New, more detailed climate projections show New Zealand is set to get hotter, faster than previously thought.
The country was already expected to keep heating, but Niwa chief scientist for climate, atmosphere and hazards Andrew Tait said updated and more refined projections showed warming accelerating even faster.
He said it was "very important" communities used the latest data to prepare for what was coming.
If countries were slow to cut greenhouse gases, New Zealand could be 3C hotter by the end of the century than it was today. Temperatures were on average 1.1C higher than at the beginning of the 20th century already.
If emissions plunged fast, New Zealand would still be 0.8C hotter by the end of the century, on top of the heating it has already experienced to date.
Niwa climate modellers used a supercomputer to run several of the most up-to-date global climate models from the latest global report from the Intergovernmental Panel on Climate Change (IPCC). They increased the resolution to enable them to zoom down to 5km x 5km squares of New Zealand, and added local topography to the mix.
The result was more heat under all emissions scenarios, and a generally dryer country than scientists previously expected from climate change.
"The new projections we've just released today are all a little bit warmer than we've had in the past," Tait said.
"The overall pattern that New Zealand is getting warmer everywhere is still consistent with what we had [published] before, but the rate of warming is increased."
Tait said scientists were still exploring what was behind the added heating.
"Generally speaking, the global climate models used for the latest IPCC assessment are also showing a bit more warming than their predecessors."
For a middle-of-the road scenario for carbon emissions, the latest information added "quite a bit" of heat compared with past efforts.
"By mid century, New Zealand on average is around about 0.2C warmer than what we thought before, and by end of century that's up to about 0.9C warmer, so that's quite a bit and it does matter - particularly when you start looking at the number of hot days or the number of frost days."
In a middle-of-the-road scenario, emitters globally would rein in pollution enough to keep the planet to about 2.7C total heating since preindustrial times, by the end of the century. The world would miss the 1.5C-2C target countries have signed up to. Even the middle path of around 2.7C would require carbon emissions to be sucked from the air and stored somewhere, after 2050, to bring CO2 in the atmosphere back down.
Tait said it was hugely important for communities to use the latest projections, incorporating the added heat.
"That's important to our understanding for extreme events like heatwaves, which can have an impact on human and animal health and things like crop growing, as well as the risk of extreme events like wildfire.
"The warming we've seen already going back to the beginning of the industrial revolution, this is in addition to that, so it's a bit scary when you think about how it relates to targets like keeping global temperatures to 1.5 or 2C above preindustrial… when we've already had 1.1C above that."
Drying out
Tait said the newest modelling also showed a subtle "general drying" compared with previous expectations.
"The overall pattern of rainfall changes is still consistent from the previous round of [modelling], which is basically that we're expecting wetter conditions in the west coast and south of the South island and drier conditions in the North Island and east of the South island.
"That pattern is still reflected in the new projections, which is really good because it means a lot of work that's already been done to understand the impact, particularly in the primary sector, is still relevant for things like water security.
"But some interesting things are starting to pop up. One is that… overall those numbers are all a bit drier, so not as wet in the west and south as we projected before and drier than we projected in the east and north.
"That is important for things like water security and our natural freshwater environment as well as extraction of water for irrigation and potable water use."
Unfortunately, Tait said, the drier overall conditions did not translate into lower risk of extreme floods.
"The other thing people need to understand and prepare for is that very heavy rainfall," he said. "These are the rainfall events that cause flooding and landslides… these are one of the biggest impacts of climate that we have in New Zealand.
"We are still seeing the same sort of picture we had before where, as the temperature warms, the ability of the atmosphere to hold more water during these big events is increasing.
"So there is a continually increasing risk of heavy rainfall events through these storms, [and that] is still consistent with the message we've had in the past."
Tait said although the news was not good, it was better to be informed.
"The more information people have about these changes, the better. It is all about resilience. Whilst these aren't great headlines, there is much to be gained from understanding these changes and preparing for them so we can protect our communities."
The Ministry for the Environment has opened the latest projections to the public, on a website which allows users to select whether they were interested in drought, rainfall, wind or temperature, which season they are interested in and whether they would like to see projections for low, medium or high greenhouse gas emissions.
The ministry said councils, iwi and businesses will be able to use the data to prepare for climate change in their communities, for example to see what average temperatures might look like in Invercargill in 50 years, or how much rainfall could be expected in Taranaki by 2100.
The new public tool was expected to help councils and businesses get access to better data, for their own climate change planning.
