A fifth assessment report from the Intergovernmental Panel on Climate Change (IPCC) states that climate change is irrefutable, is mostly caused by man and must be tackled immediately if the worst effects are to be averted.
The report underlines the challenges facing the world in stark terms and recommends key actions to limit warming.
Scientists say power generated from fossil fuels should be phased out and the world should switch to low-carbon energy sources, including nuclear.
They also warn that emissions may need to drop to zero by 2100 for the world to have a chance of keeping the global temperature rise within the internationally agreed danger level of 2C higher than in pre-industrial times.
Failure to act could lock the world on a trajectory with “irreversible” impacts on people and the environment, and will cost “much more” than precautionary measures, the report said.
Impacts including rising sea levels, warmer and more acidic oceans, melting glaciers and Arctic sea ice, and increasingly frequent and intense heatwaves are already being observed.
UN secretary-general Ban Ki-moon said: “Science has spoken. There is no ambiguity in their message. Leaders must act. Time is not on our side.”
Amid its grim projections, however, the report also offered hope. The tools needed to set the world on a low-emissions path are there but society must end reliance on oil, coal and gas to power the global energy system in order to stop polluting the atmosphere with heat-trapping carbon dioxide, the report said.
IPCC chairman Rajendra Pachauri said: “We have the means to limit climate change. All we need is the will to change, which we trust will be motivated by knowledge and an understanding of the science of climate change.”
Scottish environment minister Paul Wheelhouse said: “The scientific evidence could not be clearer. Climate change is happening now and greenhouse gas emissions from mankind are extremely likely to be the dominant cause.”
He said a new international agreement due to be adopted at a climate change conference next year in Paris would be crucial to limiting warming.
“Anything less than an ambitious global deal could be devastating to mankind and our environment,” he said.
Climate campaigner Mary Church, of Friends of the Earth Scotland, said the report highlighted the danger of “business as usual” and she condemned UK government support for exploiting unconventional gas.
“The science is clearer than ever, that unless we rapidly shift away from dirty energy, our planet is on course for devastating climate change,” she said. “World leaders have agreed that we shouldn’t let temperatures rise by above a dangerous 2C, yet scientists agree that we are on course for an absolutely catastrophic 7.8C by the end of the century if we continue business as usual. Climate science and global justice demand that we leave the vast majority of known fossil fuels in the ground and focus instead on transforming our energy systems to run on clean, renewable sources.
“In this context, the pursuit of the fracking industry is just about the most irresponsible thing governments could be doing. We urge the Scottish Government to listen to the science and use their existing powers to stop unconventional gas drilling and fracking in Scotland, focusing instead on our abundant renewable resources.”
Scotland has world-leading climate ambitions, aiming for a 42 per cent reduction in greenhouse gas emissions by 2020.