El mundo sin humanos

La revista Scientific American publica una entrevista con el Alan Weisman que acaba de publicar un libro titulado The World Without Us que explora la pregunta:

¿Qué le pasaría a este hermoso planeta si desaparecieran los seres humanos de su faz?

En el libro no especula qué podría causar el fin de nuestra especie, sino que describe cómo cambiaría la tierra si abruptamente desapareciéramos.

El monumento más perdurable de nuestra estadía milenaria seria el PLASTICO.

Todo lo demás: autopistas, rascacielos, monumentos, libros, pinturas, autos, instrumentos musicales, cables, computadoras, ropas, fábricas, iglesias, teléfonos, barcos, naves espaciales, hospitales, llaves, cuchillos… se desintegraría en relativamente poco tiempo.

Y acá estamos aun, consumiendo el mundo…

Dos libros de ficción que también exploran este escenario, pero con los humanos haciendo un patético/heroico ultimo intento de sobrevivir en un mundo pos apocalíptico:

The Road de Cormac McCarthy -Y- Plop de Rafael Pinedo.

Ver también: Imagine Earth Without People, (New Scientist, October 12, 2006)

Futuratrónics cross-index: Escenarios: Un mundo sin humanos

Foto: fuente

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Andrés Hax dijo...

June 17, 2007

An Earth Without People

A new way to examine humanity's impact on the environment is to consider how the world would fare if all the people disappeared

By Steve Mirsky

TIMELINE: The Fall of New York City
VIDEO: The Earth Without Humans

Editors’ Introduction
It’s a common fantasy to imagine that you’re the last person left alive on earth. But what if all human beings were suddenly whisked off the planet? That premise is the starting point for The World without Us, a new book by science writer Alan Weisman, an associate professor of journalism at the University of Arizona. In this extended thought experiment, Weisman does not specify exactly what finishes off Homo sapiens; instead he simply assumes the abrupt disappearance of our species and projects the sequence of events that would most likely occur in the years, decades and centuries afterward.

According to Weisman, large parts of our physical infrastructure would begin to crumble almost immediately. Without street cleaners and road crews, our grand boulevards and superhighways would start to crack and buckle in a matter of months. Over the following decades many houses and office buildings would collapse, but some ordinary items would resist decay for an extraordinarily long time. Stainless-steel pots, for example, could last for millennia, especially if they were buried in the weed-covered mounds that used to be our kitchens. And certain common plastics might remain intact for hundreds of thousands of years; they would not break down until microbes evolved the ability to consume them.

Scientific American editor Steve Mirsky recently interviewed Weisman to find out why he wrote the book and what lessons can be drawn from his research. Some excerpts from that interview appear on the following pages.

The Interviewee

Alan Weisman is author of five books, including the forthcoming The World without Us (St. Martin’s Press, 2007). His work has appeared in Harpers, the New York Times Magazine, the Los Angeles Times Magazine, Discover, the Atlantic Monthly, Condé Nast Traveler, Orion and Mother Jones. Weisman has been heard on National Public Radio and Public Radio International and is a senior producer at Homelands Productions, a journalism collective that produces independent public radio documentary series. He teaches international journalism at the University of Arizona.

Q&A With Alan Weisman

If human beings were to disappear tomorrow, the magnificent skyline of Manhattan would not long survive them. Weisman describes how the concrete jungle of New York City would revert to a real forest.

“What would happen to all of our stuff if we weren’t here anymore? Could nature wipe out all of our traces? Are there some things that we’ve made that are indestructible or indelible? Could nature, for example, take New York City back to the forest that was there when Henry Hudson first saw it in 1609?

