Fires can devastate HOT forests.
© Getty Images
New theory shows that high performance needn’t mean high risk.
For man-made systems such as machines and markets, catastrophe lurks somewhere between high risk and high performance. US physicists may have found a way to strike the optimal balance1.
This trade-off is familiar to the financial world. Brokers develop investment portfolios to provide the best returns within a specified level of risk. Mark Newman and co-workers at the Santa Fe Institute in New Mexico have borrowed some ideas from the economic theories of risk aversion to create a general prescription for avoiding ruin.
But a HOT system has an Achilles’ heel. It is typically fragile under perturbations for which it was not designed. For example, if the distribution of fire breaks or sparks alters slightly, a forest can become highly susceptible to fires and give a poor yield.
Newman’s team now points out that HOT designs have another drawback. The cost of an optimal performance is a high chance of a ruinous collapse. Catastrophic fires that burn nearly all the trees are rare, but not as rare as one would expect if fire size were random. The optimal state is a high-risk state: it gives good returns at the price of possible ruin.
Most engineers don’t want to run this risk. So Newman’s group has calculated how to design a system to optimize performance and almost eliminate the probability of ruinous events. They call this design principle ’constrained optimization with limited deviations’, or COLD.
Surprisingly, a COLD state can completely remove the danger of total ruin while sacrificing only a few per cent of the average yield relative to a HOT state. Newman and colleagues say that, as we are generally risk-averse, we are more likely to prefer COLD designs than HOT ones.
Nature, apparently, is more short-sighted. Ecosystems, for example, are often in HOT states They are catastrophically susceptible to rare disturbances not accounted for by natural selection - such as meteorite impacts.
PHILIP BALL | © Nature News Service
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