Predicting natural eventsThe study of thresholds and pattern dynamics may be a key to unlocking our understanding of catastrophic climate change and other types of complex systems. Key
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Key textThroughout 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. Related Nova topics: Monitoring the white death – soil salinity Coral bleaching – will global warming kill the reefs?
Box 1: Defining thresholdsScientists predict weather and climate using mathematical models. The underlying processes – involving sunshine, winds, heat, moisture, clouds, ocean currents and vegetation on land – are described by mathematical equations and solved on a computer. One of the difficulties faced by scientists who predict weather and climate in this way is that many aspects of the weather, such as rainfall, are not smooth but behave in an erratic or 'spiky' fashion. The yearly average of rain may occur in one or two heavy falls, with very little rain between major downpours. Such erratic events are difficult to predict.Thresholds and pattern dynamics focuses on what happens at the upper and lower limits representing rare events. With enough data and understanding, patterns emerge, the threshold can be identified and predictions of the likelihood and behaviour of extreme events can be made. Resilience is another feature of systems that have thresholds. Resilience is the amount of disturbance that a system – such as climate – can tolerate before it crosses the threshold and moves to a different state. Analysis of systems that have thresholds, and are well understood, shows that the rate of progress towards the tipping point may be gradual or rapid. Once the threshold or boundary has been met, the change from one state to the next may be quite sudden. Thresholds can be altered or influenced by human activities. They are often created when humans introduce a problem to an otherwise stable system by trying to control it or utilise the resource, such as water. Some thresholds are reversible, so a system will return to the same state it was in before crossing the threshold after a recovery period. Examples of reversible thresholds are lake eutrophication, overgrazing of vegetation and predator regulation of prey. Other thresholds, such as dryland salinity and extreme coral bleaching, are difficult or impossible to reverse. The reversal may involve 'hysteresis', in which a system that has crossed a threshold must be reversed a long way beyond the threshold point before it flips back to its original, pre-threshold state. An example is lake eutrophication caused by excessive nutrient input which generate massive algal blooms. The nutrients often take the form of nitrogen and phosphorus in runoff from farmland or urban areas. A lake may turn eutrophic when the nutrient level exceeds some threshold, but will only revert to its original non-eutrophic state when the nutrient input is reduced to a very much lower level. This illustrates the importance of having good predictive tools to minimise the possibility of causing an irreversible negative change to the environment.
Box 2: Avoiding a climate crashAlthough the climate appears to be changing slowly, the climate has changed quite rapidly in the past. Abrupt climate change is an example of a threshold phenomenon. Scientists are looking at evidence of abrupt climate change in the past to understand present climate change. They ask questions such as: Are there thresholds? What drives the thresholds? Can crucial patterns and interactions be identified? and How will things be different if we cross the threshold? to get some idea of how likely rapid climate change is in the near future. If these questions can be answered, we might be able to figure out how close we are to the threshold and determine the probability of tipping our climate into a different state.Atmospheric researchers at the CSIRO are analysing past abrupt climate changes to try and identify the external forces that might cause our present climate patterns and ecosystems to collapse. An example is the drought which has lately afflicted eastern Australia. The subtle difference from past dry periods is the interaction between drought and warming. While this drought is similar to past events in its lack of rainfall, a new feature this time is heat. It is by far the hottest drought on record. This combination is pushing many parts of the landscape, including deep-rooted trees, beyond the threshold of no return. By identifying the external forces that drive such events, it may be possible to predict critical changes and either prevent them, or else manage the consequences better. The researchers analyse well-understood systems – like fires and stock markets – to understand the drivers behind climate change. This approach has revealed hallmarks common to other complex systems which indicate there may be universal factors that can be used to analyse all systems. This research opens the way to better manage human actions that do the greatest damage to the environment and natural ecosystems. It means that we are not completely powerless to influence the trajectory of anthropogenic climate change, provided we are capable of a truly global response to a global challenge. Related sites
Activities
Further readingATSE Focus March 2006 The complexities of modelling climate change (by Michael Manton) Attempts to explain what is involved with modelling climate change.
