Thursday, September 24, 2009

Climate change may trigger earthquakes and volcanoes

FAR from being the benign figure of mythology, Mother Earth is short-tempered and volatile. So sensitive in fact, that even slight changes in weather and climate can rip the planet's crust apart, unleashing the furious might of volcanic eruptions, earthquakes and landslides.

That's the conclusion of the researchers who got together last week in London at the conference on Climate Forcing of Geological and Geomorphological Hazards. It suggests climate change could tip the planet's delicate balance and unleash a host of geological disasters. What's more, even our attempts to stall global warming could trigger a catastrophic event (see "Bury the carbon").

Evidence of a link between climate and the rumblings of the crust has been around for years, but only now is it becoming clear just how sensitive rock can be to the air, ice and water above. "You don't need huge changes to trigger responses from the crust," says Bill McGuire of University College London (UCL), who organised the meeting. "The changes can be tiny."

Among the various influences on the Earth's crust, from changes in weather to fluctuations in ice cover, the oceans are emerging as a particularly fine controller. Simon Day of the University of Oxford, McGuire and Serge Guillas, also at UCL, have shown how subtle changes in sea level may affect the seismicity of the East Pacific Rise, one of the fastest-spreading plate boundaries.

The researchers focused on the Easter microplate - the tectonic plate that lies beneath the ocean off the coast of Easter Island - because it is relatively isolated from other faults. This makes it easier to distinguish changes in the plate caused by climate systems from those triggered by regional rumbles. Since 1973, the arrival of El Niño every few years has correlated with a greater frequency of underwater quakes between magnitude 4 and 6.

The team is confident that the two are linked. El Niño raises the local sea level by a few tens of centimetres, and they believe the extra water weight may increase the pressure of fluids in the pores of the rock beneath the seabed. This might be enough to counteract the frictional force that holds the slabs of rock in place, making it easier for faults to slip. "The changes in sea level are tiny," says Day. "A small additional perturbation can have a substantial effect."

Small ocean changes can also influence volcanic eruptions, says David Pyle of the University of Oxford. His study of eruptions over the past 300 years with Ben Mason of the University of Cambridge and colleagues reveals that volcanism varies with the seasons. The team found that there are around 20 per cent more eruptions worldwide during the northern hemisphere's winter than the summer (Journal of Geophysical ResearchDOI: 10.1029/2002JB002293). The reason may be that global sea level drops slightly during the northern hemisphere's winter. Because there is more land in the northern hemisphere, more water is locked up as ice and snow on land than during the southern hemisphere's winter.

The vast majority of the world's most active volcanoes are within a few tens of kilometres of the coast (see map). This suggests the seasonal removal of some of the ocean's weight at continental margins as sea level drops could be triggering eruptions around the world, says Pyle.

The suggestion that some volcanoes erupt when sea levels drop does not necessarily mean that sea levels rising under climate change will suppress volcanism. In Alaska, Mount Pavlof erupts more often in the winter months, and previous research by Steve McNutt of the Alaska Volcano Observatory puts this down to a local sea level rise of 30 centimetres every winter due to low air pressure and high storm winds. Pavlof's location means that the extra weight of the adjacent sea could be squeezing magma towards the surface.

In other regions, additional ocean weight at continental margins as sea levels rise could bend the crust, reducing compressional conditions, says McGuire. Magma may then find it easier to reach the surface at adjacent volcanoes.

All these examples may seem contradictory, but the crucial point is that any change in sea level may alter regional stresses at continental margins enough to trigger eruptions in a volcano already primed to erupt, he says.

Small changes in rainfall can also trigger volcanic eruptions. In 2001, a major eruption of the Soufrière Hills volcano on the Caribbean island of Montserrat was set in motion by particularly heavy rainfall. This destabilised the volcano's dome enough for it to collapse and unleash magma within. Now it seems even typical tropical rain showers could trigger an eruption. And climate models suggest that many regions, including parts of the tropics, are likely to get wetter with climate change.

Adrian Matthews of the University of East Anglia, UK, and colleagues measured the minute-by-minute response of Montserrat's volcano after more than 200 bouts of precipitation over three years. The team found that these events, which Matthews says were typical of tropical weather, were followed by two days of increased volcanic activity.

A rainy day increased the likelihood of dome collapse from 1.5 per cent to 16 per cent. "It wouldn't have to be spectacularly heavy rainfall," says Matthews. "You don't have to have a hurricane." (Journal of Volcanology and Geothermal ResearchDOI: 10.1016/j.jvolgeores.2009.05.010)

Perhaps the greatest geological hazards during climate change will be the result of melting ice sheets. Apart from the risk that loose sediments exposed by melted ice could slip into the sea as tsunami-generating landslides, the removal of heavy ice could also trigger volcanic eruptions. "Even thinning of a few tens of metres could make a difference," says Andrew Russell of the University of Newcastle in the UK.

For example, Iceland's Vatnajökull ice cap sits over a plate boundary and several volcanoes. That ice is likely to disappear within the next two centuries. "If that happens you'll get rid of an awful lot of weight that will allow an increase in volcanic activity," says Russell. In the wake of the last ice age, volcanism was up to 30 times greater in northern Iceland compared with today (Earth Surface Processes and LandformsDOI: 10.1002/esp.1811).

Icy eruptions could reverberate round the world. In 1783, the Icelandic volcano Laki sent a sulphurous smog over Europe, plunging it into an extreme winter that killed thousands.

For now, it is unclear just how much climate change will affect the frequency and intensity of quakes and eruptions, says McGuire, because Earth's sensitivity to climate is only now emerging. There is not yet enough data to build predictive climate models linking the two systems. But it's crucial that we consider just how easily our actions could provoke the planet, he argues. "It's serious science, not scaremongering."

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