The hottest year on record globally in 2015 could be an average year by 2025 and beyond if carbon emissions continue to rise at the same rate, new research has found.
An Australian study published today in the Bulletin of the American Meteorological Society indicated that human activities had already locked in this new normal for future temperatures - but immediate climate action could prevent record extreme seasons year after year.
Its lead author, Dr Sophie Lewis of the Australian National University, said if the world continued with business-as-usual emissions, extreme seasons would inevitably be the norm within decades and Australia was the "canary in the coal mine" that would experience the change first.
"If we don't reduce our rate of emissions the record hot summer of 2013 in Australia - when we saw temperatures approaching 50 degrees Celsius in some areas - could be just another average summer season by 2035," she said.
"This research tells us we can potentially prevent record-breaking seasonal temperatures from becoming average at a regional level."
The idea of a "new normal" had been used repeatedly when talking about climate change but had never been clearly defined until Lewis and colleagues developed a scientific definition for the term.
"Based on a specific starting point, we determined a new normal occurred when at least half of the years following an extreme year were cooler and half warmer," she said.
"Only then can a new normal state be declared."
This process was also used to determine new normal conditions for seasonal and regional changes to the climate.
Using a supercomputer to run climate models, the researchers explored when new normal states would appear under the UN Intergovernmental Panel for Climate Change's four emissions pathways.
The research team examined seasonal temperatures from December to February across Australia, Europe, Asia and North America.
"The results revealed that while global average temperatures would inevitably enter a new normal under all emissions scenarios, this wasn't the case at seasonal and regional levels," Lewis said.
"We found that with prompt action to reduce greenhouse gases a new normal might never occur in the 21st century at regional levels during the Southern Hemisphere summer and Northern Hemisphere winter."
Victoria University climate scientist Dr James Renwick said the expression "new normal" was heard often today, but when the study worked out what that meant, the numbers were "sobering".
"If the global community lives up to the Paris Agreement and we cap warming at two degrees above pre-industrial globally, we'll see about another degree of temperature rise this century."
Even at that rate, by the 2040s, every year would be warmer than the current warmest year on record, 2015.
In another 30 years, it would take a major volcanic eruption combined with a string of La Nina events to produce a year as cool as 2015, he said.
In New Zealand, 2016 was shaping up to be a degree above the 1971-2000 normal - and more than 1.5C above pre-industrial temperatures.
"We are already seeing increases in heat waves, heavy rainfall and flooding events around the globe," Renwick said.
Adding another degree to global temperatures this century would only bring much more of the same.
"There is an urgent need to find ways to reduce greenhouse gas emissions, as that's the only way to slow the warming and the related changes in climate."
Climate change and New Zealand
• Under present projections, the sea level around New Zealand is expected to rise between 50cm and 100cm this century, while temperatures could also increase by several degrees by 2100.
• Climate change would bring more floods (around two thirds of Kiwis live in areas prone to flooding); make our freshwater problems worse and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world.
• Climate change is also expected to result in more large storms compounding the effects of sea level rise.
• New Zealand - which this year reported a 23 per cent increase in greenhouse gas emissions between 1990 and 2014 - has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.