Search

Rapid climate change was a major contributor to the end-Permian extinction (EPE). Although well constrained for the marine realm, relatively few records document the pace, nature, and magnitude of climate change across the EPE in terrestrial environments. We generated proxy records for chemical weathering and land surface temperature from continental margin deposits of the high-latitude southeastern margin of Gondwana. Regional climate simulations provide additional context. Results show that Glossopteris forest-mire ecosystems collapsed during a pulse of intense chemical weathering and peak warmth, which capped ∼1 m.y. of gradual warming and intensification of seasonality. Erosion resulting from loss of vegetation was short lived in the low-relief landscape. Earliest Triassic climate was∼10–14 °C warmer than the late Lopingian and landscapes were no longer persistently wet. Aridification, commonly linked to the EPE, developed gradually, facilitating the persistence of refugia for moisture-loving terrestrial groups. This research was also funded by U.S. National Science Foundation (NSF) grants EAR-1636625 (C.R. Fielding and D. Frank) and EAR-1636629 (A.M.E. Winguth and C. Winguth), and the Royal Swedish Academy of Sciences. We acknowledge NSF-sponsored high-performance computing support from Cheyenne provided by the U.S. National Center for Atmospheric Research (NCAR) Computational and Information Systems Laboratory.

Hurricanes and other extreme precipitation events can have devastating effects on population and infrastructure that can create problems for emergency responses and evacuation. Projected climate change and associated global warming may lead to an increase in extreme weather events that results in greater inundation from storm surges or massive precipitation. For example, record flooding during Hurricane Katrina or, more recently, during Hurricane Harvey in 2017, led to many people being cut off from aid and unable to evacuate. This study focuses on the impact of severe weather under climate change for areas of Harris County, TX that are susceptible to flooding either by storm surge or extreme rainfall and evaluates the transit demand and availability in those areas. Future risk of flooding in Harris County was assessed by GIS mapping of the 100-year and 500-year FEMA floodplains and most extreme category 5 storm tide and global sea level rise. The flood maps have been overlaid with population demographics and transit accessibility to determine vulnerable populations in need of transit during a disaster. It was calculated that 70% of densely populated census block groups are located within the floodplains, including a disproportional amount of low-income block groups. The results also show a lack of transit availability in many areas susceptible to extreme storm surge exaggerated with sea level rise. Further study of these areas to improve transit infrastructure and evacuation strategies will improve the outcomes of extreme weather events in the future.

Past studies of the end-Permian extinction (EPE), the largest biotic crisis of the Phanerozoic, have not resolved the timing of events in southern high-latitudes. Here we use palynology coupled with high-precision CA-ID-TIMS dating of euhedral zircons from continental sequences of the Sydney Basin, Australia, to show that the collapse of the austral Permian Glossopteris flora occurred prior to 252.3 Ma (~370 kyrs before the main marine extinction). Weathering proxies indicate that floristic changes occurred during a brief climate perturbation in a regional alluvial landscape that otherwise experienced insubstantial change in fluvial style, insignificant reorganization of the depositional surface, and no abrupt aridification. Palaeoclimate modelling suggests a moderate shift to warmer summer temperatures and amplified seasonality in temperature across the EPE, and warmer and wetter conditions for all seasons into the Early Triassic. The terrestrial EPE and a succeeding peak in Ni concentration in the Sydney Basin correlate, respectively, to the onset of the primary extrusive and intrusive phases of the Siberian Traps Large Igneous Province., The continental record of the end Permian mass extinction is limited, especially from high paleolatitudes. Here, Fielding et al. report a multi-proxy Permo-Triassic record from Australia, resolving the timing of local terrestrial plant extinction and the relationship with environmental changes.

Dissolved oxygen (DO) concentration in the ocean is an important component of marine biogeochemical cycles and will be greatly altered as climate change persists. In this study a global oceanic carbon cycle model (HAMOCC 2.0) is used to address how mechanisms of oxygen minimum zone (OMZ) expansion respond to changes in CO2 radiative forcing. Atmospheric pCO2 is increased at a rate of 1 % annually and the model is stabilized at 2 ×, 4 ×, 6 ×, and 8 × preindustrial pCO2 levels. With an increase in CO2 radiative forcing, the OMZ in the Pacific Ocean is controlled largely by changes in particulate organic carbon (POC) export, resulting in increased remineralization and thus expanding the OMZs within the tropical Pacific Ocean. A potential decline in primary producers in the future as a result of environmental stress due to ocean warming and acidification could lead to a substantial reduction in POC export production, vertical POC flux, and thus increased DO concentration particularly in the Pacific Ocean at a depth of 600–800 m. In contrast, the vertical expansion of the OMZs within the Atlantic is linked to increases POC flux as well as changes in oxygen solubility with increasing seawater temperature. Changes in total organic carbon and increase sea surface temperature (SST) also lead to the formation of a new OMZ in the western subtropical Pacific Ocean. The development of the new OMZ results in dissolved oxygen concentration of ≤ 50 µmol kg−1 throughout the equatorial Pacific Ocean at 4 times preindustrial pCO2. Total ocean volume with dissolved oxygen concentrations of ≤ 50 µmol kg−1 increases by 2.4, 5.0, and 10.5 % for the 2 ×, 4 ×, and 8 × CO2 simulations, respectively.