“I had a fascinating time talking to engineers and maintenance people in New York City about what it takes to hold off nature. I discovered that our huge, imposing, overwhelming infrastructures that seem so monumental and indestructible are actually these fairly fragile concepts that continue to function and exist thanks to a few human beings on whom all of us really depend. The name ‘Manhattan’ comes from an Indian term referring to hills. It used to be a very hilly island. Of course, the region was eventually flattened to have a grid of streets imposed on it. Around those hills there used to flow about 40 different streams, and there were numerous springs all over Manhattan island. What happened to all that water? There’s still just as much rainfall as ever on Manhattan, but the water has now been suppressed. It’s underground. Some of it runs through the sewage system, but a sewage system is never as efficient as nature in wicking away water. So there is a lot of groundwater rushing around underneath, trying to get out. Even on a clear, sunny day, the people who keep the subway going have to pump 13 million gallons of water away. Otherwise the tunnels will start to flood.


“There are places in Manhattan where they’re constantly fighting rising underground rivers that are corroding the tracks. You stand in these pump rooms, and you see an enormous amount of water gushing in. And down there in a little box are these pumps, pumping it away. So, say human beings disappeared tomorrow. One of the first things that would happen is that the power would go off. A lot of our power comes out of nuclear or coal-fired plants that have automatic fail-safe switches to make sure that they don’t go out of control if no humans are monitoring their systems. Once the power goes off, the pumps stop working. Once the pumps stop working, the subways start filling with water. Within 48 hours you’re going to have a lot of flooding in New York City. Some of this would be visible on the surface. You might have some sewers overflowing. Those sewers would very quickly become clogged with debris—in the beginning the innumerable plastic bags that are blowing around the city and later, if nobody is trimming the hedges in the parks, you’re going to have leaf litter clogging up the sewers.

“But what would be happening underground? Corrosion. Just think of the subway lines below Lexington Avenue. You stand there waiting for the train, and there are all these steel columns that are holding up the roof, which is really the street. These things would start to corrode and, eventually, to collapse. After a while the streets would begin cratering, which could happen within just a couple of decades. And pretty soon, some of the streets would revert to the surface rivers that we used to have in Manhattan before we built all of this stuff.

“Many of the buildings in Manhattan are anchored to bedrock. But even if they have steel beam foundations, these structures were not designed to be waterlogged all the time. So eventually buildings would start to topple and fall. And we’re bound to have some more hurricanes hitting the East Coast as climate change gives us more extreme weather. When a building would fall, it would take down a couple of others as it went, creating a clearing. Into those clearings would blow seeds from plants, and those seeds would establish themselves in the cracks in the pavement. They would already be rooting in leaf litter anyhow, but the addition of lime from powdered concrete would create a less acidic environment for various species. A city would start to develop its own little ecosystem. Every spring when the temperature would be hovering on one side or the other of freezing, new cracks would appear. Water would go down into the cracks and freeze. The cracks would widen, and seeds would blow in there. It would happen very quickly.”

How would the earth’s ecosystems change if human beings were out of the picture? Weisman says we can get a glimpse of this hypothetical world by looking at primeval pockets where humanity’s footprint has been lightest.

“To see how the world would look if humans were gone, I began going to abandoned places, places that people had left for different reasons. One of them is the last fragment of primeval forest in Europe. It’s like what you see in your mind’s eye when you’re a kid and someone is reading Grimm’s fairy tales to you: a dark, brooding forest with wolves howling and tons of moss hanging off the trees. And there is such a place. It still exists on the border between Poland and Belarus. It was a game reserve that was set aside in the 1300s by a Lithuanian duke who later became king of Poland. A series of Polish kings and then Russian czars kept it as their own private hunting ground. There was very little human impact. After World War II it became a national park. You go in there and you see these enormous trees. It doesn’t feel strange. It almost feels right. Like something feels complete in there. You see oaks and ashes nearly 150 feet tall and 10 feet in diameter, with bark furrows so deep that woodpeckers stuff pinecones in them. Besides wolves and elk, the forest is home to the last remaining wild herd of Bison bonasus, the native European buffalo.

“I also went to the Korean DMZ, the demilitarized zone. Here you have this little stretch of land—it’s about 150 miles long and 2.5 miles wide—that has two of the world’s biggest armies facing off against each other. And in between the armies is an inadvertent wildlife preserve. You see species that might be extinct if it weren’t for this one little piece of land. Sometimes you’ll hear the soldiers screaming at one another through loudspeakers or flashing their propaganda back and forth, and in the middle of all this tension you’ll see the flocks of cranes that winter there.

