3. September 2009 16:44
Tipping points - the universal warning signals for sudden change
A study of critical thresholds in complex systems ranging from the human body to financial markets has found that regardless of the details, the dynamics of each system near its 'tipping point' share similar symptoms. The research is published today in Nature and was in part funded by a European Young Investigator (EURYI) Award to co-author Jordi Bascompte of the Consejo Superior de Investigaciones Scientificas (CSIC) Spain.
Led by Martin Scheffer of Wageningen University in the Netherlands, the international team of researchers found similar early-warning signals indicating a critical threshold of change across a surprisingly wide range of scenarios. Abrupt changes in ocean circulation and Earth's climate, shifts in wildlife populations and ecosystems, crashes in the global finance market, and asthma attacks and epileptic seizures.
In systems in which we can observe transitions repeatedly, such as lakes, ranges or fields, and such as human physiology, we may discover where the thresholds are. If we have reason to suspect the possibility of a critical transition, early-warning signals may be a significant step forward in judging whether the probability of an event is increasing.
A key factor characterising tipping points is that catastrophic bifurcations, a diverging of ways, propel a system toward a new state once a certain threshold is exceeded. A system follows a trail for so long, then comes to a switchpoint where it will strike out in a completely new direction. That system may be as tiny as the alveoli in human lungs or as large as global climate.
It comes down to what scientists call squealing, or variance amplification near critical points, when a system moves back and forth between two states. A system may shift permanently to an altered state if an underlying slow change in conditions persists, moving it to a new situation. Eutrophication in lakes, shifts in climate, and epileptic seizures all are preceded by squealing.
Squealing, for example, announced the impending abrupt end of Earth's Younger Dryas cold period some 12,000 years ago, the researchers believe. The later part of this episode alternated between a cold mode and a warm mode. The Younger Dryas eventually ended in a sharp shift to the relatively warm and stable conditions of the Holocene epoch. The increasing climate variability of recent times, state the paper's authors, may be interpreted as a signal that the near-term future could bring a transition from glacial and interglacial oscillations to a new state - one with permanent Northern Hemisphere glaciation in Earth's mid-latitudes.
In ecology, stable states separated by critical thresholds of change occur in ecosystems from rangelands to oceans. The way in which plants stop growing during a drought is an example. At a certain point, fields become deserts, and no amount of rain will bring vegetation back to life. Before this transition, plant life peters out, disappearing in patches until nothing but dry-as-bones land is left.
Early-warning signals are also found in exploited fish stocks. Harvesting leads to increased fluctuations in fish populations. Fish are eventually driven toward a transition to a cyclic or chaotic state. Humans aren't exempt from abrupt transitions. Epileptic seizures and asthma attacks are cases in point. Our lungs can show a pattern of bronchoconstriction that may be the prelude to dangerous respiratory failure, and which resembles the pattern of collapsing land vegetation during a drought. Epileptic seizures happen when neighboring neural cells all start firing in synchrony. Minutes before a seizure, a certain variance occurs in the electrical signals recorded in an EEG.
Shifts in financial markets also have early warnings. Stock market events are heralded by increased trading volatility. Correlation among returns to stocks in a falling market and patterns in options prices may serve as early-warning indicators.
The EURYI awards scheme was jointly created by the European Science Foundation and the European Heads of Research Councils (EuroHORCs) to attract outstanding young scientists from any country in the world to create their own research teams at European research centres. Launched in 2003 through the EC Sixth Framework Programme (FP6) these awards have effectively been replaced by starting grants through the European Research Council (ERC). The research was also funded by the National Science Foundation in the USA, Institute Para Limes and the South American Institute for Resilience and Sustainability Studies, and the Netherlands Organization of Scientific Research among others.
Notes to editors
This story has been released by National Science Foundation and Wageningen University.
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