Human influence on climate 'clear'
Scientists are more certain than ever that humans are causing the majority of climate change - with significant impact for the planet, a key report has shown.
The first part of the Intergovernmental Panel on Climate Change's (IPCC) fifth assessment report shows that global warming is "unequivocal" and human influence on the climate is clear.
Experts said the report was a "wake-up call" that activities such as burning fossil fuels would have a profound effect on society today and in the future and campaigners called for immediate action to cut greenhouse gas emissions.
The report, which has been published in Stockholm after line-by-line scrutiny by scientists and policymakers, found i t is "extremely likely", or 95% certain, that the majority of the warming since the 1950s is down to human activity.
The likelihood is up from a 90% certainty in the last IPCC study in 2007.
As a result of the warming, ice sheets are losing mass, glaciers are shrinking, sea ice cover has reduced in the Arctic and the permafrost is thawing in the northern hemisphere, the report - which draws on thousands of scientific papers - warns.
Temperatures are set to rise by more than 1.5C above pre-industrial levels by the end of the century without ambitious action to tackle emissions, and could rise by more than 4C if emissions continue to increase, the study found.
Scientists warned it will take significant and swift action to reduce greenhouse gases in order to keep temperatures from rising more than 2C above pre-industrial levels - the threshold beyond which dangerous climate change is likely to occur.
The assessment, which considered different scenarios for emissions and their impacts, predicts this century the world is likely to see temperature rises of between 0.3C and 4.8C, depending on what action is taken on greenhouse gases.
Sea levels will rise by between 26cm (10in) and 82cm (32in) by the end of the century, r ainstorms will become more intense and frequent, and the oceans will become more acidic, the report projects.
One of the scientists leading the first section of the IPCC's fifth assessment, which looks at the science of climate change and its causes, Thomas Stocker, said: "Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.
"Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
"Global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5C relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2C for the two high scenarios.
"Heatwaves are very likely to occur more frequently and last longer. As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions," he said.
In the run-up to the publication of the IPCC report, questions have been raised about the slowdown in temperature rises in the past 15 years, with climate "sceptics" claiming it undermines the theory of climate change.
The report acknowledges that there has been a reduction in the rate of warming in the past 15 years, between 1998 and 2012, and suggests it is the result of natural variation and the impacts of volcanoes and changes in the strength of the sun.
Scientists said the slowdown was temporary and they would expect to see a return to substantial warming in coming decades.
The report also found that each of the last three decades has been successively warmer than any preceding decade since 1850, and in the northern hemisphere 1983-2012 is likely to have been the warmest 30-year period in the last 1,400 years.
Even if emissions of carbon dioxide were to stop, the result of past and present emissions means the world is locked into some level of climate change, with effects that will persist for many centuries, the experts warned.
Michel Jarraud, secretary general of the World Meteorological Organisation, said: "This report confirms, and you'll hear a lot more about it, with even more certainty than in the past, that it is extremely likely that the changes in our climate system for the past half a century are due to human influence.
"And it should serve for yet another wake-up call that our activities today will have a profound impact on society, not only for us, but for many generations to come."
He added: "Many of the extremes of the last decade were unprecedented."
Mr Jarraud said the decade from 2001-2010 was the warmest on record, and said more temperature records were broken than in any previous decade.
He said: "The IPCC report demonstrates that we must greatly reduce global emissions in order to avoid the worst effects of climate change."
Achim Steiner, executive director of the United Nations Environment Programme, acknowledged some people may focus on the questions raised in the report, but said: "For humanity to take decisions, perfect knowledge can never be the condition."
He said that while scientists may not know everything, they know enough about "the risks of not acting".
"This is not about ideology. This is not about self-interest. This is about the common interest of the international community, the planet, and ultimately our economies and society."
In the wake of the report, six of the world's biggest environmental charities and aid agencies, along with the International Trade Union Confederation issued a call for governments to bring in pollution controls.
Greenpeace, Friends of the Earth, WWF, ActionAid, Christian Aid, Oxfam and ITUC demanded governments stop investing in fossil fuels and invest in renewables.
Climate Change Secretary Ed Davey said: "The message of this report is clear - the Earth's climate has warmed over the last century and man-made greenhouse gases have caused much of that global warming.
"The gases emitted now are accumulating in the atmosphere and so the solutions must be set in motion today. The risks and costs of doing nothing today are so great, only a deeply irresponsible government would be so negligent.
"Without urgent action to cut greenhouse gas emissions, this warming will continue, with potentially dangerous impacts upon our societies and economy.
"This strengthens the case for international leaders to work for an ambitious, legally-binding global agreement in 2015 to cut carbon emissions."
Countries around the world have agreed to negotiate a legally-binding global deal in 2015 in Paris to tackle climate change, but already some are predicting the move will fail, as efforts did in Copenhagen in 2009, to reach agreement.