“But to really understand a world without humans, I realized I would have to learn what the world was like before humans evolved. So I went to Africa, the place where humans arose and the only continent where there are still huge animals roaming around. We used to have huge animals on all the other continents and on many of the islands. We had enormous creatures in North and South America—giant sloths that were even bigger than the mammoths; beavers the size of bears. It’s controversial as to what actually wiped them out, but a lot of indications point the finger at us. The extinctions on each landmass seemed to coincide with the arrival of humans. But Africa is the place where human beings and animals evolved together, and the animals there learned strategies to avoid our predation. Without humans, North America would probably become a giant deer habitat in the near term. As forests would become reestablished across the continent, eventually—in evolutionary time—larger herbivores would evolve to take advantage of all the nutrients locked up in woody species. Larger predators would evolve accordingly.”

Thinking about an earth without humans can have practical benefits. Weisman explains that his approach can shed new light on environmental problems.

“I’m not suggesting that we have to worry about human beings suddenly disappearing tomorrow, some alien death ray taking us all away. On the contrary, what I’m finding is that this way of looking at our planet—by theoretically just removing us—turns out to be so fascinating that it kind of disarms people’s fears or the terrible wave of depression that can engulf us when we read about the environmental problems that we have created and the possible disasters we may be facing in the future. Because frankly, whenever we read about those things, our concern is: Oh, my God, are we going to die? Is this going to be the end? My book eliminates that concern right at the beginning by saying the end has already taken place. For whatever reason, human beings are gone, and now we get to sit back and see what happens in our absence. It’s a delicious little way of reducing all the fear and anxiety. And looking at what would happen in our absence is another way of looking at, well, what goes on in our presence.

“For example, think about how long it would take to wipe out some of the things we have created. Some of our more formidable inventions have a longevity that we can’t even predict yet, like some of the persistent organic pollutants that began as pesticides or industrial chemicals. Or some of our plastics, which have an enormous role in our lives and an enormous presence in the environment. And nearly all of these things weren’t even here until after World War II. You begin to think there’s probably no way that we are going to have any kind of positive outcome, that we are looking at an overwhelming tide of geologic proportions that the human race has loosed on the earth. I raise one possibility toward the end of the book that humans can continue to be part of the ecosystem in a way that is much more in balance with the rest of the planet.

“It’s something that I approach by first looking at not just the horrible things that we have created that are so frightening—such as our radioactivity and pollutants, some of which may be around until the end of the planet—but also some of the beautiful things that we have done. I raise the question, Wouldn’t it be a sad loss if humanity was extirpated from the planet? What about our greatest acts of art and expression? Our most beautiful sculpture? Our finest architecture? Will there be any signs of us at all that would indicate that we were here at one point? This is the second reaction that I always get from people. At first they think, This world would be beautiful without us. But then they think, Wouldn’t it be sad not to have us here? And I don’t think it’s necessary for us to all disappear for the earth to come back to a healthier state.”

More to Explore
Plastics and the Environment. Edited by Anthony Andrady. John Wiley & Sons, 2003.

Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America. Paul S. Martin. University of California Press, 2005.

Extinction: How Life on Earth Nearly Ended 250 Million Years Ago. Douglas H. Erwin. Princeton University Press, 2006.

The Revenge of Gaia. James ­Lovelock. Allen Lane/Penguin Books, 2006.

© 1996-2007 Scientific American, Inc. All rights reserved.
Reproduction in whole or in part without permission is prohibited.

Andrés Hax dijo...

October 12, 2006
Imagine Earth without people

Humans are undoubtedly the most dominant species the Earth has ever known. In just a few thousand years we have swallowed up more than a third of the planet's land for our cities, farmland and pastures. By some estimates, we now commandeer 40 per cent of all its productivity. And we're leaving quite a mess behind: ploughed-up prairies, razed forests, drained aquifers, nuclear waste, chemical pollution, invasive species, mass extinctions and now the looming spectre of climate change. If they could, the other species we share Earth with would surely vote us off the planet.

