Floods a glimpse of our future
Torrential rain and flooding in the south of the South Island this week provided a glimpse of events likely to become more frequent as the climate continues warming.
This first week of February delivered about 1000mm of rain in 60 hours to Milford Sound.
That huge total in such a short time badly damaged and closed State Highway 94 between Te Anau and Milford Sound and caused landslides, including one that smashed a hut on the Routeburn Track, just north of the highway.
On Wednesday morning, Southland communities, including residents of Gore, Mataura and Wyndham, were advised to evacuate homes because the Mataura River had breached its banks.
A metre of rain in less than three days is a seriously impressive rainfall, even though Fiordland is a very wet place, as are the western slopes of the Southern Alps.
In March last year, the Cropp River site farther north in the Southern Alps and inland from Hokitika, recorded a record two-day rainfall of 1086mm.
Moisture-laden air is constantly crossing the Tasman Sea, bringing frequent rain to the western South Island as the air rises up the slopes of the alps, cools and loses its freight of moisture.
As the climate warms, those heavy rains are getting heavier. That’s because the amount of moisture (water vapour) the air can hold goes up rapidly with rising temperature.
The 1-degree Celsius or so of warming the globe has experienced in the past century has gone along with an average increase of a few per cent in the amount of moisture that the atmosphere contains.
When a storm develops, air flows towards the storm centre (known as ‘‘convergence’’), concentrating some of that moisture.
Rising air, in a storm or flowing over mountains, cools and causes condensation, ‘‘wringing out’’ water from the air through clouds and rain.
The science tells us that a 1C rise in temperature increases atmospheric moisture by 7 per cent, at most. However, when a storm draws air in to converge, the increase in moisture can be much more than that.
For high-intensity rains lasting only a few hours, the increase per degree of warming can be up to 20 per cent or more, according to the latest results from the National Institute of Water and Atmospheric Research (Niwa).
For one-day rainfalls, the increase is at least 10 per cent per degree of warming over most of the country.
So, as the climate warms, it becomes easier and easier for record rains to occur, as there’s just more moisture in the air.
Experiencing 1000mm of rain in two or three days will become more common over time (on the West Coast at least), and new records of 1100mm, then 1200 and 1300mm will be set in future West Coast storms.
That poses obvious risks to life and limb, to roads and buildings, and increases the occurrence of land slips and river floods.
To mark the turn of the millennium back in late 1999, I was asked to prepare a weather forecast for January 1, 3000, another 1000 years into the future.
I envisaged a northwest storm, with up to 1000mm of rain in a day in Fiordland, thinking that was fanciful enough for such a distant future.
But given this week’s events, I am thinking we will not have to wait that long at all, less than 100 years perhaps. That is, unless we knock climate change on the head by cutting emissions of greenhouse gases and getting to the zerocarbon economy as fast as possible.
When humanity stops emitting carbon dioxide into the atmosphere, the warming will stop soon after. The future is in our hands.