Throughout history, humans have been searching for a way to foretell the future. Predictions were made by peering into sheep’s entrails, gazing into crystal balls, consulting astrologers or reading tea leaves. In recent times the quest has been taken up by science. In the case of major natural events – such as earthquakes, volcanic eruptions, violent floods, disease epidemics, tsunami, drought and soil erosion – failure to predict the future can mean death, suffering and loss for millions of people. Even though natural disasters have occurred many times in the past, we still have difficulty predicting when they will occur. But many events have tell-tale build-up signs which, if correctly interpreted, can help predict the timing and scale of the impending event – and provide early warning to those who may be affected. The science of predicting such catastrophic events uses several conceptual ideas. Two of the most important are thresholds and pattern dynamics. A threshold is a point beyond which a particular outcome – perhaps a catastrophe – becomes inevitable. The threshold may lie some distance from the catastrophe itself, so that if the threshold point can be recognised and avoided, then the catastrophe can be averted. A tool for recognising thresholds is pattern dynamics. This involves identifying the characteristic behaviour of a system as a set of patterns in space and time, and finding the 'fingerprints' of these patterns near threshold points.
Scientists working with thresholds and pattern dynamics use mathematics and computers to model the factors that drive rare but important events. By running the models forward in time, scientists can identify the characteristic patterns that occur near threshold points, which give warning signs that thresholds are about to be crossed – when things will shift dramatically from their present state to a possibly dangerous or unstable one. As a simple example, think about rain falling on soil. When it reaches a point where the soil has absorbed as much water as it can, a threshold is crossed, and the water begins to run off and causes flooding. Another example of a threshold is erosion, when the power of the wind or floodwater reaches a point where it can dislodge soil particles and sweep them away. A third case is when an apparently stable environment like farmland is hit by rising saline groundwater – and the vegetation suddenly dies. For a long time, while the saline groundwater is rising, you see nothing. When the salty water reaches the surface or root zone of plants – the threshold – you see sudden death across a wide area. This is due to a relatively subtle shift in the level of the groundwater. Before any meaningful predictions of future events can be made, features that define a threshold need to be identified (Box 1: Defining thresholds). A field of research where scientists use thresholds and pattern dynamics is in predicting the effects of climate change. Although scientists agree that the climate is changing, they differ in their predictions of the rate of change and the potential impacts of climate change (Box 2: Avoiding a climate crash). Predicting the impact of droughts and floods With climate change increasing the frequency of extreme weather events, the chance of major events causing damage to the landscape is rising. Hydrologists – scientists who study the water cycle – are now using thresholds and pattern dynamics to predict the impact of more frequent and extreme floods and drought that are likely to occur due to global warming. In may seem strange, but drought increases the impact of floods on Australia's land- and water-based ecosystems. Heavy floods can cause severe erosion of the land and degrade water quality. The damage is caused by soil being swept up off the land by floodwaters, exposing infertile and unworkable soils. The soil and nutrients deposited in rivers, lakes and dams by floodwaters can lead to silt accumulation and outbreaks of algal bloom. Researchers at the CSIRO are investigating the factors that influence the impact of floods on land and water quality. They have studied the history of floods and erosion in Australia. They looked into gully erosion over the past 10,000 years and found that while it is a natural process of the Australian continent, it has increased in intensity and frequency over the past 200 years. Major floods are forecast to become more intense due to climate change, but it is the condition of the land that determines the impact of floods. Good vegetation cover is the key to preventing erosion, but drought, fires and overgrazing cause the loss of protective vegetation. These degraded habitats take years to recover their natural resistance to erosion and during this time are at high risk of losing their soil. Special care and management is required to prevent this. The CSIRO researchers aim to predict the impact of flood events so that their risks can be better managed. Studies of thresholds and pattern dynamics are used to identify the point at which flooding reaches sufficient strength to move soil particles and so erode the landscape. The ideal way for a drought to break is with gentle winter rains and steady vegetation growth. If major floods occur, they can have severe effects anywhere land cover is poor. In these conditions, changes in land management practices may offset the increased risks posed by floods under climate change. Patterns and thresholds in human behaviour Natural disasters are not the only events that involve thresholds. Researchers are studying thresholds in human affairs too – booms and crashes in money markets, sudden shifts in public opinion, changes in community behaviour, the explosion in the use of the world-wide web and even the outbreak of wars. Boxes
Related Nova topics: Monitoring the white death – soil salinity |
     |
|||||||||
Posted September 2005.
The Australian Foundation for Science is a supporter of Nova.
This topic is sponsored by the Sir Mark Oliphant International Frontiers of Science and Technology Conference Series, funded by the Australian Government under the International Science Linkages programme.
© Australian Academy of Science