“15,589 Number of species threatened with extinction”

Now just suppose they got their wish. Imagine that all the people on Earth - all 6.5 billion of us and counting - could be spirited away tomorrow, transported to a re-education camp in a far-off galaxy. (Let's not invoke the mother of all plagues to wipe us out, if only to avoid complications from all the corpses). Left once more to its own devices, Nature would begin to reclaim the planet, as fields and pastures reverted to prairies and forest, the air and water cleansed themselves of pollutants, and roads and cities crumbled back to dust.

"The sad truth is, once the humans get out of the picture, the outlook starts to get a lot better," says John Orrock, a conservation biologist at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California. But would the footprint of humanity ever fade away completely, or have we so altered the Earth that even a million years from now a visitor would know that an industrial society once ruled the planet?

“9.7: Average eco-footprint of a US citizen, in hectares”

If tomorrow dawns without humans, even from orbit the change will be evident almost immediately, as the blaze of artificial light that brightens the night begins to wink out. Indeed, there are few better ways to grasp just how utterly we dominate the surface of the Earth than to look at the distribution of artificial illumination.


By some estimates, 85 per cent of the night sky above the European Union is light-polluted; in the US it is 62 per cent and in Japan 98.5 per cent. In some countries, including Germany, Austria, Belgium and the Netherlands, there is no longer any night sky untainted by light pollution.

“18.7 Percentage of Earth's surface affected by light pollution” PHOTO: NASA

"Pretty quickly - 24, maybe 48 hours - you'd start to see blackouts because of the lack of fuel added to power stations," says Gordon Masterton, president of the UK's Institution of Civil Engineers in London. Renewable sources such as wind turbines and solar will keep a few automatic lights burning, but lack of maintenance of the distribution grid will scuttle these in weeks or months. The loss of electricity will also quickly silence water pumps, sewage treatment plants and all the other machinery of modern society.

The same lack of maintenance will spell an early demise for buildings, roads, bridges and other structures. Though modern buildings are typically engineered to last 60 years, bridges 120 years and dams 250, these lifespans assume someone will keep them clean, fix minor leaks and correct problems with foundations. Without people to do these seemingly minor chores, things go downhill quickly.

Abandoned village near Pripyat

The best illustration of this is the city of Pripyat near Chernobyl in Ukraine, which was abandoned after the nuclear disaster 20 years ago and remains deserted. "From a distance, you would still believe that Pripyat is a living city, but the buildings are slowly decaying," says Ronald Chesser, an environmental biologist at Texas Tech University in Lubbock who has worked extensively in the exclusion zone around Chernobyl. "The most pervasive thing you see are plants whose root systems get into the concrete and behind the bricks and into doorframes and so forth, and are rapidly breaking up the structure. You wouldn't think, as you walk around your house every day, that we have a big impact on keeping that from happening, but clearly we do. It's really sobering to see how the plant community invades every nook and cranny of a city."

With no one to make repairs, every storm, flood and frosty night gnaws away at abandoned buildings, and within a few decades roofs will begin to fall in and buildings collapse. This has already begun to happen in Pripyat. Wood-framed houses and other smaller structures, which are built to laxer standards, will be the first to go. Next down may be the glassy, soaring structures that tend to win acclaim these days. "The elegant suspension bridges, the lightweight forms, these are the kinds of structures that would be more vulnerable," says Masterton. "There's less reserve of strength built into the design, unlike solid masonry buildings and those using arches and vaults."

Old cars abandoned and untouched in Nicosia since 1974

But even though buildings will crumble, their ruins - especially those made of stone or concrete - are likely to last thousands of years. "We still have records of civilisations that are 3000 years old," notes Masterton. "For many thousands of years there would still be some signs of the civilisations that we created. It's going to take a long time for a concrete road to disappear. It might be severely crumbling in many places, but it'll take a long time to become invisible."

