Natural hazards in Australia : floods

Floods are caused by a number of interacting factors, making it remarkably difficult to explain changes in flood hazard. This paper reviews the current understanding of historical trends and variability in flood hazard across Australia. Links between flood and rainfall trends cannot be made due to the influence of climate processes over a number of spatial and temporal scales as well as landscape changes that affect the catchment response. There are also still considerable uncertainties in future rainfall projections, particularly for sub-daily extreme rainfall events. This is in addition to the inherent uncertainty in hydrological modelling such as antecedent conditions and feedback mechanisms. Research questions are posed based on the current state of knowledge. These include a need for high-resolution climate modelling studies and efforts in compiling and analysing databases of sub-daily rainfall and flood records. Finally there is a need to develop modelling frameworks that can deal with the interaction between climate processes at different spatio-temporal scales, so that historical flood trends can be better explained and future flood behaviour understood

Hydrological extremes and consequences of climate change

© 2014 Dr. Dörte JakobIn the design of infrastructure, risk has been – and often still is – assessed on the basis of long-term averages. Information on variation in hydrological extremes is required as the basis for informed decision-making, preparedness and possible adaptation. Long-term trends are fairly well understood for temperature but less well for precipitation. While climate models have become sophisticated tools for projecting future changes in our climate, their ability to replicate observed variations in precipitation is limited and it is therefore prudent to complement climate models through analysis of historical observations. Design rainfall is used as one of the required inputs for hydrological models in the design of structures such as dams and bridges. Design rainfall estimates are supplied in form of intensity-frequency-duration curves. Rainfall frequency analysis is almost invariably based on the assumption of a stationary climate. Sub-daily durations are of particular interest for urban applications. This thesis was strongly driven by the motivation to provide guidance to decision makers who have to account for non-stationarity in rainfall extremes. Non-stationarity in rainfall extremes comes about as a conflation of climate change and climate variability. Unlike for temperature extremes, rainfall extremes for Australia as a whole exhibit no clear increase or decrease in intensity over time but strong association with the El Niño-Southern Oscillation (ENSO). This has implications for the choice of suitable analysis techniques, e.g. sophisticated non-parametric techniques. Depending on the planning horizons both climate change and climate variability may have to be accounted for. The association of rainfall extremes with ENSO leads to an opportunity to develop statistical models to support decision-making on shorter time scales. Analysis of seasonality in frequency and magnitude of rainfall extremes revealed considerable variation across a set of sites in the southeast of Australia, implying different dominating rainfall-producing mechanisms and/or interactions with local topography. The strongest signal for an increase in extreme precipitation is found for short durations. Changes in rainfall extremes come about through a combination of changes in thermodynamical and dynamical variables. To assess large-scale changes in circulation, a classification technique (self-organising maps, SOM) was applied and synoptic types were identified. Rainfall extremes were then related to the synoptic type under which they occurred, to assess observed changes in the frequency of rainfall extremes. Rainfall extremes are typically preceded by conditions that are much wetter (both in absolute and relative terms) and warmer than the climatological average. These anomalies tend to be larger for shorter durations, and for rarer events. Given that increase in humidity exhibits strong regional variability and that it may be counteracted by changes in dynamics, it appears simplistic to state categorically that climate change will lead to an increase in extreme rainfall events and observed trends in rainfall extremes show a picture that is more complex. In summary, the combination of changes in thermodynamic and dynamic variables will define the change in frequency and intensity of rainfall extremes. The factors that are most relevant for the effect of climate change on rainfall extremes depend on geographical location

Variability and long-term change in Australian temperature and precipitation extremes

Risks from weather and climate extremes to governments, industries and communities are increasing and, at present, are not well quantified. In the presence of climate variability and long-term change, it may not be appropriate to base an assessment of the likelihood of climate hazards on the long-term averages. Many weather and climate extremes have increased in frequency and/or intensity in recent decades with climate model projections showing that several trends are likely to continue. The analysis of historical records is complementary to the use of climate models in understanding the changing nature of extremes. In this study, we apply a consistent methodology to examine the modulation in the probability of extremes in temperature and rainfall as they are influenced by climate change over the past century and natural climate variability and present summary tables and charts for a comprehensive comparison. The daily temperature data – maximum (Tmax) and minimum (Tmin) air temperature – are from a ‘high-quality’ dataset developed at the Australian Bureau of Meteorology – the Australian Climate Observations Reference Network (ACORN) Surface Air Temperature (SAT) dataset. In the absence of a comparable dataset for daily rainfall, we analysed rainfall data at ACORN-SAT locations. Our analyses of extremes are based on annual maxima (annual minima for Tmin) of daily time series from 58 Australian sites over the period 1910–2009. We found statistically significant long-term increases in extreme maximum temperatures but with marked regional and seasonal variations. The increase in the lowest minimum temperature extremes typically exceeds the increase in the extremes of maximum temperature. Daily precipitation extremes rarely exhibit long-term change over the century but are strongly modulated by the El Niño Southern Oscillation (ENSO). The relative importance of long-term change and climate variability therefore depends on the variable or index. We conclude that in assessing the likelihood of climate hazards, one needs to consider the modulation of climate extremes due to both long-term change and climate variability. Our findings imply that when planning for adaptation, different emphasis needs to be given to changing temperature and precipitation extremes

