Geological disasters might influence climate, for instance when volcanic debris blots out the sun. But climate cannot disrupt geology. Right? Well, actually no, says a British geologist Bill McGuire, in a troubling new book,Waking The Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanoes.
There is, McGuire argues, growing evidence to incriminate changing climate in the planet's most destructive geological events. Melting ice sheets and changes in sea level can, he maintains, set off the largest earthquakes and volcanic eruptions. Indeed, thanks to climate change, a human hand may already be at work. Potentially, McGuire's argument adds a whole new dimension to why we should be worried about climate change.
The most solid evidence for climatic influence on geology comes from the end of the last ice age, around 12,000 years ago, says McGuire, who is a volcanologist and professor of geophysical and climate hazards at University College London. Analysis of volcanic deposits, published in the past decade by several authors, has found that this period of rapid climate change, when ice sheets retreated from much of the planet, coincided with a sudden outburst of geological activity. The incidence of volcanic eruptions in Iceland increased around 50-fold for about 1,500 years, before settling back to previous levels.
What happened? McGuire makes the case that during the long preceding glaciation, the weight of ice some two kilometers thick over Iceland maintained high pressures underground that kept magma at the root of volcanoes solid and suppressed eruptions. But as the ice melted, the huge weight was released and the land surface lifted, sometimes by hundreds of meters. This reduced the pressure below. He cites Freysteinn Sigmundsson at the Nordic Volcanological Center at the University of Iceland in Reykjavik, who says: "Reduction of pressure enabled mantle rocks to melt, creating a zone of magma upwelling underneath Iceland." Magma production increased 30-fold – that magma, the argument goes, burst out in a spectacular epidemic of volcanic eruptions.
Similar, though less pronounced, surges in volcanic activity occurred at that time across much of the planet, wherever large ice sheets or small tropical glaciers melted, says Hugh Tuffen, a volcanologist at the University of Lancaster in England. From the Eifel mountains of Germany to the Chilean Andes, and from California to Kamchatka, volcanoes were awakened, says McGuire, who chaired a conference on climate change and geology at the Royal Society in London in 2009.
While the planet's volcanoes have been relatively peaceful during the long stable climate since then, McGuire warns that we need to watch out as the world starts to warm once more. "Volcanoes can be incredibly sensitive to tiny changes to their external environment, constantly teetering on the edge of stability," he says.
Defrosting the planet's cold regions has for some years been implicated in a range of "natural" disasters. The rapid melting of glaciers creates dangerous lakes of meltwater, perched high in the valleys of the Himalayas and Andes.
Thawed soil unleashes landslides. Christian Huggel, a geographer at the University of Zurich, Switzerland, found in a study of mountain slope failures in Alaska, New Zealand and the European Alps that "all the failures were preceded by unusually warm periods," lasting days or weeks. There are also concerns that warming will release the potent greenhouse gas methane from permafrost and continental shelves, creating a dangerous feedback to global warming itself.
But McGuire is talking about changes deep in the Earth's crust, caused by the lifting or imposing of the weight of ice and ocean water at the surface. And the concern relates to earthquakes as well as volcanoes. For earthquakes, the evidence points to changes in sea levels, as well as the melting of ice.
Many geological fault lines are on a knife-edge, awaiting any nudge to send their seismic mayhem to the surface, says McGuire. His University College London colleague Serge Guillas has found that, over the past 40 years, El Nino cycles in the tropical Pacific Ocean have triggered a regular seismic response as the pressure of water has changed with short-term sea level fluctuations. There are more earthquakes in the eastern Pacific in the months after the cycle lowers sea levels in the area by a few centimeters, which flexes the plates beneath.
A 2009 study co-authored by Selwyn Sacks, a geophysicist at the Carnegie Institution in Washington D.C., concluded that something as seemingly insignificant as low atmospheric pressure in the heart of typhoons was sufficient to trigger slow earthquakes in strata east of Taiwan.
The evidence from volcanoes of short-term influences is even more startling. According to Oxford University geologists Ben Mason and David Pyle the planet has volcanic seasons. In the Northern Hemisphere, eruptions happen most frequently between November and April. The reason, they say, is shifts in water round the globe. This movement of water slightly squashes or releasing the land beneath, at times pushing magma to the surface rather like toothpaste in a tube.