The Varosha quarter of Famagusta, closed off since 1974, is reverting to nature

The lack of maintenance will have especially dramatic effects at the 430 or so nuclear power plants now operating worldwide. Nuclear waste already consigned to long-term storage in air-cooled metal and concrete casks should be fine, since the containers are designed to survive thousands of years of neglect, by which time their radioactivity - mostly in the form of caesium-137 and strontium-90 - will have dropped a thousandfold, says Rodney Ewing, a geologist at the University of Michigan who specialises in radioactive waste management. Active reactors will not fare so well. As cooling water evaporates or leaks away, reactor cores are likely to catch fire or melt down, releasing large amounts of radiation. The effects of such releases, however, may be less dire than most people suppose.

The area around Chernobyl has revealed just how fast nature can bounce back. "I really expected to see a nuclear desert there," says Chesser. "I was quite surprised. When you enter into the exclusion zone, it's a very thriving ecosystem."

The rotting Alpha Napthol unit in Union Carbide's factory in Bhopal, abandoned since 1984 PHOTO: DAN SINHA

The first few years after people evacuated the zone, rats and house mice flourished, and packs of feral dogs roamed the area despite efforts to exterminate them. But the heyday of these vermin proved to be short-lived, and already the native fauna has begun to take over. Wild boar are 10 to 15 times as common within the Chernobyl exclusion zone as outside it, and big predators are making a spectacular comeback. "I've never seen a wolf in the Ukraine outside the exclusion zone. I've seen many of them inside," says Chesser.

The same should be true for most other ecosystems once people disappear, though recovery rates will vary. Warmer, moister regions, where ecosystem processes tend to run more quickly in any case, will bounce back more quickly than cooler, more arid ones. Not surprisingly, areas still rich in native species will recover faster than more severely altered systems. In the boreal forests of northern Alberta, Canada, for example, human impact mostly consists of access roads, pipelines, andother narrow strips cut through the forest. In the absence of human activity, the forest will close over 80 per cent of these within 50 years, and all but 5 per cent within 200, according to simulations by Brad Stelfox, an independent land-use ecologist based in Bragg Creek, Alberta.

Nature is claiming back the abandoned Union Carbide plant in Bhopal PHOTO: PAUL BULLIVANT

In contrast, places where native forests have been replaced by plantations of a single tree species may take several generations of trees - several centuries - to work their way back to a natural state. The vast expanses of rice, wheat and maize that cover the world's grain belts may also take quite some time to revert to mostly native species.

At the extreme, some ecosystems may never return to the way they were before humans interfered, because they have become locked into a new "stable state" that resists returning to the original. In Hawaii, for example, introduced grasses now generate frequent wildfires that would prevent native forests from re-establishing themselves even if given free rein, says David Wilcove, a conservation biologist at Princeton University.

Feral descendants of domestic animals and plants, too, are likely to become permanent additions in many ecosystems, just as wild horses and feral pigs already have in some places. Highly domesticated species such as cattle, dogs and wheat, the products of centuries of artificial selection and inbreeding, will probably evolve back towards hardier, less specialised forms through random breeding. "If man disappears tomorrow, do you expect to see herds of poodles roaming the plains?" asks Chesser. Almost certainly not - but hardy mongrels will probably do just fine. Even cattle and other livestock, bred for meat or milk rather than hardiness, are likely to persist, though in much fewer numbers than today.

“3.3bn: Global population of cattle, sheep and goats”

What about genetically modified crops? In August, Jay Reichman and colleagues at the US Environmental Protection Agency's labs in Corvallis, Oregon, reported that a GM version of a perennial called creeping bentgrass had established itself in the wild after escaping from an experimental plot in Oregon. Like most GM crops, however, the bentgrass is engineered to be resistant to a pesticide, which comes at a metabolic cost to the organism, so in the absence of spraying it will be at a disadvantage and will probably die out too.

Doomed cheetahs, many species would die out regardless, because their habitat is already destroyed

Nor will our absence mean a reprieve for every species teetering on the brink of extinction. Biologists estimate that habitat loss is pivotal in about 85 per cent of cases where US species become endangered, so most such species will benefit once habitats begin to rebound. However, species in the direst straits may have already passed some critical threshold below which they lack the genetic diversity or the ecological critical mass they need to recover. These "dead species walking" - cheetahs and California condors, for example - are likely to slip away regardless.

