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Thursday, 14 February 2013

Even the Earth




 even the earth under our feet …
by Chris Bird, author of ‘Local Sustainable Homes’. 

We are depressingly familiar with the impact of climate change on weather and sea levels but how about the very earth under our feet, the hard bits that make up the crust of the planet and scientists call the lithosphere. Surely these won’t be affected by global warming? Actually they will and probably already are.  Global warming is likely to cause increased seismic activity such as earthquakes and volcanic eruptions, landslides, tsunamis and other things that are not good for people. So how can this happen and where is the evidence?
The simple answer is that global warming reduces the weight of ice over land and increases the weight of water over the seabed. As the last Ice Age ended around 20,000 years ago around 52 million cubic kilometres of water were redistributed about the planet. Find yourself a calculator and spend a few minutes working that out in tonnes and you won’t be surprised to learn that this colossal weight has an effect on the movement of tectonic plates and the behaviour of magma under volcanoes. Geologists have found evidence that this caused major earthquakes in high latitudes previously covered by ice and a spectacular rise in volcanic activity.
A recent paper by Marion Jegen in the journal Geology looked at a million years of climate history as shown in the geology of Central and South America. Periods of glacial melting were followed by 5-10 fold increases in volcanic activity. On the plus side there was a lag of around 2,500 years between glacial melting and volcanic activity. However, it seems to be the speed rather than the total amount of melting that predicts how intensively eruptions increase and we are busy causing very rapid global warming!
There is lots more research published in this field and the link between climate change and potentially hazardous geological events in the past is pretty well established for everyone who doesn’t believe the world was made in seven days. But sea levels rose by 130 metres after the last ice age and we are currently already in a warm period so how much more change can we expect? What about now? Even the most dire predictions suggest only a 2m rise by the end of this century and surely this won’t precipitate geological responses?
Sorry, bad news again. It seems that the Earths crust is exquisitely sensitive to change. For example, El Nino phenomenon in the Pacific cause slight variations in sea levels that seem to trigger increased seismic activity in the East Pacific. Furthermore, the past 300 years show a seasonal correlation between volcanic activity and a range of environmental conditions. The Earth is not as thick-skinned as we are!
As we saw on Boxing Day in 2004 and in Japan in 2011, seismic events under sea can cause devastating tsunamis. I’ve just watched some YouTube footage to remind myself how shocking those images were – and the thought that future tsunamis could be man made is horrendous.
Tsunamis may also be caused by the collapse of undersea slopes as a result of the destabilisation of gas hydrate deposits in marine sediments. Gas hydrates are ice-like solids of water and gases such as methane, a powerful greenhouse gas. Their stability is dependent on pressure and temperature. As sea temperatures rise these solids can dissociate with the release of methane and the possible collapse of undersea slopes. We can only hope that increasing pressures as sea levels rise will counter the effect of increasing sea temperatures to avoid this potential double whammy of a tsunami and the release of methane.
Not all geological hazards associated with climate change stem from changes hidden in the earth’s crust. Climate change is already associated with more extreme weather and this can have disastrous geological consequences. In 1999 heavy rainfall lead to the collapse of a mountain slope in northern Venezuela. 30,000 people died. There are numerous other examples closer to home. In the European Alps, for example, floods leading to land movements killed 37 people. A few years later, in 2002, the collapse of part of a mountain on to the Kolka glacier in Russia caused an avalanche that travelled 24 km and reached speeds of around 300km/hour.  100 people died. The combination of melting glaciers and extreme rainfall are likely to make such events more common.
High altitude lakes held back by natural rock dams of glacial debris pose another hazard. As glaciers melt these lakes increase in size and threaten to sweep away the natural dams that contain them. There have already been instances of such collapses in the Himalayas with Nepal being particularly at risk. Tsho Rolpa Lake has grown six-fold since the 1950s and is fed by a glacier currently retreating at around 100m every year. A sudden dam collapse here would overwhelm 10,000 people.
Climate related geological events also have less direct impacts on human activity. Reduced ice thickness over volcanoes in Iceland could well lead (is leading?) to increased activity. The ash plume from Iceland’s erupting Eyjafjallajokull volcano in 2010 caused major disruption to air traffic in Europe and further melting of the Vatnajokull ice cap could make such events more common. We may look forward to air traffic disruption but there are other possible consequences. A six-month long Icelandic eruption in 1783 spread a toxic sulphurous haze over much of Europe with a significant death toll.
There is no clear and certain picture about how climate change will impact on the not so solid ground beneath our feet. In some areas glacial melting could reduce volcanic activity and rising sea levels could stabilise rather than destabilise has hydrates. But we mess with the world at our peril and, just as we are already seeing with the atmosphere, global warming is likely to bring more geological hazards than benefits. We have even more reasons to limit climate change and prepare for an uncertain future.
Chris Bird
This article is based on evidence presented by 33 scientists in ‘Climate Forcing of Geological Hazards’, Edited by Bill Maguire and Mark Maslin. Wiley-Blackwell 2013.
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