Climate change: Parts of NZ could be getting hotter and drier in summer faster than we think

In a just-launched study, scientists are investigating whether an intriguing feedback loop being observed elsewhere in the world because of climate change may also be quietly unfolding here. Photo / Christine Cornege

Parts of New Zealand could be growing hotter and drier in summer faster than scientists first thought, as new data suggests 2023 is likely to be our region’s warmest year on the books.

In a just-launched study, scientists are investigating whether an intriguing feedback loop being observed elsewhere in the world because of climate change may also be quietly unfolding here.

New Zealand’s national average temperature has risen by about 0.10C every 10 years since 1909 - but that rate has quickened to about 0.31C per decade in the past 30 years, and the number of days hitting 25C and above has also jumped in a raft of locations.

Waikato University climate scientist Dr Luke Harrington said large-scale models typically showed global land regions warming faster than air over oceans, with average temperatures of extra-hot days climbing yet more quickly.

Source / Dr Luke Harrington, University of Waikato

“These regions, like southern Africa or the Mediterranean, also experience clear declines in summertime rainfall with rising temperatures, which in turn creates a feedback loop between the land and the atmosphere,” he said.

“In essence, it takes a lot of energy to evaporate the moisture contained within soils during the summertime, so when soils dry out increasingly early each summer, there is more energy available to further heat the atmosphere.”

In contrast to fast-heating continental hotspots, the UN’s latest science stocktake showed maritime New Zealand was among the few land regions where warming was happening in step with surrounding oceans and global temperature averages.

“And yet, we’ve found evidence that some parts of the country are experiencing changes in the frequency of locally extreme hot days much more quickly than expected over recent decades,” Harrington said.

“Alongside this, there’s growing evidence that the northern half of the North Island will experience reductions in summertime rainfall with additional climate change.”

Harrington suspected the reason behind this could be the same land-atmosphere feedback pattern seen in the Mediterranean – and he and colleagues will explore that in what will be the largest data analysis of its kind yet carried out here.

Their study would ultimately show whether local temperature shifts to date owed to decade-to-decade variability or long-term trends occurring with global warming.

Either way, it’d eventually provide unprecedented estimates of summertime warming rates for different parts of the country, and under a range of future scenarios.

“It’s equally important for the primary sector and the health sector that we understand what’s really going on with summertime warming rates across the country.”

The project, supported with a million-dollar Marsden Fund grant, came as scientists were tracking where the calendar year would finish up in temperature records.

Rising temperature extremes have implications for human health - as well as heat stress in our livestock. Photo / Andrew Warner

Climate scientist Professor Jim Salinger calculated it’d likely been the warmest year in the wider 4.2-million sq km region around New Zealand, with combined average temperatures sitting at around 0.83C above the 1990-2020 average.

It’d been an especially warm year around the Chatham and Campbell Islands, with sea temperatures of 1.5C and 0.9C above normal respectively.

Salinger attributed the trend to marine heatwaves, the warm and wet signal of a La Nina climate pattern over the first part of the year, fewer southerly systems reaching New Zealand - and background climate change.

Globally, it’s likely that 2023 – which in July brought Earth’s hottest month in 120,000 years – will go down as the planet’s warmest to date, just after the UN’s latest climate summit wrapped up in Dubai.

“Despite all the talking, global warming is going ahead: it’s now very clear the planet is giving us signals, with more frequent extremes,” Salinger said.

Flooding in Mangawhai in February. The first six months of the year were by far the wettest in recorded history for a host of spots. Photo / Shane Wiski Whitmore

National Institute of Water and Atmospheric Research (Niwa) meteorologist Ben Noll said for New Zealand’s land temperatures alone, 2023′s final figure was likely to land close to last year’s record-setting average of 13.7C.

And there’s plenty of heat yet to come in the month, as a warm air mass from Australia arrived just before the holiday period.

“We could be seeing 30C-plus temperatures around Wednesday and Thursday, especially in eastern areas, where we’re expecting northwest winds,” Noll said.

“So, it certainly seems like we’re in for a hot end to the calendar year, here.”

The first six months of 2023 made for New Zealand’s second-warmest start to a year – but also easily the wettest for a host of centres, with Kaikohe, Whangārei, Tauranga, Napier, Gisborne and multiple locations in Auckland all receiving a year’s worth of rainfall.

Niwa’s latest hotspot watch showed that compared with normal figures for the time of year, the driest soils across the North Island were currently found in the western Waikato and on the Kāpiti Coast, while the wettest were in southern Gisborne, northern Hawke’s Bay and parts of the Far North.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.