Download Long-Term Climate Change

The Geography Magazine
Long-term climate
change
Davyth Fear
There is a common saying in Geology
that “the present is the key to the past”,
because it is only by studying how rocks
and landforms are created nowadays that
one can find out about past environments.
It might equally be said that, in studying
climate change, “the past is the key to the
present”, because we can only put the
present industrially-induced global warming
into context, and extrapolate what future
effects are likely to be by looking at what
has happened in past times.
For the purposes of this article, long-term
climate change is taken to mean time
periods greater than the last 150 years.
The pattern of temperature rise over the last
150 years has been described as a ‘hockey
stick’, with a period of relative stability
followed more recently by a more sudden rise.
Climate change since 1880
1945 Global
temperatures begin
a slow 30-year
slide. One likely
cause: sun-blocking
pollutants from postwar industry
Average global
temperature
0.6
CO2 level (data from
ice cores)
2005 Kyoto Protocol
goes into force;
global temperatures
match 1998 record
1991 Mount
Pinatubo erupts
in Philippines,
cooling Earth
by about 0.2°C
(0.36°F) for two
years
390
CO2 level (measured in
atmosphere)
0.4
0.2
380
1930s Flooding in China kills
over 400,000; dust bowl rages
across US; global temperatures
hit global highs
370
1972 Intense drought
hits the Africa Sahel
region; millions die.
More drought follows in
the mid-1980s
360
350
1896 Swedish chemist Svante
Arrhenius calculates that a
doubling of carbon dioxide
could warm the planet 5°C (9°F)
340
330
0.0
- 0.2
1957-58 Interstate highways and
suburbia sweep the US; Charles
Keeling begins measurements of
CO2 in Hawaii.
- 0.4
1880
1
2003 More
than 40,000
Europeans die in
unprecedented
heat wave
1900
1920
1940
1960
1976-77 Historic
drought in Europe;
severe winters in US;
global temperature
bottoms out
1997-98 Kyoto
Protocol finalizes
after intense
negotiations;
“El Niño of the
century” triggers
floods and drought,
helps make 1998
record-warm year
320
310
300
290
280
270
1980
2000
Atmospheric CO2 level (parts per million)
Global temperature (change from 1951-1980 mean )°C
0.8
1988 Mississippi
River dries up;
Yellowstone
burns; James
Hansen testifies
before US
Congress that
human-induced
warming has
likely begun
The future The UN’s
intergovernmental
Panel on Climate
Change predicts
a temperature
rise of between
1.1 and 6.4°C (2.0
and 11.5°F) by 2080–
2099 compared
to 1980–1999,
depending on how
much the world
reduces or increases
its emissions of
greenhouse gases,
and how sensitive
the climate proves
to be to those gases
The Geography Magazine
But how does this compare with the past, and why did the
climate change in the past if recent change is because of human
interference? There are several reasons, and scientists have
been finding out more and more about them using evidence
from tree rings, pollen, deep sea oozes, ice cores, silt, rocks
and patterns of vegetation.
Reasons for change in the climate, in essence cause and effect,
are termed ‘forcing’ and ‘response’ by climate scientists.
There are three fundamental kinds of natural climate forcing
in existence, all operating on different timescales. These are:
• changes to the strength of the Sun’s radiation occurring
over the age of the Earth, but also shorter period cycles,
with change over decades and centuries.
• changes to tectonic processes or plate tectonics, operating
over millions of years.
• and changes to the Earth’s orbit, operating over tens to
hundreds of thousands of years.
This mystery, called the faint young Sun paradox, means
that something must have kept the Earth warm whilst the Sun
was not so active. The answer is that the level of greenhouse
gases must, at one time, have been much higher than now.
A comparison of Earth and Venus shows that although
both planets have similar amounts of carbon, the Earth now
stores most of it in rocks, while on Venus it is stored in the
atmosphere, creating the fiercely hot conditions there today.
The intense volcanism of the early Earth pumped large
amounts of carbon dioxide into the atmosphere, so how and
when was it removed?
It was chemical weathering that helped remove carbon from
the atmosphere, creating clay minerals which were then stored
in the crust, or it was removed through the processes of plate
tectonics, within the mantle.
Earth’s thermostat?
The faint young Sun paradox
In the 4.55 billion years since the formation of the Solar
System, the Sun has gradually been converting hydrogen
to helium by nuclear fusion and has therefore been getting
brighter and larger than in its early days (between 25% and
30% according to most theories).
