A Statistical Evaluation of Atmosphere-Ocean General Circulation Models: Complexity vs. Simplicity

The principal tools used to model future climate change are General Circulation Models which are deterministic high resolution bottom-up models of the global atmosphere-ocean system that require large amounts of supercomputer time to generate results. But are these models a cost-effective way of predicting future climate change at the global level? In this paper we use modern econometric techniques to evaluate the statistical adequacy of three general circulation models (GCMs) by testing three aspects of a GCM's ability to reconstruct the historical record for global surface temperature: (1) how well the GCMs track observed temperature; (2) are the residuals from GCM simulations random (white noise) or are they systematic (red noise or a stochastic trend); (3) what is the explanatory power of the GCMs compared to a simple alternative time series model, which assumes that temperature is a linear function of radiative forcing. The results indicate that three of the eight experiments considered fail to reconstruct temperature accurately; the GCM errors are either red noise processes or contain a systematic error, and the radiative forcing variable used to simulate the GCM's have considerable explanatory power relative to GCM simulations of global temperature. The GFDL model is superior to the other models considered. Three out of four Hadley Centre experiments also pass all the tests but show a poorer goodness of fit. The Max Planck model appears to perform poorly relative to the other two models. It does appear that there is a trade-off between the greater spatial detail and number of variables provided by the GCMs and more accurate predictions generated by simple time series models. This is similar to the debate in economics regarding the forecasting accuracy of large macro-economic models versus simple time series models.

On the external forcing of global eruptive activity in the past 300 years

The decryption of the temporal sequence of volcanic eruptions is a key step in better anticipating future events. Volcanic activity is the result of a complex interaction between internal and external processes, with time scales spanning multiple orders of magnitude. We review periodicities that have been detected or correlated with volcanic eruptions/phenomena and interpreted as resulting from external forces. Taking a global perspective and longer time scales than a few years, we approach this interaction by analyzing three time series using singular spectral analysis: the global number of volcanic eruptions (NVE) between 1700 and 2022, the number of sunspots (ISSN), a proxy for solar activity, the polar motion (PM) and length of day (lod), two proxies for gravitational force. Several pseudo-periodicities are common to NVE and ISSN, in addition to the 11-year Schwabe cycle that has been reported in previous work, but NVE shares even more periodicities with PM. These quasi-periodic components range from ~5 to ~130 years. We interpret our analytical results in light of the Laplace's paradigm and propose that, similarly to the movement of Earth's rotation axis, global eruptive activity is modulated by commensurable orbital moments of the Jovian planets, whose influence is also detected in solar activity

On synchronous supereruptions

The Youngest Toba Tuff (YTT) supereruption from Toba Caldera in Sumatra at ca. 74,000 years BP is the largest volcanic event recorded in the Pleistocene. Intriguingly, recent radioisotopic dating of the near antipodal Los Chocoyos (LCY) supereruption from the Atitlán caldera in Guatemala finds an identical age within uncertainties to that of YTT. This opens the question of whether these synchronous supereruptions may be a coincidence or could be a consequence of each other? Using the known eruptive record from the past 2 Myr, we find that the likelihood of having two near antipodal supereruptions (>1,000 km3 tephra volume) within centuries (<400 years), as suggested by volcanic proxies and annual counting layer chronology in the ice core records, is very small (0.086%), requiring a non-random cause and effect. Considering this analysis, we speculate that one potential physical mechanism that could explain the temporal relationship between these supereruptions is that seismic energy released during YTT eruption focused on the antipodal region, where concentrated stresses ultimately promoted the eruption of the perched LCY magma system (or vice versa). This supereruption “double-whammy” may thus be the more compelling source of the significant environmental impacts often attributed individually to the YTT supereruption. Improving the existing age information of YTT and LCY, and a better understanding of caldera collapse events will enable further testing of the hypothesis that synchronous supereruptions do not result by pure chance

Regional geochemical and geophysical surveys in the Berwyn Dome and adjacent areas, north Wales

