Why New Zealand summers are so much hotter

Our summers are getting warmer and as average temperatures rise, the likelihood of extreme heat episodes skyrockets, reports Marc Daalder

ANALYSIS: This summer will be hotter than average, according to the latest unsurprising NIWA projections.

“Temperatures are very likely above average in Aotearoa in New Zealand with a period of particularly warm conditions starting around the second week of November,” the seasonal climate forecast forecast.

This is just the latest step in the trend of a warming world – and a warming New Zealand. In fact, a newsroom analysis of eight decades of summer temperature data from New Zealand’s five largest cities found that average summer temperatures rose 2 degrees in Auckland, Christchurch and Hamilton.

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While this may not seem like much, NIWA senior climatologist Sam Dean says that even a slight increase in average temperature can correspond to a much larger increase in the frequency and intensity of extreme heat.

“If an extreme is above 28 degrees in Auckland, it would be less in the 1960s and a lot more in the 2010s, and a much bigger increase than just changing the means,” he said. at Newsroom.

“It’s really important to understand that the way you experience climate change is often going to go through extremes. This is when you really notice it. And they are increasing proportionately more than average.”

The theory is quite simple – if you plot the temperatures on a curve and then move that curve, a larger proportion of the curve will now exceed the extreme threshold.

Newsroom’s analysis revealed that this model also exists for New Zealand cities. However, with each city subject to its own unique and local climate effects – such as the effect of urban heat, ocean temperatures and soil moisture – the trend does not show up as uniformly.

“It won’t necessarily be a linear response locally. You can get feedback like soil moisture – so if you have a prolonged drought and the soil gets really dry then you can lose the latent cooling that can occur. produce with evaporation, so that can change the shape of your temperature [curve]”Dean said.

In Auckland in the 1930s, temperatures only rose above 25.6 degrees for four or five days each summer. By the 2010s, that number had grown to 20 days per summer, or more than one in five.

This is despite the average maximum temperature increasing only 2 degrees since the 1930s, on average.

Hamilton experienced a similar effect, with summer days when the mercury reached 27 degrees or more occurring more than four times more frequently than in the 1940s.

In Christchurch, the effect is somewhat less pronounced – unusually hot days have dropped from 5 percent of summer days in the 1930s to 13 percent in the second decade of this century.

The pattern does not appear in the capital at all, due to the southerly sea breezes helping to regulate Wellington’s temperature. Newsroom’s analysis shows that unusually hot summer days were about as common in the 2010s as they were in the 1930s.

“In Wellington, say, you have cold upwelling off the coast and a southerly sea breeze picks up when the temperature gets too warm. If you’re close enough to shore, that [southerly sea breeze] can still be a powerful modulator that will make your change less symmetrical, ”Dean said.

“But if you get warmer seawater in Auckland, it’s also able to do the opposite, reducing the effect of the sea breeze that keeps a lid on Auckland’s extreme temperatures. can get really strong extreme temperature in Auckland because you ‘I managed to stop the sea breeze with the pressure gradients and that means you can’t get the cooling.

“It gets really complicated when you get really local. But the basic concept, yes, if you’re just going to move the distribution by an average temperature change, then you’re going to get more extreme events.”

As policymakers in Glasgow discuss global average temperatures – hoping to limit the rise to 1.5 degrees – these are the extremes we are really feeling, Dean said.

“This is what people really notice and feel and worry about. Hard nights to sleep.”

Newsroom’s analysis also looked at summer nighttime temperatures. In Auckland, unusually warm nights were almost three times more frequent in the 2010s than in the 1930s.

Tauranga has seen the number of summer nights when temperatures exceed 17.8 degrees soar, from four or five times per summer in the 1940s to 24 in the 2010s.

Extreme heat will be one of the most important things to consider as we adapt to the impacts of climate change, Dean said. Citing the electrical system as an example, he said New Zealand is not used to dealing with high demand for electricity to run air conditioning in the summer, but that could change.

“Right now in New Zealand your big consumption is in the winter, in the evening, when everyone is home and turning on their heat. But as the climate changes, it’s possible for big spikes that put strain on the electrical system occur. in summer during heat waves, ”he said.

“If the extremes change, the system might go down more often.”