A new bridge has come to symbolise Brazil's most challenging and urgent issue: balancing the demands of economic development with environmental protection

In the shadow of a giant bridge under construction, waiting in her dented Fiat for a ferry, Jandira Costa has no qualms about development in the Amazon. "We can't wait for it to open," she says of the 3.5km-long road over the Amazon's major tributary, the Rio Negro. Without a bridge, it takes Costa and her family at least half an hour to queue up and cross to the other side. Worse, crossing the river costs up to 100 real (around £37). When the bridge is completed in November, it be quicker, more convenient, and - most importantly for Costa - free.

The bridge - the first on the world's greatest river system symbolises the surging development at the heart of the world's largest rainforest and will bring much-needed economic opportunities for those living on the far bank. But environmentalists fear that the bridge, combined with new gas pipelines, roads and rising populations, could open up the rainforest to further destruction.

Manaus is the steamy and sprawling industrial capital of the vast Amazonas state, manufacturing the latest flatscreen TVs and mobile phones for the whole of Brazil. It has been an island of wealth for 200 years, but it is now opening up in all directions.

The bridge runs from north to south, to the undeveloped towns of Iranduba, Manacapuru and Novo Airão and towards the untouched jungle. To the west, a 600km gas pipeline will next month begin powering a huge new electricity power station by bringing energy from a pristine part of the forest at Urucu into the city.

To the south, the planned re-paving of an impassable 900km-long highway could break Manaus's isolation from the rich and populous south, and to the east, a new electricity line will connect the city to the national grid, giving an outlet for the fossil fuel and hydroelectric riches of the Amazon, such as the recently approved controversial Belo Monte dam.

While the 25m people in the Amazon and their local politicians welcome the opening of new horizons the question troubling scientists, environmental policymakers and campaigners is whether all these riches can be delivered while leaving the ancient trees of the Amazon standing. Their continued loss woulddrive climate change, and deprive the world of its most diverse store of animal and plant life.

"Now is a transition moment," said Izabella Teixeira, Brazil's environment minister. She says the country must find ways of improving the lives of those across the Amazon and exploiting its natural resources, such as hydroelectricity, without causing environmental damage. "I cannot forget this region. We have an economy there that needs to be developed," she says, pointing out the example of the Amazonian town of Labrea, which was a leper colony just 20 years ago.

The new BR-319 road south to Porto Vehlo, officially planned to open by 2012 if government funds are allocated, is also very popular locally. It is backed by Alfredo Nascimento, who stood down as Brazil's transport minister in March to run for the governorship of Amazonas. Locals even joke that, given that Manaus will host some games in the 2014 football World Cup, fans should have the right to drive to the game, rather than fly in.

But Professor Philip Fearnside, an ecologist at the National Research Institute for the Amazon in Manaus, fiercely opposes the opening of the road in the absence of enforceable laws to protect the forest around it. He has lived in the Amazon for 34 years and has seen it all before. In 1982, the World Bank funded the BR-364 highway from east to west across the southern Amazon. "The consequence was near total deforestation," says Fearnside. In the last decade, 75% of all deforestation in the Amazon is estimated to have taken place within 50km of roads, which provide access for the loggers. Mauro Pires, director of government's department of policy to combat deforestation agrees: "The arc of destruction is exactly where the roads are."

The tension between economic development and protecting a global resource for the good of all is a huge challenge for Brazil, and far from easy in an area so vast, poor and frequently corrupt. President Ignácio Lula has been vehement in his defence of development. He has said people should not suffer simply because they live in the Amazon and that other countries need to pay for the benefits of a healthy rainforest. "I don't want any gringo asking us to let an Amazon resident die of hunger under a tree," Lula said last November.

The government is now developing ways in which people can earn more without destroying the Amazon rainforest. The Ambé project in Para state, 700km downstream from Manaus, allows the felling of selected trees in a 25-year cycle to enable new trees to flourish. The project, the most advanced in the country, has doubled the incomes of the farmers taking part, and community leader, João Rocha, confirms "life is better now". But it employs just 280 people, and non-wood products like cosmetic oils, "bio-jewellery" made from seeds and latex toys make up just 5% of the income. The vast size of the Amazon makes it hard to connect people and scale up businesses.

