"[It is] telling that nobody in that room adopting the text was happy. That's how weak it is," European Commissioner for Climate Action Connie Hedegaard wrote on Twitter. "And they all knew. Disappointing."
The 53-page statement -- containing no defined "sustainable development" goals and no defined time horizon for setting them -- was greeted with boos by some delegates when it was unveiled at a preparatory gathering Tuesday, the Canadian newspaper The Globe and Mail reported.
"[It has] too much 'take note' and 'reaffirm' and too little 'decide' and 'commit,'" Hedegaard wrote on Twitter. "Big task now for U.N. nations to follow up."
Former Soviet President Mikhail Gorbachev said Monday the plan appeared to backslide on commitments for deep cuts in emissions of carbon-dioxide and other greenhouse gases -- reductions scientists say are needed to avoid tipping into a danger zone of climate-related floods and droughts.
More than 115 presidents, prime ministers and other officials, including Russian President Vladimir Putin and French President Francois Hollande, were to arrive Wednesday at the U.N. Conference on Sustainable Development in Rio de Janeiro, the biggest U.N. conference ever, with about 50,000 delegates from 190 countries.
Delegates also include development experts, bankers, academics and activists.
U.S. President Barack Obama, preoccupied by domestic politics and the global financial crisis, did not plan to attend the conference, the White House said.
Summit delegates said global economic constraints not only dampened summit hopes but also refueled conflicts between industrialized and developing countries, The New York Times reported.
U.N. Secretary-General Ban Ki-moon, who has been pushing to move beyond political promises and begin accomplishing genuine goals, said, "This is a once-in-a generation opportunity."
"If we don't completely rethink and radically accelerate the plans to reverse global warming, we will, in all likelihood, create catastrophic climate change in our lifetime," State of the World Forum President Jim Garrison told United Press International ahead of an earlier conference on developing "climate leadership."
"For the first time in history we must take personal responsibility for our climate," he told UPI. "We are all responsible for global warming. We must all share the leadership to solve it."
The U.N. summit is popularly known as Rio+20 or the Rio Earth Summit 2012 because it commemorates the 20th anniversary of a landmark U.N. Earth Summit that produced a global framework on climate change and an agreement to protect biodiversity.
Both agreements were binding but have yielded results that most scientists and environmental groups consider modest at best.
The 1992 summit also paved the way for the 1997 Kyoto Protocol on greenhouse gases, which is to expire this year.