The world needs an "unprecedented" response to cap warming at 1.5C and time is running out, according to a landmark report.
The Nobel Prize-winning Intergovernmental Panel on Climate Change warns that the goal set in Paris three years ago is fast slipping out of our reach.
At the current rate, the world will pass the 1.5C mark as early as 2030 and - at the latest - in 2052, the report says with "high confidence".
To have even a 50-50 chance of staying under 1.5C, the world has to be carbon neutral by 2050, with emissions of carbon dioxide falling sharply from 2020.
But that is a big ask, considering we released more carbon dioxide into the atmosphere last year than ever before.
Parts of the world are already seeing the results - deadly heatwaves becoming more common and islands particularly in the Pacific experiencing rising water levels.
If the temperature rise can be kept under 1.5C above pre-industrial levels, then those impacts - from extreme weather to rising seas - will be less severe.
Cornell University climate scientist Natalie Mahowald, a lead author on the report, said: "For some people this is a life-or-death situation, without a doubt.
"We have a monumental task in front of us, but it is not impossible," Ms Mahowald said. "This is our chance to decide what the world is going to look like."
Other scientists were less optimistic.
Appalachian State University environmental scientist Gregg Marland said: "I just don't see the possibility of doing the one and a half."
Joerj Roeglj, one of the study's lead authors from the Imperial College in London, said the Paris promises were "clearly insufficient to limit warming to 1.5C in any way".
In Paris, countries set two goals: 2C and 1.5C, the latter being after pleas from vulnerable countries. US president Donald Trump has announced he will pull out of the agreement .
The world has already warmed 1C since pre-industrial times and, based on the promises made in Paris, we're heading for 3C by 2100.
The report was prepared by more than 90 scientists and review editors from 40 countries.
It includes more than 6,000 scientific references, 133 contributing authors and more than 42,000 expert and government review comments.
To even start to try and reach this goal, the world would need "rapid and far-reaching" changes in how we use energy and land; and how we design our cities and transport systems, the report adds.
Switching away from coal, oil and gas would also be expensive but it could avoid more than 100 million premature deaths this century.
Staying under 1.5C could mean 420 million fewer people being frequently exposed to extreme heatwaves, and about 65 million fewer people being exposed to exceptional heatwaves.