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (

A range of proxy observations have recently provided constraints on how Earth's hydrological cycle responded to early Eocene climatic changes. However, comparisons of proxy data to general circulation model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised. In this work, we undertake an intercomparison of GCM-derived precipitation and P − E distributions within the extended EoMIP ensemble (Eocene Modelling Intercomparison Project; Lunt et al., 2012), which includes previously published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure, and precipitation-relevant parameterisation schemes. We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to the preindustrial conditions. This is primarily due to elevated atmospheric paleo-CO2, resulting in elevated temperatures, although the effects of differences in paleogeography and ice sheets are also important in some models. For a given CO2 level, globally averaged precipitation rates vary widely between models, largely arising from different simulated surface air temperatures. Models with a similar global sensitivity of precipitation rate to temperature (dP∕dT) display different regional precipitation responses for a given temperature change. Regions that are particularly sensitive to model choice include the South Pacific, tropical Africa, and the Peri-Tethys, which may represent targets for future proxy acquisition. A comparison of early and middle Eocene leaf-fossil-derived precipitation estimates with the GCM output illustrates that GCMs generally underestimate precipitation rates at high latitudes, although a possible seasonal bias of the proxies cannot be excluded. Models which warm these regions, either via elevated CO2 or by varying poorly constrained model parameter values, are most successful in simulating a match with geologic data. Further data from low-latitude regions and better constraints on early Eocene CO2 are now required to discriminate between these model simulations given the large error bars on paleoprecipitation estimates. Given the clear differences between simulated precipitation distributions within the ensemble, our results suggest that paleohydrological data offer an independent means by which to evaluate model skill for warm climates.

What role did changes in marine carbon cycle processes and calcareous organisms play for glacial-interglacial variation in atmospheric pCO2? In order to answer this question, we explore results from an ocean biogeochemical ocean general circulation model. We make an attempt to systematically reconcile model results with time dependent sediment core data from the observations. For this purpose, simulated sensitivities of oceanic tracer concentrations to changes in governing carbon cycle parameters are fitted to measured sediment core data.We assume that the time variation of the governing carbon cycle parameters follows the general pattern of the glacial-interglacial deuterium anomaly. Our analysis provides an independent estimate of a maximum mean sea surface temperature drawdown of about 5 °C and a maximum outgassing of the land biosphere by about 430 PgC at the last glacial maximum as compared to preindustrial times. The overall fit of modelled paleoclimate tracers to observations, however, remains quite weak indicating the potential of more detailed modelling studies for full exploitation of the information as stored in the paleo-climatic archive. It can be confirmed, however, that a decline in ocean temperature and a more efficient biological carbon pump in combination with changes in ocean circulation are the key factors for explaining the glacial CO2 drawdown. The analysis suggests that potential changes in the export rain ratio POC:CaCO3 may not have a substantial imprint on the paleo-climatic archive. The use of the last glacial as an inverted analogue to potential ocean acidification impacts thus may be quite limited. A potential strong decrease in CaCO3 export production could contribute to the glacial CO2 decline in the atmosphere but remains hypothetical.

Differences in the Atlantic meridional overturning circulation (AMOC) from the Younger Dryas (YD) to the Holocene can be explained by, but not limited to, factors relating to atmospheric greenhouse gas concentrations, discharge of freshwater into the surface ocean, and changes in Earth's orbital parameters. Utilizing the Community Earth System Model (CESM1.0.5) with moderate resolution, this study evaluates how Younger Dryas seasonal and annual radiative forcing affect the climate change and variability. The Younger Dryas to Holocene changes in radiative forcing are mostly attributed to change in orbital parameters and to lesser extent to the relatively small rise in atmosphere pCO2, which is supported by a comparison of model simulations with proxy reconstructions of sea surface temperature and oceanic δ18O. These factors led to increased precipitation and reduced transport of water masses in the North Atlantic Ocean. Atmospheric pCO2 and orbital parameter changes are not substantial enough to explain the transition to the Younger Dryas northern hemispheric cooling. Younger Dryas to Holocene changes in the Monsoonal circulation over the African continent appears to be more affected by changes in orbital parameters than in atmospheric pCO2 but underestimated compared to observed reconstructions from ice and sediment cores. Thus, additional mechanisms such as fresh water hosed-cooling and/or ice sheet-albedo effect need to be considered to explain the Younger Dryas to Holocene climate change and variability.

The Permian–Triassic boundary (PTB, ~ 252.3 Ma) marks the largest mass extinction of the Phanerozoic, with a loss of more than 90% of marine organisms, and is characterized by lethally hot surface temperatures. The PTB global warming has been linked to greenhouse gas emissions from the Siberian Traps and associated coal-bed intrusions and likely led to severe environmental consequences, such as ocean acidification, a decline in the marine productivity, and extensive hypoxia. In order to understand these changes, feedbacks in the climate system have been explored with sensitivity climate simulations and compared to temperature proxies from the sedimentary record. The response of the PTB ocean circulation to an atmospheric perturbation of ~ 5000 Pg C, comparable to Earth's total fossil fuel inventory, leads to a global temperature increase by 3–4 °C and an increase in ocean stratification. The pole-to-equator temperature gradient decreases by 2 °C with an increase in CO2-radiative forcing, predominately due to snow albedo feedbacks over Gondwana. The greenhouse-induced warming would have led to a weakening of the Hadley cell and an associated decrease in the trade winds and equatorial primary productivity. These climatic changes might have been amplified by cloud-feedback processes. A reduced concentration of cloud condensation nuclei due to a biologic decline of dimethylsulfide caused by the temperature stress or changes in airborne mineral particles could have reduced the cloud albedo, particularly in high latitudes. Results from a climate simulation with reduced cloud albedo suggest a polar warming of up to 7 °C and a reduction of the pole-to-equator temperature gradient by ~ 4 °C in addition to the reduction caused by the increase in greenhouse-induced radiative forcing. The scenario with reduced cloud albedo further leads to an increase in ocean stratification and widespread low-oxygen concentrations in the Panthalassa during the Early Triassic.

