Christine Lagarde, the head of the International Monetary Fund, has warned that the world risks a triple crisis of declining incomes, environmental damage and social unrest unless countries adopt a more sustainable approach to economic growth.

Ahead of the Rio+20 Earth summit later this month, she said the rich should restrain their demands for higher incomes while there are still 200 million people worldwide looking for a job and poverty is on the rise.

Giving her clearest backing yet to green taxes and a range of measures to protect the environment, she argued for taxes on petrol-guzzling cars among a range of green measures to tackle climate change.

“It has been 20 years since world leaders first went to Rio to commit to the noble goal of protecting the planet for future generations. And now, 20 years on, we will be journeying back to Rio to affirm our commitment to sustainable development – the idea that we should strive for economic growth, environmental protection and social progress at the same time,” she said in a speech in Washington on Tuesday.

“The idea that different economic, environmental and social objectives can be seen as distinct aspects of a single vision, essential parts of a connected whole.”

But she said the current economic crisis in Europe and slowing growth worldwide, coupled with the growing threat from climate change and social tensions could wreck the efforts of leaders to chart a more sustainable future.

“Over the past four years, we have been mired in the worst economic crisis since the Great Depression. And we are not out of it yet.

“In fact, tensions are on the rise again, and financial stability risks have once more moved front and centre. Great uncertainty hangs over global prospects.

“Too many regions today are still stuck in a trap of low growth and high unemployment,” she said.

“Right now, 200 million people worldwide cannot find work, including 75 million young people trying to take their first step on the ladder of success.

“So we need a strategy that is good for stability and good for growth – where stability is conducive to growth and growth facilitates stability.”

Lagarde, a right-wing former French finance minister, recently caused a storm of controversy after she accused Europeans of blocking progress to end the current financial crisis. Asked if she sympathised with Greeks impoverished by austerity measurers demanded by Brussels, she said the children of Niger were more her concern. It also emerged that Lagarde pays no tax on her $467,940 (£298,675) a year salary.

Ahead of the summit, she said taxes on petrol and other carbon fuels could raise billions of dollars for green investment projects. “Right now, less than 10% of worldwide greenhouse gas emissions are covered by formal pricing programmes. Only a handful of cities charge for the use of gridlocked roads. Farmers in rich countries are undercharged – if charged at all – for increasingly scarce water resources.”

She added: “Many countries continue to subsidise polluting energy systems. These subsidies are costly for the budget and costly for the planet. Countries should reduce them. But in doing so, they must protect vulnerable groups by tightly focusing subsidies on products used by poorer people, and by strengthening social safety nets.”

Nowhere is the global push to restore degraded land likely to be more important, complex and expensive than in China, where vast swaths of the soil are contaminated by arsenic and heavy metals from mines and factories.

Scientists told the Guardian that this is likely to prove a bigger long-term problem than air and water pollution, with potentially dire consequences for food production and human health.

Zhou Jianmin, director of the China Soil Association, estimated that one-tenth of China’s farmland was affected. “The country, the government and the public should realise how serious the soil pollution is,” he said. “More areas are being affected, the degree of contamination is intensifying and the range of toxins is increasing.”

Other estimates of soil pollution range as high as 40%, but an official risk assessment is unlikely to be made public for several years.

The government has spent six years on a soil survey involving 30,000 people, but the academics leading the project said they have been forbidden from releasing preliminary findings.

Chen Tongbin of the Chinese Academy of Sciences said the worst contamination was in Yunnan, Sichuan, Hunan, Anhui and Guizhou, but there were also parts of Beijing where the soil is tainted.

Unlike in Europe where persistent organic pollutants are the main concern, Chen said China’s worst soil contamination is from arsenic, which is released during the mining of copper, gold and other minerals. Roughly 70% of the world’s arsenic is found in China – and it is increasingly coming to the surface with horrendous consequences.

“When pollution spills cause massive die-offs of fish, the media usually blames cadmium, but that’s wrong. Arsenic is responsible. This is the most dangerous chemical,” he said. The country’s 280,000 mines are most responsible, according to Chen.

But the land – and food chain – are also threatened by lead and heavy metals from factories and overuse of pesticides and fertilisers by farmers. The risks are only slowly becoming well known. The Economic Information Daily reported this week that pollution ruins almost 12bn kilograms of food production each year, causing economic losses of 20 billion yuan.

Chen estimated that “no more than 20% of China’s soil is seriously polluted”, but he warned that the problem was likely to grow because 80% of the pollutants in the air and water ended up in the earth.

“The biggest environmental challenge that China faces today is water pollution, but there are efforts underway to control that. In the future, the focus must be on soil pollution because that is much harder to deal with. Soil remediation is an immense and growing challenge.”

Calls for a clean-up of the land are slowly gaining prominence. Huang Hongxiang, a researcher from the Institute of Agricultural Resources and Regional Planning, warned earlier this year that China needed to widen its focus from production volumes.

