CL1.14

CO₂ is the most important greenhouse gas in the Earth’s atmosphere and has fluctuated considerably over geological time. However, proxies for past CO₂ concentrations have large uncertainties and are mostly limited to Devonian and younger times. Consequently, CO₂ modelling plays a key role in reconstructing past climate fluctuations. Facing the limitations with the current CO₂ models, we aim to refine two important forcings for CO₂ levels over the Phanerozoic, namely carbon degassing and silicate weathering.

Silicate weathering and carbonate deposition is widely recognized as a primary sink of carbon on geological timescales and is largely influenced by changes in climate, which in turn is linked to changes in paleogeography. The role of paleogeography on silicate weathering fluxes has been the focus of several studies in recent years. Their aims were mostly to constrain climatic parameters such as temperature and precipitation affecting weathering rates through time. However, constraining the availability of exposed land is crucial in assessing the theoretical amount of weathering on geological time scales. Associated with changes in climatic zones, the fluctuation of sea-level is critical for defining the amount of land exposed to weathering. The current reconstructions used inmodels tend to overestimate the amount of exposed land to weathering at periods with high sea levels. Through the construction of continental flooding maps, we constrain the effective land area undergoing silicate weathering for the past 520 million years. Our maps not only reflect sea-level fluctuations but also contain climate-sensitive indicators such as coal (since the Early Devonian) and evaporites to evaluate climate gradients and potential weatherablity through time. This is particularly important after the Pangea supercontinent formed but also for some time after its break-up.

Whilst silicate weathering is an important CO₂ sink, volcanic carbon degassing is a major source but one of the least constrained climate forcing parameters. There is no clear consensus on the history of degassing through geological time as there are no direct proxies for reconstructing carbon degassing, but various proxy methods have been postulated. We propose new estimates of plate tectonic degassing for the Phanerozoic using both subduction flux from full-plate models and zircon age distribution from arcs (arc-activity) as proxies.

The effect of revised modelling parameters for weathering and degassing was tested in the well-known long-term models GEOCARBSULF and COPSE. They revealed the high influence of degassing on CO₂ levels using those models, highlighting the need for enhanced research in this direction. The use of arc-activity as a proxy for carbon degassing leads to interesting responses in the Mesozoic and brings model estimates closer to CO₂  proxy values. However, from simulations using simultaneously the revised input parameters (i.e weathering and degassing) large model-proxy discrepancies remain and notably for the Triassic and Jurassic.

How to cite: M. Marcilly, C., Torsvik, T. H., Domeier, M., and Royer, D. L.: Revising key parameters for long-term carbon cycle models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2794, https://doi.org/10.5194/egusphere-egu21-2794, 2021.

The Permo–Triassic Boundary (PTB) marks a time of profound climatic change. Near the PTB (~252 Ma), the largest known mass extinction occurred with more than 90% of marine species and 70% of terrestrial species became extinct. The mass extinction is linked to a massive warming event at the PTB, where tropical regions became too hot for survival ofspecies. The increase in atmospheric CO₂ during the Permian and the PTB is mainly attributed to the decrease of chemically weatherable fresh silicate rock due to orogenesis, and the CO₂ released in the atmosphere from the Siberian Traps. In this study, we use the UK Met Office fully coupled HadCM3L General Circulation Model (GCM) to perform Permo-Triassic climate simulations with different atmospheric CO₂ values that encompass most of the estimates of atmospheric CO₂ concentration during this time, to provide more insights about the climate changes during the end Permian – early Triassic. Specifically, we focus on: a) the spatial extension of dry conditions/lethally hot temperatures under different CO₂ conditions, b) the seasonal surface temperature difference and precipitation changes at higher latitudes and c) the effects of increased atmospheric CO₂ on the large-scale wind and monsoonal circulation. 

How to cite: Zoura, D., Hill, D. J., Hunter, S. J., Haywood, A. M., and Wignall, P. B.: The effects of CO2 increase and its link to the mass extincton at the Permo-Triassic boundary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8367, https://doi.org/10.5194/egusphere-egu21-8367, 2021.

