But Rajendra Pachauri, head of the United Nation's body tasked with leading the fight against climate change, also questioned the value of a new global climate deal without such a US pledge.
He said political constraints such as creating new jobs made it impossible for the new president to announce the measures that scientists believe are necessary.
His comments made at the climate summit in Copenhagen came as scientists warned that the modest IPCC estimates of likely sea level rise this century need to be increased.
Extra melting in Greenland could drive sea levels to more than a metre higher than today by 2100, they said.
Obama has said the US will work to reduce its greenhouse gas emissions to 1990 levels by 2020. Europe has pledged to cut them by 20-30 per cent on 1990 levels in the same period.
The IPCC said developed nations should aim for 25-40 per cent cuts by then to avoid dangerous climate change.
Speaking on the fringes of a high-level scientific conference on climate change in Copenhagen, Mr Pachauri told the Guardian: "He [Obama] is not going to say by 2020 I'm going to reduce emissions by 30 per cent. He'll have a revolution on his hands. He has to do it step by step."
Mr Pachauri's remarks echo those of Todd Stern, the US president's new chief climate negotiator, who said last week that it was "not possible" for the US to aim for 25-40 per cent cuts by 2020.
Such a stance could threaten attempts to agree a new global deal to regulate carbon emissions to replace the existing Kyoto protocol, the first phase of which expires in 2012. Campaigners say a new treaty must be agreed at UN talks in Copenhagen this December.