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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.
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.
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