Rajendra Pachauri, chairman of the UN scientific network on climate change, said the Obama administration was "showing what it can do, even while legislation is pending".
"It also sends a powerful signal to Congress. It shows a degree of resolve on the part of the president," he told the Associated Press news agency.
The environment minister for Sweden - which currently holds the EU presidency - said the outcome of the summit depended mostly "on what will be delivered by the United States and China".
Andreas Carlgren said he would be "astonished" if US President Barack Obama did not offer further concessions when he arrives at the summit next week.
The BBC's Mark Mardell in Washington said the US announcement had been expected for some time, but still sends an important signal to leaders attending the summit that Mr Obama is intent on passing legislation to curb emissions.
As the Copenhagen summit opened, Mr Rasmussen told delegates the world was looking to them to safeguard humanity.
He said a "strong and ambitious climate change agreement" was needed.
US-LED COPENHAGEN DEAL
No reference to legally binding agreement
Recognises the need to limit global temperatures rising no more than 2C above pre-industrial levels
Developed countries to "set a goal of mobilising jointly $100bn a year by 2020 to address the needs of developing
On transparency: Emerging nations monitor own efforts and report to UN every two years. Some international checks
No detailed framework on carbon markets - "various approaches" will be pursued
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