This report describes stream sediment and gravity surveys carried out across the Berwyn Dome and adjacent areas. The gravity survey confirmed the presence of a broad regional Bouguer anomaly low in the central part of the Dome, on which is superimposed several smaller irregular highs and lows. Some of these local anomalies possibly reflect small igneous bodies but more detailed gravity surveys would be needed to determine their form. Near Corwen the Bryneglwys Fault coincides with a 4.5 mGa1 anomaly but southwards the two features diverge, suggesting that the density interface is related either to a splay fault or to the eastern margin of the Lower Palaeozoic Montgomery trough. Some other structural trends are weakly reflected on the Bouguer anomaly and aeromagnetic maps, but there is no clear correlation with known base metal mineralisation. The Bouguer anomalies cannot be attributed to particular structures with any certainty but are probably due to a number of factors, including variation in the Precambrian basement and changes in the lithology and thickness of Lower Palaeozoic sedimentary rocks. There is no evidence for a large granitic body in Lower Palaeozoic rocks underlying the mineralisation at Llangynog. The aeromagnetic map suggests the presence of a magnetic basement at a depth of 3-4 km centred beneath the northwestern margin of the Dome. The stream sediment survey involved the collection of a - 100 mesh stream sediment, panned concentrate and water sample from each of the 399 sites sampled. The sample density was 1 site per 1.5 km*. Cu, Pb, Zn, Ba, Fe, Mn, Co, V, Cr, Ni, Zr, MO and Sn were determined in the stream sediments, Cu, Pb, Zn, Ba, Fe, Mn, Ce, Sn, Sb, Ti, Ni and As in the panned concentrates and Cu, Pb and Zn in stream waters. Major variations in the results are related to (i) hydrous oxide precipitation processes, (ii) contamination from human activities, (iii) base metal and baryte mineralisation, (iv) monazite concentrations in panned concentrates, (v) hitherto unrecorded gold mineralisation and (vi) lithological variations. The latter were related principally to shale-sandstone variation, but groups of elements attributable to the presence of basic intrusions, phosphatic rocks, coal measures, sandstones, limestones and volcanics were also discerned. Threshold levels were established from cumulative frequency curve analysis, and some anomalous sites were examined in the field. Anomalies did not form prominent coherent groups and were generally weak and scattered, with a wide variety of element groupings reflecting a range of causes. Many anomalous panned concentrates were examined mineralogically to try to . determine whether anomalies were related to chemically extreme background lithologies, contamination, or mineralisation. All the anomalies were related to one or more of the major causes of variation, although because of the very limited amount of follow-up work carried out the precise cause of many anomalies remains uncertain. No anomaly is considered to represent a strong prospect but several deserve further limited investigation, notably those associated with (i) gold mineralisation in the northwest of the area, (ii) baryte, perhaps accompanied by base metal ’ mineralisation, associated with Caradocian volcanics and phosphatic rocks at several localities, (iii) mineralisation associated with Llandeilian limestones and volcanic rocks north of Llanrhaeadr, and (iv) copper mineralisation associated with intrusives near the eastern margin of the Dome, where survey data is most incomplete

Climate in Earth history

Complex atmosphere-ocean-land interactions govern the climate system and its variations. During the course of Earth history, nature has performed a large number of experiments involving climatic change; the geologic record contains much information regarding these experiments. This information should result in an increased understanding of the climate system, including climatic stability and factors that perturb climate. In addition, the paleoclimatic record has been demonstrated to be useful in interpreting the origin of important resources-petroleum, natural gas, coal, phosphate deposits, and many others

Radio-echo layering in polar ice sheets

This thesis is concerned with layered reflections observed in the Antarctic and Greenland ice sheets during radio-echo sounding. It describes in detail layering seen with 60 and 300 MHz equipment _in the Antarctic ice sheet during three field seasons betwe~n 1974 and 1979, and discusses the effects of glaciological and equipment factors, particularly radio pulse length. The effects of changing pulse length and accumulation rate on layer echo separation are analysed, and the importance of closely spaced groups of reflectors assessed. Reflection coefficient vs depth profiles are presented for layering observed at nearly twenty sites in the Antarctic and Greenland, and are used to demonstrate the existence of two separate reflection mechanisms: changes in (a) ice density, and (b) loss tangent. Supporting density and conductivity data from ice cores are summarised. It is concluded that most layer echoes arise from layers of ice containing acidic impurities of volcanic origin~ Layer reflectivity variations_are observed over distances of tens of kilometres. Short-period fading is also observed, from which estimates of layer reflector roughness are made using the autocorrelation function and variance of the received power. The roughness estimates are shown to be consistent with a depositional origin for the surfaces. Relevant theory is summarised and a procedure developed for the remote estimation of elevated acidity levels in ice from radio-echo sounding. Such estimates are shown to compare well with direct ice core measurements. The method is used to present estimated elevated acidity profiles for the Antarctic (to ~100 kaBP) and Greenland (to ~30 kaBP), which are interpreted in terms of variations in the input of volcanogenic acid impurities to the ice sheets. The use of layering as isochronous horizons in ice flow studies is discussed in the light of new measurements, with particular emphasis on t he zone close to bedrock

