Primers: What
is Transition? Peak
Oil and Mid-Wales Climate change and Mid-Wales
Climate
Change: how will it affect the Mid-Wales area?
John Mason has a look and attempts some predictions...
The Earth's climate is in continual fluctuation. Its
properties are dependant on a number of variables, known
as forcing mechanisms because they force, or drive,
change. These include:
* Fluctuations in
solar output, affecting the amount of sunlight
(insolation) reaching Earth's surface
* Fluctuations in
Earth's orbit, affecting how close we are to the Sun
* Geographical
changes due to continental drift, affecting ocean
currents that transfer heat about the globe
* Atmospheric
composition, affecting the amount of heat (infra-red)
radiated back into space
For the past few
decades, solar output has remained steady within known
parameters, such as the 11-year sunspot cycle. No major
orbital wobbles are known to have occurred. The current
arrangement of continents favours polar ice-caps (which
form more readily when there are continental landmasses
at or very close to the poles). The major change is in
the increase of certain gases, especially carbon dioxide,
within our atmosphere and at the same time, global
average temperatures are steadily increasing.
Carbon dioxide is able to absorb radiation within the
infra-red part of the spectrum. This IR radiation is the
heat given off by the planet's surface as it absorbs
energy obtained from sunlight. The darker-coloured the
surface the more energy it can absorb and the more
infra-red it consequently gives off - a tarmac surface on
a sunny summer's day will feel hot to the touch and
that's why. Conversely, a white surface absorbs less
sunlight. Instead it reflects much of it back off the
surface. The ability of a substance to absorb or reflect
sunlight is known as its albedo.
Since carbon dioxide absorbs infra-red, it is
unsurprising that if you increase the amount of it in the
atmosphere then more heat is absorbed. This was first
noted as a potential agent of climate change in the
following paper:
Arrhenius, S.A.
1896: On the Influence of Carbonic
Acid in the Air upon the Temperature of the Ground.
London, Edinburgh, and Dublin Philosophical Magazine and
Journal of Science (fifth series), April 1896. vol 41,
pages 237-275.
Observations of measured carbon dioxide concentration
over the last fifty years show a steady increase as shown
in the graph below:
Atmospheric
carbon dioxide monthy mean mixing ratios. Data prior to
May 1974 are from the Scripps Institution of Oceanography
(SIO, blue), data since May 1974 are from the National
Oceanic and Atmospheric Administration (NOAA, red). A
long-term trend curve is fitted to the monthly mean
values. Credit: NOAA
380 parts per million doesn't sound a lot to some people:
however it should be noted that that concentration of
hydrogen sulphide ("bad egg gas") in air would
be fatal, and some nerve agents are effective at much
lower concentrations.
The need to reduce carbon dioxide emissions by curbing
the combustion of fossil fuels has been in the news a lot
just recently. Carbon naturally recycles itself on Earth:
various processes create the gas (e.g. respiration,
forest fires, volcanic activity) and others recapture it
(e.g. dissolution in water, reaction with calcium ions to
form carbonate rocks, photosynthesis). What we are doing
is adding, to this cycle, tremendous quantities of carbon
that has been fixed in rock strata over hundreds of
millions of years. This we have accomplished by burning
fossil fuels for energy and by processing carbonates
(i.e. limestone) to make cement. Carbon dioxide is a key
by-product in both instances.
Understanding how various forcing agents can change our
climate - and how quickly they accomplish this - is the
key to figuring out what is going to be the result of us
putting all of this extra carbon dioxide into the system.
This is why past climate changes, as recorded in
ice-cores, for example, are so intensively studied. The
studies allow past climate changes to be modelled and the
models can then be used in an attempt to predict what the
future holds.
Models are never perfect: this is the same in weather
forecasting, but what they can do is predict broad trends
with some success. The current changes we are seeing are
as many models have predicted: so can they predict what
we here in Mid-Wales can expect on a regional level?
To an extent they can, because in addition to them we
have a good understanding of our current climate and the
weather-patterns that it produces, both at surface and in
the upper atmosphere. So I'll run through some of the key
weather-types and how they may change.
Rainfall
To have rain, we
need to have moisture in the atmosphere. Many of our
worst flooding events occur when air that has originated
over the tropics, far to the south-west of the UK, flows
north-east towards and over us. Such airflows are known
as Tropical Maritime (TM). Rainfall may occur within such
airstreams by dynamic processes (such as air being forced
up over the Welsh mountains, where it cools and its
moisture condenses out as steady heavy rain) or by
convective processes (the development of cumulonimbus
clouds that bring thunderstorms). These rainfall-types
respectively cause widespread river-basin flooding and
more localised but extremely violent flash-floods.
As tropical seas become warmer, they will lose more
moisture to the air by increased evaporation. That
increased moisture means the potential for greater
rainfall intensities in weather-patterns dominated by
tropical maritime airstreams.
