The untold – and terrifying – story behind the earthquake that devastated Nepal last Saturday morning begins with something that sounds quite benign. It's the ebb and flow of rainwater in the great river deltas of India and Bangladesh, and the pressure that puts on the grinding plates that make up the surface of the planet.
Recently discovered, that causal factor is seen by a growing body of scientists as further proof that climate change can affect the underlying structure of the Earth.
Because of this understanding, a series of life-threatening "extreme geological events" – earthquakes, volcanoes and tsunamis – is predicted by a group of eminent geologists and geophysicists including University College London's Bill McGuire, professor emeritus of Geophysical and Climate Hazards.
It sounds like a pitch for a Hollywood apocalypse-fest – indeed the movie 2012 featured the Earth's crust collapsing after a rapid heating of the Earth's core. The mechanism here is rather more mundane, though potentially no less devastating.
Evidence from the end of the last Ice Age has already shown that the planet's uneasy web of seismic faults – cracks in the crust like the one that runs along the Himalayas – are very sensitive to the small pressure changes brought by change in the climate. And a sensitive volcano or seismic faultline is a very dangerous one.
The disappearing ice, sea-level rise and floods already forecast for the 21st century are inevitable as the earth warms and weather patterns change – and they will shift the weight on the planet. Professor McGuire calls this process "waking the giant" – something that can be done with just a few gigatonnes of water in the right – or wrong – place.
"These stress or strain variations – just the pressure of a handshake in geological terms – are perfectly capable of triggering a quake if that fault is ready to go," he tells Newsweek.
Any schoolchild geographer knows the underlying cause of earthquakes like that in Nepal: it is the uneasy grinding of the continent-sized plates that float over the Earth's molten core. This process that went into overdrive when the ice sheets started withdrawing 20,000 years ago, destablising the "mantle". The latest event in that endless process came just before midday local time on the 25 April, when the section that holds up India slipped under the Eurasian plate.
The effects were immediate and horrific – buildings collapsed over the region, leaving nearly 4,000 dead and many more injured. As Newsweek went to press, huge aftershocks were causing more chaos.
But the quake was widely predicted. The Himalayas themselves are the collateral damage of the endless shoving match between the two parts of the Asian continent. Earthquakes in Nepal have been charted for at least 700 years, and this one was an almost exact repeat of a 1934 event that killed 16,000 people in Nepal and northern India. Mahatma Gandhi, after visiting the stricken communities, said it was a providential punishment on Indians for failing to do away with the caste system.
What neither Gandhi nor 1930s scientists knew was that the rain that fills the huge rivers that rise in the Himalayas and run down to the Sea of Bengal is a crucial part of this process. Dr Pierre Bettinelli was the scientist who in 2007 first showed how this vast flush of rainwater, second only to that of the Amazon basin, affects earthquakes in the Himalayas. He spoke to Newsweek from a base in the Algerian desert where he is researching the effects of oil-well drilling – another man-made cause of earthquake.
"Imagine a piece of wood on water – that's the Indian plate – push down on it with your foot and you create compression, disturbance, in the water beside it. That you see in the increased number of seismic events at the edge of the plate."
With this insight – which has been generally accepted by scientists in the field – Bettinelli explained the seasonal differences in occurrence of earthquakes in the Himalayas. Quite simply, the coming and going of the weight of the monsoon rains was causing energy to build up at the edge of the plate.
"This effect could certainly have made the Nepal earthquake come sooner," says Professor Roland Burgmann, of the Department of Earth and Planetary Science at the University of California, Berkeley.
Meanwhile, of course, climate change has been shown to be causing enormous and disturbing changes in the size and shape of the South Asian monsoon, while human tampering has played a part in floods.
UCL's Professor Bill McGuire has few doubts that recently discovered effects like this warn of catastrophe. In a recent book, Waking the Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanoes, he ponders the effects of the 100m rise of sea-levels that's threatened should all the remaining ice on the planet melt.
That clearly makes the 150 gigatonnes of extra water that collects in Bangladesh after a heavy monsoon season, tilting the Indian plate, no more than a literal drop in the ocean.
"Across the world," McGuire writes, "as sea levels climb remorselessly, the load-related bending of the crust around the margins of the ocean basins might – in time – act to sufficiently 'unclamp' coastal faults such as California's San Andreas, allowing them to move more easily; at the same time acting to squeeze magma out of susceptible volcanoes that are primed and ready to blow."
Of course, that may be some centuries or even millennia away. Even the worst-case scenarios predicted for the 21st century imagine sea-level rises of no more than five metres. But already McGuire and colleagues have seen the effects of quite small sea-level rise on one of Alaska's faults.
"There's a volcano in Alaska, Pavlov, that only erupts during the autumn and winter. The 10cm or 15cm rise in sea level during the winter months, when low pressure comes over, is enough to bend the crust and squeeze magma out. That's an example of how tiny a change you need," he said.
Meanwhile, geologists modelling the effect of retreating ice sheets in the northern hemisphere predict more volcanic activity as pressure is released on fault lines. McGuire points to three eruptions in five years in Iceland – "You can't say that's statistical proof but … it makes you think."
For Europeans and North Americans, these volcanoes are far off. But the collapse of coastal ones would be likely to trigger tsunamis, causing devastation all around the North Atlantic.
Some of McGuire's colleagues believe he overstates the earthquake risk of sea-level rise and changing rainfall. There is just not enough data yet to prove the hypothesis, says Professor Burgmann. But he is convinced Maguire is right when he talks about volcanic eruptions.
"Ice unloading at the end of the ice ages produced a flurry of volcanic eruptions. That makes sense to me – it's very true that if you take pressure off a magmatic system that can activate eruptions. There's solid evidence of that in Iceland."
What can we do? McGuire thinks there's little, other than mapping the "coiled spring" that is the world's seismic faults with an eye to where climate and sea-level change may trigger events. Then, you can only prepare people for the earth's grumblings, from California to the Canary Islands to Nepal.
"There are geological systems all around the planet with unstable volcanoes that are susceptible: when it comes to risk, I'm afraid there's a very, very long list."