It would be "absurd" to claim the risks of climate change are small, economic expert Lord Stern said before the publication of a key scientific report on global warming.
The latest international assessment of climate science makes it crystal clear the risks are "immense", and it would be extraordinary and unscientific to ignore the evidence and argue for a delay in addressing the problem, he said.
The former World Bank chief economist and author of the key 2006 Stern review on the economics of climate change also warned that scientific projections and economic predictions were underestimating the risks of global warming.
While scientists recognised some potential impacts such as the melting of permafrost, which would release powerful greenhouse gas methane, could be very damaging, they were left out of models because they were hard to quantify.
Many economic models, meanwhile, "grossly underestimate the risks" because they assumed that growth will continue and the costs of climate change will be relatively small, he said.
"Both assumptions trivialise the problem and are untenable given the kind of change that could take place," Lord Stern warned.
Temperature rises of 3C or 4C above pre-industrial levels by 2100 would put humans "way outside" the conditions in which civilisation developed, and could cause major disruptions that would damage growth.
Hundreds of millions or even billions of people could be forced to move from where they lived, causing conflict, there could be large-scale destruction of infrastructure and important services provided by nature could collapse, he warned.
His comments come as scientists and government officials meet in Sweden to finalise the latest Intergovernmental Panel on Climate Change (IPCC) report assessing the evidence for climate change and its causes, to be published on Friday.
"The IPPC report makes crystal clear that the risks from climate change are immense," Stern said.
The IPCC has come under fire from climate change sceptics since its last assessment in 2007 over mistakes discovered in that report.
Stern said any assessment of thousands of sources of information, such as the IPCC carries out, was likely to contain some errors, but the message from the scientific information was remarkably consistent.
The IPCC assessment showed a dangerous underlying trend, based on laws of physics known for 200 years, of the rising concentrations of greenhouse gases and their impact on temperatures, with immense risks as a result.
Stern said: "It would be extraordinary and unscientific to argue on the basis of 200 years of evidence that you're confident that the risks are small.
"Those who would have us delay have to argue they're confident the risks are small. It would be an astonishing statement to make in light of all this evidence.
He added: "It would be absurd to say you are confident that the risks are small."
There was a growing recognition from governments of the need to take action to address that risk, and he said he expected more countries to take the lead set by the UK and establish domestic laws to cut emissions.
Countries ranging from the biggest emitters, the US and China, through to Ethiopia and Mexico, were already taking action to tackle climate change and reduce greenhouse gases.
Stern called on the European Union to set a target to cut the bloc's emissions by half on 1990 levels by 2030 to show leadership on the issue.
He criticised moves within the UK government to rethink its five-year carbon target up to 2027 and its failure to set a goal to slash emissions from the power sector by 2030as undermining the certainty needed to invest in a low-carbon economy. More