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Talk of ‘one-in-100-year’ weather can be disastrously misleading – Expert Q&A

Record rainfall and flooding caused by Cyclone Gabrielle across Hawkes Bay has been frequently described as a ‘one-in-100 year’ event.

With the cyclone’s anniversary this week, the SMC asked a social psychologist and emergency management expert how this framing of extreme weather events affects people’s risk perception.

Dr Lauren Vinnell, Lecturer of Emergency Management, Joint Centre for Disaster Research, Massey University:

What does it mean when an extreme weather event is described as a ‘one-in-100-year’ event?

“A ‘one-in-100 year’ extreme weather event is an event the type and scale of which we’d expect to occur, on average, once every 100 years. If we looked over a time scale of 1,000 years, we would expect about 10 ‘one-in-100 year’ events, but there could be more or fewer, and they could happen at any time during that window. There could be hundreds of years without an event, or multiple years in a row where an event occurs. Saying that something is a ‘one-in-100’ year event absolutely does not mean that the event will only happen once every 100 years.”

How does misunderstanding of the ‘one-in-100 year’ term 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 occur towards the end of the time window. This might mean that people do not prepare for a ‘one-in-100’ year event because they assume it will be decades before the event occurs, when in reality it could occur at any time. We have a tendency to focus on risks which we see as more immediate and more likely to occur sooner rather than later, so referring to a 100-year timeframe can make the risk seem less immediate, and therefore less of a priority for reduction, mitigation, and preparedness.

“Relatedly, using this term can lead to a false sense of safety among those who experience a ‘one-in-100’ year event as they assume that the risk has essentially passed for another 100 years. It’s vital to communicate that a ‘one-in-100’ year event could happen at any time, and one event occurring does not mean another won’t in the near future. These sorts of assumptions could affect a range of decisions relating to resilience, from an individual choosing whether to buy in a flood-prone area to larger-scale policy and land-use planning decisions.”

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

“A common alternative is to talk about the chance that a particular sized event will occur 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 makes it clear that there is a chance of the event occurring in any given year, rather than being 100 years away. If we think about it like dice, there’s a one in six chance of rolling a 6 each time. Rolling one 6 does not affect the probability of getting another one on the next roll. It’s (largely) the same with weather events, where an event occurring one year does not significantly change the probability of a similar scale event occurring the next year.

“An issue with talking about chance is that we often see something expressed using a percentage as less risky than something expressed with numbers. It might be that it’s easier to convince ourselves that a 1% annual probability is close enough to zero and therefore the event won’t occur, whereas saying ‘one-in-100 years’ implies that the event will happen, the question is instead when.

“Where we can, it might help to present both the ‘one in 100 year’ terminology as well as the annual chance of a particular extreme weather event to convey that the risk of the event should not be discounted, but also that it is immediate and not influenced by what’s happened before (for example, in 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 both short term and long-term forecasts. That is, how well can we forecast what’s going to happen over the next few days once the signs of a system start to appear, compared to forecasting what might happen over a season, a year, or longer. Often with extreme weather events there are also different components to consider. For example, forecasting cyclones includes frequency but also the potential path as well as the intensity. Improvements have been made on these components at somewhat differing rates.

“Climate change definitely adds some complexity as it impacts local, regional, and global weather patterns and the conditions which form extreme events such as cyclones. It’s possible that what we call a ‘one-in-100 year’ event will become more frequent, whether it’s because systems like cyclones develop more often or more likely that more cyclones which form become intense enough to be at the scale of what we would currently call a ‘one-in-100 year’ event.”

No conflict of interest.