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Extreme rainfall in a warming world – Expert Reaction

Auckland is currently challenging the record for its wettest month ever. More than 769% of its rainfall in a normal January has been recorded so far, with another round of downpours likely to hit again tomorrow night.

The SMC asked experts to discuss what we know about the link between a warming climate and such extreme rains.

Dr Luke Harrington, Senior Lecturer in Climate Change, University of Waikato, comments:

“The impossible is now becoming plausible, thanks to climate change.

“The Australian bushfires of 2019/20. The British Columbia heatwave of June 2021. Devastating floods in south-west Germany and China’s Henan province, both in July 2021. 40°C recorded across the UK for the first time in July 2022. A third of Pakistan underwater in August. Seoul’s extreme one-day downpour on August 8th. All these events were record-shattering. All were made more likely and more intense because of human-caused climate change.

“Now we add Auckland to that unfortunate list. January 27th saw records smashed, with total rainfall over 24 hours at Auckland Airport beating the previous record by a whopping 60% (most of which fell within three hours).

“Record-shattering heatwaves show by far the largest climate change signal (with events up to hundreds of times more likely to occur). Bushfires often have much smaller signals of change (the 2019/20 Australian bushfire was found to be 30% more likely because of climate change) while extreme rainfall events generally sit somewhere in the middle (events are often 50-200% more likely than in a pre-industrial climate, but with large uncertainties).

“Because of the different mechanisms involved, hourly rainfall extremes are intensifying more rapidly with climate change than equally rare downpours which persist for three or five days. Depending on the location, every degree of global warming could add between 5 and 15% to the total rain falling, possibly more. It is worth remembering, though, that even if we trimmed 20% off the top of the rain gauge records witnessed last Friday, there still would have been widespread devastation.

“Rainfall this intense would challenge many cities worldwide. Even so, the impacts of the next event can be significantly reduced if we improve stormwater system capacity and make additional space for nature-based drainage solutions.

“It’s also worth noting a strong connection exists between the speed of global warming and the chances of witnessing record-shattering events in the future. Any efforts to reduce that warming speed – by reducing our emissions – will help to reduce the number of events which really catch us off guard, like what we’ve seen in recent days.”

No conflict of interest.

Dr Sam Dean, Principal Scientist – Climate, NIWA, comments:

“Auckland has just experienced a truly exceptional rainfall event with some rain gauges recording over 200mm in under 6 hours. Such intense rainfall is responsible for the rapidly developing flash flooding that occurred. Understanding exactly how much of that rainfall, flooding and damage was caused by climate change will require dedicated attribution studies by climate scientists in the coming months. However, it is now widely understood that a warmer atmosphere can hold more moisture, which in turn causes more extreme rain.

“Based on our knowledge of how much New Zealand has warmed over the past 150 years as a result of climate change, and other studies we have done on New Zealand flood events, we can make an initial estimate that the amount of rain that fell in the event would be expected to be between 10% and 20% more than what would otherwise have occurred without the emission of planet warming greenhouse gases.

“This is a big number, even if it doesn’t seem it. We know that climate change affects short duration storms like that which caused Friday’s flash flooding in Auckland the most strongly of all. We also know that this percentage increase in the rainfall will feed through into a proportionately larger effect on the extent and depth of flooding, making climate change ultimately a much bigger factor in terms of damages and costs. Figuring out exactly how much is something we will be looking at in the months to come.

“Natural variability in influences of climate, like La Niña, will continue to affect when and how often big events strike. But it is important to understand that the world we live in now has been irrevocably affected by our continuing emission of greenhouse gases and we will at times be brutally confronted by the consequences. Thanks to our emissions to date we have thirty more years of climate change already baked in regardless of any climate mitigation action. Nearly everyone in New Zealand is at risk of flash flooding events like just occurred in Auckland, and we must all work together to adapt to a new reality that will require many hard choices.”

No conflicts of interest.

Professor James Renwick, School of Geography, Environment and Earth Sciences, Victoria University of Wellington-Te Herenga Waka, comments:

“The maximum amount of water vapour in the air increases exponentially with temperature. Hence, as the climate warms, we have the potential for increasingly heavy rainfalls. Not that every storm brings a deluge, but as we warm the climate, we constantly weight the dice towards heavier rainfall events.