We thank NERC grant NE/N006828/1 for providing funds for the first DeepMIP meeting in Boulder, Colorado, USA, in January 2016. Daniel J. Lunt acknowledges the NERC grant “Cretaceous–Paleocene–Eocene: Exploring Climate and Climate Sensitivity” (NE/K014757/1), and advanced ERC grant “The Greenhouse Earth System” (T-GRES, project reference 340923), awarded to Rich Pancost. Matthew Huber acknowledges funding from NSF OCE-0902882. Michiel L. J. Baatsen, Henk A. Dijkstra, and Anna S. von der Heydt acknowledge support by the Netherlands Earth System Science Centre (NESSC), financially supported by the Ministry of Education, Culture and Science (OCW), 024.002.001. Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (> 800ppmv) atmospheric CO2 concentrations. Although a post hoc intercomparison of Eocene (∼ 50 Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the early Eocene and the latest Paleocene (the EECO, PETM, and pre-PETM). Together with the CMIP6 pre-industrial control and abrupt 4 × CO2 simulations, and additional sensitivity studies, these form the first phase of DeepMIP-the Deep-time Model Intercomparison Project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, astronomical configuration, solar constant, land surface processes, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological data sets, which will be used to evaluate the simulations, will be developed. Publisher PDF

Anthropogenic global warming affects marine ecosystems in complex ways, and declining ocean oxygenation is a growing concern. Forecasting the geographical and bathymetric extent, rate, and intensity of future deoxygenation and its effects on oceanic biota, however, remains highly challenging because of the complex feedbacks in the Earth-ocean biota system. Information on past global warming events such as the Paleocene-Eocene Thermal Maximum (PETM, ~55.5 Ma), a potential analog for present and future global warming, may help in such forecasting. Documenting past ocean deoxygenation, however, is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column. As yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones (OMZs). We apply a novel proxy for paleoredox conditions, the iodine to calcium ratio (I/Ca) in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans, and compared our reconstruction with modeled oxygen levels. The reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the Atlantic, Indian Oceans, and possibly the Pacific Ocean, as well during the PETM, due to vertical and potentially lateral expansion of OMZs.

A frequently cited atmospheric CO2 threshold for the onset of Antarctic glaciation of ~780 ppmv is based on the study of DeConto and Pollard (2003) using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO2 concentrations passed through this threshold across the Eocene–Oligocene transition ~34 Ma. However, atmospheric CO2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from 7 different climate models with Eocene boundary conditions (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, and as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter.

Hourly surface temperature differences between Dallas–Fort Worth, Texas, metropolitan and rural sites have been used to calculate the urban heat island from 2001 to 2011. The heat island peaked after sunset and was particularly strong during the drought and heat wave in July 2011, reaching a single-day instantaneous maximum value of 5.4°C and a monthly mean maximum of 3.4°C, as compared with the 2001–11 July average of 2.4°C. This severe drought caused faster warming of rural locations relative to the metropolitan area in the morning as a result of lower soil moisture content, which led to an average negative heat island in July 2011 of −2.3°C at 1100 central standard time. The ground-based assessment of canopy air temperature at screening level has been supported by a remotely sensed surface estimate from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite, highlighting a dual-peak maximum heat island in the major city centers of Dallas and Fort Worth. Both ground-based and remotely sensed spatial analyses of the maximum heat island indicate a northwest shift, the result of southeast winds in July 2011 of ~2 m s−1 on average. There was an overall positive trend in the urban heat island of 0.14°C decade−1 in the Dallas–Fort Worth metropolitan area from 2001 to 2011, due to rapid urbanization. Superimposed on this trend are significant interannual and decadal variations that influence the urban climate.

Evidence from stratigraphic sections of the Panthalassa, Paleo-Tethys and Neo-Tethys suggests that the oceans experienced widespread anoxia during the Late Permian, which likely contributed to the extinction of ~ 90% of marine and ~ 70% of terrestrial species. The Late Permian and Early Triassic were also characterized by significant carbon isotope excursions implying that considerable perturbations in the carbon cycle occurred. Bathymetric features of the Panthalassa during this period are not well known since most of the ocean floor has been subducted; however, tectonic reconstructions suggest that active marine subduction zones surrounded Pangea. Thus, it is reasonable to assume that there was an active mid-ocean ridge system located in Panthalassa during the Late Permian. In this study, the impact of such a spreading center within Panthalassa on the climate and carbon cycle is investigated using a comprehensive climate system model for the end-Permian. This is a novel approach because a majority of previous simulations assumed a flat bottom for the Panthalassa deep-sea. The mid-ocean ridge (MOR) simulation enhanced vertical mixing and topographic steering of the currents near the ridge-axis but in comparison with the simulation using a flat bottom, changes in the global distribution of water masses and circulation in the Panthalassa were insignificant. Dissolved oxygen concentrations were not considerably affected by the implementation of the mid-ocean ridge. Thus the approximation of using a flat-bottom topography in ocean models for the Late Permian remains valid. In a second sensitivity study, the effect of a sill between the deep Paleo-Tethys and Panthalassa on water mass distribution and oxygen content has been investigated. Model results suggest that the introduction of a sill led to enhanced stratification, as well as an increase in salinity and temperature in the Paleo-Tethys. An associated reduction of the dissolved oxygen concentration to dysoxic to near-anoxic conditions below 1800 m suggests that the changes in sill height between the Paleo-Tethys and Panthalassa may have been a contributing factor of regional importance to the Permian-Triassic mass extinction.

