West Coast rivers early climate change sentinels
Rivers on the West Coast will likely give early and clear signals that climate change is upon New Zealand, a new study has found. This will probably start happening after 2050.
Rivers on the east side of the South Island will also give early signals, especially those that rise in the Southern Alps, the study by hydrologist Dr Daniel Collins found.
Rivers elsewhere in the South Island and on the North Island will be less noteworthy as climate change sentinels. Some rivers will show few or no effects from climate change, the model predicted.
Collins’ study looked at river flows, not floods or coastal storm surges. He also didn’t look at biological effects of climate change in New Zealand rivers.
Higher river flows are predicted for the West Coast and South Island rivers because increased precipitation is predicted to fall on the Southern Alps as a result of climate change, particularly in winter.
Meanwhile, higher air temperatures will have a ‘‘disproportionate effect’’ on river flow in snow-affected alpine areas. Indeed, runoff is predicted to shift from winter to spring.
All that water will flow to the West Coast and into rivers such as the Clutha River/mata-au, Rakaia and Waimakariri that rise in the Alps and flow east and south.
‘‘Mean winter flows [are] the most extensive sentinel of climate change in New Zealand rivers,’’ wrote Collins in the Hydrological Sciences Journal recently.
His model looked at river flows in spring, summer, autumn, and winter, as well as mean annual flows and low flows, to identify which are ‘‘projected to emerge first and most extensively’’.
Scientists and governments have been much concerned by the ‘‘time of emergence’’ of climate change – the point when a climate change signal becomes distinguishable from background effects.
It is very difficult to do. ‘‘It’s like eavesdropping on a conversation in a noisy bar,’’ Collins said in an interview.
Depending on the variety of factors, the time of emergence could be soon, late century and not this century. Faced with this uncertainty, Collins went looking for the ‘‘order of emergence’’; which hydrological metric is likely to emerge first and where.
This reduced uncertainty around timing and told decisionmakers what to look for. ‘‘Some things in nature will show the
fingerprints of climate change earlier than others, and we should pay attention,’’ he said.
He found, ‘‘very little emergence occurs by midcentury for any metric, and in some instances no substantial emergence occurs during the entire simulation period,’’ which ended in 2099.
But higher river flows on the South Island start turning up some time after 2050.
Decision-makers need to think about higher waters now because infrastructure built or improved now will last many decades, he said. Road bridges in the affected areas were an obvious concern.
Collins also singled out irrigation schemes. Those in Canterbury and Otago may have more water available to them, but it may come earlier in the year and more storage may be needed.
Similar scenarios – more water in winter – face hydroelectricity generators.
He did not find that mean winter river flow was the only early or best sentinel of climate change. Sea-level rise and glacier retreat were also sentinels.
Collins did most of his modelling while at Niwa. He is now at Lincoln University studying environmental policy and management.
Some things in nature will show the fingerprints of climate change earlier than others, and we should pay attention.