It's happened before: An asteroid colliding with the Earth could extinguish much of the planet's life.
Credit: Photolibrary
OUR DAYS ARE NUMBERED. We're not likely to go extinct any time soon, but we can be pretty certain we're not going to last forever. So the number of days we have left might end up being very large – human civilisation could even expand beyond Earth into the wider cosmos and last for billions of years – or the number might be alarmingly small.
There are countless calamities that could befall us and bring humanity, or at the very least, modern civilisation to its knees. We know it's happened before. Our remarkable lack of genetic diversity suggests Homo sapiens have already clung to the precipice of extinction at least once in our distant past, when our numbers dwindled to perhaps as few as 1,000 breeding pairs.
The last several millennia have also seen many civilisations rise, make a big splash and then fall. From small island communities to mighty empires spanning half the globe, none have been immune to calamity and ultimate collapse. So what makes us think our civilisation is any different?
Jared Diamond, a geographer at the University of California in Los Angeles and author of Collapse: How Societies Choose to Fail or Succeed, has intensively studied the decline and fall of civilisations.
He thinks our modern society is significantly more robust than any that have come before, primarily because we wield the power of information. With the accumulated knowledge of thousands of years of human history, we can better understand the causes of societal collapse in the hopes of avoiding them. Yet along with our great strength comes a crucial weakness – one that could be fatal.
"Today the world no longer faces just the circumscribed risk of another Easter Island society or Maya homeland collapsing in isolation, without affecting the rest of the world," says Diamond. "Instead, societies today are so interconnected that the risk we face is of a worldwide decline." So even a local calamity might drag the rest of the world down with it.
Some of the threats we face are of natural origin, others are of our own folly or design. But few are inevitable, and it should be with great interest that we work to prevent a preventable catastrophe.
CLIMATE CATASTROPHE
WE'VE ALL HEARD THE HORROR STORIES of future climate catastrophes: rising sea levels, rampant droughts, thousands of species under threat of extinction. And we've all heard the sceptics warning us not to get carried away by the panicked cries of climate alarmists.
Well, it might actually be time for alarm. Because recent findings by climate scientists such as Pieter Tans, from the U.S. National Oceanic and Atmospheric Administration, suggest we're pumping carbon dioxide into the atmosphere faster than we thought. "When they started measurements in the 1960s average growth was 0.7 ppm [parts per million] a year," he says. "But in the last five years it's been at 2 ppm, which is more or less proportional to the rate at which we are burning fossil fuels."
As a result, our temperature trajectory is looking even more dramatic than the worst-case scenarios posited in 2007 by the United Nations Intergovernmental Panel on Climate Change (IPCC), a scientific body made up of thousands of senior scientists who evaluate the risk of climate change caused by human activity. This makes it even more likely that we'll see warming of more than a few degrees this century.
This alone would be costly to our society, but there's an even more troubling scenario that has climate scientists like Tans sweating. And that's if a feedback mechanism kicks in that drives temperatures even higher.
One such mechanism is the prospect of widespread melting of Arctic ice, which would rapidly release vast amounts of stored carbon and methane into the atmosphere. This could increase temperatures by as much as 10°C by the end of the century – and no amount of cuts in fossil fuel use or emissions can stop the process once it starts.
The thing is, no one knows at what point this feedback mechanism might kick in. It might happen after 5°C of warming, or it might start after just 3°C … or even less.
Beyond this tipping point, the impact on us would be dramatic, says Mark Lynas, environmental campaigner and author of Six Degrees, winner of the prestigious Royal Society Science Book Prize. "If positive feedbacks send the warming process spinning out of control, then the consequences for life on Earth as a whole will be catastrophic," he says.
"Human society depends on a relatively intact biosphere for its continued survival. It would not take even a worst-case scenario warming to give human civilisation a knockout punch."
