It's late July and the hottest day of the year. You're dressed in black and stuck between the samba band and a line of heavily armoured riot police opposite the Houses of Parliament. Your mask keeps slipping off, sweat's trickling down your back and your last mouthful of water has just been used to wash tear gas from the eyes of a friend. Does London always have to be like this? Will you ever be able to escape the noise, the fumes and the overcrowded streets?
"Hell is a city much like London," wrote Shelley, one hundred and eighty years ago. But it hasn't always been this way. Let your mind wander ten times further back, one thousand eight hundred years before Shelley's time, when the vista in Parliament Square was very different. Where you are now a sweet stream is flowing down from the hills of Hampstead. Just ahead are the reedy shores of the meandering Thames. On the right, where Westminster Abbey will stand, is Thorney Island, named after its wild abundance of brambles. Behind are peaceful meadows fringed with willow.
Even a thousand years ago, when Edward the Confessor was considering Thorney Island as the site for his great abbey, this was a tranquil, rustic place. "A delightful spot, surrounded with fertile lands and green fields", one monk wrote in his life of Edward.
Could the clock be turned back and London once more be a sylvan paradise? If you've ever been filled with a secret wish to see the traffic vanish, the buildings tumble down and London's hills and valleys once more filled with flowers and trees and birdsong, you are not alone. Towards the close of the last century, naturalist Richard Jefferies was so disgusted by the capital's filth that he wrote After London, in which Londoners are mercifully extinct and the city rapidly reverts to marsh.
But what would really happen if London went back to nature? Let's suppose that this weekend Londoners flee after a Chernobyl style nuclear accident. Or that tonight's news reveals that a genetically-engineered virus has been set loose, as in the recent film 28 Days Later. Or that Londoners suddenly all get so sick of city life that they take Shelley's advice to flee "to the wild wood and the downs". How long before abandoned London turned back to a rural paradise? How would it look in 5, 50 or 500 years? How would nature take control?
Most cities already have the biological potential - microorganisms, soil nematodes, earthworms, up through the larger vertebrates - to rapidly begin the natural processes that humans interfere with just by being around," says John Hadidian, director of the Urban Wildlife Protection Program of The Humane Society of the United States.
The initial changes are familiar to every city dweller who goes into battle against invading weeds and shrubs. Within the first year, dandelions and other weeds begin growing in the gutters and emerge from the cracks caused by frost and flooding in concrete, paving slabs and walls.
But they only exploit existing weaknesses. Shrubs like buddleia are far more aggressive. Its roots are powerful enough to penetrate bricks and mortar to find moisture, says botanist Anthony Bradshaw, formerly of the University of Liverpool. Buddleia grows fast, and its light seeds are easily dispersed by the wind. Brought to Britain from the Himalayas to adorn Victorian gardens, buddleia is already everywhere in London, poised to rid the city of its concrete and brick.
The litter from these plants, plus the thin layer of mosses and lichens, gradually settles as a fine layer of soil on top of the concrete and tarmac, allowing other plants to spread. Within five years, roads, pavements, parking places, and the great squares of the city are carpeted with weeds and a rich turf of clover.
Nitrogen-fixing plants like clover flourish first because the soil contains much sand and detritus from disintegrating brick and concrete and is still poor in nutrients. For the same reason, alder, which can also fix nitrogen, would be one of the first trees to establish itself. These plants gradually make room for less adaptable species. An understorey of grasses and shrubs gradually spreads over the city. As the soil layer builds up, deeper-rooting plants take hold. Trees start to grow and their roots smash through what's left of the pavement and tarmac that has sealed the earth from the sun while humans held sway.
The city of Pripyat, near Chernobyl, shows how quickly nature can take its revenge. Pripyat was the most modern of Soviet towns, built with little expense spared for nuclear workers at Chernobyl. Now, says Donald Bruce, a former nuclear inspector who visited Pripyat in April, the concrete paving stones in one of the city's squares have been smashed and, in places, pushed up almost a metre by tree roots, as if a giant earthquake had struck.
The pace of change speeds up after London is hit by fire and flood. Early autumn around five years after abandonment is a likely time for fire. The streets have built up a shallow litter of grasses and fallen leaves. A dry spell and a lightning strike sets the city ablaze. Fire guts the buildings that still dominate the London landscape. As the houses burn and roofs come crashing down, nutrients are released from their timbers and from leaf litter, providing the fertiliser to speed London's return to its past. "The plants would really charge in, taking advantage of the nitrogen available from burnt material," says Hadidian.
Before the fire, London is merely derelict. In the next five years, plants are poised to really take over as another powerful force of nature - flood - begins to hit the city.