The evolution of continents over the past 250 million year is remarked by the breakup of the Pangea supercontinent. The changes of continents must have important influences on regional and global monsoon systems because monsoons are primarily a result of land-sea thermal contrast.

To study how the monsoon system had been evolved with continent changes over the past 250 million years, we carried out a series of climate simulations, using the Community Earth System Model (CESM). Changes in continents, mountain building, solar radiation, and carbon dioxide (CO2) are all considered in the simulations. In the present talk, we will present our preliminary simulation results of how the mega-monsoon associated with the supercontinent Pangea evolved into the six regional monsoons at the present over the past 250 million years. We will also demonstrate ocean circulation changes with different continent distributions, such as ENSO, and its influences on regional monsoons. Monsoon impacts on land-surface processes and the associated carbon-cycle will be also presented.

How to cite: Hu, Y., Guo, J., Li, X., Lan, J., Lin, Q., Han, J., Zhang, J., Liu, Y., and Yang, J.: Evolution of the monsoon system over the past 250 million years, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1459, https://doi.org/10.5194/egusphere-egu21-1459, 2021.

For modern Earth, the annual-mean equatorial atmosphere is flowing from east to west or called easterly winds. This is mainly due to the deceleration effect of the seasonal cross-equatorial flows of the Hadley cells, against the acceleration effect of equatorial Rossby and Kelvin waves excited from tropical convection and latent heating release. In this work, we examine the evolution of equatorial winds during the past 250 million years (Ma) using the global Earth system model CESM1.2.2. Three climatic factors different from the modern Earth, solar constant, atmospheric CO2 concentration, and land-sea configuration, are considered in the simulations. We find that the equatorial winds in the upper troposphere change the sign to westerly flows or called atmospheric superrotation in certain eras. The strength of the superrotation is comparable to the magnitude of the present easterly winds, several meters per second, not strong. This phenomenon occurs when the waves are relatively stronger and/or the Hadley cells are relatively weaker, which in turn are due to the changes in the three factors.

How to cite: Lan, J., Yang, J., and Hu, Y.: Weak Equatorial Superrotation During the Past 250 Million years, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1503, https://doi.org/10.5194/egusphere-egu21-1503, 2021.

Dust in the atmosphere can affect climate by directly absorbing and scattering solar radiation. In present day, most of dust is emitted from the dry regions over North Africa and Arabian Peninsula, it has been shown that they impact on global mean surface temperature, African monsoon, number of tropical cyclones over the Atlantic Ocean, ENSO variability and the strength of Atlantic meridional ocean circulation (AMOC). During the Mesozoic, the continental configuration was very different from the present, the supercontinent Pangea gradually broke up and Atlantic Ocean formed during this time period. On a different continental configuration, the area and location of dry regions may be very different, so the dust emission and atmospheric dust loading is different too. In this work, we use the global Earth system model CESM1.2.2 to examine the influence of dust on climate during the Mesozoic. Specifically, we simulate the dust and climate at two time slices, 250 million years ago (Ma) and 80 Ma. Results show that the atmospheric dust loading in both periods was much higher than that of present day. Such dust induced significant cooling of the surface climate, especially over polar regions.

How to cite: Lin, Q. and Liu, Y.: Influence of Dust on Climate during the Mesozoic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6803, https://doi.org/10.5194/egusphere-egu21-6803, 2021.