Stephen Schneider, a climatologist at Stanford University in California, agrees that an increase in global temperatures of even 3 to 5°C would have a massive impact on civilisation. "We'd see a dramatic increase in sea level; an increase in the intensity of hurricanes; more massive forest fires in warmer climates; health problems associated with heat; movement of diseases into new regions; and widespread water shortages." Agriculture is another area of concern.
"We could see major reductions in food production, perhaps tens of per cent of yields, and this wouldn't be good in a world of nine billion people," he says.
There is hope, however. Unlike other catastrophes in this article, we actually have a chance to prevent climate change. "If global emissions can peak and fall within the next decade then we still have a reasonable chance of avoiding the worst," says Lynas.
Schneider agrees, but he's far from optimistic. "I think it's likely we'll do way too little and wait way too long, and we'll see catastrophic events that will kick us into action," he says. "Then we'll try to reverse it but we'll end up overrunning CO2 by a considerable level."
Ultimately, climate change will play out slowly over decades or even centuries, but there are other threats to our civilisation that might strike sooner.
ASTEROID IMPACT
IT'S HAPPENED BEFORE. And when it did some 65 million years ago, it wiped three quarters of all species, including the dinosaurs, from the face of the Earth. And if it happens again, those plucky little mammals that made it through might not be so lucky.
Perhaps there aren't many 10-km asteroids around, like the behemoth that ended the age of the dinosaurs, but it might not take such a monster to be lethal to us today says Duncan Steel, one of the world's leading experts on asteroid threats, who works for the defence research company QinetiQ in Canberra, Australia.
"A one- to two kilometre strike would be a global catastrophe," he says. "It would totally obliterate an area about the size of a European country. But the main concern is not the impact itself, but the knock-on effects."
These include hurling some 100 cubic kilometres of pulverised rock into the atmosphere, forming a pall of dust that would block out radiation from the Sun. The dust could take a long time to settle, and might cool the entire planet by several degrees.
"We'd lose a few years of agriculture across the globe, which means a lot of people will die of starvation," says Steel. "The result would be the death of around 25 per cent of humankind." That's 1.5 billion people; a body count that could be high enough to critically destabilise civilisation as a whole.
The good news is that kilometre-sized bodies strike Earth about once every few hundred thousand years, so their average frequency is low. When and where they strike, though, is completely unpredictable: it could be in the next million years or next Tuesday.
There's also the threat of smaller objects, which drop into our neighbourhood with much greater frequency. One of these is thought to be responsible for the devastation at Tunguska in Siberia in 1908, when an object estimated at between 35 to 40 m in diameter exploded several kilometres above the surface with a force equivalent to one of our most powerful hydrogen bombs.
The result: around 2,000 km2 of forest was flattened. Luckily, Siberia is quite remote; it could have hit the sea and triggered a tsunami or struck closer to a population centre.
A similar object exploding over a populated city would kill millions, says Steel. Plus, we don't know very much about them, or how often they drop by. "Such objects could hit the Earth once every 100 or 500 years, we just don't know," he says. This is because most efforts at spotting nearby asteroids in the inner Solar System focus on objects significantly larger, so the smaller rocks slip under the radar.
Another problem that Steel points out is that many of the most dangerous objects might not originate from our immediate neighbourhood. They may be visitors from the outer reaches of the Solar System – and they might not be predictable. "Currently we're only looking for short period objects, but if the object enters the inner Solar System only once every 50 years, or once every 1,000 years, we're unlikely to find it with a 10-year search."
And what if we do spot one coming? "If you had 23 years' warning, and you could change the object's speed by just 1 cm/sec, that gives you a change in direction equivalent to the radius of the Earth – and then some," says Steel. "But if we only had a few years' warning we may not be able to get to it in time, even if we had a rocket on the launch pad ready to go." So should we worry? Yes, says Steel. But he's personally more worried about supervolcanoes.
SUPERVOLCANO
ON 18 MAY 1980, Mount St Helens in Washington State, USA, exploded with terrible force. It lifted more than a cubic kilometre of hot magma, rock and ash into the atmosphere and left a crater more than two kilometres wide. Fifty-seven people died as a direct result of the eruption, and its cost to the U.S. economy was more than US$1 billion. In the scheme of things, that was a small eruption.