With humans gone, floods are inevitable. On 12th January 1996, the Thames flood barrier was closed for three consecutive tides to hold back damaging surges generated by a combination of a major storm and spring tides. Without the flood barrier, says Mervyn Littlewood, a tidal engineer at Hydraulics Research, Wallingford, a surge tide could charge into central London and damage the embankments that protect it. "Then in a comparatively short time you would get regular inundation from spring tides." Even without a storm, says Littlewood, neglect ensures that the river's earth embankments are slowly eroded or damaged by subsidence. Several times a year, and then more frequently, land along the banks of the Thames floods and gradually reverts to marshland.
At the close of the first decade after abandonment, the wildlife really begins to come into its own. Familiar weeds, like rose bay willowherb that grew in places endlessly disrupted by humans, yield to the true flowers of nature. Orchids once more bloom in central London. The river, railways and canals, which provide broad avenues linking central London to the surrounding countryside, help the old inhabitants spread back into town.
The advance parties are already here. The Camley Street Natural Park beside the Regent's Canal in north London is in an area that was once part of the Middlesex forest. Andy Littlewood, who manages the park for the London Wildlife Trust, says that it contains five common spotted orchids whose seeds probably arrived via the canal from some distant source. It also contains a large pond whose fish population is sustained by fry from the canal. Should London ever be evacuated, Littlewood believes the park itself would act as a seed store as well as an animal pool for colonisation further afield.
With the plants come butterflies, bees and other insects and invertebrates. Larger insects, birds and finally mammals follow as the food chain rebuilds itself. Soon, even the centre of the city is full of butterflies as well as kestrels, foxes, hedgehogs, bats and - where the floods leave pools and marsh in the area closest to the river - toads, frogs and newts. Birch trees flourish in the developing soil.
But not all abandoned London's wildlife is entirely natural. The two grey wolves in London Zoo have upped and bred with pet dogs gone wild. Bernt Jones of Uppsala University in Sweden, says that larger dogs like German shepherds could well survive in the wild and prove suitable mates for the wolves. Smaller breeds of dog would simply provide their lunch.
Cats do better. A 1993 study of the stomach contents and behaviour of feral cats showed that 75 percent of their diet came from food put out for them, while the rest came from scavenging. But other studies have shown that urban cats kill a lot of prey which they do not eat - mostly birds, followed by small mammals such as rats and mice. According to Hadidian, it would be much easier for cats to "rediscover their wild genes" than for dogs.
Many animals that we think of as 'wildlife' vanish in the first decade, as they depend on the food and shelter provided by humans. Trafalgar Square is bereft of its vast flocks of pigeons and house mice disappear altogether. Sewer rats fare no better. "The reason we've got rats is because we've got people," says Dave Cowan of the Ministry for Agriculture, Fisheries and Food Central Science Laboratory in Slough. Voles, fieldmice and other rural species would return to fill the niche, says Oliver Gilbert, a reader in landscape architecture at the University of Sheffield.
But as birch saplings encroach on the Mall, and much of central London is full of burnt out buildings, overrun with creepers and with shrubs growing out from ledges and cracks in their walls, the besieged city still holds out against the onslaught of nature. The concrete and steel office blocks in the City financial district and out east to Canary Wharf are immensely strong. Although the streets have turned green, the concrete buildings merely look neglected. Windows are broken, or have fallen from their frames, and the concrete is stained by the smoke of fires. But their structures are in as good shape as ever - or even better.
Without people, there'd be no pollution from vehicles or industry, says Tim Burstein of the Department of Materials Science at the University of Cambridge, and rain would be less acidic than it is now. That helps preserve concrete, which is alkaline. The condition of the buildings is good news for cliff-nesting birds. At the end of the first decade, the complex of pipes and stairwells that adorn the Lloyd's building supports a huge population of kestrels, sparrowhawks and even a few rough-legged buzzards from Scandinavia.
Some other great steel structures - the bridges of London - would not find life after humans so congenial. Blackfriars bridge and some of the other metal bridges that span the Thames need regular repainting and thirty years would be enough for them to fall into disrepair, says Littlewood.
The massive masonry piers hold up for a long time, but fifty years after abandonment the bridges are collapsing. "You would end up with a series of weirs down the river where bits and pieces had fallen in," says Littlewood. Salmon returned to the Thames in the 1970s, after it was cleaned up, but have not yet spawned there. In the new pollution-free London, salmon leap their artificial weirs on the way to the spawning grounds upriver.
Twenty to thirty years after the humans leave, birch woodland would rapidly fill the open spaces, says David Goode, director of the London Ecology Unit. Elsewhere an impenetrable understorey of elder thicket - perhaps rising to five metres in height - dominates. As the birch matures and some trees fall, creating even more ground litter, sycamore and maple move in.
"The whole built townscape would change quite dramatically in something like 30 years," Goode says. Ivy, carried by birds from the Victorian cemeteries where it proliferates, is growing down from the roofs of skyscrapers, giving Canary Wharf and Centre Point a spreading green cap. Ivy also clambers up from the ground, probably reaching a maximum height of 40 metres, says Gilbert.
