Climate change: Small amount of sea level rise exposes hundreds more farms to flooding

Even a small amount of sea level rise could expose hundreds more dairy farms near our coasts to flooding, new modelling reveals.

Rising oceans pose one of New Zealand’s biggest threats, with an estimated 750,000 Kiwis and 500,000 buildings near rivers and coasts already vulnerable to inundation.

Studies have been telling us how that risk could worsen faster than we think: one recent analysis showed higher seas could make once-in-a-century coastal storms an annual event in our two biggest cities and other places as early as 2040.

But, until now, less had been known about the potential impacts to our economically-vital productive agricultural land, of which nearly 1500sq km is today considered exposed to coastal flooding from 100-year events.

In just-published research, carried out as part of the collaborative Deep South National Science Challenge, researchers took a closer look at the threat to our multi-billion-dollar dairy sector.

Specifically, they sought to quantify the impacts to farms from flood-making extreme sea level events (ESL) - like king and storm tides - with relative sea level rise (RSLR), or the height of the ocean relative to land at different places.

Using freshly-developed tools, they modelled ESL events under 10, 100, 200 and 500-year annual recurrence intervals - or the average number of years between an event happening again - together with RSLR, at 50cm increments up to two metres.

They then compared that data against the location and production capacity distribution of farms, to give the percentage and quality of farming land inundated.

The results were sobering.

Even with just another 50cm of sea level rise – which could come within a few decades – the number of farms affected during a severe one-in-10-year event doubled to more than 800.

At current sea levels, 472 farms were potentially exposed to a one-in-10-year extreme event, while 1276 farms were at risk of a one-in-500-year event.

With seas two metres higher, the number of farms exposed to one-in-10 and one-in-500-year events climbed to 1276 and 1344 respectively.

Flooding at Hikurangi Swamp, north-west of Whangarei, in March 2007. New research finds sea level rise-driven flooding could add to the climate change risks our valuable dairy sector already faces. Photo / Brett Phibbs

Within that small relative difference, study lead author Dr Heather Craig said, lay one of the study’s most important findings.

It showed the rise in danger wasn’t driven by the extreme events themselves – but the fact that background sea level rise caused inundation to occur much more frequently.

“This means that we have one of our main economic contributors becoming significantly more exposed to extreme sea level events in the near future.”

Those regions with the highest proportion of affected dairy farms were found to be Otago, Northland, and Bay of Plenty.

There, between 15 and 20 per cent of farms were found to be vulnerable to one-in-10-year events under a two-metre scenario – or to one-in-100-year events with a metre of sea level rise.

In Waikato, particularly, more than 250 farms were at risk from only 50cm of rise.

“For many dairy farms - particularly in the Northern part of the Waikato Region, Northland and the Bay of Plenty - the portion of the farm potentially exposed to coastal inundation with RSLR is often the most productive,” Craig added.

“That is the part that requires the least amount of modification, such as fertilisation, to be highly productive. This could have significant consequences if repeated coastal inundation events cause salination of these soils.”

Further, the study identified that at least $224m worth of milking sheds exposed to one-in-10-year events with two metres of sea level rise.

When they looked at livestock affected, an estimated 175,000 were located on dairy farms at least partially exposed to a one-in-10-year event at current sea levels, while at two metres of rise, some 510,000 would be on farms exposed to a one-in-500-year event.

Craig said some farms affected by these events would likely need support during and after, which might include financial assistance from government agencies.

“We’d like to further explore what these impacts will mean for the dairy industry, both at a farm-scale and in terms of the national economy.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.