Scientists are to tell the international community that they are at least 95 per cent sure that human activity is the main cause of climate change, according to one of the most authoritative reports on the subject.
The UN's Intergovernmental Panel on Climate Change (IPCC) will say next week that certainty has increased from "very likely" to "extremely likely" that human activity has caused more than half of the observed temperature rise from 1951 to 2010, in a large part due to fossil fuels and deforestation.
The Fifth Assessment Report, known as AR5, is the IPCC's most definitive yet, and will run to thousands of pages. It will be released in several stages over the coming year after contributions from more than 800 scientists in 85 countries.
It is set to be finalised by a group of scientists before its release on 27 September, but draft pages show that in addition to temperature rises, changes are being observed throughout the climate system.
Findings from the IPCC are expected to show that the world's oceans are becoming warmer and more acidic, with sea levels rising. They also indicate that the potential for weather to become more extreme depends on which of a number of potential scenarios come to pass.
Sea levels have risen by 19cm in the past century, with the pace accelerating due to melting ice and sea water expanding as the world warms. They project that sea levels will rise between 26cm and 81cm by 2100, depending on the severity of the temperature increase – with projections showing temperatures are now "likely" to rise by 2–4C by 2100 unless emissions are cut.
The oceans have absorbed 93 per cent of the heat trapped in the climate system by greenhouse gases between 1971 and 2010, according to the draft report, with the top 75m warming by at least 0.1C a decade.
Leaks from the draft report have also caused controversy. The IPCC's findings show that the rate of warming has slowed down in the past 15 years with the rise dropping from 0.12C per decade between 1951 and 2012, to 0.05C between 1998 and 2012, which has been seized on by climate change sceptics. However, experts state that such a drop can be explained by natural variations in the climate and factors such as volcanic eruptions that spew ash into the air, which can dim sunlight and cool temperatures. Such a "hiatus" is not expected to last, they add.
The IPCC also cites the case of Antarctic sea ice, which has increased by 1.5 per cent per decade, with the reasons for this not fully understood. However, the report states with confidence that global ice sheets are losing mass and that the pace of melting is increasing.
The draft report has been welcomed by climate change activists, with the potential it brings to push governments into action, but some doubt that the political will exists to make the change. "The report confirms what we already know, rather than offering radical new insights," said Dr Doug Parr, the chief scientist of Greenpeace UK. "One would hope that the report will give impetus to the political process. The excuses for inaction are dying away and the IPCC report helps that process."
He added: "This report forms the bedrock of showing that there is still a problem and that things have got to change."
But Tom Burke, the founding director of E3G, a non-profit organisation that aims to accelerate the global move towards sustainable development, who is also currently an environmental policy adviser to the mining giant Rio Tinto, claimed that "political will has declined" in recent years over climate change.
"I think politicians are distracted by the economic crisis, but you also have trillions in investment in fossil fuels through the next decade and basically if you solve the issue you are going to take the value out of that," he said.
Mr Burke added that this "was not a criticism of the IPCC report" and that we would still see events "drive public opinion" on the issue of climate change as the 2015 deadline for a new deal – set by the UN Climate Change Conference in Doha last year – draws ever closer.