The petrology and chemistry of the Setberg volcanic region and of the intermediate and acid rocks of Iceland

The tertiary to Mid-Quaternary Setberg volcano in western Iceland consists of two centres. The northern, Centre 1 is tholeiitic and defined by cone sheets, a caldera and a gabbro intrusion. The southern, Centre 2 is chemically transitional between a tholeiitic and an alkalic basalt series, and contains a cone sheet swarm, gabbroic ring features and a major granophyric cone sheet. Each centre can be related to a minor magmachamber at a depth of 2-3 km. General hydrothermal alteration around both centres has resulted in aureoles of epidotization and the development of greenstones, containing garnet in the central areas. Late-Quaternary volcanism in the Setberg area has produced an alkalic basalt succession, ranging from alkalic olivine baalts to benmoreites. This activity is related to the E-W Snaefellsnes volcanic zone and its causes are explained in terms of Icelandic plate tectonics. The tholeiitic seies of centre 1, form a differentiated sequence from olivine tholeiites to rhyolites, contaning a Ca-poor pyroxene and augite in the basic and intermediate rocks, and a large compositional interval devoid of olivine. Plagioclase is the sole feldspar phase. The transistional series of Centre 2 is olivine-bearing throughout, with augite and rare phenocrysts of orthopyroxene in the basalts. Two lineages of differentiated rocks of this series are typified either by iron-enrichment (to icelandites) or by alkali enrichment (to rhyodacites and alkalic-rhyolites). The latter contains anorthoclase and potassic oligoclase or sanidine, as well as sodic pyroxene. Rocks of the alkalic series are olivine and pyroxene-bearing, with anorthoclase appearing in the benmoreites, while phlogopite and hornblende have been found in some of the lavas. Apatite occurs as a phenocryst in members of both the transitional and alkalic series. The origin of the transitional basalt magma is discussed according to two hypotheses. Its formation is accounted for by solidification from above in a magma reservoir of great vertical dimensions, leading, to confinement of the magma to depths where orthopyroxene replaces olivine on the liquidus, resulting in a trend towards undersaturation. The chemistry of the Late-Quaternary alkalic basalts of Setberg and Snaefeilsnes is related to magma generation at greater depths than in the case of the tholeiitic basalts of the Icelandic central volcanic zone. The volume, bimodal distribution and diversity of acid and intermediate magmas in Iceland are discussed. The high proportion of acid to basic volcanic rocks in eastern Iceland is contrasted with 3-4% in the rest of the country. Two types of acid centres are identified: centres of Thingmuli type, where tholeiitic basalts are associated with low-alkalic rhyolites; and alkalic centres, containing comendites, quartz- trachytes, alkalic-rhyolites and associated alkalic basalts or transitional basalts. Peralkaline acid rocks are identified and described from Iceland for the first time. Their genesis is discussed and related to fractionation from alkalic-rhyolites and trachytes by the "plagioclase effect". No evidence has been unearthed in this study which contravenes the hypothesis that the Icelandic acid rocks are a product of fractionation from basic magma

Probabilistic seismic hazard assessment framework for Uganda: a stochastic event-based modelling approach

Uganda lies between the eastern and western arms of the East African Rift System, the largest seismically active rift above sea level. With increasing population, urbanisation and rapid construction, seismic risk in the country is escalating fast and is compounded by the high vulnerability of the building stock and inadequate disaster prevention and mitigation strategies. Hence, there is an urgent need to assess Uganda’s resilience against seismic risks. This paper presents a Monte-Carlo based probabilistic seismic hazard model for Uganda, as the first step towards the development of a seismic risk and resilience assessment framework for the country. In addition to fault segment data, earthquake catalogues are compiled for the period between 1900 and 2022 to estimate recurrence parameters for source zones in the area of interest. Area source zones incorporating focal mechanisms are used to stochastically model a national hazard framework for Uganda. A logic tree approach is applied to implement four ground motion prediction equations for both stable continental and active shallow crust geologies. Mean hazard curves, uniform hazard spectra, earthquake disaggregation and spectral pseudo-accelerations for major Ugandan cities are derived in addition to hazard maps for the country. The findings are largely consistent with previous regional studies and confirm that western Uganda is exposed to the highest level of seismicity. The model presented herein can be used to kick-start the update and continuous improvement of Uganda Seismic Design Code and the National Policy for Disaster Preparedness and Management