The peace prize is inevitably the most controversial of the awards made by those charged with picking recipients of Alfred Nobel’s munificence. The prizes chosen by the Swedish Academy for achievements in medicine, chemistry, physics, economics and literature are usually made to men and women whose work has, over time, changed the understanding of their subject. With peace, it is different. The prize is more closely linked to the state of the world today and the actions of those trying to better it. The prize is awarded to those who, by definition, provoke controversy. And the definition of peace by the Norwegian parliamentary committee is far broader than it was in Nobel’s time, when his will stipulated that it should promote “fraternity between nations”. But even for those used to controversy, the award to Al Gore may provoke division.

It is certainly unusual rehabilitation for a defeated presidential candidate. Yet it is not the first time a politician who lost at the ballot box has won in Oslo. To Jimmy Carter, David Trimble, Yassir Arafat and F. W. de Klerk the prize was no guarantee of political longevity. Some recipients have been buoyed by the international recognition: others – Betty Williams and Mairead Corrigan, Rigoberta Menchú Tum and Wangari Maathai – have disappeared from view.

The main difficulty is that the prize is meant as much to encourage the recipient’s cause as to reward achievement. Rarely does it do both. A few giants continue to enjoy international prestige: Nelson Mandela, the Dalai Lama and Mother Teresa, for example. Others have found their cause may have been harmed by their award. Aung San Suu Kyi’s detention was probably prolonged because of the spotlight on her defiance; and Shirin Ebadi, the Iranian human rights activist who won in 2003, has attracted the intense suspicion and hostility of Iran’s hardline clerics.

Encouraging a worthwhile cause is always a fine judgment. There is no global consensus; the prize reflects the values of the Norwegian jury. Many will see the award to Mr Gore – shared with the Intergovernmental Panel on Climate Change – as a none-too-subtle rebuff to President Bush, a sceptic on the issue. It would be the second time that a prize has been seen as a rebuke to Washington: the award to the International Atomic Energy Agency in 2005 looked like a response to Mr Bush’s decision not to wait for the final report of the weapons inspectors before launching the Iraq war. Questions will also be asked about Mr Gore’s own principles and detachment. Was he as enthusiastic for the cause when, as Vice-President, he did not challenge Bill Clinton’s reservations about the Kyoto Protocol? And has he not now swung so far the other way that his pronouncements are now both propagandist and unnecessarily alarmist? Certainly the judge who found An Inconvenient Truth too unbalanced and too full of errors to show to British schoolchildren without a factual health warning believes that he lacks credibility. The judge’s rigour certainly contrasts with those whose reflex was to applaud the sentiments of Mr Gore’s documentary without even thinking of checking the science on which it rested.

Nevertheless, it is surely wrong to criticise the Nobel committee for seeing climate change as a potential threat to peace. Nobel’s strict definition of peace as reducing standing armies has been broadened to include human rights violations, racial oppression, poverty and dictatorship among the threats to peace. The IPPC scientists, mandated to gather data in a neutral way, have plausibly outlined the possible link between global warming and large-scale migration, competition for resources and the risk of war. If this is so, few people have done more to raise awareness than Mr Gore. And, crucially, no one has had such an influence in the US, the one country most able to give a lead or make a difference. For this he deserves credit. All campaigners risk overstating their case simply in order to be heard. Thanks partly to Mr Gore, climate change is now central to political debate. There is therefore all the more reason why, with the prize achieved, he should take care not to spoil his case by overstatement or the cavalier and selective use of facts.