No one seemed to be expecting Barack Obama to win the Nobel Peace Prize, despite the fact that the bookies were giving odds of seven to one, the same as they had on Morgan Tsvangirai, who was widely touted as the favourite. But perhaps more surprising is the degree of disapproval with which many have greeted the news.

Critics have said that the award is premature and complained that the new US President has so far done nothing more than make a few inspirational speeches. The prize, they say, should not be for effort but for results. Some have even said that he has merely got the award just for not being George W Bush. The Nobel decision-makers will look pretty foolish, they complain, if before he picks up the prize in December, Obama decides to send another 40,000 US troops to Afghanistan.

Such a response is misguided. His nomination for the prize may have been submitted less than two weeks after he took office in February. But in the time between then and the announcement yesterday Barack Obama has wrought a sea change in the international political climate. Not being George Bush was a good start. For Bush represented an arrogant, belligerent, unilateralist style of American foreign policy which served the world ill. Obama arrived with a heart for peace and an openness of mind to other nations which was in itself a huge transformation. He may see no alternative to fighting the war against al-Qa'ida in Afghanistan but everywhere else he has reasserted the importance of the United Nations and of multilateral diplomacy. He replaced military threats with dialogue with Iran and North Korea. He has begun talks with Russia over nuclear disarmament.

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He has prioritised peace in the Middle East. He has reached out a hand of friendship to the Muslim world. He has thrown Washington's recalcitrant attitude to global warming into reverse. All change begins with a change of mind by one individual and Obama has been that person.

The world now expects an awful lot more from him. The Nobel award is a recognition of that too. It is just not true that in the past the prize was awarded only where work for peace had reaped concrete proofs. Many times it has been given as an encouragement to see the effort through. In 1976, the award to the Peace Women was intended to send a signal to the two battling communities in Northern Ireland. In 1991, Aung San Suu Kyi was already in jail when she won the prize in a well thought-out gesture of support for democracy in Burma. In 1994 the Nobel committee hoped to maintain the momentum to peace in the Middle East by handing the prize to Yasser Arafat, Shimon Peres and Yitzhak Rabin. The prize tries to enhance the future as well as celebrate past achievements.

In his will, the founder Alfred Nobel said that the prize should go "to the person who shall have done the most, or the best, work for fraternity between the nations and the abolition or reduction of standing armies and the formation and spreading of peace congresses". It is hard to think of anyone of whom that is truer last year than Barack Obama.

The prize may make life difficult for him domestically, giving his right-wing critics another stick with which to beat him over healthcare. But to the rest of the world the US President is an inspiration. The Audacity of Hope, he called one of his books. Rarely has a single individual in recent times given so much of the world cause to dare to anticipate that a better world can yet be made.