Never before has the survival of so much rainforest depended on one person. But that is where President Rousseff of Brazil finds herself. The Brazilian congress just passed a forest code that puts the Amazon and other forests in jeopardy.

Dilma Rousseff's imminent decision on whether to pass or veto the bill will have huge ramifications. If approved, it would give loggers and farmers free rein to chop down 190m acres of forest. A territory the size of France and Britain combined will be at risk. It would open forests and rivers up for grabs, putting 70% of Brazil's river basins at risk. It would also give amnesty to anyone previously charged with illegal deforestation.

This bill would be a catastrophe not just for Brazil, but for the world and all our futures. Brazil is home to 40% of the world's last remaining rainforest – a lung that provides the earth with one fifth of our oxygen. So why is the congress passing such a destructive bill? And why would Rousseff not just veto it right away? Simple: industrial farmers and loggers have a stranglehold on congress and this powerful lobby claims current legislation is freezing development in Brazil. Others say forest must be converted into farmland to tackle rising food prices in Brazil.

None of these arguments hold water. The incredible development of Brazilian agriculture in the past decade is due to investment in more efficient farming and has been fuelled by the rising price of food commodities over 10 years. It has nothing to do with needing more access to forests. In Brazil, 200 million cattle roam over 500m acres. More efficient farming will free more land without any need for deforestation.

Every threat to the Amazon is a threat to indigenous life. The forest code would allow deforestation in previously protected areas. The interests of those that have lived in the forests for generations are being put second to those of commercial land speculators. Environmentalists who have spoken out to protect the forest have been harassed, threatened and even killed by thugs.

But this is not just a dispute between businessmen and environmentalists. More than 79% of Brazilians reject the new bill. All former environment ministers , whatever their political leaning, have joined forces to express their strong opposition to this issue and recently, even some of the top businessmen in Brazil came out against the forest code. More than 2 million people have signed a global Avaaz campaign calling on Rousseff to use her veto. Tens of thousands have signed the petition and thousands have called Rousseff's office and Brazilian embassies across the world. This bill is now as important to people living in the islands of São Tomé as it is for those in São Paulo.

The government has a proud record of protecting the environment: in the past few years Brazil vastly reduced deforestation rates, achieving a 78% decline between 2004 and 2011. Rousseff came to office promising to firmly oppose any amnesty to the destroyers of the forest. It is now up to her to stick to her promises and maintain the environmental records of her predecessor.

Brazil's track record made it the natural host of next month's critical Earth summit – the most important global environmental summit in 20 years. More than 50,000 people from all over the world will come to Rio and discuss the fate of the planet and how to accelerate the fight against environmental destruction, the collapse of biodiversity, and climate warming.

Rousseff will host the summit – a massive responsibility that requires legitimacy. But if she allows this bill to pass, Brazil will not be seen as a credible host of Rio+20.

A veto by Rousseff will be an act of global leadership, a gesture desperately needed to win the fight against climate change. An approval by her will cast a dark shadow over her presidency and Brazil's authority in these global forums. Worse still, a victory for big business profits over the planet's future will set a frightening precedent for the protection of the last remaining forests across our world. Brazil is seen by many countries as a model of 21st century development. This is a crucial moment to define what kind of model Brazil wants to be.

Millions of people will be watching Rousseff as she comes to a decision on this forest code. It is a decision that will have an impact on all our futures.

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