The world faces severe, widespread and irreversible impacts of climate change without urgent action to cut emissions, a major international report has warned.
While there are solutions to climate change, experts warned there was little time before the window of opportunity to limit dangerous temperature rises closed, and that delaying action would greatly increase costs.
Prompt, significant cuts to greenhouse gases are needed, the report says, with the phasing out by the end of the century of fossil fuel power plants without technology to capture and store carbon, and a shift from investment in oil, coal and gas towards renewables.
The stark warning comes from the UN climate body the Intergovernmental Panel on Climate Change (IPCC), in its final report on its latest assessment on the science of climate change, which draws together three studies published in the past year.
Launching it in Copenhagen, 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."
But he added: "The good news is that if we act now, immediately and decisively, we have the means to build a better and more sustainable world. Many tools and technologies are already available, renewable energy sources are increasingly economically competitive, energy efficiency has long proven its value.
"Action on climate change can contribute to economic prosperity, better health and more liveable cities while reducing the risks of future environmental degradation."
The report comes as efforts build towards securing a new global treaty on climate change, which it is hoped can be agreed in Paris at the end of next year.
It prompted calls from environmentalists for a shift away from fossil fuels, while business leaders asked governments for an ambitious deal to give them certainty to invest in a low-carbon future. The Synthesis Report brings together the three parts of the IPCC's fifth assessment on climate change. This analysis involved thousands of scientists and drew on 30,000 scientific papers, and has been subject to scrutiny and approval by governments.
Rajendra Pachauri, chairman of the IPCC said: "We have the means to limit climate change. The solutions are many and allow for continued economic and human development. 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 Scottish Government are committed to playing our part by working with our partners, as we have done since the 2009 Climate Change Scotland Act, to deliver on Scotland's ambitious greenhouse gas emissions targets."
The IPCC report showed global warming was "unequivocal" and human influence on the climate was clear, saying it was "extremely likely" or more than 95 per cent certain that most of the warming since the 1950s was caused by humans. Climate change is impacting on human health, farming and wildlife, and increasing weather extremes, the Synthesis Report said.
Massive cuts to greenhouse gas output is needed in the coming decades to keep temperature rises to no more than 2°C - the level at which dangerous impacts of climate change are expected.
A failure to take more action than already planned over greenhouse gases leaves the world at risk of temperatures soaring by 4.8°C or even higher by 2100.
Rising temperatures and sea levels would cause extinctions, heatwaves and intense storms, worsening health and poverty, water shortages and damage to food security, the report warned.
Pledges by countries to cut greenhouse gases by 2020 are not enough, and delaying action until 2030 would make it much harder to keep the temperature down, it said.
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