The government is also beginning to solve land rights disputes and granting concessionary plots to local families in the region. Domingos dos Santos Rodriguez, manager of the Tapajós natural forest park in Para, is proud of the protection his park has afforded but acknowledges it is "quite common" for people to find ways to acquire more than the one concession allowed.

Another question being raised is whether the rewards brought by new roads and other infrastructure will benefit the majority of the 25m people who live in the Amazon, or simply boost the wealth of the rich. Raquel Carvahlo, Greenpeace Amazon campaigner in Manaus, says the city, with its tax breaks for manufacturers, is already very attractive. "It will see an increase in population which will probably overburden public services – medical care, sewage treatment, housing etc - but the truth is the workers needed must have some level of technical ability so the major portion of migrants survive in the black economy." Fearnside believes the large cattle ranchers, responsible for most of past deforestation, will gain far more than landless labourers who sit at the bottom of Brazil's yawning wealth divide.

The destruction of forest has dropped dramatically since its peak in 2004 and was 75% lower in 2009. But it is difficult to unpick how much of that fall is due to increased government enforcement - meaning deforestation is under permanent control - or fluctuations in beef and soy prices, or the rising value of Brazil's currency or increases in the productivity of cleared land.

Prof Troy Beldini, at the large-scale Biosphere-Atmosphere programme centre 85km south of Santarem, is working to turn the scientific research of his team into practical protective policies. "The Amazon will one way or another be used because it provides jobs," he says. "The deforestation trend will go up unless there is a strong monitoring presence. We have satellite technology in Brazil, what we need is a [stronger] ground presence."

Fearnside also believes the Amazon will be developed but argues that strong and enforceable protections must be in place before the chainsaws move in. In the past, he says, protection plans were prepared in parallel and then forgotten when building began. He adds: "If you are going to spend a billion reals to improve life in the interior of the Amazon you do not build a road [the BR-319], you build hospitals and schools where the people already live."

The upcoming presidential election in October is not likely to change Brazil's direction of travel. Both leading candidates - Dilma Rouseff and José Serra - are seen as developmentalists, though the presence of Marina Silva, the ex-environment minister who resigned in 2008 in frustration at development plans, is giving campaigning a green tinge.

However, a serious political threat is a proposed change to the Forest Code that would massively increase the proportion of a farmer's land that can legally be cleared of trees. It is opposed by Teixeira and Brazil's environment agency Ibama, but has passed the committee stage in Brazil's congress and will be voted on later this year. Its architect, the populist Communist Aldo Rebello, frequently talks of an international conspiracy by rich countries to suppress Brazil's growth.

The biggest hope for a game-changing moment that will protect the Amazon's thick blanket of stored carbon is the agreement of a global regime, known as Reducing Emissions from Deforestation and Forest Degradation (REDD), where forest saved from felling can be used to create international carbon credits, bought by nations seeking to offset their own emissions. Norway has already promised Brazil $1bn by 2015 for a parallel scheme.

"You cannot polarise the debate. Brazil is a big player in the timber trade and it will be in the future," says Teixeira. "You have to discuss what are the economic requirements to prevent deforestation. REDD is essential to this new step of development."

The failure to agree a global climate deal in Copenhagen has slowed the founding of a forest based carbon credit scheme. Hopes remain that the next meeting, in Mexico in November, could see a breakthrough. But disagreements remain with Brazil wary of a full market-based system, arguing grants must be part of the package, and also expecting more of the money to go to poorer countries with tropical forests such as Congo.

Making trees worth more alive than dead is one of the least expensive ways of tackling global warming. The colossal amount of carbon stored in the Amazon means it could provide billions of dollars of funding for development while keeping the forest intact.

The Manaus-Iranduba bridge in numbers

At 3,595m long, it is one of the longest bridges in Brazil

It has 74 supporting columns, with the central one standing 187m high

The height of the road rises to 55m in the middle

The cost is an estimated $400m and will be toll-free

Currently 2,500 vehicles a week are ferried across the river, at fares up to R100 (£37).

Its construction consumed 1m bags of cement - around 50,000 tonnes

• Damian Carrington's travel expenses were paid for by the Brazilian government. They had no say in the content of this article

• This article was amended on 6 August 2010. The original stated that the Manaus-Iranduba bridge is the longest in Brazil. This has been corrected.

