Identifying climatic analogs for Wisconsin under 21st-century climate-change scenarios

There is a deep disconnect between scientific and public concern about climate change. One reason is that global climate change is a fairly abstract concept with little perceived relevance, so a key challenge is to translate climate-change projections into locally concrete examples of potential impacts. Here we use climate analog analyses as an alternative method for identifying and communicating climate-change impacts. Our analysis uses multiple downscaled general circulation models for the state of Wisconsin, at 0.1 decimal degree resolution, and identifies contemporary locations in North America that are the most similar to the projected future climates for Wisconsin. We assess the uncertainties inherent in climate-change projections among greenhouse gas emission scenarios, time windows (mid-21st century vs. late 21st-century) and different combinations of climate variables. For all future scenarios and simulations, contemporary climatic analogs within North America were found for Wisconsin’s future climate. Closest analogs are primarily 200–500 km to the south-southwest of their Wisconsin reference location. Temperature has the largest effect on choice of climatic analog, but precipitation is the greatest source of uncertainty. Under the higher-end emission scenarios, the contemporary climatic analogs for Wisconsin’s end-21st-century climates are almost entirely outside the state. Climate-analog analyses offer a place-based means of assessing climate impacts that is complementary to the species-based approaches of species distributional models, and carries no assumptions about the characterization and conservatism of species niches. The analog method is simple and flexible, and can be readily extended to other regions and other environmental variables.     

Causes and consequences of errors in determining sorption capacity of coals for carbon dioxide at high pressure

Recent comparisons of CO₂ sorption by coals at high pressures have shown major differences between the results obtained by different laboratories. These need to be resolved for laboratory estimation of CO₂ sequestration in coal seams to be useful. A compilation of potential sources of error in determination of sorption characteristics and their impact on sorption measurements is provided here. A series of tests is also provided that can be used to identify and reduce such errors in measurement. For example, an error in temperature produces a characteristic distortion of the sorption curve for carbon dioxide, which can be corrected to some extent. A negative value for excess sorption at high pressure is almost certainly diagnostic of either a cell volume that has been overestimated or that some part of the substrate that is inaccessible to the gas is accessible to helium. The major source of variation between results from the different laboratories that supplied the closest sorption values was found to be variations in the assumed free space volume, which could be due to discrepancies in determined helium density or measured cell volume. Including a term in the sorption model that is proportional to gas density will markedly reduce the influence of such errors in estimating sorption capacity or heats of sorption. The influence of swelling and moisture on sorption isotherms is also quantified here. Correction for swelling of coals in carbon dioxide changes the estimated sorption capacity by less than 1%, if a term that is proportional to gas density is included as a free parameter in the model fitting the isotherm.     

High-resolution carbon isotope changes in the Permian–Triassic boundary interval, Chongqing, South China; implications for control and growth of earliest Triassic microbialites

High-resolution δ¹³C_CARB analysis of the Permian–Triassic boundary (PTB) interval at the Laolongdong section, Beibei, near the city of Chongqing, south China, encompasses the latest Permian and earliest Triassic major facies changes in the South China Block (SCB). Microbialites form a distinctive unit in the lowermost 190 cm above the top of the Changhsing Formation (latest Permian) at Laolongdong, comparable to a range of earliest Triassic sites in low latitudes in the Tethyan area. The data show that declining values of δ¹³C_CARB, well-known globally, began at the base of the microbialite. High positive values (+3 to 4 ppt) of δ¹³C_CARB in the Late Permian are interpreted to indicate storage of ¹²C in the deep waters of a stratified ocean, that was released during ocean overturn in the earliest Triassic, contributing to the distinctive fall in isotope values; this interpretation has been stated by other authors and is followed here. The δ¹³C_CARB curve shows fluctuations within the microbialite unit, which are not reflected in the microbialite structure. Comparisons between microbialite branches and adjacent micritic sediment show little difference in δ¹³C_CARB, demonstrating that the microbialite grew in equilibrium with surrounding seawater. The Early Triassic microbialites are interpreted to be a response to upwelling of bicarbonate-rich poorly oxygenated water in low latitudes of Tethys Ocean, consistent with current ocean models for the PTB interval. However, the decline of δ¹³C_CARB may be due to a combination of processes, including productivity collapse resulting from mass extinction, return of deep water to ocean surface, oxidation of methane released from methane hydrate destabilisation, and atmospheric deterioration. Nevertheless, build-up of bicarbonate-rich anoxic deep waters may be expected as a result of the partial isolation of Tethys, due to continental geography; release of bicarbonate-rich deep water, by ocean upwelling, in the earliest Triassic may have been an inevitable consequence of this combination of circumstances.     

