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?
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.
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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