The faint young Sun paradox
(A) The most plausible explanation of the faint
young Sun paradox is that the weakness of the
early Sun was compensated for by a stronger
carbon greenhouse in the atmosphere. (B) When
the Sun later strengthened, increased chemical
weathering deposited the excess greenhouse
carbon in rocks, and the greenhouse effect
weakened enough to keep Earth’s temperatures
moderate.
Astrophysical models of the Sun’s evolution indicate
it was 25% to 30% weaker early in Earth’s history (left
above). Climate models show this situation would
have produced a completely frozen Earth for well over
half its early history if the atmosphere had the same
composition as it has today (right above).
Only a very small decrease in the Sun’s present strength would
be required to cause global freezing of all surface water. An
analysis of the Earth’s early history via the geological record,
sparse though it is, indicates that the creation of sedimentary
rocks by rivers has always been important, suggesting
temperatures were roughly similar to today.
2
Weathering acts as a negative feedback loop, regulating the
temperature of the atmosphere. Some researchers believe that
evolution has helped in this process because, when the Sun
was weaker, simple organisms played a much smaller role in
the process of chemical weathering, while the development of
more advanced organisms accelerated the weathering process
and enabled the removal of more carbon from the atmosphere.
This idea is called the Gaia hypothesis.
The Geography Magazine
Plate tectonics and supercontinents
The theory of plate tectonics, the idea that the
world’s land surfaces could come together as
supercontinents, then break up and re-assemble,
means that the global patterns of ocean currents,
winds, rain and temperatures could also change
over time.
Scientists can track the positions of continents
by analysing traces of magnetism preserved in
rocks when they form. In this way, when large
land masses are situated near the poles, major
ice sheets can develop (the Polar Position
Hypothesis). However, this is not enough in
itself to produce ice ages. Continents have been
on or near the South Pole for most of the last
250 million years, mostly without producing ice
ages.
Two more recent theories, the spreading
rate hypothesis and the uplift weathering
hypothesis fit the available evidence more
completely. The first links the speed of formation
of new crust at mid-ocean ridges to the input of
carbon dioxide to the atmosphere.
The carbon dioxide is removed by weathering
and deposited in the ocean as carbonate rocks,
being returned to the upper mantle via subduction. This process has a time lag of several tens of millions of years. The second
theory states that the global rate of chemical weathering depends upon the amount of fresh rock available, as well as upon the
usual factors of temperature, precipitation and vegetation. Mass-movement in mountain ranges, the frequency of earthquakes
in fold mountain chains and the high precipitation rates of mountain areas all combine to produce very rapid weathering rates
in high mountain areas.
Greenhouse and Icehouse Earth
By 100 mya, our Earth could rightly be called a greenhouse world. No record of permanent ice is to be found in the geological
record. Global sea levels were 200 metres above present levels, flooding low-lying continental margins.
Mid Cretaceous 120 million years ago
3
The Geography Magazine
During this geological era (the Cretaceous), chalk and
limestone were deposited in many shallow seas. Temperatures
at that time were warmer at all latitudes, but especially at
polar ones. Atmospheric CO2 concentrations were believed
to have been between 4 and 10 times higher than today, and
temperatures were over 5°C warmer than now. It is thought
that the extra CO2 came both from active spreading ridges and
from methane hydrates stored in cold ocean seabeds.
Over the last 55 million years (from the mid Eocene period),
the Earth has been cooling down, with sea levels dropping and
continents reaching their present position. During the middle
of the Pleistocene Ice Age, CO2 levels had decreased to reach
a low of about 50% of present day levels. Mid-ocean ridge
spreading rates slowed down, but perhaps more importantly,
the Tibetan Plateau started rising as India and Asia collided.
South America and Antarctica separated circa 20 mya and, with
the circulation of air and water around Antarctica unimpeded,
permanent ice formed on Antarctica about 13 mya. After
the joining of North and South America nearly 4 mya, the
currents of the Atlantic and Pacific shifted, intensifying the
movement of warm water northwards, via the Gulf Stream
and away from Antarctica.
Changes in the Earth’s orbit
During the Pleistocene Ice Age, we can clearly see another
climate forcing mechanism, i.e. changes to the Earth’s orbit.
The Ice Age is divided into dozens of glacial and inter-glacial
periods, and these are linked to the complicated series of
changes that describe the Earth’s orbit through space.
Scientists have known for a long time that the gravity of the
Sun, Moon and other planets cause changes to the Earth’s
angle of tilt, the eccentricity of orbit and the relative positions
of the solstices and equinoxes within the orbit.