Consider the eruption at Lake Toba in Indonesia. When it went off, it ejected more than 2,800 cubic kilometres of material – that's more than 2,000 times what Mount St Helens managed – and it left a crater which today is a lake 100 km long and 30 km wide. Good thing it happened around 70,000 years ago.
But it wasn't so good for our ancestors, says Stanley Ambrose, an archaeologist at the University of Illinois in the USA. He thinks the Lake Toba eruption could account for the severe genetic bottleneck that humans went through at around the same time. He suggests our ancestors were reduced in numbers to just a few thousand individuals. That's about as close to extinction as any species ever wants to get.
Could such an eruption occur again? "It is more likely that the Earth will next experience a super-eruption than an impact from a large meteorite greater than one kilometre in diameter," says Stephen Self, geologist at the Open University in Britain.
Jacob Lowenstern, a vulcanologist at the U.S. Geological Survey, agrees. "The likelihood of another super-eruption is about one in 200 over the next 1,000 years." Lowenstern is based at the picturesque Yellowstone National Park in Wyoming, on the Pacific west of the USA, which just happens to be a supervolcano itself.
Many of the day wanderers and picnickers don't realise they're walking atop a slumbering volcanic titan that has erupted several times in the past, occasionally rivalling Lake Toba in ferocity.
What would happen if it went off today? Utter devastation, says Lowenstern, particularly since the thousand-odd cubic kilometres of hot magma ejected during the eruption has to land somewhere.
"About a third would remain close to the source as pyroclastic flows, clouds of pumice and ash, burying anything in its path. Another third would be distributed distally within 50 km of Yellowstone. And the remainder would fall in progressively lesser amounts up to about 500 km from the source," he says.
But again, that might not be the worst of it. "You'd get a lot of particles in the atmosphere and that would cool the Earth significantly," he says. "It would have a massive impact on biomass and particularly agriculture and crops over a five- to ten-year period globally."
Self thinks the knock-on effects of such an eruption could be devastating. "Major disruption of services that society depends upon can be expected for periods of months to years after the next very large explosive eruption," he says. "And the cost to global financial markets will be high and sustained."
All this is made worse by the simple fact there's no known way to prevent a supervolcano eruption, even if vulcanologists like Lowenstern think we can predict the eruptions days or weeks before they occur. But given all this, Lowenstern isn't overly worried. "This is not something that is very likely," he says. "I worry more about pandemics and nuclear war than volcanic eruptions."
PANDEMIC
OFTEN THE MOST PROFLIGATE KILLERS are the least visible. While asteroids and supervolcanoes are spectacularly destructive, there's a potentially more deadly threat that's far less flashy: the humble flu.
Throughout the 20th century, over 100 million people lost their lives in three flu pandemics alone. That's about the same number of deaths as occurred in both World Wars combined. And there are many other diseases that could be even more lethal still.
Could such a pandemic outbreak occur again? In 2002 it almost did, says Gregory Härtl, from Epidemic and Pandemic Alert and Response at the World Health Organisation (WHO) in Geneva, Switzerland. "SARS was a pandemic that almost happened," he says.
SARS, or Severe Acute Respiratory Syndrome, struck at the end of 2002 and caused more than 770 deaths with a mortality rate of nearly 10 per cent. By comparison, the influenza's mortality rate is usually less than one per cent. Had local governments and organisations like the WHO not responded immediately to quarantine and blunt the SARS outbreak, the outcome could have been grave indeed. "We got lucky with SARS," says Härtl.
We may have dodged a bullet with SARS, but the gun is still loaded and pointed in our direction. For this reason, many experts in infectious diseases believe it's not a matter of if we'll have another pandemic outbreak, but when. "My personal opinion is that there's a 100 per cent chance of another influenza pandemic in the next 100 years," says Martin Meltzer, senior health economist at the Centres for Disease Control and Prevention in Atlanta, USA.