Wooden constructions would be the first to vanish completely, says Hari Srinivas of the Department of Social Engineering at Tokyo Institute of Technology, followed by the materials that glue a building together - partitions, insulation - materials that insects destroy by nesting in them. House sparrows, pigeons and other birds that nest inside city buildings are usually accompanied by insects that live in their nests. When the birds leave, the insects colonise carpets, fur and plants.
The remains of houses built of brick and stone are still clearly visible amid the growing forest. Steel pipes and copper cables are rusted but still recognisable. But as trees grow and take root among the rubble, more and more walls come crashing down. Leaf litter begins to pile up over the brick and rubble, softening its hard angles. Several centuries more are needed before they are reduced to mysterious hummocks beneath the turf and the giant towers of concrete, still dominating the skyline above the trees, are brought down.
London's brick buildings have been gutted by fire, undermined by water, battered by storms and infiltrated by plant roots and insects. Those nearest the rivers - the Thames and its tributaries, including London's many underground rivers such as the Fleet, which flows alongside Farringdon Road - are the first to go, says Heather Viles, a lecturer in physical geography at the University of Oxford.
Now concrete and steel structures, too, are succumbing. After 200 years many buildings are crumbling, if not on the brink of collapse. While the concrete remained alkaline, the steel bars that reinforce it held fast against corrosion. But carbon dioxide dissolved in rain has gradually carbonated the surface of the concrete and edged its way in, while acid from decaying organic matter in the ground has infiltrated concrete foundations.
Once the steel corrodes, the end is swift. The corrosion products take up about three times the volume of the steel itself, says Burstein, so as the steel rusts, it expands until its concrete covering crumbles off. "This is probably the major failure you would see in buildings like Canary Wharf and in the City, as that steel reinforcement begins to corrode," explains Alan Poole of the Geomaterials Unit at Queen Mary and Westfield College, London.
Add to that the effects of rising groundwater. Already, says Viles, London has problems largely because the exodus of industry from the city means that less groundwater is being pumped out for commercial use. As the water rises even further, large buildings with concrete foundations in the clay sink and tilt.
The foundations of Canary Wharf are a huge mass of concrete and reinforcing steel. Two or three hundred years after abandonment, as the clay becomes waterlogged and the steel corrodes, the whole tower begins to tilt. In the mid 24th Century, the Great Leaning Tower of Canary Wharf is a major tourist attraction. The great grandchildren, many times removed, of those who abandoned London now take ecocruises along the river, its banks lined with elder and willow and the flood plain beyond with poplar and ash. in the distance, a huge oak forest spreads over the low hills towards Hampstead Heath. The low-lying Isle of Dogs has reverted entirely to marsh. Rearing up above the reeds and mud is the vast, ivy-clad tower, tilting at a crazy angle, a monument to the hideous city that was.
The river dominates the landscape. From above, a circling buzzard sees an utterly changed London. After 500 years without human intervention, the Thames is nothing like it is now. "The river through central London is narrower than it should naturally be, because of reclamations at the side," says Mervyn Littlewood. "If it was allowed to look after itself, it would probably be shallower and wider." Eventually it reaches the width of pre-Roman London when Southwark, at the south side of London Bridge, was marsh and mudflats. Much of the city reverts to flood-plain forest.
Adventurous hikers now follow the woodland trails through central London. But the naturalists among them can detect the work of humans. London has never reverted completely to its true origins. Humans brought too many plants and animals from abroad. "Since man arrived with his trade and interest in horticulture and crop plants, the rate of immigration has speeded up enormously and species from all over the world have arrived," Gilbert says.
The forest of Greater London contains many of the exotic species introduced by humans that could adapt and survive without them, such as the hybrid "super grasses" bred for disease resistance and persistence. In the drier areas, the oak forest is interspersed with foreign species such as sycamore, Norway maple, Turkey oak and some conifers.
The birds are not all native. Ring-necked parakeets, which originally came from Asia, are doing well. Wolves, or wolf-German shepherd hybrids, roam the forests preying on roe, muntjac and sika deer and feral pigs descended from the stocks at London Zoo and city farms. In the suburbs, the descendants of sweet chestnuts that once lined the streets are flourishing.
Some time, perhaps five hundred years after abandonment, the Great Leaning Tower of Canary Wharf finally crashes down. "Eventually it would collapse, probably one stormy night," says Poole. It's unlikely that it, or any modern building would last as long as some great stone medieval constructions. "Look at Ely Cathedral - 1,000 years old, slight lean, but remarkably stable," says Burstein. "The superstructure might be shaky but the substructure is surprisingly resilient." In a flood plain like London's, inundation of foundations and natural soil movements would leave very few buildings standing after 1,000 years. By that time, both the oak and the flood plain forests would be mature and the rubble of Canary Wharf would have sunk into the marsh. London is no longer a blot on the landscape..