At Greater Wellington Regional Council’s Wairarapa Committee last week, a brief but important conversation was had about how scientific terminology could be confusing for the general public.
One committee member made a point that the term “one-in-50-year flood” could be somewhat confusing, leading people to believe that a flood of that intensity could only happen once every 50 years. In fact, a flood of that intensity can occur twice in a week, month, or year.
The “one-in-50-year flood” is a description of the likelihood of the weather event happening, not a prescription of when it will happen. It gives that event a 1 per cent chance of occurring within 50 years.
The same goes for any weather-related anomaly, and with a changing climate, those floods that we may have expected once in 50 years could be seen more than once a year.
The flood, in a way, is no longer a “one-in-50-year flood” but simply a flood.
The baseline for extremity had been lifted, so should the classification too?
Andy Pitman, director of the ARC Centre of Excellence for Climate System Science at UNSW Sydney, along with colleagues Anna Ukkola and Seth Westra, explained some of the modellings in an article in The Conversation.
“First, let’s clear up a common misunderstanding about what a one-in-100-year event means. It does not mean the event will occur exactly once every 100 years or that it will not happen again for another 100 years,” they said.
“Over a period of 1,000 years, you would expect the one-in-100-year event would be equalled or exceeded ten times.”
They said several of those events could happen in just one year.
Additionally, they made the point that the likelihood is based on historical data collected over the past 100 years.
Greater Wellington Regional Council [GWRC] commissioned the National Institute for Water and Atmospheric Research [Niwa] in 2021 to create a report on Wairarapa’s climate change predictions.
In that report, climate scientists say that because extreme weather is rare, a very long record is needed before a climate change signal becomes detectable in the number of events: “for instance, a period much longer than 50 years would be needed before the shift of 1-in-50-year events to 1-in-30-year events became evident”.
They said while risks associated with some extreme events might have changed substantially because those events have become more or less likely, it might still be too early to be able to discern those changes in the observational record.
Although we are observing these big rainfall events more often, it’s not enough to quantify on any scientific scale. We know that they’re happening more often, but it will take time for what would normally be called a one-in-50-year flood to become a one-in-30-year flood.
This all points to the importance of science communication; a better understanding of what these things mean will eventually lead to a better-informed general public.
In the face of climate adversity, knowledge is by far, the most powerful weapon.