“784 Number of species that have gone extinct in the wild since 1500 AD”

Other causes of species becoming endangered may be harder to reverse than habitat loss. For example, about half of all endangered species are in trouble at least partly because of predation or competition from invasive introduced species. Some of these introduced species - house sparrows, for example, which are native to Eurasia but now dominate many cities in North America - will dwindle away once the gardens and bird feeders of suburban civilisation vanish. Others though, such as rabbits in Australia and cheat grass in the American west, do not need human help and will likely be around for the long haul and continue to edge out imperilled native species.

“388 Number of species listed on the invasive species database”

Ironically, a few endangered species - those charismatic enough to have attracted serious help from conservationists - will actually fare worse with people no longer around to protect them. Kirtland's warbler - one of the rarest birds in North America, once down to just a few hundred birds - suffers not only because of habitat loss near its Great Lakes breeding grounds but also thanks to brown-headed cowbirds, which lay their eggs in the warblers' nests and trick them into raising cowbird chicks instead of their own. Thanks to an aggressive programme to trap cowbirds, warbler numbers have rebounded, but once people disappear, the warblers could be in trouble, says Wilcove.

On the whole, though, a humanless Earth will likely be a safer place for threatened biodiversity. "I would expect the number of species that benefit to significantly exceed the number that suffer, at least globally," Wilcove says.

On the rebound

In the oceans, too, fish populations will gradually recover from drastic overfishing. The last time fishing more or less stopped - during the second world war, when few fishing vessels ventured far from port - cod populations in the North Sea skyrocketed. Today, however, populations of cod and other economically important fish have slumped much further than they did in the 1930s, and recovery may take significantly longer than five or so years.

Overfishing has done for the cod

The problem is that there are now so few cod and other large predatory fish that they can no longer keep populations of smaller fish such as gurnards in check. Instead, the smaller fish turn the tables and outcompete or eat tiny juvenile cod, thus keeping their erstwhile predators in check. The problem will only get worse in the first few years after fishing ceases, as populations of smaller, faster-breeding fish flourish like weeds in an abandoned field. Eventually, though, in the absence of fishing, enough large predators will reach maturity to restore the normal balance. Such a transition might take anywhere from a few years to a few decades, says Daniel Pauly, a fisheries biologist at the University of British Columbia in Vancouver.

With trawlers no longer churning up nutrients from the ocean floor, near-shore ecosystems will return to a relatively nutrient-poor state. This will be most apparent as a drop in the frequency of harmful algal blooms such as the red tides that often plague coastal areas today. Meanwhile, the tall, graceful corals and other bottom-dwelling organisms on deep-water reefs will gradually begin to regrow, restoring complex three-dimensional structure to ocean-floor habitats that are now largely flattened, featureless wastelands.

Coral reefs will recover their beauty and thrive

Long before any of this, however - in fact, the instant humans vanish from the Earth - pollutants will cease spewing from automobile tailpipes and the smokestacks and waste outlets of our factories. What happens next will depend on the chemistry of each particular pollutant. A few, such as oxides of nitrogen and sulphur and ozone (the ground-level pollutant, not the protective layer high in the stratosphere), will wash out of the atmosphere in a matter of a few weeks. Others, such as chlorofluorocarbons, dioxins and the pesticide DDT, take longer to break down. Some will last a few decades.

The excess nitrates and phosphates that can turn lakes and rivers into algae-choked soups will also clear away within a few decades, at least for surface waters. A little excess nitrate may persist for much longer within groundwater, where it is less subject to microbial conversion into atmospheric nitrogen. "Groundwater is the long-term memory in the system," says Kenneth Potter, a hydrologist at the University of Wisconsin at Madison.

