ERTH20001 Dangerous Earth Lecture Summaries

Transcription

1 ERTH20001 Dangerous Earth Lecture Summaries

2 Introduction to Natural Hazards Natural Hazards: Geological and climatic events that pose a threat to human populations, property and activities. Typically uncontrollable events, but may be predictable at least in general terms Generally not unique events, but tend to reoccur intermittently through time Does not include biological or industrial hazards: Plague, disease, infestations But these may accompany other hazards Pollution, nuclear accidents, chemical spills, explosions, fires But natural disasters may lead to some of these Impact of Natural Disasters Impact of natural disasters is increasing Number of reported disasters is increasing Climatic hazards occur more frequently and have the greatest impact (over human timescales) Geological processes may occur on much longer timescales Some geological hazards are rare, but potentially extremely devastating Australia is prone to some kinds of hazards Population Density 1 in 3 Australians live in either Sydney or Melbourne; 2 in 3 Australians live in major cities 90% live in an urban area (town > 1000 people) Why are hazards increasing? Unprecedented human population growth Unprecedented concentration of populations in cities Many large cities are in dangerous areas Many of the largest population centres are also poor and lacking resources Environmental problems may directly increase the frequency and severity of some hazards E.g. over-cultivation and deforestation Global warming is increasing cyclones and floods Greatest number of deaths occur in the Third World Greatest property damage in Developed Countries Timing and Scale of Natural Disasters Individual disasters vary greatly in size Largest disasters: Typically very rare events Smaller events: Occur more frequently; but have less impact Frequency of a hazard: The number of events of a given magnitude in a particular period of time Recurrence Interval: The average time between events of a certain size Hazard Assessment Looks at the physical aspects of the hazard When and where previous events have occurred The severity of past events and likelihood of future magnitudes Determining the frequency of events (to varying precision) Conveying this information to decision makers Risk Assessment Combines the probability of an event with the potential losses and damage Takes into account things like: Hazard assessment Vulnerability Exposure Location of resources, infrastructure, etc. Incorporates social and economic considerations in addition to scientific factors

3 Combined with hazard assessment, should lead to evaluation, informed decision making and policy and appropriate resource allocation Prediction and Warning Prediction depends on each hazard type Sometimes good, sometimes not Prediction: A statement of probability based on scientific observation Typically years to decade time scales Forecast: A short-term prediction of the specific magnitude and time of occurrence of an event Usually days to months ahead Fore-warning May be based on precursor events or related events Understanding Hazards Hazard: The physical effects of an event Frequency Severity or magnitude Distribution Duration Risk: The likely impact from an event Includes social and economic factors Hazard probability Vulnerability and susceptibility Location Exposure Natural events become hazards only when people place themselves in harms way Effects of Hazards 1. Primary Damage caused directly by the event itself, e.g. building damage from a tornado or earthquake; water damage from a flood 2. Secondary Consequences caused by the main event, e.g. fire caused by a lava flow; broken gas mains in an earthquake 3. Tertiary Long-term or permanent changes caused by the main event, e.g. shoreline changes after an earthquake; new river channel after flooding Definitions Natural hazard: The probability of occurrence of a damaging phenomenon within a specified period of time in a particular area Vulnerability: The degree of loss resulting from the occurrence of a natural hazard of a given magnitude Risk: The probability that a particular hazard will be harmful R = H x V Elements at Risk (E): The population, property, and economic activity in a given area Total risk (R T ): The expected loss of life, injuries, property damage, and disruption of economic activity from a particular hazard R T = R x E Avalanches Highly variable in magnitude and frequency Relatively predictable Can be controlled to a significant degree Where Regions where snow accumulates, generally on slopes between When Can occur after rain or warmer days, followed by heavy snow In 90% of all avalanche accidents, the avalanche is triggered by the victim

4 Mass Extinctions Extinctions All other natural disasters affect individuals Fossils show that periodically entire species die off and become extinct, often abruptly Extinction is a normal part of the history of life on Earth Most species that have ever lived are now extinct The average life span for a species is around 4 million years Mass Extinctions Occasionally large numbers of species become extinct simultaneously These occur sufficiently often and we can be sure they will happen again Biggest known natural disasters Provide evidence for geological calamities on a huge scale Humans are almost certainly experiencing/causing one now Extinctions through Time Many major boundaries in the ecological time scale are marked by mass extinctions Five times in the last 500my over 65% of all species have been wiped out After each event life takes on a different form as surviving species rapidly radiate Examples Cretaceous-Tertiary (K-T) Extinction of dinosaurs (except birds) 60% of all species; 80% of marine microorganisms Permian-Triassic (P-T) 90% of all species extinct Possible Causes Plate movements Slow movements of tectonic plates: Changes the distribution of land and sea Changes in sea-floor spreading rates changes sea levels Cause pressure on living populations: Destroying habitat and creating new ones Bringing competing populations together Clearly can lead to changes in patterns of life, but only very slowly Rates of plate tectonic processes generally far too slow to account for mass extinctions Climate change The most often invoked explanations for mass extinctions No doubt that rapid climate change places populations under pressure, e.g. Expansion of ice caps Changes in sea level Advantages of this explanation: Globally synchronised Can be quite extreme But major disadvantages Does not effect all climatic zones equally Many plants and animals survived huge climate changes in the Quaternary Unlikely to be sufficient by itself Changes in ocean chemistry At present ocean water are well mixed by circulation Bottom waters are rich in oxygen and nutrients Able to support diversity of life During very warm climates Circulation may cease and mixing may stop

5 Bottom waters become oxygen depleted and poisonous to life Possible venting of noxious hydrogen sulphide gas Several global-scale anoxic events are known Many organisms become extinct sea and land Overturn events may cause more widespread and rapid extinctions Some are now correlated with mass extinctions Produce a distinctive signature in carbon isotopes Large-scale volcanic activity Major periods of volcanic outpouring have occurred Large flood basalt provinces, individual flows >2000 km 3 Millions of km 3 of lava erupted in a few million years At east two such episodes occur at time of mass extinctions Siberian Flood Basalts at P-T Boundary ~249 Ma Deccan Traps of India at K-T Boundary ~65 Ma Several other extinctions also correlate with these events Mechanism still not clear: Such eruptions are not themselves very destructive They produce relatively little dust and ash Huge outpourings of gas possible climatic effect Even a few million years may be too slow to explain mass extinctions Biological causes, e.g. disease Possible mechanisms include disease and over-predation Over-predation can certainly lead to extinctions E.g. Effect of feral cats on small mammals in Australia However predators are not global Major problems with this idea: Such processes are occurring all the time Diseases mostly affect only a single species Diseases hardly ever kill all individuals Extinction would also kill off the disease itself, as it is also a living organism One probable exception is the late Quaternary Mass Extinction (in last 50,000 years) Possibly due to us, Homo sapiens, a global predator Large extra-terrestrial impacts N.B. Possibly several causes working together Late Quaternary Mass Extinction Populations of large mammals (Megafauna) were found on many continents and islands Mammoths and Mastodons in N. America Diprotodons, giant kangaroos, and emus in Australia Moas in New Zealand All these large mammals had survived major climate changes of the Pleistocene ice ages They disappeared over last 50,000 years Mostly soon after the arrival of the first humans Strongly suggests human predation was the cause the first global predator Extinctions still going on through habitat destruction Extra-terrestrial Impacts Clues: The K-T Boundary Layer Thin brownish clay layer precisely at the K-T Boundary Found initially at Gubbio, Italy; Stevns Klint, Denmark Now known to be worldwide Distinctive characteristics: Highly enriched in iridium Quartz grains with shock textures High pressure forms of mineral grains