If political leaders have one duty above all others, it is to protect the security of their people. Thus it was, according to the prime minister, to protect Britain's security against Saddam Hussein's weapons of mass destruction that this country went to war in Iraq. And yet our long-term security is threatened by a problem at least as dangerous as chemical, nuclear or biological weapons, or indeed international terrorism: human-induced climate change.

As a climate scientist who has worked on this issue for several decades, first as head of the Met Office, and then as co-chair of scientific assessment for the UN intergovernmental panel on climate change, the impacts of global warming are such that I have no hesitation in describing it as a "weapon of mass destruction".

Like terrorism, this weapon knows no boundaries. It can strike anywhere, in any form - a heat wave in one place, a drought or a flood or a storm surge in another. Nor is this just a problem for the future. The 1990s were probably the warmest decade in the last 1,000 years, and 1998 the warmest year. Global warming is already upon us.

The World Meteorological Organization warned this month that extreme weather events already seem to be becoming more frequent as a result. The US mainland was struck by 562 tornados in May (which incidentally saw the highest land temperatures globally since records began in 1880), killing 41 people. The developing world is the hardest hit: extremes of climate tend to be more intense at low latitudes and poorer countries are less able to cope with disasters. Pre-monsoon temperatures this year in India reached a blistering 49C (120F) - 5C (9F) above normal.

Once this killer heat wave began to abate, 1,500 people lay dead - half the number killed outright in the September 11 attacks on the World Trade Center. While no one can ascribe a single weather event to climate change with any degree of scientific certainty, higher maximum temperatures are one of the most predictable impacts of accelerated global warming, and the parallels - between global climate change and global terrorism - are becoming increasingly obvious.

To his credit, Tony Blair has - rhetorically, at least - begun to face up to this. In a recent speech he stated clearly that "there can be no genuine security if the planet is ravaged by climate change". But words are not enough. They have to be matched with adequate action. The recent announcement of a large-scale offshore wind generating program was welcome, but the UK still lags far behind other European countries in developing renewables capacity.

The latest report on energy and climate change by the royal commission on environmental pollution addressed the much more demanding global reductions in greenhouse gas emissions that will be required over the next 50 years (in addition to the Kyoto agreement) and how these could be achieved. Given that the UK needs to take its share of the global burden the commission recommended that we should aim for a cut in these emissions of 60% by 2050.

It also pointed out the urgent need for an adequate mechanism for negotiating each country's emission target and advocated a globally implemented plan known as "contraction and convergence". The energy white paper published earlier this year accepted the royal commission's 60% reduction target, but it is disturbing that it provided no clarity on UK policy regarding the framework for international negotiation.

Any successful international negotiation for reducing emissions must be based on four principles: the precautionary principle, the principle of sustainable development, the polluter-pays principle and the principle of equity. The strength of "contraction and convergence" is that it satisfies all these principles. But it also means facing up to some difficult questions.

First, world leaders have to agree on a target for the stabilization of greenhouse gases in the atmosphere at a sufficiently low level to stave off dangerous climate change. Second, this target, and the global greenhouse gas budget it implies, has to form the framework for an equitable global distribution of emissions permits, assigned to different countries on a per-capita basis. Countries with the largest populations will therefore get the most permits, but for the sake of efficiency and to achieve economic convergence these permits will need to be internationally tradable.

This is the only solution likely to be acceptable to most of the developing world, which unlike us has not had the benefit of over a century of fossil fuel-driven economic prosperity. And it also meets one of the key demands of the United States, that developing countries should not be excluded from emissions targets, as they currently are under the Kyoto protocol.

Nowadays everyone knows that the US is the world's biggest polluter, and that with only one 20th of the world's population it produces a quarter of its greenhouse gas emissions. But the US government, in an abdication of leadership of epic proportions, is refusing to take the problem seriously - and Britain, presumably because Blair wishes not to offend George Bush - is beginning to fall behind too. Emissions from the US are up 14% on those in 1990 and are projected to rise by a further 12% over the next decade.

It is vital that Russia now ratifies the Kyoto protocol so that it can at last come into force. But while the US refuses to cooperate, it is difficult to see how the rest of the world can make much progress on the much tougher longer-term agreements that will be necessary after Kyoto's mandate runs out in 2012.

Nor does the latest science provide any comfort. The intergovernmental panel on climate change has warned of 1.4C to 5.8C (2.5F to 10.4F) temperature rises by 2100. This already implies massive changes in climate, and yet the current worst-case scenarios emerging from the Met Office's Hadley Center. envisage even greater rises than this - a degree and speed of global warming the consequences of which are hard to quantify or even imagine.

So Blair has a challenge. The world needs leadership, and the British prime minister is well placed to stand at the head of a new "coalition of the willing" to tackle this urgent problem. He is also uniquely placed to persuade Bush to join in this effort, given their joint commitment to making the world safe from "weapons of mass destruction".

But even if he fails to persuade him, there are other allies who would still respond to his leadership - even if this means opposing the US until such time as it no longer has an oilman for president. If Blair were to assume this mantle, history might not only forgive him, but will also endorse Britain's contribution to long-term global security.

· Sir John Houghton was formerly chief executive of the Meteorological Office and co-chair of the scientific assessment working group of the intergovernmental panel on climate change. He is the author of 'Global Warming: the Complete Briefing'.

© Guardian Newspapers Limited 2003

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