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Converting probabilities between time-intervals

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This is the first in an irregular sequence of snippets about some of the slightly more technical aspects of uncertainty and risk assessment.  If you have a slightly more technical question, then please email me and I will try to answer it with a snippet. Suppose that an event has a probability of 0.015 (or 1.5%) of happening at least once in the next five years. Then the probability of the event happening at least once in the next year is 0.015 / 5 = 0.003 (or 0.3%), and the probability of it happening at least once in the next 20 years is 0.015 * 4 = 0.06 (or 6%). Here is the rule for scaling probabilities to different time intervals: if both probabilities (the original one and the new one) are no larger than 0.1 (or 10%), then simply multiply the original probability by the ratio of the new time-interval to the original time-interval, to find the new probability. This rule is an approximation which breaks down if either of the probabilities is greater than 0.1. For exa...
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1-in-200 year events

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You often read or hear references to the ‘1-in-200 year event’, or ‘200-year event’, or ‘event with a return period of 200 years’. Other popular horizons are 1-in-30 years and 1-in-10,000 years. This term applies to hazards which can occur over a range of magnitudes, like volcanic eruptions, earthquakes, tsunamis, space weather, and various hydro-meteorological hazards like floods, storms, hot or cold spells, and droughts. ‘1-in-200 years’ refers to a particular magnitude. In floods this might be represented as a contour on a map, showing an area that is inundated. If this contour is labelled as ‘1-in-200 years’ this means that the current rate of floods at least as large as this is 1/200 /yr, or 0.005 /yr. So if your house is inside the contour, there is currently a 0.005 (0.5%) chance of being flooded in the next year, and a 0.025 (2.5%) chance of being flooded in the next five years. The general definition is this: ‘1-in-200 year magnitude is x’ = ‘the current rate for eve...
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The Global Goals: How on Earth can geologists make a difference?

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Image credit: Geological Society On the 30 th October the Bristol Geology for Global Development (GfGD) group trekked off to London to the grandeur of the Geological Society  for the 3 rd annual GfGD conference . Joel Gill, the director of GfGD, opened the conference with the bold claim: “Probably the world’s first meeting of geologists to discuss the Global Goals.” And it’s not an overstatement. Despite first appearances, geology has a crucially important role to play in many of the 17 goals internationally agreedby World Leaders in September this year . So why aren’t we talking about it? The conference acted as a platform for these discussions, it gave geologists a chance to learn how they can actually contribute to the success of these international development targets and it introduced us to new ways in which geology can help make a difference. . @JoelCGill describing the importance of geology & geologists in meeting UN Sustainable Development Goals #GfGDConf p...
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