“Many of the heaviest rainfall events in New Zealand and elsewhere are associated with ‘atmospheric rivers’: vast corridors of moisture that extend from the tropics to higher latitudes. The recent flooding in Auckland is an example of this. As the atmosphere becomes more loaded with water vapour, these ‘rivers’ are set to transport more moisture. Already, a significant atmospheric carries as much water as the Amazon does, around 350 times the flow in the Clutha or over 600 times the flow in the Waikato. The latest IPCC report states that ‘there is high confidence that the magnitude and duration of atmospheric rivers are projected to increase in future, leading to increased precipitation.’

“Exactly how much increase we’ll see depends on how much more greenhouse gas is emitted into the global atmosphere. If emissions reduce according to the Paris Agreement and global warming is limited to less than 2°C, we may see an overall increase in atmospheric moisture of 10% or so. When concentrated into a storm, an atmospheric river, or a thunderstorm, that could easily translate to 20-30% or more rainfall than would have occurred without the warming.

“The flooding event in Auckland will be studied closely no doubt, to determine just how rare an event it was, and how much of an influence climate change played in its intensity. We need to remember there was also an influence from ‘natural variability’ in the form of a La Niña and the associated warmer seas around New Zealand and the western Pacific, plus the influence of a ‘Madden-Julian event’ passing our longitudes in the tropics. By researching this event, we can better understand the role of different components of the climate system on extreme rainfalls in New Zealand and how they are changing, something we still have plenty to learn about.”

Conflict of interest statement: “I receive funding from MBIE and other agencies to study climate change. I was an author on the past three Assessment Reports of the Intergovernmental Panel on Climate Change. I am a Commissioner with the New Zealand Climate Change Commission.”

Professor Ilan Noy, Chair in the Economics of Disasters and Climate Change, Victoria University of Wellington:

“The costs of the recent Auckland floods is high, and the human toll is horrible. This deluge was unprecedented, but we should not conclude from that the catastrophe was inevitable (what insurers call an Act of God). This flood highlights two issues for me as an economist: The first is that we need to invest more in infrastructure that will enable us to be better prepared for what a changing climate is likely to throw at us. In many places, our public infrastructure has been neglected for too long. And second, that in some locations, new defensive infrastructure may not be enough (or might be too costly). In those cases, we need to relocate some households and communities out of harms way. Together, we need to develop the mechanisms to be able to do both these things, and to decide when each one might be more appropriate. Doing nothing, however, should no longer be considered a viable option for dealing with the increased risk of flooding in many locations across Aotearoa.”

No conflicts of interest declared.

Dr Daniel Kingston, School of Geography, University of Otago, comments:

“The rainfall over Auckland this weekend was certainly extreme: over 750% of the Auckland monthly average rain fell, and over a third of Auckland’s entire annual average – with further substantial rain expected in the coming days. Both events are linked to atmospheric rivers funnelling warm moist air down from the tropics. The severity of the weekend event was linked to its relatively slow passage over the country – similar to the slow-moving atmospheric river that devastated the top of the South Island in August last year.

“While it’s not generally possible to say that climate change caused particular events, we can increasingly say how climate change has modified them compared to preindustrial conditions. For instance, the extreme rainfall that led to the Canterbury 2021 flood was estimated as being 10-15% more intense as a result of climate change. This is because a warmer atmosphere can hold more water vapour – meaning that when heavy rain events do occur, there is more water available to be rained out.

“As we move into the future these effects will increase, although not necessarily in a linear fashion. This is because extreme rainfall is not just dependent on the amount of water in the atmosphere, but also how larger scale atmospheric circulation changes and the impacts this has on the number of rain-generating weather systems that pass over us. This makes location-specific projections uncertain, but at a global level we can make some generalisations. For instance, we can expect the maximum daily precipitation for a 50-year event (in other words, an event we have a 2% chance of exceeding in any given year) to be almost 15% greater compared to preindustrial conditions under 2°C of global warming, and over 30% higher at 4°C.”

No conflict of interest.