Flooding likely to become commonplace

Silage bales float in a paddock on the Taieri during last week’s flooding. Photo: Peter McIntosh
Silage bales float in a paddock on the Taieri during last week’s flooding. Photo: Peter McIntosh
Last week’s flooding in Otago may become a regular occurrence, writes Niwa climate scientist Daniel Collins.

The recent deluges across Otago and the West Coast have brought more rain to some places than other parts of New Zealand receive in a year. Roads were cut off, a bridge collapsed, farmland was swamped, and one person lost their life. This was a tragedy, but all too real for us all. It is also a poignant reminder of things to come.

Flooding across the West Coast, Otago and Southland is only going to get more severe in the coming decades with climate change. As global emissions of heat-trapping gases continue and the world warms, westerly airflow across New Zealand will ramp up. This will bring more water to an already wet West Coast, the headwaters of the Clutha, and other rivers fed by westerly and southerly storms. The rain will fall and rivers will rise.

While climate change is liable to affect flooding anywhere in New Zealand, current research suggests the West Coast, Otago and Southland are the first in line for larger and more frequent river floods. This threat is compounded for coastal communities by sea level rise, and compounded in the north by increasing intensity of extra-tropical cyclones.

What we can expect, more or less, is that the largest floods will become larger and floods of all sizes will become more frequent. More rivers may be in flood at any one time, further stretching emergency response resources and recovery budgets.

How much the world warms, and how much climate change affects flooding in the South Island and elsewhere, depends largely on collective global actions to mitigate climate change.

Any mitigation would be good, and achieving net zero carbon by the middle of the century would significantly help avoid the worst flood impacts otherwise expected for much of New Zealand. Where mitigation is too little or too late, adaptation may still be an option.

Communities may build up flood defences or build resilience to floods. They may retreat from the flood threats, albeit with economic and social costs of their own, and likely spurred on by declining insurance coverage.

For some communities and taonga, however, it may already be too late.Whatever the course of action, climate change means we must change how we think about flood risk.

We cannot simply assume that flood risk will be the same in the future as in the past, which engineers and planners have done for decades. And while climate and hydrology models give insight about what the future may bring, they do not provide blueprints. Nor can we wait for the floods to come, because by then it will be too late.

Coping with flooding in a changing climate requires robust, precautionary planning that is agile and responsive enough to track the changing climate, and attentive to community needs. This is now required by the Ministry for the Environment for coastal adaptation, and should be needed for river flood management soon.

On the West Coast, the pressures of climate change put communities between a rock and a hard place. Over the coming decades they will become squeezed between rising seas and rising rivers, between increasing flood costs and shrinking glaciers and coal exports.

Elsewhere in the South, communities may also have to contend with more severe drought or shifting microclimates for grapes, or benefit from more reliable hydropower. Whatever the future, we must recognise that climate change will affect everyone differently, yet we must all work together to reduce the harm to those most exposed and vulnerable.

- Dr Daniel Collins is a Niwa climate scientist.

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