[Texgreen] The post-human planet

[Roger Baker]

http://www.newscientist.com/channel/life/mg19225731.100

Imagine Earth without people
Exclusive from New Scientist Print Edition.
Printed on Thu Dec 28

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The human impact on earth

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.
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?

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 (see  
Graphic). 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.

"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.

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."

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 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 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.

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.

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.

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.

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.

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.

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.

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