US President Barack Obama has changed his plans for the UN summit on climate change, and will be in Copenhagen at the end of the conference rather than the beginning.
The White House says that, after talks with other leaders and after seeing the progress that has already been made to give momentum to negotiations, Mr Obama has moved his arrival from 9 December to 18 December.
It says he believes that "continued US leadership can be most productive through his participation at the end of the Copenhagen conference".
The summit aims to draw up a treaty to succeed the 1997 Kyoto Protocol.
UN organisation to investigate climate row
Meanwhile, the United Nations' climate science organisation will look at claims that British scientists manipulated data on global warming.
Dr Rajendra Pachauri, head of the Intergovernmental Panel on Climate Change (IPCC), told the BBC that the claims were serious and he wants them investigated.
The row broke out last month when hundreds of emails between scientists at the University of East Anglia's climatic research unit and other scientists around the world were put on the internet.
Some observers allege that one of emails suggest a wish on the part of the unit's head, Professor Phil Jones, to exclude certain papers from the UN's next major assessment of climate science - a claim he strenuously denies.