The Intergovernmental Panel on Climate Change (IPCC) report shows unprecedented changes were needed across society to limit global warming to 1.5C above pre-industrial levels.
But doing so will reduce the severity of climate impacts ranging from extreme weather to rising seas, and in the wake of the report, campaigners urged rapid action.
Matthew Spencer, Oxfam's director of campaigns and policy said: "Climate change has set our planet on fire, millions of people are already feeling the impacts, and the IPCC is clear that things could get much worse without immediate action.
"The faster governments phase out coal, embrace the renewable energy revolution and move to protect communities at risk, the more lives and livelihoods will be spared."
He warned the world was already seeing the beginning of "massive displacement and a shocking rise in hunger" - and unless temperatures stayed below 1.5C, island nations would disappear beneath rising seas.
Rachel Kennerley, climate campaigner for Friends of the Earth, said: "Just like ignoring credit card statements so that repayments only become sharper and steeper, this report shows that weak responses will make it harder in the long-run.
"Right now it's difficult, but not impossible, to contain climate chaos, but the window of opportunity will close for good the longer we delay."
She warned the predicted loss of all coral reefs if governments could not contain warming would mean a massive loss of fish that people rely on for food and costing lives and livelihoods.
She said: "That is the kind of reality we must face if governments don't take notice of this report."
Dr Stephen Cornelius, chief adviser on climate change at WWF, said the world was already seeing the loss of natural habitats and species, shrinking ice caps and rising sea levels.
He said: "We know what is needed to limit global warming to 1.5°C and we can do it relying mostly on proven technologies such as decisively scaling up renewable energy and halting deforestation.
"We have the targets, we have the solutions and the difference between impossible and possible is political leadership. WWF calls on leaders to accelerate climate action immediately."
The Elders - a group of senior world figures originally brought together by Nelson Mandela - said the report highlighted that the issue was a "ticking time-bomb".
Gro Harlem Brundtland, acting chairwoman and former prime minister of Norway, said: "Climate activists have been calling for decades for leaders to show responsibility and take urgent action, but we have barely scratched the surface of what needs to be done. Further failure would be an unconscionable betrayal of the planet and future generations."
Mary Robinson, former president of Ireland and ex-UN envoy on climate change, added that it was time for action, saying: "Leaders need to step up, serve their people and act immediately."
Stephanie Pfeifer, chief executive of the Institutional Investors Group on Climate Change (IIGCC) which represents investors with trillions of pounds of assets under management, said the report showed limiting global warming to 1.5C is what was needed.
"We also know that bold action to address climate change offers 26 trillion US dollars [Â£20 trillion] in economic benefit across the global economy through to 2030.
"Now isn't the time to be fazed by the scale of the far-reaching change required, but instead to focus on the benefits this will deliver in safeguarding global prosperity," she urged.