A set of deep-time climate simulations each 10Ma years from 250Ma to PI are conducted by using the NCAR-CESM1.2, for understanding the evolution of the ocean monsoon regions driven by tectonic dynamics over the past 250 million years and exploring its variation mechanisms. In recent years, scientists have proposed the concept of a global monsoon system, which includes not only typical monsoon regions (such as the African monsoon region and South Asian monsoon region), but also the atypical Northwest Pacific Ocean monsoon region. Research on the ocean monsoon is very limited, especially in the field of paleoclimate. The results in this paper show that the horizontal gradients of the thickness of the ocean mixed layer may be more important for the formation of the ocean monsoon than land-sea thermal contrast, which is confirmed by the aquaplanet simulations with various gradients of the ocean mixed-layer thickness. Near the Pacific monsoon region in the northern hemisphere, the thickness of the ocean mixed layer has obvious meridional and zonal gradients, which will correspond to the meridional and zonal thermal contrasts. In addition, there are obvious seasonal reversals in the gradients of the ocean mixed-layer thickness in summer and winter, and the corresponding longitudinal and zonal thermal contrast produce seasonal reversals. Over the past 250 million years, the thickness of the ocean mixed layer on the east side of the Pacific Ocean Basin in the Northern Hemisphere has deepened, and the corresponding ocean monsoon area on the east side of the Pacific Ocean has decreased. The changes in the thickness of the ocean mixed layer are closely related to the changes in the surface wind field. Examining the changes in the atmospheric circulations, we find that the Pacific subtropical high strengthens and extends from east to the west bank of the ocean basin, where the atmospheric low-level anticyclonic circulation causes the ocean surface layer to converge and sink and thus causes the ocean mixed layer to deepen. The changes in the Pacific subtropical high are related to changes in the continental monsoon region. Since the 170Ma, the Pangea supercontinent splits up, causing the supercontinent's inland water vapor to increase, the land monsoon area to increase, and the ocean monsoon area to decrease. According to the "monsoon-desert mechanism" of Rodwell and Hoskins, we can understand the relationship between the strengthening of land monsoon condensation heating and the formation of subtropical high over the western ocean.

How to cite: Han, J., Hu, Y., and Liu, Y.: Evolution of the ocean monsoon regions over the past 250 million years, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3690, https://doi.org/10.5194/egusphere-egu21-3690, 2021.

How to cite: Zhu, C.: East‐Central Asian Climate Evolved With the Northward Migration of the High Proto‐Tibetan Plateau, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1878, https://doi.org/10.5194/egusphere-egu21-1878, 2021.

The early Eocene greenhouse represents the warmest interval of the Cenozoic and therefore provides a unique opportunity to understand how the climate system operates under elevated atmospheric CO₂ levels similar to those projected for the end of the 21st century. Early Eocene geological records indicate a large increase in global mean surface temperatures compared to present day (by ~14°C) and a greatly reduced meridional temperature gradient (by ~30% in SST). However, reproducing these large-scale climate features at reasonable CO₂ levels still poses a challenge for current climate models. Recent modelling studies indicate an important role for shortwave (SW) cloud feedbacks to drive increases in climate sensitivity with global warming, which helps to close the gap between simulated and reconstructed Eocene global warmth and temperature gradient. Nevertheless, the presence of such state-dependent feedbacks and their relative strengths in other models remain unclear.

In this study, we perform a systematic investigation of the simulated surface warming and the underlying mechanisms in the recently published DeepMIP ensemble. The DeepMIP early Eocene simulations use identical paleogeographic boundary conditions and include six models with suitable output: CESM1.2_CAM5, GFDL_CM2.1, HadCM3B_M2.1aN, IPSLCM5A2, MIROC4m and NorESM1_F. We advance previous energy balance analysis by applying the approximate partial radiative perturbation (APRP) technique to quantify the individual contributions of surface albedo, cloud and non-cloud atmospheric changes to the simulated Eocene top-of-the-atmosphere SW flux anomalies. We further compare the strength of these planetary albedo feedbacks to changes in the longwave atmospheric emissivity and meridional heat transport in the warm Eocene climate. Particular focus lies in the sensitivity of the feedback strengths to increasing global mean temperatures in experiments at a range of atmospheric CO₂ concentrations between x1 to x9 preindustrial levels.

Preliminary results indicate that all models that provide data for at least 3 different CO₂ levels show an increase of the equilibrium climate sensitivity at higher global mean temperatures. This is associated with an increase of the overall strength of the positive SW cloud feedback with warming in those models. This nonlinear behavior seems to be related to both a reduction and optical thinning of low-level clouds, albeit with intermodel differences in the relative importance of the two mechanisms. We further show that our new APRP results can differ significantly from previous estimates based on cloud radiative forcing alone, especially in high-latitude areas with large surface albedo changes. We also find large intermodel variability and state-dependence in meridional heat transport modulated by changes in the atmospheric latent heat transport. Ongoing work focuses on the spatial patterns of the climate feedbacks and the implications for the simulated meridional temperature gradients.

