Effects: increasingly extreme weather events
Image: NASA
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Summary
A global average increase in temperatures of 1.5°C above pre-industrial levels doesn’t seem like much, but averages don’t generally kill you or destroy your home, your farm, or your town.
The effects of rising temperatures are becoming more evident, but how do we know if a specific weather-related event is due to climate change? ‘Event Attribution‘ is the term used by scientists to work out what percentage, if any, climate change is responsible for the frequency and scale of extreme events such as the 2020 Australian wildfires, the 2021 Canterbury floods and 2021 Pacific Northwest heatwave that killed an estimated billion marine animals and over 500 people, Cyclone Gabrielle in Aotearoa in February 2023, the 2025 Los Angeles fires, the unprecedented 2026 heatwave across Europe, or the catastrophic August 2026 Nepal-Tibet floods.
For too long, weather’s randomness has kept events such as these from being blamed squarely on climate change… Now, we can specify increased chances for specific events. This extends to forecasts: we can identify the places that are more likely to see wildfires, mudslides and fish die-offs. Such calculations dent both climate denial and a false sense of security. They take away the argument that ‘extreme weather happens anyway, so we don’t need to worry about it’. Extreme weather happens—and these metrics pinpoint what is becoming more likely, by how much and why… Such evidence is also useful for legal proceedings when citizens call corporations or governments to account for their role in climate change. – Richard A. Betts
If we understand how likely an event occurs because of climate change versus natural fluctuations or how much climate change has turbo-charged the effects of large scale weather patterns such as El Niño, we can better plan to mitigate the risks and future climate costs. Many events are caused by compounding factors. For example, monoculture pine forests can’t grip the soil as well as native plants and slash from forestry is left where it lies because it’s not cost effective to remove it. In alpine regions warming temperature are melting glaciers, but less visible is the melting permafrost that holds large chunks of mountains together. Heavy rain may be the trigger events that results in an avalanche, but the underlying cause was warming temperatures.
Insurance underwriters use climate attribution tools to help calculate the cost of insurance and to decline your insurance; for example if you live in an area at risk from floods or rising sea levels.
Extreme vs. average weather-related events: Aotearoa
The chances of getting a warm year are increasing all the time. The chances of getting a cold year are decreasing all the time. When you look at shorter time frames, from day to months, it’s really about the chances of getting a very hot event, extreme high temperatures, or heavy rainfall, are what’s really impacting both ecosystems and human activities. It’s how extreme are changing that’s really important…As temperatures rise, the number of rain days decrease. So moderate rain decreases while extreme rain increases…more and more extreme rainfall events and floods, with concurrent impacts, including erosion and landslips. – Prof. James Renwick 2022
When Cyclone Gabrielle struck Hawkes Bay in January 2023 it provided some insight into the limitation of climate models. The 2020 NIWA report, Climate change projections and impacts for Tairāwhiti and Hawke’s Bay projected that under the RCP8.5 (worst-case) scenario the average maximum annual 5-day rainfall by the year 2081 would be 151.3 (+14.8)mm at Glengarry.
During the cyclone, the Glengarry site recorded 546mm of rainfall with almost 400mm falling in 12 hours at a maximum intensity of 56mm/hour.
Over New Zealand, an average two to three fold rise in frequencies of extremes occurs irrespective of seasons due to anthropogenic influence, with a mean temperature increase close to 1°C. – Thomas et al, October 2022
NIWA’s long-running ‘seven-station’ series shows NZ’s average annual temperature has increased by about 1°C over the past 100 years. – Annual Climate Summary 2025, Earth Sciences New Zealand
Extreme weather events are rising at a pace which exceeds expectations based on thermodynamic arguments only, changing the way we perceive our climate system and climate change issues…. The additional evaporation and rainfall tends to end up in heavy rain rather than alleviating drought: half of it comes down in the wettest 6 d(ays) each year. Even once global warming is stopped, we will see unprecedented extremes for a long time to come.
Just think of a former once-in-5000-year event which at 1.5°C warming may have become a once-in-50 year event. – Di Capua and Rahmstorf 2023
At many locations studied, the one-in-100-year extreme sea level event is now occurring at least once per decade. Sea levels are continuing to rise so coastal inundation events will be happening even more frequently over time from here… This week’s extreme wave event on the Wellington south coast reinforces the idea that we need to act now and develop response plans urgently. I trust our policymakers and politicians are listening. – Prof. James Renwick 2026
The takeaway message is clear. Existing climate models and weather forecasts are not always able to predict the magnitude of extreme (acute) events. This should be considered when looking at flood hazard mapping tools as the statistical probability of any AEP1% event may no longer be valid because they were calculated for a climate that no longer exists. In some locations, the probability of recurrence may increase to decadal (AEP10%) or even annual (AEP100%). That is, they may occur every year.
Event Attribution
The April 2023 Podcast from The Conversation that takes you inside the UN’s era-defining climate report via the hearts and minds of the scientists who wrote it. The presenters delve into one of the major shifts in the public communication of climate change: the attribution of extreme weather events to climate change.
In Video 1, Dr Fredi Otto, leading climate scientist and co-lead of World Weather Attribution (WWA) at Imperial College London, and Laura Tobin, broadcast meteorologist and weather presenter explain the science behind these phenomena, the real-world impacts, and how we can better adapt to a rapidly changing climate.
Figure 1 links to the WWA report on Cyclone Gabrielle.
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