Global climate change is “very likely” to have a human cause, an influential group of scientists has concluded.
The Intergovernmental Panel on Climate Change (IPCC) said temperatures were probably going to increase by 1.8-4C (3.2-7.2F) by the end of the century.
It also projected that sea levels were most likely to rise by 28-43cm, and global warming was likely to influence the intensity of tropical storms.
The findings are the first of four IPCC reports to be published this year.
“We can be very confident that the net effect of human activity since 1750 has been one of warming,” co-lead author Dr Susan Soloman told delegates in Paris.
The report, produced by a team tasked with assessing the science of climate change, was intended to be the definitive summary of climatic shifts facing the world in the coming years.
The agency said that it would use stronger language to assess humanity’s influence on climatic change than it had previously done.
In 2001, it said that it was “likely” that human activities lay behind the trends observed at various parts of the planet; “likely” in IPCC terminology means between 66% and 90% probability.
Now, the panel concluded that it was at least 90% certain that human emissions of greenhouse gases rather than natural variations are warming the planet’s surface.
They projected that temperatures would probably rise by between 1.8C and 4C, though increases as small as 1.1C (2F) or as large as 6.4C (11.5F) were possible.
In 2001, using different methodology, the numbers were 1.4 (2.5F) and 5.8C (10.4F).
On sea level, there has been a more fundamental debate.
Computer models of climate do not generally include water coming into the oceans as ice caps melt. So the IPCC had to decide whether to exclude this from its calculations, or to estimate the effect of a process which scientists do not understand well but which could have a big impact.
They used the former, more conservative approach, projecting an average rise in sea levels globally of between 28 and 43cm. The 2001 report cited a range of nine to 88cm.
As for climate change influencing the intensity of tropical storms in some areas of the world, the IPCC concluded that it was likely – meaning a greater probability than 66% – that rising temperatures were a factor.
Dr Rajendra Pachauri, the IPCC chairman, said: “It is extremely encouraging in that the science has moved on from what was possible in the Third Assessment Report.
“If you see the extent to which human activities are influencing the climate system, the options for mitigating greenhouse gas emissions appear in a different light, because you can see what the costs of inaction are,” he told delegates in Paris.
Achim Steiner, executive director of the United Nations Environment Programme (Unep), said the findings marked a historical landmark in the debate about whether humans were affecting the state of the atmosphere.
“It is an unequivocal series of evidence [showing that] fossil fuel burning and land use change are affecting the climate on our planet.”
He added: “If you are an African child born in 2007, by the time you are 50 years old you may be faced with disease and new levels of drought.”
He said that he hoped the IPCC report would galvanise national governments into action.
But a study published on the eve of the IPCC report suggested that the international body’s previous reports may have actually been too conservative.
Writing in the journal Science, an international group of scientists concluded that temperatures and sea levels had been rising at or above the maximum rates proposed in the last report, which was published in 2001.
The paper compared the 2001 projections on temperature and sea level change report with what has actually happened.