How to cite: Steinig, S., Zhu, J., and Feng, R. and the DeepMIP team: Drivers of (non-)linear Eocene surface warming in the DeepMIP ensemble, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14127, https://doi.org/10.5194/egusphere-egu21-14127, 2021.

The early Eocene is a warm period with very high atmospheric CO₂ levels, which receives many interests from climate modelling aspects. To simulate the early Eocene paleoclimate, a realistic reconstruction for land-sea distribution, paleotopography and paleobathymetry is the fundamental step. Here, we present global paleogeographic reconstructions for the early Eocene (~55 Ma), based on integrated paleogeographic data set, the Plate-tectonic reconstruction software (GPlates) and Geographic Information System software (ArcGIS). Comparing with previous paleogeographic reconstructions, we improve the reconstructions incorporated many latest geologic data and data set, including: (1) better representations of the Tethys Sea, some marginal or inland seas in the East and Southeast Asia, Atlantic and Arctic region, and the Drake Passage and Tasmanian Gateway; (2) integrated paleoelevation data of global high plateaus and mountains, especially the paleotopography of East Asia, and adopting the latest paleotopographic reconstruction data of the Antarctic; and (3) using the updated data set of oceanic crust paleo-age and oceanic sediment thickness to reconstruct the paleobathymetry.

How to cite: He, Z., Zhang, Z., and Guo, Z.: Reconstructing early Eocene (~55 Ma) paleogeographic boundary conditions for use in paleoclimate modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10665, https://doi.org/10.5194/egusphere-egu21-10665, 2021.

The early Eocene is a warm period with a very high atmosphere CO₂ level in the Cenozoic. It  provides a good reference for our future climate under the Representative Concentration Pathway 8.5 scenario. Therefore, the early Eocene climate has received many attentions in  modeling studies, for example, the Deep-Time Model Intercomparison Project (DeepMIP). However, the early Eocene palaeogeographic conditions show remarkable contrasts to the present conditions. Meanwhile, there are a few different reconstructions for the early Eocene palaeogeography, which may cause further model spreads in simulating the early Eocene warm climate. Here, we present a series of experiments carried out with the NorESM1-F, under the framework of DeepMIP. In these experiments, we consider three different palaeogeographic reconstructions for the early Eocene. We also compare our simulations with climate proxy records, to validate which palaeogeographic reconstructions can reproduce simulations that agree better with the climate proxy records.

How to cite: Zhang, Z., Zhang, Z., and Guo, Z.: Early Eocene simulations with three different palaeogeographic conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14704, https://doi.org/10.5194/egusphere-egu21-14704, 2021.

Geological evidence shows that the Asian inland environment experienced enhanced aridity from the Early to the Late Eocene. The underlying mechanism for this enhanced Eocene aridity in the Asian inland is still not well illustrated and varies between global cooling and early Tibetan Plateau uplift. In this report, we evaluate the climate impact of three factors, global cooling, topographic uplift and land–sea reorganization, on the enhanced Eocene aridity in Asian inland, in the perspective view from paleoclimate modeling. Paleoclimate modeling supports the Eocene aridification in Asian inland explored by paleoclimate reconstruction. Both the early uplift of Tibetan Plateau and global cooling induced by atmospheric CO₂ reduction contributed to the enhanced aridity in Asian inland in the late Eocene. The Eocene land sea redistribution caused the precipitation increase in Asian inland and hence didn’t contribute to the enhanced aridity there. The uplift of the central Tibetan Plateau during the early stage of the India–Asia collision is emphasized more to be responsible for the long-term Asian inland aridification during the Eocene, playing at least an equally important role as the global cooling induced by decrease in atmospheric CO₂. The variation of atmospheric CO₂ is likely more important in modulating the regional aridity, leading to the short-term fluctuations in this Eocene Asian inland aridification.

How to cite: Li, X., Zhang, Z., Zhang, R., and Yan, Q.: The roles of global cooling and early Tibetan Plateau uplift on the enhanced aridity in Eocene Asian inland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3578, https://doi.org/10.5194/egusphere-egu21-3578, 2021.