What no one knowsis whether the next outbreak will be cut short like SARS, or whether it'll unfold into a global pandemic, perhaps even more devastating than the pandemic of 1918, which claimed more than 40 million lives. The problem is, even with our advanced ability to detect and treat disease and communicate rapidly, our globalised nature means that an outbreak could spread around the world in a matter of days.
Also, increased population densities and more intermingling with animals means there are more opportunities for diseases to make the leap from animals to humans. "We have more people, more pigs and more poultry since the last pandemic. Therefore, in terms of the mixing needed to produce a new strain, we probably face a greater risk than in 1968 [when more than 40 million died]," says Meltzer.
So what would be the impact of a new pandemic? The WHO estimates a 'best case' scenario as likely to cause up to 7.4 million deaths, while a worst-case scenario could be even worse than the 1918 pandemic. And the economic cost? According to the World Bank, a major outbreak leading to 70 million deaths would cost around five per cent of global GDP, or up to US$2,000 billion.
The good news is that while another outbreak is virtually inevitable, there's a lot we can do to prepare or even prevent it, says Härtl. As with the SARS outbreak, we know that containment can work, as long as there's broad cooperation between governments and international organisations.
DOOMSDAY DEVICE
ALL OF THE SCENARIOS we've considered so far have to some extent been 'natural' disasters (even if humanity is ultimately responsible for climate change). But is the greatest threat to civilisation natural? Or is it us?
"The biggest threats to humanity arise from humanity," says Nick Bostrom, a philosopher at Britain's University of Oxford and cofounder of the World Transhumanist Association. "Our species has survived volcanic eruptions, meteoric impacts, and other natural hazards for tens of thousands of years. It seems unlikely that any of these old risks should exterminate us in the near future. By contrast, human civilisation is introducing many novel phenomena into the world, ranging from nuclear weapons to designer pathogens to high-energy particle colliders."
We need only reflect on the notion that the closest humanity has recently come to annihilation – with the possible exception of the Lake Toba supervolcano eruption 70,000 years ago – was during the Cold War, when the U.S. and former Soviet Union engaged in games of nuclear brinkmanship, particularly over Cuba in 1962.
And as technology advances, allowing the development of even more destructive weapons, and time makes it ever more likely they'll fall into even less responsible hands, the 21st century could prove even more dangerous than the 20th.
Diane Bretherton, associate professor at the Australian Centre for Peace and Conflict Studies at the University of Queensland in Brisbane, has studied the psychology of human conflict and has seen how potentially benign scenarios can end up escalating into violence.
"It can happen all too easily, I'm afraid," she says. "Once it's started, it's difficult to back out." She suggests we have a tendency to create the conditions for violence even when it's the very thing we're trying to avoid. One example is the use of threats to keep others in control, whether by bouncers in a nightclub, or one government threatening sanctions against another. Yet such threats are often countered with an aggressive response, and if no-one backs down, violence can ensue.
But it's not all bad news. Bretherton believes that despite our proclivities for conflict, we're hardwired towards cooperation, especially when we have the opportunity to understand our potential opponents better. "One of the things that helps check violent behaviour is having a number of different perspectives, so increased communication does help."
SO SHOULD WE BE OPTIMISTIC about our future? Bostrom is, although with a proviso: "If we survive intact for 500 years, then we might well survive for billions of years," he says. The trick is making it through 500 years in a world full of increasingly powerful nuclear, biological and chemical weapons, as well as relatively new threats like runaway nanotech or rogue artificial intelligences.
If there's anything to be learned from our lightning tour of civilisational calamity, it's that complacency could be our worst enemy. Our civilisation, as robust as it is, is not unassailable, and it's only through a sober acknowledgement of the threats that face us, and a determination to confront them, that we might avoid the fate of the many societies that have come, and gone, before us.
Tim Dean is a science writer based in Sydney, Australia.
