"We can't make these commitments while other countries say 'we'll see tomorrow,''' Sarkozy said.
Sarkozy hailed Brazil for being "the first developing country to put its proposals on the table''.
Brazil announced on Friday that it would voluntarily reduce carbon emissions by
36.1 per cent to 38.9 per cent by 2020, largely by slowing deforestation in the
vast Amazon region, while Europe is pledging a 30 per cent cut.
A day earlier, the country announced it had registered its biggest annual decline in deforestation, which scientists think is responsible for 20 per cent of the globe's carbon emissions.
Still to commit
So far, the US and China, the world's two biggest polluters, have no firm commitments on cuts to bring to the table.
"This puts the US in a completely isolated situation," Marcelo Furtado, Greenpeace Brazil's executive director, said.
He said China would present its climate target next week, leaving Barack Obama, the US president faced with "fulfilling his promises to fight climate change, or continuing the policies of (his predecessor George W.) Bush."
"We're worried Copenhagen will be a fiasco," Marco Aurelio Garcia, Lula's foreign affairs adviser, said before Lula left for Paris to meet Sarkozy.
"There are risks because the biggest country [the US] won't be arriving with an ambitious programme."
The Copenhagen conference is aimed at hammering out a successor to the
1997 Kyoto treaty, the climate change-fighting treaty that the US never ratified.