19 October, 2011

Tectonic Disasters and Climate Change

Shih Gang Reservoir damaged by
earthquake (source: Wikimedia Commons)
Climate Change seems to get the blame for everything these days. My Earth Scientist colleagues sometimes respond to news of a cluster of earthquakes or volcanic eruptions by joking ‘It must be caused by climate change.’ Well, a recent article in New Scientist suggests that comments like these may not be so crazy after all (New Scientist 1 Oct 2011, p.38).
Tectonic disasters include volcanic eruptions, earthquakes and tsunamis. In a nutshell, these are all examples of the Earth’s crust responding to built up tension or pressure.

The Earth’s crust consists of dozens of tectonic plates, all moving in different directions. This movement causes pressure or tension to build up at the plate boundaries until it is released by movement along a fault, resulting in an earthquake.
Most volcanoes are also located near plate boundaries. Volcanoes erupt when the pressure from the molten rock or magma beneath the surface build up to the point that it can no longer be contained by the overlying crust.
Is it possible that tectonic activity could be influenced by climate change? Maybe, if there is ‘something’ that is influenced by climate and can significantly change the pressure on the crust. Well, that ‘something’ is water. Water is pretty heavy and climate change could move a lot of it around, redistributing the pressure on the crust.

Let’s look at a simple case. Many plate boundaries are near coastlines. Many plate boundaries also have high mountains with glaciers on the land side. If the glaciers melt, then this water moves from the land to the sea. Decreasing the load on the land and increasing the load in the sea puts stress on the plate boundary faults increasing the likelihood of an earthquake.

This loading and unloading can also affect volcanic activity. Removing ice cover decreases the pressure over the volcano. This in turn results in more rock melting and more magma being produced, increasing the pressure inside the magma chamber. The decrease in overlying pressure also means that the crust is less able to suppress the increase in magma making an eruption more likely.

It is not just melting ice that could have an effect. Seasonal phenomena like El Niño can result in regional changes in sea level of up to 50 cm which can also affect the pressures on the underlying plates. A recent study of earthquakes near Easter Island found a link between El Niño related changes in sea level and earthquake activity (Philosophical Transactions of the Royal Society A, vol 368 p.2481).

So how real is the risk. No one is suggesting that climate change is behind major recent disasters like those in Japan and New Zealand. Indeed, there is no evidence of a significant increase in earthquakes or volcanic activity over the last century. But the amount of warming we have experienced so far is small compared to what is predicted to come. At the moment these effects are mainly theoretical, but if global warming increases they could become very real.

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