“If we don’t improve the quality of farmland, but only depend on increasing investment and improving technology, then – regardless of whatever super rice, super wheat and other super quality crops we come up with – it will be difficult to guarantee the sustainable development of our nation’s agriculture.”

I’m not sure I fully understand the term “psychogeography”. To me, it means the exploration of an unlikely place or a hidden aspect of a place, and whenever I hear it I think of Sunday walks in my childhood, when we would follow an overgrown and neglected path and sometimes scrape away the turf to discover a square stone with bolt holes drilled through it. As beetles hurried this way and that across its surface, my older brother would explain that the stone had once held an iron rail and that the path had once been a wagon-way, built in the 18th century to take coal from the Fife pits to a harbour on the Forth.As nobody else seemed to know or care about these facts, I felt I was sharing a historical secret. There were several of them close by: dark, deep ponds that had once been quarries; a ruined slipway built to take seaplanes; steel rings that had tethered barrage balloons; an abandoned railway tunnel where bats flew. Like a great many people in what was at that time an industrial country, I grew up in a landscape that was interestingly pockmarked with successive eras of exploitation, and all of it so commonplace that beyond a mention of its origins, Watt’s engine or Crompton’s spinning mule, it never found a place in the history books.

Almost all of that Fife landscape has now been buried without ceremony by motorways and housing estates, but equivalents can be found elsewhere, none of them grander and stranger than that part of Kent known as the Hoo peninsula, which lies between the Medway and the Thames and which, if Norman Foster and Boris Johnson have their way, could become the most vital stretch of land in Britain. As the location of Foster’s proposed Thames Hub, the Hoo peninsula will be paved with new railways and docks and the four-runway airport with which Johnson wants to replace overcrowded Heathrow. A new Thames barrier will generate electricity from the currents and tide. Passengers who land there will take ongoing flights and containers ongoing trains.

The scheme is so ambitious – Foster says it requires us “to recapture the foresight and political courage of our 19th-century forebears” – that estimating the cost beyond dozens of billions is pointless. Nevertheless, David Cameron has included it among the options to be considered when the government decides how the UK can continue to provide a hub airport for Europe: pledges to the voters of west London having ruled out Heathrow’s expansion.

If Hoo were chosen, which isn’t unlikely, the question then becomes: what would be destroyed to make way for it? The Royal Society for the Protection of Birds has, as usual, the quickest and simplest answer – the wetland habitats of visiting species – but beyond that the losses are less definable, and not so easy to raise a fuss over. Since Dickens’s day, the creeks and marshes of Hoo have had a bleak form of celebrity as the spot where Pip first met Magwitch, and where prison hulks (Magwitch had just escaped from one of them) could be occasionally glimpsed through the mist on the Medway. In fact, the countryside is prettier and hillier than you expect. On a hot day last week, workers from Poland and Bulgaria were spreading straw across fields of strawberries while the knapped flint of Hoo’s several 13th-century churches shone in the sun. There is also a 14th-century castle owned by Jools Holland and a workaday marina, about as far from Cowes in its social atmosphere as it’s possible to get.

The main impression is of tremendous utility. Power lines sag west towards London to take electricity from the power stations at Kingsnorth and Grain, whose chimneys stand solid against the sky. A diesel rumbles along a single-track freight line with a train of containers from the dock near the peninsula’s tip. And beside this present activity lies the evidence of older industries come and gone. A good guide will point out the hollows in the tidal reaches that were dug out in the 19th century when Medway mud was loaded into sailing barges by labourers called “muddies”, taken to kilns and mixed with chalk to provide the London building boom with cement. What he needn’t point out are the barges, which rot as nicely shaped timbers where the highest tide has left them and are in their way picturesque.

This is also a place of blighted ambition. The railway, for instance, was built for a glamorous purpose it only briefly fulfilled. Trains would take cross-Channel passengers to a pier with a hotel attached called Port Victoria, where they could catch steamers to Belgium and cut a few minutes from journey times offered by rival companies. But only Victoria, the monarch, found much use for it and long before the second world war the Hoo line had become a little-used byway. It last saw a passenger 50 years ago. Port Victoria has been buried under oil pipelines and mud.

Then on Hoo’s northerly coast, there is Allhallows-on-Sea, the Ozymandias of seaside resorts. Developed by the Southern Railway, which built a branch to it in the 1930s, Allhallows was intended to have 5,000 houses, several hotels, a zoo and Britain’s largest swimming pool with a wave-making machine. Then the war intervened. Postwar Londoners failed to return as holidaymakers and the railway closed. Today a big, echoing 1930s pub, the British Pilot, stands at the end of a cul-de-sac, beyond which is a park of holiday chalets and a sea wall with views across the estuary to Southend. Retired couples spend their summers there and winters in Goa or Cyprus, dividing the money released by the sale of their old homes between a chalet in Allhallows and a flat in the sun. “We don’t do cold,” says a tanned woman in her 60s, talking of these annual switches; while another wonders what will happen if her husband dies before her and she, a non-driver, is left alone in this inaccessible place.