Carbon dioxide, the biggest worry in today's world because of its leading role in global warming, will have a more complex fate. Most of the CO2 emitted from burning fossil fuels is eventually absorbed into the ocean. This happens relatively quickly for surface waters - just a few decades - but the ocean depths will take about a thousand years to soak up their full share. Even when that equilibrium has been reached, though, about 15 per cent of the CO2 from burning fossil fuels will remain in the atmosphere, leaving its concentration at about 300 parts per million compared with pre-industrial levels of 280 ppm. "There will be CO2 left in the atmosphere, continuing to influence the climate, more than 1000 years after humans stop emitting it," says Susan Solomon, an atmospheric chemist with the US National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. Eventually calcium ions released from sea-bottom sediments will allow the sea to mop up the remaining excess over the next 20, 000 years or so.

Even if CO2 emissions stop tomorrow, though, global warming will continue for another century, boosting average temperatures by a further few tenths of a degree. Atmospheric scientists call this "committed warming", and it happens because the oceans take so long to warm up compared with the atmosphere. In essence, the oceans are acting as a giant air conditioner, keeping the atmosphere cooler than it would otherwise be for the present level of CO2. Most policy-makers fail to take this committed warming into account, says Gerald Meehl, a climate modeller at the National Center for Atmospheric Research, also in Boulder. "They think if it gets bad enough we'll just put the brakes on, but we can't just stop and expect everything to be OK, because we're already committed to this warming."

That extra warming we have already ordered lends some uncertainty to the fate of another important greenhouse gas, methane, which produces about 20 per cent of our current global warming. Methane's chemical lifetime in the atmosphere is only about 10 years, so its concentration could rapidly return to pre-industrial levels if emissions cease. The wild card, though, is that there are massive reserves of methane in the form of methane hydrates on the sea floor and frozen into permafrost. Further temperature rises may destabilise these reserves and dump much of the methane into the atmosphere. "We may stop emitting methane ourselves, but we may already have triggered climate change to the point where methane may be released through other processes that we have no control over," says Pieter Tans, an atmospheric scientist at NOAA in Boulder.

No one knows how close the Earth is to that threshold. "We don't notice it yet in our global measurement network, but there is local evidence that there is some destabilisation going on of permafrost soils, and methane is being released," says Tans. Solomon, on the other hand, sees little evidence that a sharp global threshold is near.

All things considered, it will only take a few tens of thousands of years at most before almost every trace of our present dominance has vanished completely. Alien visitors coming to Earth 100,000 years hence will find no obvious signs that an advanced civilisation ever lived here.

Yet if the aliens had good enough scientific tools they could still find a few hints of our presence. For a start, the fossil record would show a mass extinction centred on the present day, including the sudden disappearance of large mammals across North America at the end of the last ice age. A little digging might also turn up intriguing signs of a long-lost intelligent civilisation, such as dense concentrations of skeletons of a large bipedal ape, clearly deliberately buried, some with gold teeth or grave goods such as jewellery.

And if the visitors chanced across one of today's landfills, they might still find fragments of glass and plastic - and maybe even paper - to bear witness to our presence. "I would virtually guarantee that there would be some," says William Rathje, an archaeologist at Stanford University in California who has excavated many landfills. "The preservation of things is really pretty amazing. We think of artefacts as being so impermanent, but in certain cases things are going to last a long time."

Ocean sediment cores will show a brief period during which massive amounts of heavy metals such as mercury were deposited, a relic of our fleeting industrial society. The same sediment band will also show a concentration of radioactive isotopes left by reactor meltdowns after our disappearance. The atmosphere will bear traces of a few gases that don't occur in nature, especially perfluorocarbons such as CF4, which have a half-life of tens of thousands of years. Finally a brief, century-long pulse of radio waves will forever radiate out across the galaxy and beyond, proof - for anything that cares and is able to listen - that we once had something to say and a way to say it.

But these will be flimsy souvenirs, almost pathetic reminders of a civilisation that once thought itself the pinnacle of achievement. Within a few million years, erosion and possibly another ice age or two will have obliterated most of even these faint traces. If another intelligent species ever evolves on the Earth - and that is by no means certain, given how long life flourished before we came along - it may well have no inkling that we were ever here save for a few peculiar fossils and ossified relics. The humbling - and perversely comforting - reality is that the Earth will forget us remarkably quickly.

From issue 2573 of New Scientist magazine, 12 October 2006, page 36-41

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