Trace greenhouse gases are a fundamentally important component of Earth's global climate system sensitive to global change. However, their concentration in the pre-Pleistocene atmosphere during past warm greenhouse climates is highly uncertain because we lack suitable geochemical or biological proxies. This long-standing issue hinders assessment of their contribution to past global warmth and the equilibrium climate sensitivity of the Earth system (E(ss)) to CO(2). Here we report results from a series of three-dimensional Earth system modeling simulations indicating that the greenhouse worlds of the early Eocene (55 Ma) and late Cretaceous (90 Ma) maintained high concentrations of methane, tropospheric ozone, and nitrous oxide. Modeled methane concentrations were four- to fivefold higher than the preindustrial value typically adopted in modeling investigations of these intervals, even after accounting for the possible high CO(2)-suppression of biogenic isoprene emissions on hydroxyl radical abundance. Higher concentrations of trace greenhouse gases exerted marked planetary heating (> 2 K), amplified in the high latitudes (> 6 K) by lower surface albedo feedbacks, and increased E(ss) in the Eocene by 1 K. Our analyses indicate the requirement for including non-CO(2) greenhouse gases in model-based E(ss) estimates for comparison with empirical paleoclimate assessments, and point to chemistry-climate feedbacks as possible amplifiers of climate sensitivity in the Anthropocene.
Abstract. The first agricultural societies were established around 10 ka BP and had spread across much of Europe and southern Asia by 5.5 ka BP with resultant anthropogenic deforestation for crop and pasture land. Various studies have attempted to assess the biogeochemical implications for Holocene climate in terms of increased carbon dioxide and methane emissions. However, less work has been done to examine the biogeophysical impacts of this early land use change. In this study, global climate model simulations with HadCM3 were used to examine the biogeophysical effects of Holocene land cover change on climate, both globally and regionally, from the early Holocene (8 ka BP) to the early industrial era (1850 CE). Two experiments were performed with alternative descriptions of past vegetation: (i) potential natural vegetation simulated by TRIFFID but no land-use changes, and (ii) where the anthropogenic land use model, KK10 (Kaplan et al., 2009, 2011) has been used to set the HadCM3 crop regions. Snapshot simulations have been run at 1000 year intervals to examine when the first signature of anthropogenic climate change can be detected both regionally, in the areas of land use change, and globally. Results indicate that in regions of early land disturbance such as Europe and S.E. Asia detectable temperature changes, outside the normal range of variability, are encountered in the model as early as 7 ka BP in the June/July/August (JJA) season and throughout the entire annual cycle by 2–3 ka BP. Areas outside the regions of land disturbance are also affected, with virtually the whole globe experiencing significant temperature changes (predominantly cooling) by the early industrial period. Large-scale precipitation features such as the Indian monsoon, the intertropical convergence zone (ITCZ), and the North Atlantic storm track are also impacted by local land use and remote teleconnections. We investigated how advection by surface winds, mean sea level pressure (MSLP) anomalies, and tropospheric stationary wave train disturbances in the mid- to high-latitudes led to remote teleconnections.
The growth of the Tibetan Plateau throughout the past 66 million years has profoundly affected the Asian climate, but how this unparalleled orogenesis might have driven vegetation and plant diversity changes in eastern Asia is poorly understood. We approach this question by integrating modeling results and fossil data. We show that growth of north and northeastern Tibet affects vegetation and, crucially, plant diversity in eastern Asia by altering the monsoon system. This northern Tibetan orographic change induces a precipitation increase, especially in the dry (winter) season, resulting in a transition from deciduous broadleaf vegetation to evergreen broadleaf vegetation and plant diversity increases across southeastern Asia. Further quantifying the complexity of Tibetan orographic change is critical for understanding the finer details of Asian vegetation and plant diversity evolution.
The Laurentide ice sheet, which covered Canada during glacial periods, had a major influence on atmospheric circulation and surface climate, but its role in climate during the early Holocene (9–7 ka), when it was thinner and confined around Hudson Bay, is unclear. It has been suggested that the demise of the ice sheet played a role in the 8.2 ka event (an abrupt 1–3 °C Northern Hemisphere cooling lasting ~ 160 years) through the influence of changing topography on atmospheric circulation. To test this hypothesis, and to investigate the broader implications of changing ice sheet topography for climate, we analyse a set of equilibrium climate simulations with ice sheet topographies taken at 500 year intervals from 9.5 to 8.0 ka. Between 9.5 and 8.0 ka, our simulations show a 2 °C cooling south of Iceland and a 1 °C warming between 40° and 50°N in the North Atlantic. These surface temperature changes are associated with a weakening of the subtropical and subpolar gyres caused by a decreasing wind stress curl over the mid-North Atlantic as the ice sheet lowers. The climate response is strongest during the period of peak ice volume change (9.5–8.5 ka), but becomes negligible after 8.5 ka. The climatic effects of the Laurentide ice sheet lowering during the Holocene are restricted to the North Atlantic sector. Thus, topographic forcing is unlikely to have played a major role in the 8.2 ka event and had only a small effect on Holocene climate change compared to the effects of changes in greenhouse gases, insolation and ice sheet meltwater.
Abstract This study comprehensively investigates the impacts on the mean state of the Last Glacial Maximum (LGM) climate, particularly atmospheric circulation over the Northern Hemisphere associated with the different Paleoclimate Modelling Intercomparison Project Phase 4 (PMIP4) ice sheets, ICE-6G_C, GLAC-1D, and PMIP3, using the coupled atmosphere–ocean–vegetation model HadCM3B-M2.1aD. The simulation with PMIP3 ice sheets is colder than either of the two PMIP4 ice sheets mainly because of the larger area of land ice impacting surface albedo. However, changes in the circulation impact sea ice cover resulting in the GLAC-1D simulation being almost as cold. Although the PMIP4 ice sheets also induce different responses in the atmospheric circulation, some common features are identified in all simulations, including strengthening and lateral expansion of the winter upper-level North Atlantic jet with a large southwest-northeast tilt and summertime North Pacific jet, a southward shift of the wintertime Icelandic Low and Azores High and the summertime Pacific High. Compared to terrestrial-ocean reconstructions, all the PMIP4 ice sheet experiments overestimate the LGM cooling and wet conditions. The simulation with the ICE-6G_C ice sheet provides the closest reproduction of LGM climate, while the simulation with the PMIP3 ice sheet shows the coldest LGM climate state. Our study shows that in order to "benchmark" the ability of climate models to realistically simulate the LGM climate, we need to have reliable boundary conditions to ensure that any model biases are caused by model limitations rather than uncertainty about the LGM boundary conditions.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Paleoceanography and Paleoclimatology. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]Millennial-scale climate oscillations triggered by deglacial meltwater discharge in last glacial maximum simulationsAuthorsYvan MaloRoméiDRuza FIvanoviciDLauren JGregoireSamSherriff-TadanoPaulValdesiDSee all authors Yvan Malo RoméiDCorresponding Author• Submitting AuthorUniversity of LeedsiDhttps://orcid.org/0000-0003-3343-1459view email addressThe email was not providedcopy email addressRuza F IvanoviciDUniversity of LeedsiDhttps://orcid.org/0000-0002-7805-6018view email addressThe email was not providedcopy email addressLauren J GregoireUniversity of Leedsview email addressThe email was not providedcopy email addressSam Sherriff-TadanoUniversity of Leedsview email addressThe email was not providedcopy email addressPaul ValdesiDSchool of Geographical Sciences, University of BristoliDhttps://orcid.org/0000-0002-1902-3283view email addressThe email was not providedcopy email address
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