When the world’s leaders finally meet in Copenhagen in December and try and come up with an historic agreement on climate change, they will not be able to use lack of knowledge as an excuse for inaction.

In what can only be described as a watershed moment, the 2,500 leading scientists meeting in Copenhagen this week issued a statement calling on governments to take “vigorous and widely implemented” steps to address climate change. Failure to do so would result in “significant risk” of “irreversible climatic shifts”.

Although their full findings will not be published until June, the statement was a clear signal to the world’s politicians. Scientists agreed that “worst case” scenarios were already becoming reality and that, unless drastic action was taken soon, “dangerous climate change” was imminent.

They continued: “The climate system is already moving beyond the patterns of natural variability within which our society and economy have developed and thrived. These parameters include global mean surface temperature, sea-level rise, ocean and ice sheet dynamics, ocean acidification, and extreme climatic events. There is a significant risk that many of the trends will accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts.”

The summary adds: “There is no excuse for inaction. We already have many tools and approaches – economic, technological, behavioural, management – to deal effectively with the climate change challenge. But they must be vigorously and widely implemented.”

The meeting was also addressed by Lord Stern, the economist, whose landmark review of the economics of climate change published in 2006 highlighted the severe cost to the world of doing nothing.

He now says the report underestimated the scale of the risks, and the speed at which the planet is warming. Stern asked: “Do politicians understand just how difficult it could be, just how devastating rises of 4C, 5C or 6C could be? I think, not yet.”

He continued: “A rise of 5C would be a temperature the world has not seen for 30 to 50 million years. We’ve been around only 100,000 years as human beings. We don’t know what that’s like. We haven’t seen 3C for a few million years, and we don’t know what that looks like either.”

He said that if the world was to warm by 5C over the next century, there would be dramatic consequences for millions of people. Rising seas would make many areas uninhabitable leading to mass migrations and inevitably sparking violent conflict.

“You’d see hundreds of millions people, probably billions of people who would have to move and we know that would cause conflict, so we would see a very extended period of conflict around the world, decades or centuries as hundreds of millions of people move, ” said Lord Stern. “So I think it’s very important that we understand the magnitude of the risk we are running.”

Lord Stern’s views were echoed by Danish Prime Minister Anders Fogh Rasmussen: “Business as usual is dead – green growth is the answer to both our climate and economic problems.”