Twenty years after the largest gathering of heads of state in history at the 1992 Earth Summit, the world’s nations are returning to Rio de Janeiro to negotiate further progress toward sustainable development.

The 80-page draft text that the delegates will be discussing addresses a number of important issues. Yet it is clear that not only has humanity failed to address the problems at the needed scale in the intervening years, but that “Rio+20” will fail to do so as well.

That said, it would be shortsighted to give up on Rio+20: humanity needs the building blocks that can be added by this conference to be as robust as possible.

Part of the problem is a preoccupation with the here and now. That includes the drama of economic problems in the euro zone and weakness in other large economies. In the United States, partisan politics are so polarized and poisonous, and media so fractured, that there is little mention of Rio+20. Many key leaders will not attend: Barack Obama, Angela Merkel and David Cameron (despite the change of date to avoid conflict with the Diamond Jubilee), to name three.

Another problem is the absence of any sense of urgency, even though the world’s actions today are producing an unacceptable environment for those who will be living at the century’s end. One scientific colleague, newly a grandmother, says that in addition to grasping the issues intellectually, she now takes them personally.

What in fact can be achieved at Rio+20?

First, there could be a wholehearted agreement on the draft. Key elements include important energy goals: Dubbed “Sustainable Energy for All,” they include energy access for all and ambitious but achievable goals in renewable energy and energy efficiency.

There are also important “green economy” initiatives — efforts to bring environmental value into economic decision making. Sustainable Development Goals should be negotiated to include much stronger environment elements than their predecessors, the Millennium Development Goals.

Negotiations around global governance, on which progress has been halting at best, will be tougher. Nowhere is that clearer than with the talks on a United Nations Framework Convention on Climate Change, which have been marked by wrangling over which major emitters should go first. It could almost be caricatured as a game of musical chairs were it not so deadly serious.

In the United States in particular, science has been systematically debased and questioned, as if it killing the messenger will make the problem go away. More than once I have been told that Rio+20 is about development, not environment, losing sight of the fact that environment is an integral part of sustainable development.

A mosaic approach can make some progress. Regional carbon markets involving countries or groups of states can move things in the right direction.

The private sector — which includes corporations with product dwarfing the G.D.P. of many nations — can play a leadership role. There are huge opportunities for innovation in both the nonprofit and for-profit sectors. Science can identify ways to pull carbon dioxide from the atmosphere and feed nine billion people without destroying more natural ecosystems.

There is pressing need to redefine what is reasonable. The usual approach is not reasonable if it leads to a runaway climate problem, cascading loss of biodiversity and a world increasingly resembling a toxic waste dump.

Twenty years beyond the Earth Summit, the clock is ticking. In 1992, atmospheric carbon dioxide concentrations were 357 parts per million. Today they average 395 p.p.m. and readings of 400 p.p.m. have been observed in the Arctic. Rio+20 must mark a start in managing the planet with greater seriousness of purpose than humanity has displayed so far.

Thomas Lovejoy is professor of science and public policy at George Mason University and biodiversity chairman at the H. John Heinz III Center for Science, Economics and the Environment.