By the end of the Late Permian, most continents had collided to form the supercontinent of Pangea. The associated climatic changes at the Permian–Triassic boundary coincided with the most severe mass extinction in the Phanerozoic. One extinction hypothesis favors a climatic response to an increase in large-scale volcanism resulting in ocean stagnation and widespread anoxia with fatal consequences for marine and land organisms. Recent interpretations of geochemical data suggest that orbitally-driven periodic upwelling of toxic hydrogen-sulfide rich water masses contributed to the extinction of species. In this paper, we use the Community Climate System Model (CCSM3) in order to explore the effect of eccentricity-modulated changes of the precession on the strength of Pangean megamonsoons and their impact on productivity and oxygen distribution. The climate model simulates high variability in monsoonal precipitation, trade winds and equatorial upwelling in response to precessional extremes, leading to remarkable fluctuations in the export of carbon from the euphotic zone and hence reduction in dissolved oxygen concentrations in subsurface layers. These findings are in general agreement with increased primary productivity, intensified euxinia within the oxygen-minimum zone, and decimation of the radiolarian zooplankton community as inferred from Japanese marine sections. Strong changes in river run-off linked to precipitation oscillations possibly led to a high variability in the nutrient supply to the Tethys Ocean, thus affecting regional productivity and oxygen distribution. The model results suggest that orbital variability in the sedimentary record and the associated extinction of species are related rather to periodic anoxia in near surface-to-intermediate depth than to widespread anoxic events in the Panthalassic deep-sea.

Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (> 800 ppmv) atmospheric CO2 concentrations. Although a post-hoc intercomparison of Eocene (~50 million years ago, Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the latest Paleocene and the early Eocene. Together these form the first phase of DeepMIP – the deeptime model intercomparison project, itself a group within the wider Paleoclimate Modelling Intercomparison Project (PMIP). The experimental design consists of three core paleo simulations and a set of optional sensitivity studies. The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, orbital configuration, solar constant, land surface parameters, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological datasets, which will be used to evaluate the simulations, will be developed.

Dissolved oxygen (DO) concentration in the ocean is an important component of marine biogeochemical cycles and will be greatly altered as climate change persists. In this study a global oceanic carbon cycle model (HAMOCC 2.0) is used to address how mechanisms of oxygen minimum zones (OMZ) expansion respond to changes in CO2 radiative forcing. Atmospheric pCO2 is increased at a rate of 1% annually and the model is stabilized at 2 X, 4 X, 6 X, and 8 X preindustrial pCO2 levels. With an increase in CO2 radiative forcing, the OMZ in the Pacific Ocean is controlled largely by changes in particulate organic carbon (POC) export, resulting in increased remineralization and thus expanding the oxygen minimum zones within the tropical Pacific Ocean. A potential decline in primary producers in the future as a result of environmental stress due to ocean warming and acidification could lead to a substantial reduction of vertical carbon flux and thus increased DO concentration particularly in the Pacific Ocean at a depth of 600-800 m. In contrast, the vertical expansion of the OMZs within the Atlantic and Indian Oceans are linked to reduced oxygen solubility due to rise in potential temperature and to a lesser extent changes in remineralization rates. Changes in oxygen solubility also lead to the formation of a new OMZ in the western subtrobical Pacific Ocean. The development of the new OMZ results in dissolved oxygen concentration of ≤50 μmols throughout the equatorial Pacific Ocean at 4 times preindustrial pCO2. Total ocean area with dissolved oxygen concentrations of ≤50 μmols increases by 2.5%, 4.5%, and 7.6% for the 2 X, 4X, and 8 X CO2 simulations, respectively.

The early Eocene (~55 to 50 Ma) is a time period which has been explored in a large number of modelling and data studies. Here, using an ensemble of previously published model results, making up "EoMIP" – the Eocene Modelling Intercomparison Project – and syntheses of early Eocene terrestrial and sea surface temperature data, we present a self-consistent inter-model and model–data comparison. This shows that the previous modelling studies exhibit a very wide inter-model variability, but that at high CO2, there is good agreement between models and data for this period, particularly if possible seasonal biases in some of the proxies are considered. An energy balance analysis explores the reasons for the differences between the model results, and suggests that differences in surface albedo feedbacks, water vapour and lapse rate feedbacks, and prescribed aerosol loading are the dominant cause for the different results seen in the models, rather than inconsistencies in other prescribed boundary conditions or differences in cloud feedbacks. The CO2 level which would give optimal early Eocene model–data agreement, based on those models which have carried out simulations with more than one CO2 level, is in the range of 2500 ppmv to 6500 ppmv. Given the spread of model results, tighter bounds on proxy estimates of atmospheric CO2 and temperature during this time period will allow a quantitative assessment of the skill of the models at simulating warm climates. If it is the case that a model which gives a good simulation of the Eocene will also give a good simulation of the future, then such an assessment could be used to produce metrics for weighting future climate predictions.

The biggest mass extinction in the Phanerozoic, at the end of the Permian, has been associ- ated with oceanic changes, but the exact dynamics are still debated. Intensifi ed stratifi cation, widespread anoxia, chemocline excursions, and large-scale ocean overturn events have all been invoked as contributors to the extinction. In this study the effects of possible changes in environmental conditions, such as an increase in nutrient input or dust fl uxes into the ocean or an intensifi cation of the biological pump, on Permian-Triassic ocean chemistry are inves- tigated. Series of sensitivity experiments were performed with a fully coupled climate-carbon cycle model. None of the forcings alone generates extensive low-oxygen conditions in the deep sea. These are only simulated by an intense eutrophication in combination with an enhanced biological pump, but still confi ned to the central Panthalassic, Tethys, and the eastern Boreal Oceans. Our fi ndings support the conclusions from a recent geochemical study of a Japanese deep Panthalassa section, that around the Permian-Triassic boundary, the oxygen minimum zone expanded considerably, while the deep Panthalassa remained ventilated. The warming- induced increase in low-oxygen conditions within the water column aggravated adverse exist- ing conditions and likely contributed to the extinction peak. Upwelling of toxic water was probably regionally confi ned and hence not the main cause for the end-Permian marine and terrestrial mass extinction. Widespread deep-sea anoxia, generated by a strong increase in weathering and the related enhanced nutrient input into the oceans, is probably closely linked to the delayed recovery of species in the Early Triassic.