Would it matter to the world beyond, other than to birds and ornithologists too, if Hoo became a giant airport and dock, clustered with warehouses, freight yards and car parks? It looks no more than a fitting next step for a peninsula that has for centuries been so ruthlessly used. Really, unless you live there, would you care?

And yet something important will go: wreckage, the traces of a previous era that have no official curator and are therefore delightful to find. High up one of Hoo’s creeks sits a motorised barge, built in 1915 and long defunct, but still cared for by her last skipper, Cliff Pace, who turns the pages of his old logbook smiling at what he and his barge once achieved. “We took 3,237 bags of prunes from Albert Dock to Whitstable … 5,385 cartons of corned beef from the Victoria Dock to Stroud … 163 bundles of pick-axe handles from West India Dock to Otterham Quay.” Even in the 1970s, the estuary was busy with lighters and lightermen – lovely times, says Mr Pace, but all gone. I look at his entries in the logbook and feel, just for a second, the same sensation of discovery that came when a carpet of moss was peeled from a square stone, the beetles scattered and my brother said, “Look…”

When a floating dock the size of a boxcar washed up on a sandy beach in Oregon, beachcombers got excited because it was the largest piece of debris from last year’s tsunami in Japan to show up on the US west coast.

But scientists worried it represented a whole new way for invasive species of seaweed, crabs and other marine organisms to break the earth’s natural barriers and further muck up the area’s marine environments.

And more invasive species could be hitching rides on tsunami debris expected to arrive in the weeks and months to come.

“We know extinctions occur with invasions,” said John Chapman, assistant professor of fisheries and invasive species specialist at Oregon State University’s Hatfield Marine Science Center. “This is like arrows shot into the dark. Some of them could hit a mark.”

Though the global economy has accelerated the process in recent decades by the sheer volume of ships, most from Asia, entering west coast ports, the marine invasion has been going on since 1869, when the transcontinental railroad brought the first shipment of east coast oysters packed in seaweed and mud to San Francisco, said Andrew Cohen, director of the Center for Research on Aquatic Bioinvasions in Richmond, California.

Now, hotspots like San Francisco Bay amount to a “global zoo” of invasive species and perhaps 500 plants and animals from foreign shores have established in US marine waters, said James Carlton, professor of marine sciences at Williams College.

They come mostly from ship hulls and the water ships take on as ballast, but also get dumped into bays from home aquariums.

The costs quickly mount into the untold billions of dollars. Mitten crabs from China eat baby Dungeness crabs that are one of the region’s top commercial fisheries.

Spartina, a ropey seaweed from Europe, chokes commercial oyster beds. Shellfish plug the cooling water intakes of power plants. Kelps and tiny shrimp-like creatures change the food web that fish, marine mammals and even humans depend on.

A 2004 study in the scientific journal Ecological Economics estimated 400 threatened and endangered species in the US are facing extinction because of pressures from invasive species.

It is too early for scientists to know how much Japanese tsunami debris may add to the invasive species already here.

“It may only introduce one thing,” said Cohen of the Aquatic Bioinvasions research center. “But if that thing turns out to be a big problem, we would rather it not happen. There could be an economic impact, an ecological impact, or even a human health impact.”

The dock, torn loose from a fishing port on the northern tip of Japan, was covered with 1.5 tons of seaweed, mussels, barnacles and even a few starfish. Volunteers scraped it all off, buried it above the high water line, and sterilized the top and sides of the dock with torches.

But there was no telling whether they might already have released spores or larvae that could establish a foothold in a bay or estuary as it floated along the coast, Carlton said.

One thing they know is that the bigger the debris, the more likely it has something on it.

Chapman estimated there were hundreds of millions of individual living organisms on the dock when it washed up on Agate Beach outside Newport, Oregon.

But even a small plastic float that washed up on a beach in Alaska carried a live oyster, said Mandy Lindeberg, research scientist at the NOAA Fisheries Auke Bay Laboratories in Juneau, Alaska.

Whether hitchhiking species will survive here depends in large part to randomness, it is thought. Seaweeds probably would not have survived to reproduce in the crashing surf at Agate Beach. It’s the wrong kind of environment. But if they had floated into Yaquina Bay, very similar to their home waters in Japan, they could grow and reproduce.

Lindeberg said, “The only defense for invasive species is early detection. Just like cancer.”

While monitoring is relatively cheap, say $30,000 to watch nearby waters for species from the dock, trying to stop an established invasion is expensive. California spent $7m trying to eradicate a seaweed, she said.

James Morris, a marine ecologist and invasive species specialist at the NOAA National Centers for Coastal Ocean Science, in Beaufort, North Carolina, said the idea a natural disaster like the tsunami could introduce a new avenue for invasive species is intriguing.

“It goes to show you that when it comes to invasive species, there are some things you can work to regulate and control,” he said. “And there are issues like this that come up that open up a whole different realm of possibilities.”