CLIMATE
CHANGE IN WALES -
A GEOLOGIST'S PERSPECTIVE
PART 1: EXAMINING THE PAST
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Climate change (impact
definition): Change of the climate system that is faster
than the adaptation time of social and/or ecosystems.
11,500
years ago we finally came out of the last glaciation, or
ice-age. This had lasted for over 100,000 years, during
which ice-caps formed over the Welsh mountains, covered
the Irish Sea and extended southwards almost into South
Wales.
This was but one of several lengthy glaciations within
the Pleistocene, a division of geological time that began
a little over 1.8 million years ago. Prior to that, the
climate had been considerably warmer for some time....
This graph shows average global temperature
changes (blue line) through the last 500+ million
years. The geological timescale is not
proportional, in order to fit in the more
detailed data over shorter timespans that is
available in geologically more recent time. Some
key dates (in millions of years ago) are inserted
below the blue temperature plot.
The science of interpreting ancient climates is
known as palaeoclimatology. The evidence sought
after is in the form of fossil faunas and floras
which may be interpreted to reconstruct ancient
ecosystems - for example, extensive shallow-water
coral-reefs are indicative of a warm climate.
Coupled with data which allow reconstructions of
the positions of the continents relative to the
Earth's poles, and hard geological evidence such
as tillites (sedimentary rocks deposited by
ice-flows), it has been possible to construct a
view of what the climate was like going back many
millions of years.
Above: a reconstruction of
the positions of some of the continents, 495
million years ago, when the UK was not only south
of the Equator but split in two with an ocean in
between! The continental collision that brought
the two halves together forced up the range of
mountains that includes Snowdon, Scafell and Ben
Nevis.
Due to the fates and fortunes of continental
drift, Wales was pretty much unaffected by the
earlier glaciations during the Ordovician and
Permo-Carboniferous. It was still situated in the
tropics....
Above: the cliffs at
Lavernock, on the Bristol Channel in South Wales,
record the time when Wales was an arid desert in
the Triassic Period. The rocks here are red
marls, with bands of the mineral gypsum. They
were formed in a desert plain in which saline
lakes temporarily appeared and evaporated.
It can be difficult to imagine geological
timescales and the vast changes that happen
within them. One has to combine the geological
evidence available with the imagination of the
mind's eye. I have had a go at describing what
the view might have been like from Plynlimon in
Central Wales during the Triassic:
"It is a fine summer
day and we stand on the summit of Plynlimon
looking at the strange landscape through the
heat-haze. To the north and the south low
undulating hills stretch into the far distance.
To the west the hills tail off into a great
depression where Cardigan Bay ought to be. There
is not a sign of life in this silent red land.
The open sea lies a long distance off to the
north and east, beyond the salt-lagoons of the
Cheshire Plain. The seemingly endless cycle of
sunrise and sunset over the red landscape will
continue for another ten million years. It will
take that long for the sea to return."
The tropical odyssey continued until Wales, along
with the rest of Europe, began to drift northward
in the aftermath of the breakup of the
supercontinent Pangaea, which got underway in the
Jurassic Period....
Above: the southern part of
the Northern Hemisphere in Jurassic times, just
after Pangaea started to break up.
During Cretaceous times, the global climate was
stable and considerably warmer than that of
today, especially in the higher latitudes.
Geological evidence suggests that there were no
permanent polar ice-caps present for a period
exceeding 40 million years. The situation
continued through into the early part of the
Tertiary Period, seemingly unaffected by the K-T
mass extinction event, but by mid-Tertiary times
a progressive cooling was underway.
Above: by Miocene times,
Europe was a more familiar shape, although Italy
was yet to dock with the rest of the continent.
The collision, later in the Miocene, caused the
mountain-building events that resulted in the
formation of the Alps.
By the start of the Pleistocene, the Earth had
begun to experience a series of cyclic, violent
climatic oscillations, each involving a slower
cooling to a lengthy glaciation followed by a
sharp warming into a shorter interglacial period,
where the climate was similar to, or slightly
warmer than, that of today. We are now within
such an interglacial episode.
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The
current interglacial is referred to as the
Holocene and is the most recent division of
geological time. The last glacial episode started
to retreat in earnest 14,500 years ago, but was
interrupted by another cooling phase and slight
readvance of ice. This is known as the Younger
Dryas, and its end marks the Pleistocene-Holcene
boundary. The rate of warming that ended the
Upper Dryas was extraordinary: analyses of
ice-cores have indicated rises of 10oC
in as little as a decade!
Above: the beach at
Tonfanau, on the Cardigan Bay coast, consists of
extensive boulder-fields interspersed with small
patches of sand. Many of the boulders are
rock-types that do not outcrop on mainland Wales.
They were dumped here by the ice-sheet that
flowed down the Irish Sea, and were either ripped
up from what is now the sea-bed of Cardigan Bay
or were transported from even further afield by
the ice.
The sudden warming was interrupted at times. A
cooling event well-documented from ice-cores
occurred 8200-8400 years ago and the effects were
many years of cool dry conditions across the
Northern Hemisphere; a second event 6000 years
ago had serious ecological effects in the Sahara
area where dry conditions encouraged major desert
expansion over what had previously been a fairly
well-vegetated landscape.
More recently, we have the cooling known as the
"Little Ice-age" which brought to an
end warmer conditions experienced in Medieval
times. The cooling began in the 1400s and ended
in the 19th Century. Now, the second part of this
account looks at the current warming, and
examines its potential effects in the context of
what we know about the past.
PART
TWO
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