Australasian Science April 2006, pages 26-29 Can science foretell the future (by Julian Cribb) Reports on the analysis of thresholds to predict earthquakes, floods, stockmarket crashes and other major events.
June 2005, pages 32-34 The science of complex systems (by John Finnigan) Explains how scientists are probing the complex interactions that influence the behaviour of bushfires, cyclones, the stock market and even electricity prices.
August 2004, pages 29-31 The day after tomorrow (by Peter Kershaw and Chris Turney) Provides evidence of catastrophic global climate change in the Atherton tablelands.
January/February 2005, page 4 Weather is warmer and wetter (by Stephen Luntz)
June 2004, page 37 It's getting wetter Summarises the conclusions from a meeting of climatologists that the world is getting wetter as it warms.
Cosmos 20 September 2006 Predicting the future: It's becoming a science (by Julian Cribb) Reports on the analysis of thresholds to predict earthquakes, droughts and epidemics.
Decision Point July 2008, page 7 Crossing thresholds in times of change (by Jonathan Rhodes) Provides principles for managing biodiversity in the face of climate change thresholds.
Ecos No. 130, 2006, pages 28-31 A view of dangerous climate change (by Steve Davidson) Looks at the possibility of climate change reaching a tipping point, beyond which it is impossible to manage.
Nature 2 December 2004, pages 559-560 Hot news from summer 2003 (by Christoph Schär and Gerd Jendritzky) Asks whether the European heatwave in 2003 was a rare meteorological event or a first glimpse of climate change to come.
11 September 2003, page 111 Analysis pours cold water on flood theory (by Quirin Schiermeier) Describes new analyses of historical flood data suggesting that recent floods in Germany were not evidence of a general trend towards extreme weather.
21 August 2003, pages 867-867 Heatwave underlines climate-model failures (by Declan Butler) Explains why new models are needed to predict extreme weather events.
11 October 2001, pages 591-596 Catastrophic shifts in ecosystems (by Marten Scheffer, Steve Carpenter, Jonathan A. Foley, Carl Folke and Brian Walker) This technical article suggests that strategies for sustainable management of ecosystems should focus on maintaining resilience.
New Scientist 1 March 2008, page 11 Quiet period could warn of impending earthquakes (by Kate Ravilious) Reports on the change in radio waves prior to earthquakes.
27 January 2007 Can you believe the weather? (by Michael Brooks) Looks at the science of predicting the weather.
18 January 2007 2100: A world of wild weather (by Kate Ravilious) The first maps of their kind show how climate change will turn rare disasters into regular events.
19 August 2006, pages 8-9 Glaciers heading for point of no return (by Jessica Marshall) Suggests that if Greenland's ice carries on melting at the same rate, it could reach a critical threshold by the century's end.
1 July 2006, page 50-51 You can't predict life's black swans (by Michael Bond) An interview with applied statistician Nassim Nicholas Taleb about rare, high impact events.
10 June 2006, pages 44-47 Raiders of the lost storms (by Emma Young) Looks at the long term records to answer the question 'Will a warmer world bring more hurricanes?'
26 November 2005, pages 46-48 The iceman cometh (by Fred Pearce) An interview with Lonnie Thompson about his belief that the tropics are the source of Earth's climate variability.
10 September 2005, pages 8-9 The day their luck ran out (by Andy Coghlan and Justin Mullins) Asks why New Orleans was so ill-prepared for hurricane Katrina.
29 January 2005, page 6 Quake, flood, fire. Will we be ready? (by Emma Young) Discusses the need for early warning systems for extreme natural hazards.
10 September 2003 Severe floods in Europe not rising (by Jenny Hogan and Carolyn Fry) Describes new evidence linking some of Europe's worst ever flooding to poor land management practices.
21 September 2002, page 4 Africans go back to the land as plants reclaim the desert (by Fred Pearce) Describes areas of Africa that are regreening after years of drought conditions.
PhysicsWeb June 2006 Hurricane force (by Roger Smith) Discuses whether the recent increase in the number of intense storms is linked to global warming.
Science 24 December 2004, pages 2197-2198 Ecological versus climatic thresholds (by Mark Maslin) Defines the differences between ecological and climatic thresholds.
