At this point, heatwaves, heavy rains, droughts, wildfires and coastal flooding would all become much more common across the globe.
Even worse, if we remain at our current levels of emissions, we are on a path to warming 4 C above pre-industrial levels by 2100, which if reached would trigger a chain of cataclysmic changes that include extreme heatwaves, declining global food stocks, substantial species extinctions and sea-level rising that would affect hundreds of millions of people, according to the report.
Climate scientist says UN climate claims are nonsense, coral reefs are not in danger
Experts say time is running out to prevent these and an an array of other disasters, including mosquito borne diseases such as malaria, dramatically depleted fisheries and widespread crop failures.
To have at least a 50/50 chance of staying under the 1.5°C cap, the world must become "carbon neutral" by 2050, according to the new report.
"That means every tonne of CO2 we put into the atmosphere will have to be balanced by a tonne of CO2 taken out," said lead coordinating author Myles Allen, head of the University of Oxfords Climate Research Programme.
Staying under this 1.5°C has important consequences for human life; a further 0.5°C would cause numerous added challenges.
If global warming rose to 2°C above that of the pre-industrial era, then sea level rises would put an additional 10 million people at risk of flooding.
Reducing this rise by 10cm – which would still represent an increase of half a metre relative to 1986-2005 – would have significant positive repercussions for large coastal cities such as New York, Shanghai and Osaka.
The report says a "slower rate of sea level rise enables greater opportunities for adaptation in the human and ecological systems of small islands, low-lying coastal areas and deltas".
Sea levels would be at least 10 centimetres higher by the end of the century at 2 C warming than they would at 1.5 C, causing mass migration from areas that may be flooded, warns the UN report.
In addition, eight per cent of plants and four per cent of vertebrates are projected to lose over half of their geographic range if global warming hits 1.5°C – although these numbers double if we see temperatures rise by 2°C.
This could have a dramatic impact on the food-chain, and many of these potential impacts are deemed "irreversible".
As well, there is a chance to save the coral reefs with 1.5 C warming, as opposed to no chance at 2 C, according to the report.
Coral reefs are projected to decline by a further 70–90 per cent at 1.5°C, although 2°C would kill almost all of them.
This would impact global fisheries, with one projection suggesting that the global annual catch would decrease by about 1.5 million tonnes if global warming hit 1.5°C, and more than three million tonnes for 2°C.
The report warns that "any increase in global warming is projected to affect human health, with primarily negative consequences".
These include higher risks from insect-borne diseases, such as malaria and dengue fever, as rising temperatures allow mosquitos to spread, including into northern Europe. Heatwaves in cities would also become worse, increasing deaths from heat-stroke.
At current levels of greenhouse gas emissions, we could pass the 1.5C marker as early as 2030, the Intergovernmental Panel for Climate Change (IPCC) reported with "high confidence".
But temperatures are not changing at the same rate everywhere. Warming will predominantly affect continents, although the Arctic will be hardest hit – with rises between two and three times higher due to the impact of melting snow and ice.
"The report shows that we only have the slimmest of opportunities remaining to avoid unthinkable damage to the climate system that supports life as we know it," said Amjad Abdulla, the IPCC board member and chief negotiator for the alliance of small island states.
The report said that coordinated global action is urgently required, warning that "historians will look back at these findings as one of the defining moments in the course of human affairs."
Under the Paris Agreement, adopted in December 2015, 197 countries agreed to aim to hold the rise in global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit it to 1.5°C .
Limiting global warming to 1.5C comes with a hefty price tag: some $2.4 trillion (2.1 trillion euros) of investments in the global energy system every year between 2016 and 2035, or about 2.5 percent of world GDP.
However that amount must be weighed against the even steeper cost of inaction, the report concludes. The investment can also be seen as an opportunity as it could dramatically boost trade.
A climate scientist said Australias coral reefs are not in danger in the wake of a UN report on climate change that he says is helping to overturn industrial civilisation.
An Intergovernmental Panel on Climate Change (IPCC) report released earlier this week forecast doom if coal-fired power is not ended within 32 years, worldwide.
It predicted up to 90 per cent of coral reefs would be lost if the earth warmed by 1.5 degrees Celcius by 2100, prompting fears for Australias Great Barrier Reef.
The latest IPCC report said coral such as Australias Great Barrier Reef (pictured) would vanish unless the world phases out coal power 32 years. Professor Lindzen says this is nonsense