Cosmic impact, volcanic erruptions, pinball planet or catastrophic climate change - they're all real possibilities.
Cosmic Impact
Long dismissed as a medieval fantasy, the possibility of celestial devastation is now regarded as a real threat. The change in perception came in the 1980s, following evidence of the impact of an asteroid around 10km across near present-day Mexico 65 million years ago – the time of the extinction of the dinosaurs. The threat of cosmic impacts remains. In June 1908, hundreds of square kilometres of north-eastern Siberia near the Tunguska River were devastated by the impact of a 50m-wide asteroid.
Then in 1989, a 300m-wide asteroid, 4581 Asclepius, avoided a collision with Earth by less than six hours. Had it struck, the devastation would have been equivalent to the detonation of over 1000 Hiroshima atom bombs. And if, as is most likely, it had landed in an ocean the resulting tsunami could have been large enough to engulf entire coastal cities.
This led to the launch of NASA’s Spaceguard Survey in 1998, charged with discovering and tracking 90 per cent of ‘Near-Earth Objects’, (NEOs) above 1km in diameter – big enough to cause global destruction. Yet according to a US National Academies of Sciences report published in May, the survey is still not complete – and last year an NEO over 2km across was found, indicating that planet killers could still be lurking out there unseen.
According to the NAS report, NEOs found on a collision course might be nudged onto different paths using nuclear explosions. But such manoeuvres take decades to bring about – and aren’t possible for NEOs more than a few kilometres across. Against these, the report concludes, “there is at present no feasible defence”. In the meantime, the near-misses continue: last January a 10m-wide meteor codenamed 2010 AL30 came within 122,000km of Earth. Its would have packed a punch equivalent to several times that of the atomic bomb dropped on Hiroshima.
Volcanic Eruptions
Of all the causes of future global upheaval, none is more plausible, proven and unavoidable than volcanic eruptions. Powered by the radioactive decay of elements trapped inside the Earth since its formation 4500 million years ago, volcanoes have repeatedly reshaped our planet. In the process, they have played a key role in mass extinctions, including the ‘Great Dying’ of 251 million years ago – the biggest catastrophe ever suffered by life on Earth. But unlike any other source of global upheaval, volcanoes have had devastating effects within the very recent past and could do so again at any time.
Volcanoes wreak destruction in several different ways, their only common factor being that nothing can be done to stop them. The most obvious is the direct explosion: when Mount Tambora erupted in 1815, it did so with the violence of a million Hiroshima atomic bombs, the blast effects causing over 90,000 deaths in the surrounding area.
But scientists now recognise that such titanic explosions also have far more widespread and long-lasting effects. As recently as 1991, the eruption of Mount Pinatubo in the Philippines – the most violent in living memory – blasted an estimated 10 billion tonnes of debris into the atmosphere. Over the next 15 months, sunlight levels over the whole planet fell by around three per cent, causing global temperatures to fall by around 0.5°C.
Then there’s the effect of the gases released by eruptions. The huge loss of marine life that took place during the Great Dying is now thought to be principally the result of the acidification of the oceans caused by carbon dioxide from volcanoes mixing with seawater. Sulphur dioxide is another threat: Mount Pinatubo injected 20 million tonnes of this acidic gas into the stratosphere, where it attacked the ozone layer that protects us from the Sun’s carcinogenic UV radiation.
The nightmare scenario is where a series of such mega-eruptions occur in succession. It’s happened before: the Great Dying has been linked to over 100,000 years of volcanic activity in what is now Siberia. Whether the seething radioactive cauldron beneath our feet can still muster such an apocalyptic outburst only time will tell.
Planet Pinball
In the 1950s, a Russian psychiatrist named Immanuel Velikovsky hit the best-seller lists with Worlds in Collision, describing a time when the planets careered around the Solar System like snooker balls.
Scientists dismissed Velikovsky as a crackpot.
But over 30 years after his death, Velikovsky’s “crazy” ideas no longer seem so outrageous. Computer simulations have shown that the apparently clockwork-like procession of the planets around the Sun can suffer outbreaks of cosmic chaos.
The culprit is so-called gravitational resonance, in which planets receive regular jolts from their neighbours. Over time, these add up to produce dramatic changes in the shape and size of the orbits of planets. Last year, Jacques Laskar and Mickael Gastineau of the Paris Observatory revealed just how dramatic this could be. Using a network of supercomputers, they simulated the future of the Solar System, and found that resonance effects could lead to collisions between the inner planets – with the Earth at risk of being hit by Mars, Venus and Mercury. Fortunately, the risk is far less than one per cent over the next five billion years. Which is probably just as well, as humans could only avoid such a catastrophe by leaving the Earth in search of a new home
Catastrophic Climate Change
In 1988, Prime Minister Margaret Thatcher told leading British scientists of her fear that by producing ever more greenhouse gases like carbon dioxide “we have unwittingly begun a massive experiment with the system of this planet”. Two decades later, the threat from global warming appears to have receded, with the rise in global temperature having levelled off. But scientists warn that with nations pumping ever more greenhouse gases into the atmosphere, the hiatus is likely to be temporary. According to a recent study by climate experts at the UK Met Office, global temperatures are likely to resume their upward path after this year, with at least half of the next 10 years being even warmer than 1998, currently the hottest on record.
What impact that will have on our planet remains one of the most contentious questions of our time. At the centre of the debate are so-called positive feedback mechanisms, which have the power to turn small changes into climatic upheaval that takes place too fast for society to cope with. For example, as the temperature of the Earth increases, water vapour evaporates from the oceans at an increasing rate – entering the atmosphere and trapping more of the Sun’s heat, thus driving global temperatures still higher.
In 2005, environmentalists highlighted research suggesting that increasing global warming could thaw out vast frozen stores of methane – a potent greenhouse gas – lurking underground in Siberia, triggering a positive feedback effect. According to Friends of the Earth the result “could unleash runaway global warming that will be beyond our control”.
While there is little evidence for any immediate threat of such a catastrophe, earlier this year a team led by Natalia Shakhova of the University of Alaska reported finding methane seeping into the seas around Siberia – and called for research to understand its effects.
Follow Science Focus onTwitter,Facebook, Instagramand Flipboard