This story was first published on September 8, 2022. It was republished on January 29, 2023 during the Auckland floods.
A “one-in-100-year event” was frequently used to describe the wild weather Aotearoa experienced this winter. But is this an accurate or helpful way to discuss extreme weather events in the midst of a climate emergency?
New Zealand just experienced the wettest July in recorded history, and one of the wettest overall winters, with massive storm systems associated with atmospheric rivers dumping 150% more rainfall than normal across the motu.
These rainfall events caused widespread flooding and landslides around the country that many politicians, media, and experts highlighted as “one-in-100-year events”.
The problem with this “one-in-100-year” framing is it makes you think there won’t be another storm of that scale for another 100 years, University of Canterbury professor of climate change Dave Frame says.
But during a climate emergency, Frame says extreme weather events such as these are likely to be more frequent.
“For some heat-related events, we know that what was yesterday's 100-year event in a few decades could be a once in every one- or two-year event.”
Trends indicate that our winters will get wetter and our summers will be hotter and drier, Frame says.
It’s important we don’t play what he calls “risk hot potato” where the risks linked to climate change are thrown to someone else by framing the events as an unavoidable act of nature.
“People are too casual about calling things a one-in-100-year event and then stepping away as though ‘that's not on me, that's on God.’”
Frame says while it is impossible for people to ignore the frequency and severity of these weather events, some still believe it is not caused by human-driven climate change.
“And that's where I think the work being on attribution is really, really central. It's the smoking gun.”
The smoking gun
National Institute of Water & Atmospheric Research (NIWA) climate scientist Dr Suzanne Rosier and her team have been working on confirming the extent human-driven climate change is affecting these extreme weather events.
“We look at an event and model it both as it was and also as it might have been in a hypothetical world where humans hadn't had an influence, such as emitting greenhouse gasses,” Rosier says.
The team found that as global temperatures have risen due to human activity, this has driven changes in the severity of both heat and rainfall events in Aotearoa.
“New Zealand has warmed by about 1.2 degrees since pre-industrial times, generally in line with global warming, and the extremes of temperature have also warmed similarly,” Rosier says.
“On the whole, we can be pretty confident about how temperature and its extremes are changing.
“It’s harder to be confident about how extreme rainfall is changing as this depends so much on how exactly weather patterns play out over New Zealand. However, we have often - but not always - found that rainfall is heavier in the human-influenced case, and that we might expect it to come around again sooner.”
Regular as rain
Frame says it's the static nature of the one-in-100-year event framing that's the problem.
The environment is becoming rapidly different now than it was 100 years ago and using the past as a benchmark for weather is quickly becoming irrelevant, he says.
In the past, this framing has been important for communicating to people when a weather event was out of the ordinary but climate change is going to make severe storms normal.
Frame says scientists and society need to find a new way of discussing weather events that communicates their severity, while reminding us that they are being driven by human climate change and that we need to do something about it.