Nicholas Stern has cited recent bushfires in Australia along with current flooding in the United Kingdom to argue climate change is already causing devastating destruction.
The British economist, who authored the influential 2006 study on global warming, says record rainfall and storm surges causing flooding across the UK "are a clear sign we are already experiencing the impacts of climate change".
"But it is not just here that the impacts of climate change have been felt through extreme weather events over the past few months," Stern writes in Britain's left-leaning Guardian newspaper.
"Australia has just had its hottest year on record during which it suffered record-breaking heatwaves and severe bushfires in many parts of the country.
"And there has been more extreme heat over the past few weeks."
The economist further cites recent heatwaves in Argentina, floods and landslides in Brazil and November's Typhoon Haiyan, which killed more than 5700 people in the Philippines.
The Intergovernmental Panel on Climate Change has pointed to changing extreme weather since 1950 with the Earth warming by about 0.7C.
Stern says if greenhouse gas emissions aren't cut the world faces "even more devastating consequences" with temperatures possibly rising 4C by the end of the century.
"The shift to such a world could cause mass migrations of hundreds of millions of people away from the worst-affected areas," the expert writes.
"That would lead to conflict and war, not peace and prosperity."
Stern says the "lack of vision and political will" from the leaders of many developed countries is harming their own competitiveness and undermining efforts to create a new global pact by late 2015.
He argues the UK will have to become more resilient to the impacts of climate change that can't be avoided and "a sensible way to raise money would be to implement a strong price on greenhouse gas pollution across the economy".
Australian Prime Minister Tony Abbott is scrapping the former Labor government's carbon tax and last year described attempts to link bushfires with climate change as "complete hogwash".
In the UK this week, energy secretary Ed Davey argued deniers were undermining public trust in the scientific evidence for climate change and that "we can see around us today the possible consequences of a world in which extreme weather events are much more likely".
Mr Davey's comments came after Met Office chief scientist Julia Slingo said "while there is no definitive answer for the current weather patterns that we have seen all the evidence suggests that climate change has a role to play in it".
Dame Julia was subsequently savaged by former Thatcher chancellor Nigel Lawson for making "an absurd statement".
Lord Lawson is the founder of the Global Warming Policy Foundation that former Australian prime minister John Howard addressed in late 2013.
Mr Howard in London said he "instinctively" felt warming claims were exaggerated and he only backed emissions trading before the 2007 election because he faced a "perfect storm" on the issue.
