southern Hemisphere, comprising leaf physiognomic data from Argentina, Bolivia, South Africa, Australia, New Zealand and other Pacific Islands that offers comparable precision for climate prediction to similar datasets derived from the Northern Hemisphere. (C) 2014 Elsevier B.V. All rights reserved