지구에서 인류가 사라지면?

오랜만에 '한겨레21' 사이트에 들어갔다가 눈에 띈 칼럼을 옮겨온다 제목이 '지구에서 인류가 사라지면?'이니까 '로자의 방주'라는 카테고리에 딱 맞는 테마이기도 하다. 필자는 저명한 과학칼럼니스트 김동광씨이다. 예전에 교양과학서 전문번역집단이었던 과학세대의 리더격으로 활동하기도 했는데, 요즘은 저술에도 주력하고 있는 듯하다(그와 함께 과학 저술가로서 기억해둘 만한 이름은 이인식씨이다). 그가 번역한 책들을 검색해보니 내가 갖고 있는 것만 해도 상당한 수에 달한다. 특히 눈에 띄는 건 스티븐 제이 굴드나 스티븐 호킹의 책들이다. 국내에서도 굴드급의 과학 저술가가 나오기를 은근히 기대해본다(강조는 나의 것이다).

한겨레21(06. 10. 24) 지구에서 인류가 사라지면?

진화생물학자들은 생물의 기나긴 진화 과정에서 인류가 차지하는 위치를 밝혀내려고 노력했다. 오랫동안 사람들은 생물의 진화 과정을 인간을 중심으로 생각하는 데 익숙했고, 단세포 생물에서 영장류에 이르는 과정을 일직선이나 계단처럼 매끄럽게 이어지는 단일한 경로로 간주하곤 했다. 인류의 탄생이 지극히 우연적인 과정일 수 있다는 생각이 받아들여지기 시작한 것은 극히 최근의 일이다.

‘생물 총회’가 열리면 인간은 퇴출 대상

그런데 인류가 등장한 세상은 그 이전에 견줘 너무나 크게 변화했다. 이제 인류가 등장하기 이전의 세계를 상상하기 힘들 정도이다.

△ 인간들이 생태계 깊숙이 들어가면서 자연의 자리는 갈수록 좁아지고 있다. 남아프리카공화국 더반의 해안에서 하마 떼가 떠내려가고 있다.(사진/ REUTERS)

그 변화는 대부분 부정적인 방향으로 진행됐다. 일부 추정에 따르면 인류는 자연이 만들어내는 생산성의 약 40%를 무단으로 징발해서 오로지 자신의 목적을 위해 사용한다고 한다. 상당 부분이 먹고사는 기본적인 생활을 유지하기 위한 것이 아니라 이른바 생활 수준의 향상으로 인해 우리가 누리고 있는 문화적 생활을 위해 들어가는 엄청난 양의 에너지와 물자들이 차지하고 있다. 그로 인해 불과 100여 년 전과 비교할 수도 없는 엄청난 양의 쓰레기를 쏟아낸다. 사람들이 도시를 짓고 농장을 만들면서 전유하고 있는 토지는 지구 전체 면적의 3분의 1에 이른다. 울창하던 숲과 지구의 허파이자 생물다양성의 보고로 알려진 열대우림도 이제는 간신히 그 명맥을 유지하고 있을 뿐이다. 오대양의 주요 어장들 중 상당수는 이미 오래전에 어자원이 고갈됐다. 남획과 오염, 그리고 서식지 파괴로 생존 기로에 서 있는 생태계의 다른 구성원들의 입장에서 본다면 인류는 터무니없는 탐욕과 횡포를 부리고 있는 셈이다.

어디 그뿐인가. 그것으로도 모자라서 인류는 자신들끼리의 다툼으로 시시각각 생물권 전체의 존재 가능성을 위태롭게 할 수 있는 도박판을 벌이고 있다. 미국과 소련은 냉전 종식 이후 상당량의 핵무기를 해체하면서 핵으로 인한 절멸 가능성을 크게 줄이는 듯했지만 얼마 전부터 파키스탄 등이 다시 핵무장을 하면서 일촉즉발의 위험 상황을 연출하고 있다. 더구나 북한과 미국이 핵을 둘러싸고 마주 달리는 폭주족처럼 누가 끝까지 버티느냐를 가리는 ‘겁쟁이 경기’를 벌이면서 자칫 한반도가 재앙의 터전이 될 수도 있는 위기 상황이 벌어지고 있다. 핵으로 인한 재앙은 생물권 전체의 파국으로 이어질 수 있다.

