The leaders of nearly two dozen environmental groups called on President Obama to attend next month’s Rio+20 Earth Summit in Rio de Janeiro, underscoring the uncertainty that continues to cloud the historic meeting.

More than 130 presidents, prime ministers and other heads of state have committed to attend the conference from June 20 to 22, which is also known as the United Nations Conference on Sustainable Development. But many experts question whether the session — coming 20 years after a U.N. summit in Rio that concluded with three global environmental treaties — will produce a meaningful outcome. And the White House has not said whether Obama will travel there.

Occupied with the European financial crisis, leaders such as Germany’s Chancellor Angela Merkel and Britain’s prime minister David Cameron have opted out of attending the summit. But several significant world leaders, such as Russian president Vladi­mir Putin, will come to Rio.

Johan Schaar, co-director of the World Resources Institute’s vulnerability and adaptation initiative, said that under a best-case scenario Rio would foster a discussion on how countries could grow economically without harming the environment.

“Governments do not have any appetite to make binding commitments in this forum,” Schaar said at a panel hosted last week by the Swedish Embassy and the Center for American Progress. “What would be a positive outcome of Rio would be a path toward defining sustainable development goals.”

In the letter to Obama, the heads of U.S. environmental groups including the Natural Resource Defense Council and the National Wildlife Federation noted that U.N. Secretary General Ban Ki-moon described Obama’s presence as “crucial.”

“Your presence at this summit would signal its critical importance to all Americans, demonstrate our country’s deep concern over urgent global issues that will inevitably affect our security and well-being, and highlight our nation’s determination to be a contender in the race to a low-carbon green economy,” the environmentalists wrote.

The White House could not be reached for comment on the matter Monday morning.

President George H.W. Bush attended the 1992 Earth Summit in Rio, which produced the U.N. Framework Convention on Climate Change and the U.N. Convention on Biodiversity.

Jacob Scherr, NRDC director of global strategy and advocacy, said in a conference call with reporters that this year activists are less focused on getting a negotiated text than “a cloud of commitments” that will “get countries, corporations and communities to actually take action” on some of the pledges they have made in the past.

Scherr and several of his colleagues say the United States could demonstrate leadership on issues ranging from plastic pollution in the oceans to support for renewable energy.

But some environmentalists, such as CAP director of climate policy Daniel J. Weiss, said they were doubtful Obama would attend the summit. Speaking last week at the House of Sweden, Weiss said he wished the president would attend, “even though it might not [produce] any concrete result.”