The prominent global warming event at the Paleocene-Eocene boundary (55 Ma), referred to as the Paleocene-Eocene Thermal Maximum (PETM), was characterized by rapid temperature increase and changes in the global carbon cycle in

The Paleocene/Eocene Thermal Maximum (PETM) ~ 55 Ma ago, corresponds to a time characterized by extreme global warming caused by a massive carbon input into the ocean and atmosphere. Climate proxies from sedimentary records suggest that fresh water flow from an ice-free Arctic into the remainder of the global ocean increased due to tectonic changes, enhanced runoff, and thermal expansion. In this study we use the Community Climate System Model version 3 (CCSM-3), including a carbon cycle model, to examine the sensitivity of the ocean circulation to freshwater outflow from the Arctic Ocean during the PETM, and whether these changes may have contributed to an additional warming during the PETM. Two experiments, the first with freshwater exchange between the Arctic and Pacific Oceans and the second between the Arctic and Atlantic Oceans, are compared with a reference experiment with exchange between the Arctic and Indian Oceans and with independent stratigraphic and geochemical records from the Ocean Drilling Program (ODP). As freshwater is transported from the Arctic into the North Pacific Ocean, stratification is enhanced in the North Pacific due to a significant reduction in surface salinity. As a consequence, intermediate to deep-water sources shift from both hemispheres in the Pacific Ocean to a dominant source in the South Pacific Ocean and an additional source in the northern Tethys Ocean. This simulated shift of deep-water sources during the PETM is in agreement with recent Nd isotope measurements. The circulation patterns in the Pacific are similar to those inferred from stable isotope reconstructions, but contradicting a strong North Atlantic deep-water source during the PETM. Freshwater input into the Pacific Ocean results in a warming of intermediate water masses of > 2 °C in the North Pacific. When freshwater flow is routed from the Arctic into the Atlantic Ocean, surface density changes are too small to change vertical stratification substantially, contrary to a previous study. In summary, based upon circulation patterns and temperature increases due to freshwater flux through the Bering Strait, Arctic freshwater input into the North Pacific may have contributed to methane hydrate destabilization, an event suggested to have accelerated warming during the PETM.

The Paleocene–Eocene Thermal Maximum (PETM; 55 Ma) is of particular interest since it is regarded as a suitable analog to future climate change. In this study, the PETM climate is investigated using the Community Climate System Model (CCSM3) with atmospheric CO2 concentrations of 4×, 8×, and 16× the preindustrial value. Simulated climate change from 4× to 8× atmospheric CO2 concentration, possibly corresponding to an environmental precursor of the PETM event, leads to a warming of the North Atlantic Ocean Intermediate-Water masses, thus lowering the critical depth for methane hydrate destabilization by ∼500 m. A further increase from 8× to 16×CO2, analogous to a possible massive methane hydrate release, results in global oceanic warming and stratification. The increase in the radiative surface warming, especially at high latitudes, is partially offset by a decrease in the ocean heat transport due to a reduced overturning circulation. Surface temperature values simulated in the 16×CO2 PETM run represent the closest match to surface temperature reconstructions from proxies for this period. Simulated PETM precipitation, characterized by a slight northward shift of the intertropical convergence zone, increases at higher CO2 concentrations, especially for the northern midlatitudes as well as the high latitudes in both hemispheres. Data-inferred precipitation values and gradients for North America and Spain, for instance, are in good agreement with the 16×CO2 simulation. Increasing atmospheric CO2 concentrations might also have favored the release of terrestrial methane through warmer and wetter conditions over land, thus reinforcing the greenhouse gas concentration increase.

A complex earth system model, simulating atmosphere and ocean dynamics, marine biogeochemistry, terrestrial vegetation and ice sheets, was used to study feedbacks between the terrestrial biosphere and climate with a set of long-term climate change ensemble experiments. CO2 emissions were assigned according to historical data and the IPCC SRES scenarios B1, A1B and A2, followed by an exponential decay of the emissions for the period 2100-3000. The experiments give a reasonable reconstruction of the measured CO2 concentrations between 1750 and 2000. Maximum atmospheric CO2 concentrations of 520 ppm (B1), 860 ppm (A1B) and 1680 ppm (A2) were reached between 2200 and 2500. Additional experiments were performed with CO2 emissions and suppressed climate change, as well as an experiment with a prescribed land surface. The experiments were repeated with the vegetation model driven offline, to investigate the effects of climate and CO2 changes separately. The biogeochemical and biogeophysical interactions between terrestrial biosphere and atmosphere were quantified and compared. A decrease of albedo at high latitudes was the most important biogeophysical change. For the A2 scenario experiment, it causes an additional temperature increase of 1 to 2 K for some high latitude regions by the year 3000, but the changes are minor compared to the heating due to CO2 increase. The terrestrial biosphere takes up between 15 and 30% of the CO2 emissions, depending on the scenario and the period considered. The carbon is stored in the tropics and subtropics, where carbon is stored fast, and in the high latitudes, where carbon storage, partly due to forest expansion, is Much slower. By the year 3000, the storage of terrestrial carbon results in a decrease of atmospheric CO2 concentration of 400 ppm, which in turn decreases the global temperature increase by 0.4 K. (C) 2008 Elsevier B.V. All rights reserved. [References: 46]

An adjoint model is applied to examine the biophysical factors that control surface pCO2 in different ocean regions. In the tropical Atlantic and Indian Oceans, the annual cycle of pCO2 in the model is highly dominated by temperature variability, whereas both the temperature and dissolved inorganic carbon (DIC) are important in the tropical Pacific. In the high-latitude North Atlantic and Southern Oceans, DIC variability mainly drives the annual cycle of surface pCO2. Phosphate addition significantly increases the carbon uptake in the tropical and subtropical regions, whereas nitrate addition increases the carbon uptake in the subarctic Pacific Ocean. The carbon uptake is also sensitive to changes in the physiological rate parameters in the ecosystem model in the equatorial Pacific, North Pacific, North Atlantic, and the Southern Ocean. Zooplankton grazing plays a major role in carbon exchange, especially in the HNLC regions. The grazing parameter regulates the phytoplankton biomass at the surface, thus controlling the biological production and the carbon uptake by photosynthesis. In the oligotrophic subtropical regions, the sea-to-air CO2 flux is sensitive to changes in the phytoplankton exudation rate by altering the flux of regenerated nutrients essential for photosynthesis.