Correlation of the Kimmeridgian-Tithonian (Jurassic) boundary beds exposed in the Boulonnais,France with those at Kimmeridge,Dorset, UK

Sediments of Kimmeridgian and Tithonian age are well exposed on the Boulonnais coast of northern France between Equihen and Cap Gris Nez and on the south coast of England at and adjacent to Kimmeridge Bay. Both successions were deposited on a marine shelf and lie within the Subboreal faunal province which enables detailed correlations to be made between them based on ammonite assemblages. They are, however, lithologically markedly different due to their environmental settings: close to a land area in the case of the Boulonnais and within a depositional basin in the case of Kimmeridge. The succession adjacent to the Kimmeridgian-Tithonian boundary exposed in the Boulonnais is highly condensed and laterally variable with more attenuated successions occurring close to the former Anglo-Brabant Massif land area. The boundary occurs at the end of a succession of up to six regressive-transgressive events that onlap the land area. This is in contrast to that at outcrop at Kimmeridge, where the Kimmeridgian-Tithonian boundary is marked by a correlative conformity in an unbroken basinal succession. The cliff and foreshore exposures in the Kimmeridge area provide the only unbroken succession in the Subboreal faunal province of the beds adjacent to the Kimmeridgian-Tithonian boundary.     

Age of the Cutler Dam Alloformation (Late Pleistocene), Bonneville Basin, Utah

Luminescence geochronology, especially infrared stimulated luminescence analyses on marsh mud, shows that a relatively deep lake reached its peak (1340 m above sea level) in the Bonneville basin 59,000±5000 yr ago. The age is consistent with nonfinite ¹⁴C ages and with amino acid geochronology on ostracodes. The Cutler Dam Alloformation was deposited during this lake cycle, which, like the subsequent Bonneville lake cycle, appears to have reached its maximum highstand following the peak of a global glacial stage (marine oxygen-isotope stage 4) but at a time when other records from North America show evidence for cold climate and expanded glacier ice.     

Fragmentation of magma during Plinian volcanic eruptions

 The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V _G /V _L ) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V _G /V _L in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between pumices at a given level in a deposit. These variations in V _G /V _L do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary evolution. On the timescale of Plinian eruptions, pumices with viscosities <10⁹ Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity. Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <10⁵ Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching. After accounting for these effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is a relatively constant minimum value of V _G /V _L of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >10⁵ Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes. Received: 19 April 1995 / Accepted: 3 April 1996     

A comparison of forest gap models: Model structure and behaviour

Forest gap models share a common structure for simulating tree population dynamics, and many models contain the same or quite similar ecological factors. However, a wide variety of formulations are being used to implement this general structure. The comparison of models incorporating different formulations is important for model validation, for assessing the reliability of model projections obtained under scenarios of climatic change, and for the development of models with a wide range of applicability. This paper reviews qualitative and quantitative comparisons of the structure and behaviour of forest gap models.As examples of qualitative model comparisons, the different formulations used for the heightdiameter relationship, for the maximum growth equation, and for the effects of temperature and drought on tree growth are reviewed. The variety of formulations currently in use has the potential to influence simulation results considerably, but we conclude that little is known on the sensitivity of the models in this respect.The quantitative model comparisons performed so far allow us to draw the following conclusions: (1) Gap models are quite sensitive to the formulation of climate-dependent processes under current climate, and this sensitivity is even more pronounced under a changed climate. (2) Adaptations of forest gap models to specific regions have required detailed sub-models of species life history, thus complicating model comparison. (3) Some of the complex models developed for region-specific applications can be simplified without hampering the realism with which they simulate species composition. (4) Attempts to apply the models without modification beyond the area for which they were developed have produced controversial results.It is concluded that the sensitivity of forest gap models to the exact process formulations should be examined carefully, and that more systematic comparisons of model behaviour at a range of test sites would be desirable. Such studies could improve our understanding of forest dynamics considerably, and they would help to focus future research activities with gap models.