The study from the Intergovernmental Panel on Climate Change (IPCC) warned that many impacts of rising temperatures were already being felt and spelled out risks ranging from increased floods and droughts, to threats to food security, increased conflict and economic losses.
The world is in "an era of man-made climate change" and has already seen impacts of global warming on every continent and across the oceans.
And experts warned that in many cases, people are ill-prepared to cope with the risks of a changing climate.
Dr Richard Dixon, director of Friends of the Earth Scotland, said: "Business as usual means a very grim future, especially if you live in a poor country, where floods, droughts, food shortages and rising sea levels will make life very difficult indeed.
"But nowhere will escape major impacts, as global food production is devastated and hundreds of millions of people flee homelands no longer able to support them.
"This report is a very stark reminder of why the world needs to give up on fossil fuels as quickly as possible to make sure the worst of these predictions don't come true.
"World leaders need to step up with tough emission reduction pledges at Ban Ki-moon's summit in September and agree an effective global climate deal in Paris at the end of next year."
Mark Kenber, chief executive of the Climate Group, which works with governments, business and organisations to drive action on climate change, said: "The latest report from the IPCC makes for sobering reading."