On the eve of a major gathering to discuss the state of the planet, a Washington Post poll shows that most Americans think the world’s natural environment has deteriorated over the past decade, and more than six in 10 say humans are making the problem worse.

Leaders from more than 130 nations are now meeting in Rio de Janeiro for the high-level session of the U.N. Conference on Sustainable Development, also known as the Rio+20 Earth Summit. The meeting, which happens once a decade, aims to identify how to achieve economic growth without depleting the planet’s natural resources.

Negotiators produced a final text Tuesday for heads of state to vote on this week, calling for a new emphasis on the economic value of the natural world, although it came under intense criticism from many civil society groups as being weak and lacking concrete goals and timetables.

Perhaps more significantly, two dozen major firms made new environmental commitments Monday at the conference. Coca-Cola pledged to develop plans to protect the water sources for its 200 bottling plants worldwide, while Dow Chemical said it will assess the economic value it gets from the ecosystems connected to its new bioplastics plant in Brazil.

Wednesday brought a handful of new commitments, including one from the eight largest multilateral development banks pledging to invest $175 billion over the next decade to finance more sustainable transportation systems in developing countries. Separately, Maldives President Mohamed Waheed announced Wednesday that he would make all of his country’s waters a marine reserve.

The actual negotiations in Rio have produced little of substance, beyond an abstract commitment to craft “sustainable development goals” in the future. The Brazilian government, which is hosting the meeting, took out language which would have committed countries to reaching three U.N. goals by 2030: ensuring universal access to electricity and heating; doubling the global rate of energy efficiency improvement; and doubling the share of renewable energy in the global energy mix.

“The text is disappointing to many who were hoping that Rio would be a once-in-a-lifetime event redirecting the world with a clear plan for a clean and prosperous future,” wrote Rebecca Lefton, a policy analyst at the Center for American Progress, in an e-mail. She added that she hoped future negotiations on sustainable development goals could produce more concrete commitments from countries.

Others in Rio said they understood why Americans see a global environmental decline. Nauru Ambassador Marlene Moses, who chairs the Alliance of Small Island States, noted that in her country, “food and water are more scarce now, rainfall is less predictable, and we have drought more often. It’s being threatened, due to man-made climate change.”

According to the poll, twice as many Americans think the environment will get worse over the next decade as think it will get better. More than three-quarters of those who see an eroding environment say humans have a mostly negative impact. Even among those who say the environment has not changed or has improved in recent years, a slim majority — 52 percent — say people are making things worse.

Americans’ views of the environment divide along party lines, according to the poll. More than seven in 10 Democrats and independents say human activity has had a “mostly negative” effect on the environment over the past decade; only a bare majority of Republicans agree. Democrats and independents are also more apt to say the environment has gotten worse over the past decade and are more downbeat about its prospects.

U.N. Foundation President Timothy E. Wirth, who is attending the Rio summit, said groups with an economic interest in exploiting natural resources and burning fossil fuels are helping to shape public attitudes.

“The scientific evidence for deterioration is overwhelming and almost universally shared,” Wirth wrote in an e-mail. “The polling data suggests that the deniers, who have much to gain financially by continuing to use resources in an unsustainable fashion, are having a real impact on American public opinion.”

Four in 10 Americans expect the environment to get worse in the coming decade — similar to the number who think it will hold steady — and about two in 10 think it will get better.

William K. Reilly, who attended the 1990 Rio Earth Summit as head of the Environmental Protection Agency and returned to this year’s conference, said there is a disconnect between what companies are doing to address environmental challenges and the public’s view of the planet.

“Americans are in a pessimistic mood right now, but things are happening,” Reilly said in a phone interview. “When companies are making these commitments, that has potentially profound consequences. These announcements indicate that the private sector has taken this conference more seriously than most governments.”

And Glenn Prickett, the Nature Conservancy’s chief external affairs officer, wrote in an e-mail that “the best news from Rio” is that both political and business leaders are finally accepting the idea “that a healthy environment is good for the economy. The question isn’t ‘whether’ we should take care of the environment anymore, it’s ‘how’ we can do it.”

Although the overall U.S. environmental outlook is far from rosy, pessimism has receded in recent years. A majority of the public in a 2006 ABC News-Time-Stanford poll — six out of 10 — predicted that the environment would get worse in the coming decade.

The new Washington Post poll, which surveyed 1,002 randomly selected adults by telephone June 14-17, has a margin of error of plus or minus 3.5 percentage points.

Assistant polling analyst Michael Brandon contributed to this report.