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Time is almost up. It is critical we secure a legally binding approach on climate change in Durban

The lesson the world is learning the hard way from the financial crisis is that there is only one boat and we are all in it. To stay afloat, we need rules tough enough to stop systemic risks becoming systemic collapses. This lesson is as true for the environment as it is for the economy.

A key battle in the campaign to build an effective system of global rules will shortly take place in Durban, where the UN climate negotiations reopen at the end of this month. The International Energy Agency has set the scene, with the timely warning in its new World Energy Outlook that we are way off track to avoid dangerous climate change, and that the window for effective action is closing fast.

It is fashionable to argue that a new climate treaty, based on the Kyoto architecture of legally binding carbon caps, is dead. We should, on this view, give Kyoto a decent burial and switch to plan B. This turns out to be a looser arrangement in which governments make voluntary pledges to each other. Its advocates often call themselves "realists".

The case for voluntarism was first put by those who want to try less hard to deal with climate change. It has subsequently attracted support from academics and other commentators whose concern – indeed alarm – about the climate is unquestionable. They may be desperate rather than cynical, but they tend to know more about the climate than they do about diplomacy. The problem is in the politics not the architecture.

The choice between what needs to be done but looks impossible, and what can be done but is clearly not enough, is as old as history. It lay behind the struggle between Churchill and Halifax as Britain faced Hitler's tanks on the Channel coast. Nato's success in Libya was conducted against a barrage of predictions that it would lead to years of stalemate. When there is no alternative, realism lies in expanding the limits of the possible, not in nourishing the delusion that something else might help.

There really is no plan B for the climate. A voluntary framework will not be enough to keep us within the 2C limit of manageable climate change. Unmanageable climate change will precipitate systemic collapses, including of our food and water security. Success or failure will depend on governments convincing investors that they are determined to enact the policies necessary to drive private capital towards a low-carbon future. In the boardroom a voluntary pledge from a government sounds rather like "maybe". That's why in the UK we have set legally binding carbon budgets through the Climate Change Act.

If a legally binding approach, including a round of post-2012 Kyoto commitments, falls off the table at Durban, most would see this as giving up on climate change. They would be right. The Kyoto protocol is arguably the EU's greatest diplomatic achievement. It inspired the world's largest single market to take big steps towards a carbon neutral energy system, making our economies stronger and more resilient on the way. Europeans should be proud of this approach, not embarrassed by it, even if some of our global partners are not yet ready to embrace it.

It is true that the current cycle of Kyoto commitments only covers industrialised countries, and that some of those outside the EU are reluctant to take on new commitments. Many rightly argue that an effective regime must bind all major economies, not only the EU and those in its orbit. But we do not need this all at once any more than we needed to include everyone from the start to make the GATT work.

Durban needs to send a clear signal that the world is moving rapidly in this direction and that as soon as countries become sufficiently prosperous they will accept binding caps. The deal that is both available and essential must include a second phase of Kyoto commitments for those willing to accept them, plus an unambiguous "commitment to commit" by 2020 from the other major players. This would at last unblock the path to a binding regime with full participation.

Voluntary pledges alone will not keep the global economy open, drive trade and investment, maintain financial stability, or protect peoples against food, water and energy insecurity. If we cannot summon the will to make hard promises on climate change – the first challenge we have ever faced that will affect literally everyone – it will become much harder to do so on everything else.

It's no surprise that such a complex enterprise is taking time to accomplish. The great achievements in the continuing effort to secure our mutual interests by agreeing global rules – the multilateral trade system, the regimes for arms control and nuclear non-proliferation, the European single market, the international criminal court – all took time and many steps to bring to maturity. True, on this occasion we can't afford to take as long as some of these projects did, and we cannot wait for conflict to concentrate our minds. But there is no fundamental obstacle. The technology and capital are available. The framework we need is not only compatible with the economic needs of the major economies but essential to securing them.

Arnold Toynbee warned that technology was giving us the power to destroy ourselves. If we could see through the fog of current events, we might discern a fork ahead. One path points towards chronic insecurity and conflict; the other offers a prospect of co-operation and mutual prosperity. The choice between these two paths that will be foreshadowed at Durban.

John Ashton is the Foreign Office's special representative for climate change