Greenpeace has described the latest IPCC report as "shocking" and warned that time had almost run out to prevent the IPCC's "nightmare vision" becoming reality.
"This is a glimpse into an apocalyptic future," said Stephanie Tunmore, Greenpeace International Climate and Energy Campaigner.
"The earth will be transformed by human induced climate change, unless action is taken soon and fast.
"What this report shows is that we are simply running out of time." She said governments now had to act or "there will soon be nowhere to run and nowhere to hide."
But there was still time for an "energy revolution" to create a carbon-free economy which could keep global average temperature increases well below the two centigrade level at which the IPCC predicts catastrophic effects.
"The one option that is clearly no longer open to us after this report is to continue to sit on our hands and do nothing," added Ms Tunmore.
The Director of WWF's Global Climate Change Programme, Hans Verolme, commented: "The urgency of this report, prepared by the world's top scientists, should be matched with an equally urgent response by governments.
"Doing nothing is not an option — on the contrary it will have disastrous consequences. The industrialised countries simply need to accept their responsibilities and start implementing the solutions."
Friends of the Earth International's Climate Campaigner, Catherine Pearce, said: "This report confirms that the scientific findings are stronger than ever.
"Climate change is no longer just an environmental issue. It is a looming humanitarian catastrophe, threatening ultimately our global security and survival."
European Environment Commissioner Stavros Dimas said the report supported the EU's goal of limiting global warming to no more than two degrees centigrade above the "pre-industrial" temperature.
That would prevent irreversible and potentially catastrophic changes in the global climate, such as the melting of the Greenland ice sheet, said the Commissioner — but it would not avoid all the effects.
"Today's IPCC report spells out very clearly the severe effects that climate change will have on all of us. It shows many of the serious impacts that would occur if global warming exceeded the EU's target of not more than two degrees above the pre-industrial level. The temperature today is already almost 0.8°C above that level, so the world needs to act fast if we are to succeed in stabilising climate change and thereby prevent its worst impacts."
The British Red Cross said the report confirmed worst fears about the vulnerability of poor countries.
"Climate change is a humanitarian issue as much as it is environmental, political or economic and as such illustrates just how essential is the work of organisations, such as the British Red Cross, in preparing communities for disasters," said Head of Policy David Peppiatt.
He said the British Red Cross responded effectively to disasters, but its work started locally well before trouble hit, arranging early warning systems and evacuation plans and the stocking of emergency food supplies in flood-risk areas.