Amino acid and amino sugar compositional changes during in vitro degradation of algal organic matter indicate rapid bacterial re-synthesis

Amino acids (AA) and, more recently, amino sugars (AS) in marine or lacustrine sediments have been increasingly used as paleoproxies. In order to assess AA and AS compositional changes during simulated microbial degradation, as well as to understand the importance of amino-compound re-synthesis by microbes during early diagenesis, decomposition experiments (300  days) were performed with algal (Fragilaria crotonensis) organic matter (OM)/quartz-sand mixtures under controlled redox conditions. Despite expected greater overall degradability under oxic conditions, decomposition kinetics of the bulk algal OM, as well as the total particulate AA and AS were similar under oxic and anoxic conditions, following exponential decay kinetics consistent with the observed mobilization and transfer of large parts of the particulate organic carbon (C) and nitrogen (N) into the dissolved inorganic and organic C and N pools. Carbon-normalized AA and AS yields suggest relative enrichment of amino compounds during partial decomposition, indicating the production and accumulation of microbial biomass during early diagenesis, independent of the redox environment. Moreover, AA and AS compositional changes, such as the relative enrichment of the AA glycine and the AS muramic acid (MurA), and the decrease in the molar ratio of glucosamine and galactosamine (GlcN:GalN) during degradation in both redox systems, were consistent with significant bacterial re-synthesis and the preferential preservation of bacterial biomaterial with increasing diagenesis. Large disparities between different bacterial amino-sugar based estimates of bacterial contribution indicate that bacterial end-member compositions are not currently known well enough to make these bacterial-biomarker constraints quantitative. However, the overall trends are consistent, indicating substantial turnover of eukaryotic into bacterial OM on short time scales of weeks to months. Together these results suggest that the influence of bacterial reworking in conserving sedimentary OM via its transfer into more refractory OM pools may be substantially greater than previously appreciated. We also investigated established amino-compound based indicators of OM degradation, bacterial synthesis, and sediment reactivity. Despite discrepancies, which we attribute to different susceptibilities of the respective indicators towards degradational changes on different time-scales, the tested indices were overall consistent with past data. These results therefore confirm their value as universal indicators of OM diagenesis. Together, our data highlight the vital role of bacterial reworking on the composition of sedimentary OM, with important implications for the alteration of primary geochemical signatures during early sedimentary diagenesis and their use as proxies in paleoenvironmental studies.ISSN:0016-7037ISSN:1872-953

The effect of tidal range and mean sea-level changes on coastal flood hazards at Lakes Entrance, south-east Australia

Despite being well-documented in other countries, the roles that anthropogenically induced changes and natural variability in tidal processes play in modulating coastal flood frequencies have not been investigated in Australia. Here we conduct a brief assessment of changes in tidal variability around Australia. We then apply a simple attribution framework to quantify the separate and joint effects of tidal range changes and increasing relative mean sea level on nuisance flood frequency at the location with the largest relative changes in tidal range, Lakes Entrance, Victoria. To understand how these changes in variability affect flood hazards, we consider a nuisance flood threshold based on recent coastal flood impact surveys. Results show that increases in the heights of high tides over recent years have exerted a large influence on coastal flood frequencies. These recent changes are potentially linked to changes in channel dredging regimes. We show that 93% of nuisance flood days since 2009 would not have occurred without these tidal range changes or the coincident increases in the mean sea level. We demonstrate the importance of considering tidal processes in estuarine coastal flood hazard assessments for future planning, even if these processes do not represent a substantial flood threat today. We discuss the implications of this study for future work on estuarine flood hazards and the benefits of considering impact-based thresholds in the assessment of such hazards

Briefe als Bildungsmedien

Schütze SB. Briefe als Bildungsmedien. In: Balcke D, Benecke J, Richter A, Schmid M, Schulz-Gade H, eds. Bildungsmedien im wissenschaftlichen Diskurs. Festschrift für Eva Matthes zum 60. Geburtstag. 1st ed. Bad Heilbrunn: Klinkhardt; 2022: 235-246