We're watching our future washing away

By SIMON COLLINS

New Zealand's landscape has been built up by earthquakes and volcanoes and washed down by storms and floods, which are likely to become more frequent as the global climate warms up.

Three-quarters of our rocks - an even higher proportion in the North Island - are soft sedimentary material laid down mostly by water on floodplains and out at sea, and later lifted up by earthquakes.

This young, soft and vulnerable land is then lashed by all the forces of the weather generated in the vast Pacific Ocean.

As Dr Warren Gray, a meteorological hazards researcher for the National Institute of Water and Atmospheric Research, puts it: "The difference between the feel of weather here and in, say, Britain is that there is a much more pumped-up energetic feel to our weather systems. Our weather systems charge through with force."

Professor Terry Healy, of Waikato University, says we have made things worse for ourselves by cutting down our trees, reducing the forest cover from 85 per cent of the land before humans arrived to 29 per cent - including plantations which we periodically chop down. By contrast, forests still cover two-thirds of Japan.

So when the rain falls on our soft land there are few trees to break its force and few deep roots to soak it up. Instead, the water gouges out the topsoil - and often the rock beneath it - and carries it out to sea.

During the February floods in the southern North Island, Landcare scientists stood on Palmerston North's Fitzherbert Bridge and lowered sampling devices into the Manawatu River as trees, hay bales and even cows floated past.

During the peak eight-hour period, they calculated that 28 tonnes of sediment flowed under the bridge every second, a quarter of it rich topsoil.

Even by our usually volatile standards, says Graham Hancox of the Institute of Geological and Nuclear Sciences, two big floods in a few months - first in the Manawatu and now in the Bay of Plenty - is "a little bit unprecedented".

But the outlook is for more, if the global climate is changing. "Studies have been done suggesting that what is now a one-in-100-year storm, by 2050 or 2080 might be a one-in-50-year storm," Gray says.

On top of that very long-term trend, areas such as the Bay of Plenty on the east coast of the North Island can expect more storms during the next 30 years because we have just switched from a 30-year cycle of relatively more El Nino weather events - which bring New Zealand relatively cold, dry weather - to a new cycle that is likely to be dominated by La Nina, bringing warmer, wetter weather with more storms from the north and east.

So we are going to be vulnerable. We can plant trees and cut back our carbon emissions to save our great-grandchildren from the worst, but for many years ahead the die is already cast.

In the meantime, as Gray says, all we can do is manage the risk. "Shifting where we live is one thing," he says, "but putting efforts into mitigation steps like stopbanks is a second. Having warning systems that can anticipate the damage is a third, allowing us to take actions."

Every option will be costly. As Gray says, each community will have to weigh the risks and decide how much it is willing to spend on managing them.