At 08:45 local time (02:45 GMT) on 26 December 2004, 1 h and 45 min after the Sumatra Earthquakes of magnitude 9.3, a devastating tsunami struck the east coast of Sri Lanka. Nearly 2 h and 30 min after the wave hit the coast, a weather satellite passed over Sri Lanka's coastal zone providing a rare glimpse of internal waves along the continental slope due to this tsunami. The satellite imagery indicates wave-like features from the tsunami being reflected, diffracted, and scattered off the steep continental slope and submarine canyons adjacent to Sri Lanka. The energetic wave and its modification to internal waves possibly eroded sediment from the sea floor and transported it to the sea surface. Solitary features generated by internal waves can explain the observed pattern. Future modelling approaches considering these nonlinear interactions would be required for a better understanding of the tsunami behaviour in the coastal zone, where its destructive effects are most prominent.

Transient experiments for the Eemian (128–113 ky BP) were performed with a complex, coupled earth system model, including atmosphere, ocean, terrestrial biosphere and marine biogeochemistry. In order to investigate the effect of land surface parameters (background albedo, vegetation and tree fraction and roughness length) on the simulated changes during the Eemian, simulations with interactive coupling between climate and vegetation were compared with additional experiments in which these feedbacks were suppressed. The experiments show that the influence of land surface on climate is mainly caused by changes in the albedo. For the northern hemisphere high latitudes, land surface albedo is changed partially due to the direct albedo effect of the conversion of grasses into forest, but the indirect effect of forests on snow albedo appears to be the major factor influencing the total absorption of solar radiation. The Western Sahara region experiences large changes in land surface albedo due to the appearance of vegetation between 128 and 120 ky BP. These local land surface albedo changes can be as much as 20%, thereby affecting the local as well as the global energy balance. On a global scale, latent heat loss over land increases more than 10% for 126 ky BP compared to present-day.

A new complex earth system model consisting of an atmospheric general circulation model, an ocean general circulation model, a three-dimensional ice sheet model, a marine biogeochemistry model, and a dynamic vegetation model was used to study the long-term response to anthropogenic carbon emissions. The prescribed emissions follow estimates of past emissions for the period 1751–2000 and standard IPCC emission scenarios up to the year 2100. After 2100, an exponential decrease of the emissions was assumed. For each of the scenarios, a small ensemble of simulations was carried out. The North Atlantic overturning collapsed in the high emission scenario (A2) simulations. In the low emission scenario (B1), only a temporary weakening of the deep water formation in the North Atlantic is predicted. The moderate emission scenario (A1B) brings the system close to its bifurcation point, with three out of five runs leading to a collapsed North Atlantic overturning circulation. The atmospheric moisture transport predominantly contributes to the collapse of the deep water formation. In the simulations with collapsed deep water formation in the North Atlantic a substantial cooling over parts of the North Atlantic is simulated. Anthropogenic climate change substantially reduces the ability of land and ocean to sequester anthropogenic carbon. The simulated effect of a collapse of the deep water formation in the North Atlantic on the atmospheric CO2 concentration turned out to be relatively small. The volume of the Greenland ice sheet is reduced, but its contribution to global mean sea level is almost counterbalanced by the growth of the Antarctic ice sheet due to enhanced snowfall. The modifications of the high latitude freshwater input due to the simulated changes in mass balance of the ice sheet are one order of magnitude smaller than the changes due to atmospheric moisture transport. After the year 3000, the global mean surface temperature is predicted to be almost constant due to the compensating effects of decreasing atmospheric CO2 concentrations due to oceanic uptake and delayed response to increasing atmospheric CO2 concentrations before.

A complex earth system model (atmosphere and ocean general circulation models, ocean biogeochemistry and terrestrial biosphere) was used to perform transient simulations of two interglacial sections (Eemian, 128–113 ky B.P., and Holocene, 9 ky B.P.–present). The changes in terrestrial carbon storage during these interglacials were studied with respect to changes in the earth's orbit. The effects of different climate factors on changes in carbon storage were studied in offline experiments in which the vegetation model was forced only with temperature, hydrological parameters, radiation, or CO2 concentration from the transient runs. The largest anomalies in terrestrial carbon storage were caused by temperature changes. However, the increase in storage due to forest expansion and increased photosynthesis in the high latitudes was nearly balanced by the decrease due to increased respiration. Large positive effects on carbon storage were caused by an enhanced monsoon circulation in the subtropics between 128 and 121 ky B.P. and between 9 and 6 ky B.P., and by increases in incoming radiation during summer for 45° to 70° N compared to a control simulation with present-day insolation. Compared to this control simulation, the net effect of these changes was a positive carbon storage anomaly in the terrestrial biosphere of about 200 Pg C for 125 ky B.P. and 7 ky B.P., and a negative anomaly around 150 Pg C for 116 ky B.P. Although the net increases for Eemian and Holocene were rather similar, the magnitudes of the processes causing these effects were different. The decrease in terrestrial carbon storage during the experiments was the main driver of an increase in atmospheric CO2 concentration during both the Eemian and the Holocene.

