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Human climate change '95% certain'
A major new report on climate science is expected to state that humans are to blame for global warming
Scientists are more certain than ever that humans are causing the majority of climate change, a major new report has shown.
The first part of the Intergovernmental Panel on Climate Change's (IPCC) fifth assessment report shows that warming in the climate system is "unequivocal" and human influence on the climate is clear.
The report, which has been published 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.
The study predicts that 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 over 4C if emissions continue to increase.
Storms will become more intense and frequent, sea levels will rise by between 26cm (10in) and 82cm (32in) by the end of the century and the oceans will become more acidic, the assessment 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."
The report considered a series of scenarios for greenhouse gas emissions and their impacts on the climate.
"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.
But over the long term, from 1951, climate models have matched what has happened to global temperatures, the report said.
In addition, changes in the climate can be seen through "multiple lines of independent evidence", as well as surface temperature, including the loss of ice, rising sea levels and the warming of the ocean, experts said.
Each of the last three decades has been successively warmer at the Earth's surface 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.
In a message to the conference in Stockholm, Ban Ki-moon, secretary general of the United Nations, sent his "best wishes" and "gratitude" to the IPCC, and said the "world's eyes" are on the city today.
"Since 1990 the IPCC has provided regular, unbiased assessment of the mounting impact of a warming planet.
"You are the world's authorities on climate change, recognised with a Nobel Peace Prize," he said.
He added that "we need to build resilience and seize the opportunities of a low-carbon future".
Concluding, he said: "The heat is on. Now we must act."
Michel Jarraud, secretary general of the World Meteorological Organisation, said "we have gone a long way" in the last 25 years in "our knowledge of the climate system" and "the human role in climate change".
He 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."
Mr Jarraud said the report reflects how far scientific knowledge has come since the last report six years ago.
"This is knowledge that can be used, that should be used, to produce actionable climate information and services to help society," he said.
Mr Jarraud said that "despite this really overwhelming scientific consensus", further assessment is needed.
"We should not stop our investment in that. This is essential investment for the future generations," he said.
Achim Steiner, executive director of the United Nations Environment Programme, described the report as a "dramatic reminder of both the significance, the pace, and also our ability to increasingly understand what is happening to our planet".
While he acknowledged some people may focus on the questions raised in the report, he 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."
Rajendra K Pachauri, chairman of the IPCC, said: "We've come a long way."
He said "almost 60%" of the authors in this assessment are "new to the IPCC" and so bring "fresh perspectives" and "new knowledge".
Mr Pachauri said he wanted to "pay the highest tribute" to the scientists who worked on the report.
"They worked extremely hard," he said.
He spoke of the "quality of leadership" and "the extent of co-operative and collaborative spirit" among the authors.
"We believe that the public, decision-makers, the scientific community and of course the average person on the street would be able to make use of this report effectively," he said.