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The US president abruptly altered the timing of his appearance at the conference, hoping to capitalise on steps by India and China in the effort to agree a new global warming pact, Robert Gibbs, the White House official press secretary, said.
"The president believes that continued US leadership can be most productive through his participation at the end of the Copenhagen conference on December 18th rather than on December 9th," Mr Gibbs said in a statement.
"There are still outstanding issues that must be negotiated for an agreement to be reached, but this decision reflects the president's commitment to doing all that he can to pursue a positive outcome."
Mr Obama had earlier planned to stop at the summit on Wednesday on his way to Nobel Peace Prize events in Oslo.
The move means that Mr Obama will be squeezing in a separate, 10th foreign trip before Christmas - a record pace of travel for a first-year president - as a means to giving momentum to a deal aimed at fighting global warming.
Mr Obama will now leave for Oslo late Wednesday, attend Nobel events on Thursday and return to Washington on Friday.
US president Barack Obama decided to arrive to Copenhagen climate change summit on December 18, contrasting with his previous plan to arrive on December 9.

Obama's Nobel Peace Prize events take place in Oslo around December 1. He decided not to go to Copenhagen before them, delaying his arrival instead.

The purpose of the summit is to finalize a treaty to succeed the 1997 Kyoto Protocol. Robert Gibbs, the White House official press secretary, commented, saying that presence of the USA president at later days might encourage more serious results:

"The president believes that continued US leadership can be most productive through his participation at the end of the Copenhagen conference on December 18th rather than on December 9th. There are still outstanding issues that must be negotiated for an agreement to be reached, but this decision reflects the president's commitment to doing all that he can to pursue a positive outcome."

Danish Prime Minister Lars Lokke Rasmussen welcomed Mr Obama's move, saying that "President Obama's presence is an expression of the growing political momentum towards sealing an ambitious climate deal in Copenhagen."

Greenpeace International's political climate coordinator Martin Kaiser also talked about the timetable plan change optimistically: "After a global outcry, President Obama has listened to the people and other world leaders; he has come to his senses and accepted the importance of this potentially historic meeting. Now that he has moved the date, he needs to move his targets and his financial contribution to be in line with what climate science demands."

The decision of the President came after his talk with Australian Prime Minister Kevin Rudd, German Chancellor Angela Merkel, French President Nicolas Sarkozy and British Prime Minister Gordon Brown. Some commentators say that the President could have felt the world leaders' optimistic approaches to the December forum, therefore deciding to delay his invigorative arrival to a more critical point.