이쯤 되면 유엔이 아니라 생물들의 대표가 한자리에 모여서 한반도를 둘러싼 위기 고조를 생물권 전체의 위기로 간주하고 시급한 해결책을 논의하는 생물권 임시 총회가 열릴 법도 하다. 거기에서는 사람과 도롱뇽, 귀신고래, 들국화 그리고 미생물까지 모든 생물종이 똑같은 의결권을 가진다고 가정한다면 이런 긴박한 상황에서 과연 어떤 결정이 내려질까? 진화의 역사에서 가장 늦게 무대에 등장해서 가장 빠른 시간 동안 지배적인 지위를 차지하고 온갖 포악과 전횡을 해대는 골치 아픈 막내둥이를 영원히 퇴출하기로 결론짓지 않겠는가? 사실 그런 결정이 나도 우리로서는 별로 할 말이 없는 셈이다.

인간이 퇴출되어 어느 먼 외계 행성에서 재교육을 받기 위해 강제 송환됐다면, 지구는 어떻게 될까. <뉴사이언티스트> 최근호는 ‘사람이 없는 지구를 상상해보자’라는 특집 기사를 통해 지구에서 어느 날 갑자기 사람이 모두 사라졌을 때 과연 어떤 일이 벌어질 것인지 예측했다(*이 기사는 아래에 옮겨놓았다). 미국 샌타바버라에 있는 국립생태분석종합센터의 보전생물학자인 존 오록은 사람의 자취가 사라지는 순간 지구의 생태계는 즉각적으로 호전되기 시작할 것이라고 말한다. 가장 먼저 나타나는 변화로 밤하늘에서 인공 조명이 사라지고 칠흑같이 어두운 밤이 돌아오게 된다. 지구 전체의 18.7%가 인공 불빛에 오염됐다지만, 웬만한 도시 지역에서는 별을 보기 힘든 지경이다. 한동안 재생 가능 에너지를 이용하는 자동 전등들이 빛을 내더라도, 유지·관리가 되지 않기에 지구의 밤에서 문명의 불빛이 사라지는 것은 시간문제일 뿐이다.

원전의 방사능 누출도 빨리 복원돼

건물들과 도로는 어떻게 될까? 오늘날의 건물은 약 60년, 교량은 120년, 댐은 250년가량 견딜 수 있게 설계된다고 한다. 그러나 이 수치는 누군가가 유지·관리를 하고 벌어진 틈을 메워주는 등 수리와 보수를 해줄 때 지속될 수 있고, 아무도 돌보지 않으면 훨씬 빨리 무너진다. 체르노빌 사고로 사람들이 철수한 프리프야트시가 실례이다. 올해로 체르노빌 사고가 일어난 지 꼭 20년이 된 이 도시의 건물들은 이미 붕괴가 진행 중이다. 그리고 태풍, 홍수 등의 자연재해로 입는 손상이 누적되면서 훨씬 빨리 폐허로 변할 수 있다. 그렇지만 건물이나 도로의 잔해들, 특히 콘크리트와 돌로 된 구조물의 일부는 수천 년 동안 남아 있을 것이다.

△ 지구촌의 약 19%가 인공 불빛에 오염됐다. 거대 건축물이 즐비한 이 지역은 인간이 사라진 상태에션 느리게 복구 될 수 밖에 없다(사진/ 한겨레 이정아 기자)

현재 가동 중인 원자력 발전소는 어떻게 될까? 핵폐기물 관리 전문가인 로드니 윙은 원자력발전소를 유지·관리하던 사람들이 순간적으로 사라지면 냉각수가 증발하고 결굴 원자로가 녹아내리는 참사가 빚어질 것이라고 말한다. 그로 인해 엄청난 양의 방사능이 누출된다. 그렇지만 자연의 복원력은 우리의 예상보다 훨씬 강하다고 한다. 체르노빌의 경우에도 사람들이 철수한 뒤, 몇 년 만에 생태계가 복원되기 시작해서 현재는 다른 지역에서 볼 수 없는 늑대까지 번성하고 있다.