Widespread anoxic and dysoxic deposition in marine sediments has been associated with a stagnant ocean at the Permian– Triassic (P–Tr) boundary. This P–Tr boundary is also associated with one of the largest mass extinctions of species of the Paleozoic, marking the transition to the Mesozoic. Here we review recent three-dimensional numerical model studies that have investigated possible circulation patterns at the P–Tr boundary. In addition, we conduct new sensitivity experiments with a coupled ocean–atmosphere model of intermediate complexity, including marine chemistry, to study changes of the deep-sea oxygen distribution associated with (1) a significant change in the hydrological cycle and (2) a massive release of methane gas hydrates from marine sediments. Freshening of sea surface water masses by a strong freshwater source along the west coast of south polar Pangea produces a significant local minimum in oxygen in the eastern tropical Panthalassa ocean at intermediate depth and at the sea floor. The methane release experiment with an atmospheric CO2 concentration of 18 times the preindustrial CO2 level simulates a strong increase in sea surface temperature (about 4.5 8C) and a vigorous deep-sea circulation. In contrast to the conclusions drawn from previous studies, this reanalysis of the ocean circulation at the P–Tr boundary favors a strong ventilation in most parts of the deep-sea. Thus, a significant change in the oceanic carbon inventories and a reorganization of the marine productivity associated with a stronger biological pump may be required to explain the anoxic sedimentary deposits. D 2005 Elsevier B.V. All rights reserved.

The causes of the transition from an Early Permian cold house into an Early Triassic hothouse world have been of great interest, because this time encompasses the greatest extinction of species in Earth’s history. Common hypotheses for the causes of this transition include the massive volcanic

In models of the global carbon cycle, the pCO2of the atmosphere is more sensitive to the chemistry of the high-latitude surface ocean than the tropical ocean. Because sea-surface nutrient concentrations are generally high in the high latitudes,pCO2sensitivity to high-latitude forcing also determinespCO2sensitivity to the biological pump globally. We diagnose high-latitude sensitivity of a range of ocean models using atmosphericpCO2above an abiotic ocean; cold high-latitude waters pull abioticpCO2to low values. Box models are very high-latitude sensitive, while most global circulation models are considerably less so, including a two-dimensional overturning model, two primitive equation models, the Hamburg class of large scale geostrophic (LSG) general circulation models (GCMs), and the MICOM isopycnic GCM. High-latitude forcing becomes more important in a depth-coordinate GCM when lateral diffusion is oriented along isopycnal surfaces, rather than horizontally, followingRedi [1982]. In two different GCMs (a primitive equation model and LSG), addition of the Gent and McWillams[1990] isopycnal thickness diffusion scheme had only minor impact on high-latitude sensitivity. Using a simplified box model, we show that high-latitude sensitivity depends on a high-latitude monopoly on deep water formation. In an attempt to bridge the gap between box models and GCMs, we constructed a simple slab overturning model with an imposed stream function which can be discretized at arbitrary resolution from box model to GCM scale. High-latitude sensitivity is independent of model resolution but very sensitive to vertical diffusion. Diffusion acts to break the high-latitude monopoly, decreasing high-latitude sensitivity. In the isopycnal GCM MICOM, however, high-latitude sensitivity is relatively insensitive to diapycnal diffusion of tracers such as CO2. This would imply that flow pathways in MICOM take the place of vertical diffusion in the slab model. The two nominally most sophisticated ocean models in the comparison are the isopycnal model MICOM and the depth-coordinate GCM withRedi [1982] and Gent and McWilliams [1990] mixing. Unfortunately, these two models disagree in their abiotic CO2behavior; the depth-coordinate isopycnal mixing GCM is high-latitude sensitive, in accord with box models, while MICOM is less so. The rest of the GCMs, which have historically seen the most use in geochemical studies, are even less high-latitude sensitive than MICOM. This discrepancy needs to be resolved. In the meantime, the implication of the MICOM/traditional GCM result would be that box models overestimate high-latitude sensitivity of the real ocean. This would eliminate iron dust fertilization of the ocean as an explanation for the glacialpCO2 range of 180–200 μatm [Archer et al., 2000].

Fifteen years after the discovery of major glacial/interglacial cycles in the CO2 concentration of the atmosphere, it seems that all of the simple mechanisms for lowering pCO2 have been eliminated. We use a model of ocean and sediment geochemistry, which in- cludes new developments of iron limitation of biological production at the sea surface and anoxic diagenesis and its effect on CaCO3 preservation in the sediments, to evaluate the current proposals for explaining the glacial/ interglacial pCO2 cycles within the context of the ocean carbon cycle. After equilibration with CaCO3 the model is unable to generate glacial pCO2 by increasing ocean NO3 but predicts that a doubling of ocean H4SiO4 might suffice. However, the model is unable to generate a doubling of ocean H4SiO4 by any reasonable changes in SiO2 weathering or production. Our conclusions force us to challenge one or more of the assumptions at the foundations of chemical oceanography. We can abandon the stability of the "Redfield ratio" of nitrogen to phos- phorus in living marine phytoplankton and the ultimate limitation of marine photosynthesis by phosphorus. We can challenge the idea that the pH of the deep ocean is held relatively invariant by equilibrium with CaCO3 .A third possibility, which challenges physical oceanogra- phers, is that diapycnal mixing in ocean circulation mod- els exceeds the rate of mixing in the real ocean, dimin- ishing the model pCO2 sensitivity to biological carbon uptake.

