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Rio de Janeiro's original Earth Summit more than twenty years ago was a blockbuster. It was a watershed moment in the world's efforts to tackle climate change, producing an international treaty which laid the groundwork for the system of control of carbon dioxide emissions that was enshrined in the Kyoto Protocol.
Not bad for a couple of weeks work. The problem is of course, what came after. Because precious little has been done to follow up the initiative of Rio.
No surprise then that this years Rio summit is a much more muted affair. The draft text of an agreement from heads of government has been described as "lacking ambition".
Indeed the great green rhetoric these days is more likely to come from companies than governments. Companies like the Brazilian mining giant Vale. Vale runs the largest iron ore mine in the world right in the middle of the Amazon rainforest and the company claims its operation is sustainable. David Shukman, the BBC's science editor, has been there to check out its green credentials.
Restoring rainforest is a difficult enough, but it pales into insignificance beside the biggest environmental challenge of them all - tackling climate change.
The framework for regulating emissions that sprang from the original Rio summit has been largely ignored with some of the world's biggest polluters - most notably the US and China - refusing to sign up.
And, if anything, resistance to emissions regulation is growing. The latest country to refuse new targets on cutting CO2 emissions is a european one, Poland.
Poland fears that long-term goals to cut CO2 emissions would force it to stop using coal and increase the use of natural gas. That poses a danger for the country's energy security because it would mean higher dependence on imports from Russia. Our correspondent Paul Moss has been there to find out more.
So are there other ways of persuading countries and companies to take a more sustainable path?
One of the the big themes at the Rio Summit is "green accounting" whcich tries to put a value on the natural world. Justin Rowlatt talks to Michael Jacobs, visiting professor at the Grantham Research Institute on Climate change and the Environment at the London School of Economics and who was the former environmental advisor to the British Prime Minister, Gordon Brown.