생태계의 복원 속도는 지역에 따라 다르게 나타날 것이다. 온난하고 습윤한 지역은 추운 지역에 비해 복원이 빠르게 진행된다. 사람의 토지 이용을 연구하는 생태학자 브래드 스텔폭스가 캐나다 지역을 대상으로 시뮬레이션한 결과 숲은 약 50년이 지나면 전 지역의 80%를 덮고, 200년 뒤에는 95%를 차지할 것이라고 예측했다. 반면 벼나 밀처럼 단일 품종이 경작되던 지역이 자연 상태로 돌아가기까지는 수세기가 걸릴 것이라고 한다. 보전생태학자들은 일부 지역의 경우 사람이 사라져도 원래 모습으로 돌아갈 수 없다고 말한다.

가축이나 애완동물의 운명은? 일부는 그 선조였던 야생동물의 상태로 돌아가겠지만, 오랜 세월에 걸친 인위 선택의 결과로 스스로 먹이를 찾거나 번식한 능력을 상실한 동물들은 사라지게 될 것이다. 미국 환경보호국의 제이 라이히맨은 “사람이 사라진 들판에 푸들이 떼를 지어 달릴 것이라고 상상할 수 있겠습니까?”라고 묻고는 그럴 가능성은 거의 없다고 말한다. 그러면 유전자조작(GM) 작물들은 어떻게 될까? 일부는 야생종으로 살아남겠지만, 제초제 내성을 갖도록 조작된 식물들은 제초제가 없는 상황에서 경쟁종에 비해 유리할 것이 없기 때문에 사라질 가능성이 높다.

사람이 사라지면 현재 멸종 위기에 처해 있는 동물들이 다시 번성할 수 있을까? 서식지 파괴가 멸종의 큰 원인이기 때문에 상황이 나아질 수 있지만, 일부 종은 이미 개체 수가 더 이상 복원될 수 없는 정도로 줄어들었기 때문에 크게 상황이 나아지기 힘들 전망이다. 그리고 사람들의 보호를 받으면서 명맥을 유지하던 멸종위기종 가운데 상당수는 사람이 사라지면서 더 빨리 자취를 감출 수도 있다. 그 원죄는 인간에게 있지만 사람들을 그리워할 생물종은 이들이 유일한 셈이다.

대기과학자인 수잔 솔로몬은 화석연료 사용 등으로 대기 중에 방출된 이산화탄소가 기후에 미치는 영향은 1천 년 이상 계속되고, 남아 있는 과잉의 이산화탄소를 바다가 모두 흡수하기까지는 2만 년 이상이 걸릴 것이라고 내다보았다. 모든 요소들을 종합해서 지구상에서 인류가 남겼던 자취가 완전히 사라지기까지 걸리는 시간은 대략 10만 년으로 예측된다. 먼 외계의 방문자가 지구를 찾아와 겉으로만 훑어본다면 어떤 문명의 흔적도 찾지 못할 것이다.

기후변화, 핵전쟁보다 무섭다

영원히 계속될 것처럼 생각되는 인류 문명이 사라지는 데는 그리 오랜 시간이 걸리지 않으며, 지배적인 종으로 군림하던 인간이 하루아침에 사라져도 지구와 생물권이 운행되는 데 아무런 문제도 없다. 오히려 피폐해지던 생태계는 즉각 복원 작업을 시작하고, 그 누구도 인간을 그리워하지 않을 것이다. 생태학자들은 핵전쟁보다 더 심각한 위기가 갑작스런 기후변화와 그로 인한 생태계의 교란이라고 주장한다. 요즘 한반도는 들어보지도 못한 가을 더위에 시달리고 있다. 핵위기에 떠밀려 하찮은 현상으로 치부되지만, 어쩌면 우리를 더 낯선 상황으로 몰아갈 전조일지도 모르지 않은가.(김동광 과학저술가·고려대 강사)

 

Imagine Earth without people

Bob Holmes

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 (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.

“18.7 Percentage of Earth's surface affected 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.

“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.

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.

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)

06. 10. 29.