Secretary of State Hillary Rodham Clinton will lead the U.S. delegation to the U.N. Conference on Sustainable Development in Rio de Janeiro next week, disappointing environmental activists who have been pushing President Obama to personally attend.

U.S. EPA Administrator Lisa Jackson will serve as alternate head of the U.S. team, while Special Envoy for Climate Change Todd Stern will double up on his role as top U.S. diplomat at the global warming talks to also serve as chief negotiator in Rio.

"We're of course very pleased Secretary Clinton will be there. But at the same time it's important that President Obama find ways to demonstrate that the United States remains committed to addressing climate change and sustainability issues, both around the globe and at home, and both during and after Rio+20," said Jacob Scherr, director of global strategy and advocacy for the Natural Resources Defense Council, in a statement.

The announcement on the eve of Rio+20, so named because it marks the 20th anniversary of the original Rio Earth Summit, comes amid confirmation that both German Chancellor Angela Merkel and U.K. Prime Minister David Cameron also will skip the conference. Both will be represented by others in the government.

Meanwhile, countries continue to bicker over the summit's goals with objections to everything from poverty alleviation targets to whether rich countries should commit to curb their consumption of natural resources.

"The lack of leadership at the international level is significant," Mary Robinson, the former president of Ireland and U.N. High Commissioner for Human Rights, said yesterday.

Speaking at a conference sponsored by the U.S. Agency for International Development, Robinson said a "reasonable declaration" may still emerge from Rio+20. But she also was incredulous that the conference is avoiding climate change.

Negotiations over combating climate change remain deeply fraught within the U.N. Framework Convention on Climate Change (UNFCCC). Activists have openly complained that Rio+20 organizers, in their fear of tainting the conference with the problems of the UNFCCC, have made discussion of global warming at Rio+20 about as welcome as the Ebola virus.

"The planet is at risk. Let's get real," Robinson said. "And yet, in Rio, we don't want to talk about climate, unbelievably."

IMF's Lagarde will push for a price on carbon

Meanwhile, activists are focusing on the 134 world leaders who will attend. Yesterday, International Monetary Fund Managing Director Christine Lagarde added her name to the list, saying she will make a personal appearance in Rio.

Calling green growth central to global economic stability, Lagarde said the IMF's role will be in offering research and expertise in three areas: putting a price on carbon; eliminating fossil fuel subsidies; and helping countries account for their forests, water, corals and other natural capital alongside traditional accounting measures like gross domestic product.

Insisting no fewer than five times during a talk sponsored by the Center for Global Development that the IMF does not plan to overstep its mandate, Lagarde offered no specifics for the Rio conference.

"It's not as if I have a big box full of cash and I can use it smartly," she said. Lagarde also declined to pinpoint what exactly would amount to a successful outcome in Rio, saying, "I'm not fixated on an amount, a structure, something that makes headlines." Still, she made plain that the IMF has an important role to play in generating the data to help policymakers move forward on controversial environmental policies.

"There is much work to be done at the technical level, in terms of the appropriate design of taxes and tax-like instruments to get the prices right. The IMF will play an active role in this," Lagarde said. When it comes to understanding and accounting for the value of resources and carbon pollution, she said, "At present, we are only at base camp in terms of getting the prices right."

Climate policy analysts said despite the lack of specifics, Lagarde's decision to attend the Rio+20 summit is monumental.

"For the person who has been paying attention to fixing Europe to say, 'This is important,' it gives credibility to these issues," said Michele de Nevers, visiting senior associate at the Center for Global Development, who spent nearly 30 years working on environmental issues at the World Bank.

"The IMF has so much credibility with the guys with the money," she said. "The fact that the IMF is advocating a carbon tax? That's a huge signal."

Nancy Birdsall, president of the Center for Global Development, predicted push-back from the IMF staff and board and from others uncomfortable with the managing director's taking a strong stand on climate policy. But, she argued, Lagarde "is making the point that long-term financial stability is part of reducing risks that arise because of climate change."

Birdsall said that while it is unclear what impact, if any, Lagarde's presence might have at Rio, the real work will happen after the hoopla of the U.N. conference. "This is a vehicle for making a point that has to endure far beyond Rio," she said.