Rapid, far reaching and unprecedented changes are needed in all aspects of society to limit global warming, a new report says.
The Intergovernmental Panel on Climate Change (IPCC) said limiting warming to 1.5 degrees compared to two degrees is necessary to ensure a more sustainable and equitable society.
The report provided key scientific evidence ahead of the December climate change conference in Poland, when governments review the Paris Agreement to tackle climate change.
It said every extra bit of warming matters, especially since warming of 1.5 degrees or higher increases the risk of long-lasting or irreversible changes, such as the loss of some ecosystems.
The lower limit would reduce sea level rise by 10 centimetres by the end of the century, the Arctic Ocean would be free of sea ice in summer just once a century instead of once a decade, and coral reefs would decline by about 70 percent rather than be completely lost.
It said "rapid and far-reaching" transitions were required and that carbo dioxide emissions would need to fall by 45 percent from 2010 levels by 2030.
"The report is clear that avoiding a warmer future above 1.5 degrees will bring significant benefits for millions of people who will face significantly reduced risks of flooding, food insecurity and climate stress, including Pacific communities," said Associate Professor Bronwyn Hayward from Canterbury University, one of the report's authors.
It would come as a huge shock to people that think climate change will happen in the future, Dr Hayward said.
"It will be a shock for them to realise perhaps 10 million people are going to be affected just by sea level rise alone if we can't hold the world's climate at 1.5," she said.
"Perhaps another 420 million will be affected by heat stress. These are realities for the Pacific. This will be a shock for the rest of the world because it's the end of magical thinking that we can somehow put this off."
The report agreed the world was already experiencing the consequences of one degree of global warming through more extreme weather, rising sea levels and diminishing Arctic sea ice.
The IPCC noted good news in that some of the kinds of actions needed to limit global warming to 1.5ºC were already underway, but it said they needed to accelerate, with current decisions vital to ensuring a safe and sustainable world.