If warming continued at the current rate - heating up the planet by 0.2C each decade, and having already pushed temperatures 1C above pre-industrial averages - the 1.5C threshold would be crossed at some point between 2030 and 2052.
To keep within that mark, CO2 emissions would need to be halved over the next decade, while other greenhouse gases like methane and nitrous oxide would also need to be forced down.
If the 1.5C threshold could be held, the world could escape an extra 10cm of sea level rise, over and above what's already been locked in for this century.
If the world couldn't hold the line, then the 2C threshold - the ultimate limit the Paris Agreement was built around - could be over-shot only around a decade later.
But even the difference between another 1.5C and 2C would be "earth-shattering", climate scientist Dr Jim Salinger said.
"For example, coral reefs would decline by 70 to 90 per cent with global warming of 1.5C, whereas virtually all would be lost with 2C," he said.
"With a global average temperature rise by 2C above pre-industrial values, then around 400,000 of the species that we know could go extinct, the numbers for 1.5C would probably be about a third to half this number."
Without deep and unprecedented cuts to emissions now, the world would have fewer opportunities to develop sustainably, and be forced to rely increasingly on unproven, risky and possibly socially undesirable technologies to remove carbon from the atmosphere in the future.
"But to avoid climate warming above 1.5C, we have to scale up action in unprecedented ways across all sectors of our economy and everyday life, over the next 10 years," said the University of Canterbury's Associate Professor Bronwyn Hayward, who served as a lead author on the report.
"At the very least a world warmed above 1.5C has significant implications for New Zealand national adaptation planning."
Methane - which makes up much of this country's greenhouse gas inventory, and mostly sourced from agriculture - would have to halved globally by 2040, when carbon dioxide must be about a quarter of its 2020 emissions.
The Government has yet to determine how or whether methane emissions will be dealt with under its Zero Carbon Act, but recent research found they'd have to be clawed back by nearly a quarter by mid-century to stop any additional global warming.
"New Zealand does not have the luxury in delaying its agricultural methane emissions to play its part to achieve the 1.5C target," Salinger said.
Victoria University climate scientist Professor James Renwick said the report ultimately made a compelling case for rapid decarbonisation, starting now.
"If we want to save even a fraction of unique ecosystems such as the Great Barrier Reef, we cannot afford two degrees of global warming."
Given the past trajectory of global emissions of greenhouse gases - nearly a doubling in the past 30 years - achieving the 1.5C mission represented "a truly heroic and unprecedented effort" to turn the global economy around, he said.
"Are the governments, the businesses, and the people of the world up to it?"
Thousands of groups have responded to the report by making impassioned pleas to governments and business leaders.
"We already have first degree burns from monster storms, devastating floods, and brutal droughts, and this is just the beginning," said Oxfam New Zealand's executive director, Rachael Le Mesurier.
Limiting warming to another 1.5C it would halve the number of people expected to suffer water scarcity and see significantly less impact on crop yields and fisheries, fewer deaths from extreme heat and millions fewer people forced from their homes by rising sea levels.
For New Zealand's Pacific neighbours, WWF-New Zealand senior campaigner David Tong said, climate change was happening now.
In Kiribati, he'd seen homes flooded by salt water, wells that had become undrinkable – and "incredible, vibrant people" who were leading the transition to a clean energy future.
"They're not drowning, they're fighting," Tong said.
"New Zealand has an opportunity now to show global leadership on climate change, to show our Pacific neighbours that we stand with them, and to unlock a safe climate future for all New Zealanders.
"As our government drafts the Zero Carbon Act, we urge them to make sure that it will deliver what our country and our planet need for a safe climate future."