We have introduced a simple particle field into an existing and well-documented ocean general circulation model. This enables us to investigate the advection and scavenging of particle-reactive species within the water column. As a first use of this model, we have assessed the advection and flux to sediment of 230 Th , a nuclide with a well understood marine chemistry that exhibits extreme particle reactivity. The flux to sediment of this nuclide is of interest as it is widely assumed to be related only to water depth, and therefore to act as a constant-flux indicator for marine sediments. By assuming an average settling velocity for marine particles of 3 m/d, in good agreement with observational constraints, the model generates a particle field close to that observed. Thorium-230 is scavenged onto this particle field reversibly according to Kd values constrained by observations and incorporating a particle-concentration effect. This scavenging gives a good fit to the ≈900 literature water-column measurements of 230 Th suggesting that the model is advecting and removing 230 Th realistically. An exception to this is the Weddell Sea, where the model has too little ice cover and too much lateral mixing, which prevents it from duplicating the observed high 230 Th values. The model confirms that significant advection of 230 Th occurs and duplicates the low 230 Th values seen deep in the North Atlantic due to the advection of low-230Th surface waters to depth. Model-derived maps of the 230 Th flux to the sediment indicate that ≈70% of the ocean floor receives a 230 Th flux within 30% of that expected from production. In extremely non-productive regions, the flux can fall to as low as 0.4 times that expected for in situ scavenging, while highly productive regions have fluxes up to 1.6 times that expected. An additional model run using glacial circulation fields suggests that glacial 230 Th fluxes are similar to those in the Holocene except in regions close to sea ice. This is particularly true of the North Atlantic, where appreciably more scavenging occurs in the glacial run due to advection of 230 Th from the ice-covered Arctic, and because of reduced North Atlantic Deep Water (NADW) formation. These ice-related effects mean that the area of ocean floor with 230 Th fluxes within 30% of production falls to ≈60% for the glacial. The Holocene and Glacial flux maps allow an assessment of the accuracy of 230 Th -derived sedimentation rates for existing and future studies.

A chemically reactive 10-layer sediment module was coupled to a geochemical ocean general circulation model (the Hamburg Oceanic Carbon Cycle Model). The sediment model includes four solid sediment components (CaCO3, opal, organic carbon, and clay), and five pore water substances (dissolved inorganic carbon, total alkalinity, PO43−, O2, Si(OH)4) plus corresponding species containing 13C and 14C instead of 12C. The processes, namely, particle deposition, pore water reactions, pore water diffusion and interaction with the open water column, vertical sediment advection, sediment accumulation, and bioturbation, are simulated through basic parametrizations. For the water column part the Si and C cycles are coupled by a formulation of the “rain ratio” Si:C(CaCO3):C(POC), where POC is particulate organic carbon, in biogenic particle export production, with CaCO3 frustrule production growing in parallel to a weakening of opal production during progressing deficiency of dissolved silicate in the surface layer. For two preindustrial velocity fields the model reproduces major features of observed water column and sediment tracer distributions parallel to a correct preindustrial CO2 level close to 280 ppm. The model reacts sensitively to the formulation of the POC flux parametrization, the rain ratio, as well as the solubility of opal but is fairly insensitive to changes in the bioturbation rate as well as the amount of clay deposition. A simulation of the sediment distribution by use of a velocity field, which represents the ocean at conditions during the last glacial maximum, yields realistic glacial-interglacial changes for the Atlantic Ocean, while discrepancies remain for the Indo-Pacific region. A significant decrease of the atmospheric pCO2 could be achieved through an additional change of water column inventories by a change in weathering input of Si and alkalinity.

It is predicted that by the year 2300, the atmospheric CO2 concentration will exceed ~2000 ppmv (Caldeira & Wickett, 2003; Mikolajewicz et al., 2007), corresponding to a release of 4000 x 1015 g carbon (PgC) by fossil fuel emissions and land use changes since the beginning of the industrial revolution. The anthropogenic carbon will eventually sequester on time scales of 100,000 yrs as organic carbon into the ocean and land biosphere and as CaCO3 into the geosphere (Archer et al., 1998). This carbon transfer in the atmosphere-ocean system is comparable to that at the Paleocene-Eocene boundary (55 Ma), when a massive release of carbon into the climate system led to a prominent global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). The PETM is characterized by a major (>3.0‰) negative carbon isotope excursion, documented in marine and terrestrial fossils (e.g. Koch et al., 1992; Kelly et al., 1998; Handley et al., 2008), and a worldwide seafloor carbonate dissolution horizon (e.g. Bralower et al., 1997; Lu et al., 1998; Schmitz et al., 1996; E. Thomas et al., 2000) as well as shoaling of the lysocline and carbonate compensation depth (Zachos et al., 2005). These changes are consistent with the release of more than 2000 PgC of isotopically depleted carbon into the ocean-atmosphere system within less than 10,000 years (Panchuk et al., 2008; Zachos et al., 2007, 2008), pointing to a greenhouse gasdriven warming (see Fig. 1). Recent estimates from Cui et al. (2011) indicate a slow emission rate of 0.3-1.7 PgC yr-1 as compared to the present-day emission of carbon dioxide of ~9.9 PgC yr-1 from fossil fuel emissions (Boden et al., 2010) and land-use changes (Houghton, 2008). Surface temperatures increased by 5°C in the tropics (Tripati & Elderfield, 2005; Zachos et al., 2005) and mid-latitudes (Wing et al., 2005), and by 6-8°C in the ice-free Arctic and sub-Antarctic (Hollis et al., 2009; Kennett & Stott, 1991; Moran et al., 2006; Sluijs et al., 2006, 2007, 2008a, 2011; E. Thomas et al., 2000; Weijers et al., 2007), and deep-sea temperatures increased by 4-6°C (Tripati and Elderfield, 2005; Zachos et al., 2008), relative to Paleocene temperatures (see Fig. 1). At the same time, large-scale changes in the climate system occurred, for example in the patterns of atmospheric circulation, vapor transport, precipitation (Robert & Kennett, 1994; Pagani et al., 2006a; Brinkhuis et al., 2006; Sluijs et al., 2008a, 2011; Wing et al., 2005), intermediate and deep-sea circulation (Nunes & Norris 2006; D.J. Thomas, 2004; D.J. Thomas et al., 2008) and a rise in global sea level (Sluijs et al., 2008b; Handley et al., 2011). The sea level rise is caused by various factors, including thermal