A limit of 1.5° in global warming is feasible - but will still have far greater implications than previously thought, according to the Intergovernmental Panel on Climate Change (IPCC), while inaction will have major consequences.
The IPCC report noted the world is rapidly losing any chance to limit average temperature increase to 1.5 degrees, though there is still a theoretical chance if the political will existed.
This would mean replacing petrol and diesel cars with electric vehicles or other clean alternatives and scrapping the use of gas boilers in homes in just a few decades.
Responding to the report, Agneta Rising, director general of World Nuclear Association said: "Today's IPCC report makes clear the potential benefits of limiting climate change to 1.5 degrees, the urgency for action to achieve this and the necessity of nuclear energy as an important part of an effective global response".
But meeting the more ambitious goal of slightly less warming would require immediate, draconian cuts in emissions of heat-trapping gases and dramatic changes in the energy field.
Should governments fail to do that within a decade, and temperatures rise more than 1.5 degrees C, there's one more Hail Mary option.
Can humanity cap global warming at 1.5°C? Therefore, even though urgent action is a necessity, it should be equitable and the onus of addressing climate change can not fall on the developing world. Its release has elicited calls to action from climate campaigners and policymakers the world over.
Temperatures during summer heatwaves, such as those just experienced across Europe this summer, can be expected to increase by 3 degrees C says the report.
Overall, the authors say that current greenhouse gas emissions must be reduced by 45 percent from 2010 levels by 2030. Among other measures, the IPCC says, coal needs to be all but eliminated as a source of electricity, renewable power must be greatly expanded, and "negative-emissions" strategies that suck carbon dioxide out of the atmosphere need to be adopted on a large scale, particularly if emissions reductions are delayed. A problem with the Paris accord is that the pledges made do not get the world anywhere near cutting carbon emissions to zero by mid-century, which is what is needed to curb warming to 1.5C.
Countries must take "unprecedented" action to slash carbon emissions to zero by 2050 and limit risky global warming, a key report warns.
There's no doubt that this dense, science-heavy, 33-page summary is the most significant warning about the impact of climate change in 20 years.
"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 an alliance of small island states at risk of flooding as sea levels rise.
To meet the 1.5 degree target, coal consumption would need to be cut by at least two thirds by 2030 and fall to nearly zero in electricity production by 2050.
We are nowhere near reaching the target to limit average global temperature rises to 1.5C above pre-industrial levels, and the world is on track to be 3C warmer. A huge percentage of reefs, from 70-90%, could still be lost with 1.5 degrees of warming.
Besides special reports, the IPCC has issued five major Assessment Reports that serve as the scientific foundation for United Nations climate talk.
The United States is not alone in failing to reduce emissions enough to prevent the worst effects of climate change.
The report reads, "Human activities are estimated to have caused approximately 1.0 degree Celsius of global warming above pre-industrial levels, with a likely range of 0.8 degree celsius to 1.2 degree celsius".
"We welcome the conclusions of this historic report, one that should give the global community not just a wake-up call, but also hope that we can avoid the most devastating impacts of climate change", said Bill Hare, Climate Analytics CEO. The technology to do this is in the early stages of development and many researchers say it could be hard to develop it for use on a global scale.
The report fired up activists even as critics dismissed the deadline as another arbitrary "climate tipping point", as Climate Depot's Marc Morano put it.