Global cooling, the early uplift of the Tibetan Plateau, and the retreat of the Paratethys are three main factors that regulate long-term climate change in the Asian interior during the Cenozoic. However, the debated elevation history of the Tibetan Plateau and the overlapping climate effects of the Tibetan Plateau uplift and Paratethys retreat makes it difficult to assess the driving mechanism on regional climate change in a particular period. Some recent progress suggests that precisely dated Paratethys transgression/regression cycles appear to have fluctuated over broad regions with low relief in the northern Tibetan Plateau in the middle Eocene–early Oligocene, when the global climate was characterized by generally continuous cooling followed by the rapid Eocene–Oligocene climate transition (EOT). Therefore, a middle Eocene–early Oligocene record from the Asian interior with unambiguous paleoclimatic implications offers an opportunity to distinguish between the climatic effects of the Paratethys retreat and those of global cooling.

Here, we present a complete paleolake salinity record from middle Eocene to early Miocene (~42-29 Ma) in the Qaidam Basin using detailed clay boron content and clay mineralogical investigations. Two independent paleosalimeters, equivalent boron and Couch’s salinity, collectively present a three-staged salinity evolution, from an oligohaline–mesohaline environment in the middle Eocene (42-~34 Ma) to a mesosaline environment in late Eocene-early Oligocene (~34-~29 Ma). This clay boron-derived salinity evolution is further supported by the published chloride-based and ostracod-based paleosalinity estimates in the Qaidam Basin. Our quantitative paleolake reconstruction between ~42 and 29 Ma in the Qaidam Basin resembles the hydroclimate change in the neighboring Xining Basin, of which both present good agreement with changes of marine benthic oxygen isotope compositions. We thus speculated that the secular trend of clay boron-derived paleolake salinity in ~42-29 Ma is primarily controlled by global cooling, which regulates regional climate change by influencing the evaporation capacity in the moisture source of Qaidam Basin. Superimposed on this trend, the Paratethys transgression/regression cycles served as an important factor regulating wet/dry fluctuations in the Asian interior between ~42 and ~34 Ma.

How to cite: Ye, C., Yang, Y., Fang, X., Zhang, W., Song, C., and Yang, R.: Paleolake salinity evolution in the Qaidam Basin (NE Tibetan Plateau) between ~42 and 29 Ma: Links to global cooling and Paratethys sea incursions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13526, https://doi.org/10.5194/egusphere-egu21-13526, 2021.

The Cenozoic Asian aridification has been related to the retreat of the Paratethys, the uplift of the Tibet, and/or global cooling. However, the details of the mechanisms responsible for this paleoclimate shift remain poorly constrained. Modern observations indicate that interactions between mid-latitude westerlies and the Pamir-Tian Shan Mountains significantly impact hydroclimate patterns in central Asia today, and may have played an important role in driving Asian aridification during the Cenozoic. However, the timing when this topographic-atmospheric framework was established remains poorly constrained.

Here, we present magnetostratigraphy, U-Pb geochronology, thermochronology, paleoclimatology, stable carbon and oxygen isotope geochemistry, and climate modelling techniques to the Cenozoic sedimentary sequences in the Tajik Basin. Our results show that: 1) the penultimate and ultimate retreat of the Paratethys from central Asia occurred at ~41 and ~37.4 Ma, respectively; 2) the Pamirs have experienced active deformation and accelerated exhumation during the late Oligocene to early Miocene; 3) the windward (western) side of the Pamir and Tian Shan has been characterized by a wetter climate changes, whereas, the leeward (eastern) side of the orogen has been characterized by more arid conditions since the Late Oligocene; 4)  This distinct east-west hydroclimate differences, when integrated with climate modeling results, suggests that at least part of the Pamir-Tian Shan mountains had reached elevations ≥ 3 km and acted as a moisture barrier for the westerlies since ~25 Ma. We suggest that the interactions between the westerlies and the Pamir-Tian Shan orogen played an important role in driving Asian aridification since the Late Oligocene.

How to cite: Wang, X., Carrapa, B., Zhang, X., Oimuhammadzoda, I., and Chen, F.: Uplift of the Pamir and Tian Shan set hydroclimate patterns in central Asia since the late Oligocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7781, https://doi.org/10.5194/egusphere-egu21-7781, 2021.