Our summers are getting warmer and as average temperatures rise, the probability of extreme heat events skyrockets, Marc Daalder reports
Analysis: This summer will be warmer than average, according to the unsurprising latest projections from NIWA.
"Temperatures are very likely to be above average across Aotearoa New Zealand with a period of particularly warm conditions from around the second week of November," the seasonal climate outlook forecasts.
It's only 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 has found that average summer temperatures are up 2 degrees in Auckland, Christchurch and Hamilton.
While that might not sound like much, NIWA's principal climate scientist Sam Dean says even a slight increase in the average temperature can correspond to a much larger increase in the frequency and intensity of extreme heat.
"If an extreme is anything over 28 degrees in Auckland, then there would be fewer in the 1960s and a whole lot more in the 2010s, and at a much greater increase than just the shift in the means," he told Newsroom.
"It's really important to understand that the way you experience climate change is often going to be through the extremes. That's when you'll really notice it. And they do increase proportionately more than the mean."
The theory is pretty simple - if you plot temperatures on a curve and then move that curve, a greater proportion of the curve will now be past the extreme threshold.
Newsroom's analysis found this pattern existed for New Zealand cities as well. However, with each city subject to its own unique and local climate effects - like the urban heat effect, ocean temperatures and soil moisture - the trend doesn't display quite so uniformly.
"It's not necessarily locally going to be a linear response. You can get feedbacks like soil moisture - so if you have an extended dry spell and the soil gets really dry, then you can lose the latent cooling that can occur with evaporation, so that can change the shape of your temperature [curve]," Dean said.
In Auckland, in the 1930s, temperatures would only exceed 25.6 degrees on four or five days each summer. By the 2010s, that had swelled to 20 days a summer - more than one in five.
That's despite the average peak temperature rising just 2 degrees since the 1930s, on average.
Hamilton has seen a similar effect, with summer days where the mercury hit 27 degrees or higher happening more than four times as frequently as in the 1940s. In Christchurch, the effect is somewhat less pronounced - abnormally hot days have gone from 5 percent of summer days in the 1930s to 13 percent in the second decade of this century.
The pattern doesn't show up at all in the capital, as a result of southerly sea breezes helping to regulate Wellington's temperature. Newsroom's analysis shows abnormally hot summer days were about as common in the 2010s as they were in the 1930s.
"In Wellington, say, you've got cold upwelling water off the coast and a southerly sea breeze kicks in when the temperature gets too warm. If you're close enough to the coast, that [southerly sea breeze] may always be a strong modulator that's going to make your shift look not quite so symmetrical," Dean said.
"But if you are getting warmer sea waters in Auckland, that's equally capable of doing the reverse, of reducing the sea breeze effect that keeps a lid on extreme Auckland temperatures. Sometimes, you can get a really strong temperature extreme in Auckland because you've managed to shut down the sea breeze with the pressure gradients and that means you can't get the cooling.
"It becomes really complicated when you get really local. But the basic concept, yes, if you're just going to shift the distribution by a mean temperature change, then you'll get more extreme events."
While policymakers in Glasgow are discussing global average temperatures - hoping to limit the rise to 1.5 degrees - it is the extremes that we really feel, Dean said.
"Those are what people really notice and experience and get concerned about. The nights that are hard to sleep."
Newsroom's analysis also looked at summer nighttime temperatures. In Auckland, abnormally hot nights were nearly three times more common in the 2010s than in the 1930s.
Tauranga has seen the number of summer nights where temperatures exceed 17.8 degrees skyrocket, from four or five times a summer in the 1940s to 24 in the 2010s.
Extreme heat will be one of the most important things to take into account as we adapt to the impacts of climate change, Dean said. Pointing to the electricity system as an example, he said that New Zealand isn't used to managing high demand for electricity to power air conditioning in the summers, but that might change.
"At the moment, in New Zealand, your big consumption is in the winter, in the evenings, when everybody's at home turning on their heaters. But as the climate changes, it's possible that big spikes that put pressure on the power system may occur in the summer during heat waves," he said.
"If the extremes are changing, it may well be that the system breaks more often."
