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Wellington Weather

Wellington has been windier and wetter in the past -- it's also been a beautifully warm, calm area, or so fossil pollens tell us.

By Dr Dallas Mildenhall

Wellington's climate is now warmer, and possibly wetter, than it has been over the last 300,000 to 400,000 years, according to recent studies of sediments on the floor of Hutt Valley.

Over this geologically short time period, Wellington has been subject to a variety of climates both wetter and drier than today, windier and calmer, hotter and colder. Such changes can be tracked by fluctuations in spore and pollen counts from plants.

All sediments deposited on land contain spores and pollen from plants, especially mosses, ferns, conifers and flowering plants. By comparing the ratio of spores and pollen from warmth-loving plants to those from cold-loving plants, a curve showing climate changes can be produced. A series of samples has recently been collected from Petone, covering a period of around 335,000 years.

The Petone sediments show four major cold periods and four major warm periods, each with fluctuating climates. Sometimes during the cold periods, the climate was as pleasant as today's; sometimes during the warm periods it was far less pleasant.

During the cold, glacial periods, snow would have lain around Wellington for much of the year, and erosion would have been rapid. Coarse gravel would have filled the valleys. Much of this would have come from storm-related slips and from a periglacial process called solifluction, where the alternate freezing and thawing of water causes rocks and soil to creep downhill. The gravel would also have extended well out to sea from the present coastline; the sea level would have been at least 70-120 metres lower, with much water held in ice caps and glaciers.

The vegetation consisted of grasses, shrubs and stunted trees in the valleys, and probably occasional tussock on the hills. The hilltops were probably bare, and chipped away by ice and snow which would break open rocks by freezing and expanding within cracks. Thawing would then carry away the rock fragments. The climate would have been cold, stormy and windy, but relatively dry because much of the water would come in the form of snow.

During the warm, interglacial periods, there would rarely be any ground snow at any time, even on the hilltops. Less erosion would occur and the valley sediments would be silts, muds, sands and peats from swamps and bogs. The rainfall would be higher, but the whole landscape would be vegetated, reducing any erosion effects. The vegetation consisted of large trees, like rimu, kahikatea and kauri, and ferns such as mingimingi, only found north of Lake Taupo these days.

During some of the warm periods, the climate was so warm that the soil turned red, visible in road cuttings around Wellington.

And what of the future? The spore/pollen curve suggests that Wellington is heading for another cold period. Over the last 5,000-7,000 years, some frost-tender plants have disappeared or become less common in the Wellington area. Hutu, or Ascarina lucida, and akeake, Dodonaea viscosa, are two plants which have decreased as droughts and frosts have become more common over the last 5,000 years.

This cooling climate will be offset by global warming, which will tend to increase temperature and possible rainfall and storminess in the Wellington area. Which effect wins -- the natural progression towards cooling, or the human effect towards heating -- only time will tell.

Dr Dallas Mildenhall is a palynologist with the Institute of Geological and Nuclear Sciences Ltd.