Talk of ‘one in 100 years’ weather can be disastrously misleading – Q&A for experts

Talk of ‘one in 100 years’ weather can be disastrously misleading – Q&A for experts


The record rainfall and flooding caused by Hurricane Gabriel across Hawke’s Bay is often described as a “once in a 100 year” event.

With the hurricane anniversary coming up this week, SMC asked a social psychologist and emergency management expert how this framing of severe weather events affects people’s perceptions of risk.

Dr Lauren Fennell, Lecturer in Emergency Management, Joint Center for Disaster Research, Massey University:

What does it mean to describe an extreme climate event as a “once in 100 years” event?

“A ‘one-in-100-year’ extreme climate event is an event of the type and size that we would expect to occur, on average, once every 100 years. If we look at a time scale of 1,000 years, we would expect about 10 ‘one-in-100-year’ events.” , but there may be more or less, and they can occur at any time during that period. There can be hundreds of years without an event, or several years in a row where the event occurs. Saying that something is an event “occurs once every 100 “Year” does not mean at all that the event will only happen once every 100 years.

How does misunderstanding of the term “one in 100 years” affect resilience?

“Previous research has shown that when people are told about a risk in this way, they tend to assume that the event will happen at the end of the time window. This may mean that people do not prepare for a ‘once in 100 years’ event because they assume it will take decades before it happens.” event, when in reality it could happen at any time. We tend to focus on risks that we see as more urgent and more likely to happen sooner rather than later, so referring to a 100-year time frame can make risks seem less urgent, and therefore less of a priority. To reduce and mitigate them, and prepare.

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“Consequently, use of this term can lead to a false sense of security among those facing a ‘once in 100 years’ event as they assume that the risk has essentially passed for another 100 years. It is important to note that a ‘once in 100 years’ event “year” can happen at any time, and the occurrence of one event does not mean that another event will not occur in the near future. These types of assumptions can affect a range of resilience decisions, from an individual’s choice of whether to buy in a flood-prone area to Broadly related to land use policy and planning.

Do we have any other ways of talking about extreme weather events?

“A common alternative is to talk about the probability of an event of a certain size occurring in any given year. A ‘one in 100 year’ event is equivalent to saying that the event has a 1% chance of occurring every year. The benefit of using this term is that it shows that there is a probability of the event occurring In any given year, rather than in 100 years. If we think of it like dice, there is a one in six chance of getting a 6 each time. Rolling a 6 does not affect the probability of getting another one on the next roll. It is (to an extent Large) It’s the same with weather events, where an event occurring in one year does not significantly change the probability of a broadly similar event occurring the following year.

“The problem with talking about chance is that we often see something expressed using a percentage as less risky than something expressed in numbers. It may be easier to convince ourselves that an annual probability of 1% is close enough to zero that the event will not occur, whereas Saying “one in 100 years” implies that the event will happen, the question is instead when.

“Where we can, it may be useful to introduce both ‘one in 100 year’ terms as well as the annual chance of a particular extreme climate event to make it clear that the risks of this event should not be discounted, but also that they are immediate.” It is not influenced by what has happened before (for example, on this Q&A page from Environment Canterbury).”

How well do we know how often extreme weather events occur? Is this changing under climate change?

“When we talk about weather events, we often consider short-term and long-term forecasts. That is, how well we can predict what will happen over the next few days once signs of a system start to appear, compared to predicting what might happen over a season, year or more.” “Often, with severe weather events, there are also different components that need to be taken into account. For example, hurricane forecasting includes frequency but also the likely path as well as intensity. These components have been improved at somewhat different rates.”

“Climate change certainly adds some complexity as it affects local, regional and global weather patterns and conditions that shape extreme events like hurricanes. It is possible that what we call a ‘one in 100 year’ event will become more frequent, whether that be due to the development of systems like hurricanes in It is often or likely that more hurricanes that form become intense enough to be on the scale of what we had envisioned.It is currently called a “one in 100 year” event.

There is no conflict of interest.

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(Tags for translation) Scientific Media Center

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