28 March 2003, pages 2005-2010 Abrupt climate change (by R.B. Alley, J. Marotzke, W.D. Nordhaus, J.T. Overpeck, D.M. Peteet, R.A. Pielke, Jr., R.T. Pierrehumbert, P.B. Rhines, T.F. Stocker, L.D. Talley, and J.M. Wallace) Reviews past abrupt climate change events and the role of thresholds in predicting future events.
Scientific American July 2007, pages 26-33 Warmer oceans, stronger hurricanes (by Kevin Trenberth) Looks at the evidence linking global warming to cyclone activity.
November 2006, pages 55-61 Reviving dead zones (by Laurence Mee) Looks at runaway plant and algae growth in coastal areas caused by nutrient excess, an example of an event with a threshold.
17 March 2006 Statistical analysis bolsters theory linking warmer oceans to stronger hurricanes (by David Biello) Reports that new analysis suggests a link between global warming and stronger hurricanes.
18 October 2005 Climate model predicts extreme changes for US (by Tracy Staedter) The latest climate model of the US predicts higher temperatures, heavier rainfalls and shorter winters than previously expected.
7 March 2005 New model predicts timing and intensity of solar storms Reports that a mathematical model can predict solar storms with improved accuracy.
January 2005 Punctuated disequilibrium (by Claudio Angelo) Predicts that occasional but extreme climate could turn parts of the Amazon rain forest into dry savannas.
January 2005 Climate change desiccating the planet, researchers conclude Describes a study suggesting that the portion of the planet affected by extreme drought has doubled in the last thirty years.
Useful sitesEarth Science Australia
Geography and climate special article – climate change (Australian Bureau of Statistics) Defines climate change and summarises the cause, impact and modelling
of climate change.
Mathematics in environmental science (Macquarie University, Australia) Discusses the use of mathematical models in environmental
science.
What is resilience? (Resilience Alliance, USA) Defines resilience and how it may be altered in ecosystems.
After the flood (Higher Education and Research Opportunities in the United Kingdom) Discusses the need for resilience in coastal communities facing extreme
weather events and natural disasters.
The discovery of rapid climate change (Physics Today, USA) Follows the change in scientific thinking during the 20th century
leading to the discovery of rapid climate change.
Timeline of milestones (American Institute of Physics) Provides a chronological sequence of the most important events in the
history of climate change science.
Hotter oceans, fiercer storms (NOVA scienceNOW, USA) Examines the link between rising sea surface temperature and more
intense storms.
Australian Broadcasting Corporation
Glossaryanthropogenic. Caused or induced by humans; of human origin. catchment. The area from which a river, stream, lake or other body of water receives its water. complex system. A complex system is one in which there are multiple interactions between many components. The properties of a complex system are not completely explained by an understanding of its component parts. erosion. A term that can be applied to soil or rock. Soil erosion is the gradual loss of any type of soil from the soil surface, usually caused by water and wind. Rock is eroded when it is slowly made smoother or worn down, again by wind or water. global warming. An increase in the average temperature of the Earth's surface. Global warming is one of the consequences of the enhanced greenhouse effect and will cause worldwide changes to climate patterns. hydrology. The study of water, its properties and movement through the Earth’s land and atmosphere. Hydrology includes the study of the distribution of water, the accumulation of water – in lakes, oceans and underground – and the effects of water on the earth's surface. model. Solving complex problems associated with real situations is often made easier by setting up a model of the situation – a mathematical description of the problem. To set up a model, a problem is simplified and only those aspects that can be represented mathematically are included. After the problem is solved mathematically, tentative solutions are translated back to the real situation, as possible real solutions. At this stage the inadequacy of the simple model may be revealed, and some parts of the process may need to be changed. More information on models and modelling can be found at What is modelling? (Nova: Science in the news, Australian Academy of Science). threshold. The point at which a signal can be detected or is strong enough to trigger a change. water (hydrological) cycle. A biogeochemical cycle through which the Earth’s fixed supply of water is collected, purified and redistributed from the environment to living organisms and back to the environment. The water cycle is powered by the sun causing evaporation into the atmosphere of water from plants and from the Earth’s surface.
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