In remarks to Friday's Financial Times Deutschland
(FTD), Barroso said he did not expect agreement on concrete climate protection measures at next week's G8 summit.
German Chancellor Angela Merkel, who is hosting the summit in the northern resort of Heiligendamm, has put the issue at the top of the agenda.
Bush, however, told a German newspaper he was confident that Merkel would back his initiative, put forward in a speech in Washington on Thursday.
In remarks to the Frankfurter Allgemeine
newspaper, the US president rejected speculation that disagreement between officials working on a climate change text for approval at the G8 meeting had soured US-German relations.
The full text of the interview was to be published Saturday.Special responsibility
In his remarks to the FTD, Barroso said the US had "special responsibility as a large emitter of pollutants." "It is clear that we need a more ambitious position from the US," he added.
Barroso didn't mince words
Barroso's strong criticism follows Merkel's cautious welcome to the Bush initiative.
Speaking Thursday, Merkel said Bush's speech represented "movement" on the previous uncompromising US position on setting greenhouse gas emission targets and an "important step on the road to Heiligendamm."
Barroso, who will be present at the G8 meeting, came out strongly in favor of binding emission targets.
"The US is relying strongly on market mechanisms in the battle against climate change, and rightly so," he said. "But market mechanisms only work when one has binding targets."
He expressed the hope the US would see the need to bring the United Nations into the process, adding that the G8 summit should provide a launching pad for the UN climate protection meeting in Bali in December.Beyond Bush
Barroso said Bush's initiative was being superceded by events within the US, where "public opinion on the issue is developing at breathtaking pace."
Noting that Bush leaves office in January 2009, the EU head expressed the hope for a "real breakthrough to the post-Kyoto era in 2009."
The top German official preparing the ground for Heiligendamm acknowledged that there was no common line on the issue. Bernd Pfaffenbach said "exaggerated expectations" had been generated in the run-up to Heiligendamm and did not rule out failure to reach accord on climate protection.
Pfaffenbach said Bush's idea of drawing in the large industrialized countries and developing giants like India and China could "certainly make sense," but he stressed that the German approach was to work through the UN.
China is also waking up to a looming climate catastrophe
The 1997 Kyoto Protocol, which calls for limited reductions in greenhouse gas emissions from the industrialized world, runs out in 2012. The US has not ratified the treaty and demands that rapidly developing countries like China and India be brought into the process.Overestimating her influence?
Under Merkel's leadership, the EU has set ambitious targets for cutting carbon dioxide (CO2) emissions by 20 percent from 1990 levels by 2020. A similar goal has been set for increasing the use of renewable energy resources.
In its leading article, the FTD said the German chancellor had overestimated her powers on the world stage following her success on the issue at an EU summit in Brussels in March.
The influential newspaper predicted the G8 summit, which takes place from Wednesday to Friday, would mark Merkel's "greatest foreign policy defeat" to date at the hands of Bush.