In Mid-Wales, the tendency, as seen elsewhere, to build
houses on flood-plains, is less frequently seen, which is
a good thing given the likelihood of more intense winter
floods due to moist TM airflows. More worrying is the
risk of an increase in the frequency of severe convective
flash-floods, such as those that occurred in North Wales
in July 2001 and in Boscastle in August 2004.
Drought
Droughts tend to
occur when anticyclones (high-pressure systems) become
stuck over the UK for a long time, in a
"blocked" weather-pattern. Increased warmth
during such conditions will allow greater evaporation of
surface moisture and thus greater drying of soil. The
key, and a major uncertainty, is whether such blocked
patterns will increase or decrease. The fact that
Mid-Wales is so close to the Atlantic Ocean - a major
source of moisture - is to our advantage here, but it
should be noted that there have been some notable summer
droughts in recent years, such as in 2006, when many
people relying on springs ran out of water and
borehole-drillers saw trade running at an excellent
level!
Severe storms
Increased sea
temperatures mean more energy is potentially available to
developing Atlantic depressions, but there are numerous
other factors that are critical in their development into
severe storms. Specifically, upper atmospheric conditions
must be literally spot-on: briefly, a developing storm
has to be engaged by an approaching upper trough for it
to deepen explosively. The timing is critical in
determining if the outcome is a "bomb" - a
rapidly deepening depression giving violent storm-force
winds - or a more typical squib - ordinary wet and windy
weather!
Severe
thunderstorms may produce very gusty winds, large hail
and tornadoes as well as flash-floods (covered above).
There is one particular scenario that Wales experiences
most winters - a cold N to NW Polar Maritime or, more
rarely, Polar airflow, in which, because the cold air is
moving over warm seas, it becomes destabilised, resulting
in convection and resulting showers and thunderstorms. If
the sea temperature increases, greater instability,
resulting in more vigorous convection, is likely to
result in an increase in the severity of such storms.
Tornadoes form
when vigorous convection occurs in a highly sheared
environment. The term shear refers to changes in
windspeed and direction at different heights in the
atmosphere. Shear values are often high over the UK, so
if more vigorous convection occurs, as in the above
scenario, then provided that shear is in place an
increase in tornado activity cannot be ruled out.
Sea-level rise
Coastal sites in
Mid-Wales record past rises in sea-level - e.g. the
"Fossil Forest" at Borth. Models vary with
regard to the amount of sea-level rise that can be
expected due to the changes currently underway. The two
key factors are melting of land ice (such as the
Greenland ice-cap) and the thermal expansion of the
oceans - if you warm things up they expand. Recent
reconstructions of sea-level rise during the last
interglacial suggest large rises occurred of around 1.6
metres per century. It is not entirely certain how these
can be applied to the current warming as the transition
from a glacial to interglacial climate involved the
melting of vast additional volumes of land-ice. Today we
have the situation of a warming occurring within an
interglacial.
As in any change and its ecological effect, it's not the
amount of change, but the rate of change that is
critical. If we know that a change will take place over
500 years that will involve relocating e.g. a coastal
community, then it is manageable. If however that change
occurs over just a few decades, it becomes increasingly
difficult to manage.
Above: a cross-section through Borth
Beach and the bog behind it - a classic local area in
which climate changes of the past are demonstrated. The
shingle storm-beach, fed by eroded cliff material at
Borth Head, is thought to have come into existence about
6500 years ago as sea-level rise finally stabilised long
after the last glaciation ended. It is thought that its
original position was approximately 1km seaward and it
has migrated landward in the intervening time.
Ecosystem
change
Perhaps least understood is how local ecosystems may
alter in the event of a warming climate. The simple
scenario would be a gradual advance northward of
warmth-loving species and a corresponding northward
retreat of cold-loving ones. The same scenario also
applies with regard to altitude, with an obvious
candidate for change being an eventual rise in the
tree-line.
More seriously, well-established links between species
may be thrown slightly out-of-phase, for example wild
birds breeding before sufficient food in the form of
insect-life is available, or plants flowering before
there are sufficient bees to feed on the flowers and
assist with pollination. Such occurrences could
potentially have drastic negative impacts on some
species.
Another potential problem is the northward spread of
various agricultural pests and diseases - with regard to
the latter, the migration of their insect vectors is
already a worry, for example in the case of the recent
occurrences of bluetongue in stock in the east of
England.
At sea, there are already signs of possible changes in
fish movements. In 2007, a number of tropical Amberjacks
were caught for the first time in Welsh waters and
mackerel were being caught as late as mid-November - a
month later than usual. Whether these observations
represent a long-term trend remains to be seen.
Primers:
What is Transition? Peak
Oil and Mid-Wales Climate change and Mid-Wales
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