The first Rio Earth summit in 1992 paved the way for a global treaty on biodiversity, and the 1997 Kyoto Protocol on greenhouse gases, which is due to expire this year. The Rio+20 moniker is a nod to the 1992 summit.
Heads of state and ministers, including Russian President Vladimir Putin, French President Francois Hollande and U.S. Secretary of State Hillary Clinton, will meet with diplomats representing other nations from Wednesday for three days to discuss the text and possibly make some changes to its wording.
Observers do not expect major amendments.
U.S. special envoy for climate change, Todd Stern, told reporters on Tuesday he did not expect the document to change much after heads of state meet to discuss it.
"We don't have anything that we are expecting to try to drive into the document that is not there yet," he said.
Environmental groups criticized the text, saying it omitted or watered down important proposals and challenged heads of state to act urgently to respond to climate change.
"This summit could be over before it's started. World leaders arriving tonight must start afresh. Rio+20 should be a turning point," said Oxfam spokesman Stephen Hale.