The Congolese representative spoke about half-way into the Security Council debate. "This will not be the first time people have fought over land, water and resources," he said, "but this time it will be on a scale that dwarfs the conflicts of the past". The French called it the "Number one threat to mankind". The Belgian said that in response to that threat we had to do nothing less than rethink from top to bottom how we thought about our security: we could not afford to fall into the trap that has cost the world so dear throughout history and assume that the future will look like the past. The UN Secretary General, Ban Ki Moon, said the scenarios facing us were alarming.

What was the focus of all this concern? Climate change. Our increasingly unstable climate is no longer seen as primarily an environmental or economic issue. As the threat we face has grown larger in scale and sharper in outline over the past two years, as recent scientific evidence has reinforced, and in some cases exceeded, our worst fears as to the physical impacts facing us, so it has become increasingly clear that climate change has consequences that reach to the very heart of the security agenda - flooding, disease and famine and, from that, migration on an unprecedented scale, and in areas of already high tension, drought and crop-failure and, from that, intensified competition for food; water and energy in regions where resources are already stretched to the limit; economic disruption on the scale predicted in last year's Stern Report and not seen since the end of World War II.

This is not about narrow national security - it is about our collective security in a fragile and increasingly interdependent world. And tragically, once again, it will be those who are most vulnerable and least able to cope that will be hit first. There is certainly no choice between a stable climate and the fight against poverty. Without the first, the second will certainly fail.

Anyone wanting to trace the links between what science is telling us about physical impacts and the broader ramifications for our security would do well to read a startling report that appeared on Monday. The Military Advisory Board are a group of the most respected retired admirals and generals in the United States. During their careers they have stood face to face with everything from containment and deterrence of the Soviet nuclear threat during the Cold War to the more recent struggle against terrorism and extremism. They are about as far as you can get from the old stereotype of a tree-hugging environmentalist.

And yet, in that report they state, categorically, that projected climate change poses a serious threat to America's national security. It is, they say: "a threat multiplier for instability in some of the most volatile regions of the world". In other words, an unstable climate will make the very kinds of tensions and conflicts that the Security Council deals with, day in day out, yet more frequent and even more severe.

It is those concerns, then, that lay behind the UK's decision to use our presidency of the Security Council to instigate this unprecedented debate on Tuesday. And it is those concerns that prompted 53 countries - an almost unheard of number for a meeting of this kind - to sign up to speak and take part.

Taking it to the Security Council is not an alternative to action elsewhere within the United Nations or across the international community. As the UK's lead negotiator at the UN Framework Convention on Climate Change for over five years, I am the last person who would wish to undermine those other and vital multilateral efforts.

But, charged as it is with the maintenance of international peace and security, the Security Council can make a unique contribution in the building of a shared understanding of what an unstable climate will mean for our individual and collective security. And the decisions we come to and the action we take - in whatever forum - as we begin to build a low carbon, global economy will be better, stronger and more effective because it is informed by the fullest possible understanding of all the implications of climate change - including the security imperative.

Tuesday was a landmark day. It marked the recognition of climate change as a core security issue. It demonstrated that the vast majority of the international community now see an unstable climate as an unprecedented threat that we must meet with much greater urgency and ambition. If we succeed in that shared endeavour, we will all enjoy a better prospect of security. Climate change is a threat that can bring us together if we are wise enough to stop it from driving us apart.

Ms Beckett is the British Foreign Secretary