Firstly, the angle of tilt of the Earth (23.5° at present and
slowly decreasing) may vary between 22.2° and 24.5° because
of the gravitational tug of large planets such as Jupiter. This
changes the amount of solar insolation received, especially at
high latitudes, amplifying or suppressing the seasons. As the
tilt decreases, glacial periods tend to set in because the cooler
summers cannot melt the winter’s snow. Similarly increasing
tilt can help end a glacial period.
Effects of increased tilt on polar regions
Increased tilt brings more solar radiation to the two
summer-season poles and less radiation to the two
winter-season poles.
Long-term changes in tilt
Changes in the tilt of Earth’s axis have occurred on a
regular 41,000-year cycle.
4
The Geography Magazine
The shape of the Earth’s orbit also varies between more
circular and more elliptical shapes. At present, the eccentricity
is fairly circular. There are two main cycles of change, with
one cycle having a period of 100,000 years and the other
413,000 years.
Precession of Earth’s axis
Earth’s slow wobbling motion causes its rotational
axis to point in different directions through time,
sometimes (as today) toward the North Star, Polaris,
but at other times toward other stars.
Long-term changes in eccentricity
The eccentricity (ε) of Earth’s orbit varies at periods
of 100,000 and 413,000 years.
Presently, the Earth is about 3% closer to the Sun in early
January than in early July, and this means that 7% more solar
energy reaches Earth. The last eight glacial periods have
coincided with the 100,000 year cycle, although previous to
this, the 41,000 cycle of tilt seemed to be dominant, as analysis
of microscopic shells (foraminifera) in deep sea sediments
shows. The reasons for this change will be dealt with later in
the article.
Finally, the Earth wobbles in its orbit like a spinning top.
This movement is called precession and operates on a cycle of
26,000 years. On top of this, the elliptical shape of the orbit
rotates, causing the solstices and equinoxes to slowly move
round the orbit in a cycle of 23,000 years.
5
Precession of the ellipse
The elliptical shape of Earth’s orbit slowly precesses in
space, so that the major and minor axes of the ellipse
slowly shift through time.
The Geography Magazine
These two movements combine to magnify or suppress
contrasts in Earth-Sun distance around the orbit. For instance,
10,000 years ago, the stronger summer Sun in the Northern
Hemisphere meant much wetter monsoons.
As the amount of insolation changes with these changes in
the Earth’s orbit, ice sheets are able to grow or shrink in polar
latitudes. Their response is not immediate however, but shows
a time lag calculated to be between 5,000 and 8,000 years.
Changes in greenhouse gas concentration
The record of global atmospheric CO2 and methane
concentrations also shows regular fluctuations. Analysis of ice
cores from Greenland and Antarctica enable measurements to
be made of ice layers dating back thousands of years. Methane
concentrations show regular cycles at intervals of 23,000 years,
mainly because the wetter monsoons increased the amount of
standing water in bogs, thus creating oxygen-free conditions
needed for the creation of methane.
Long-term C02 changes
A 400,000-year record of C02 from Vostok ice in
Antarctica shows four large-scale cycles at a period of
100,000 years similar to those in the marine ∂18 O (ice
volume) record.
CO2 is removed from the atmosphere during colder periods
because:
Methane and the monsoon
The methane record from Vostok ice in Antarctica
shows regular cycles at intervals near 23,000 years (left).
This signal closely resembles the monsoon-response
signal driven by low-latitude insolation (right).
CO2 concentrations do not show the same pattern as
methane. There is a general match between CO2 and ice sheet
volume, with interglacial periods showing 90 ppm greater
concentrations than glacial periods.
6
• it dissolves more readily in colder seawater
• the increased upwelling of cold water in tropical areas
stimulates the growth of plankton
• cold water in Antarctic areas stays at the surface for longer,
again stimulating higher productivity.
But is it changes in CO2 which drive ice volume change,
or do changes in ice sheet cover drive change in CO2 levels
during glacial periods? It is the latter which scientists favour
because CO2 levels are connected to changes in circulation and
carbon storage in the deep ocean, changes which can respond
relatively quickly to climatic forcing. As has already been stated
earlier, however, the Pleistocene Ice Age displayed cycles of
41,000 and 23,000 years in the expansion and contraction of
ice sheets up to 0.9 mya. Following this, a 100,000 year cycle
took over.
The Geography Magazine
from a glacial period to an interglacial one, and vice versa?
Data, initially from the deep oceans, but later from pollen
records in soils around the world, indicates that conditions
could change much more rapidly than previously envisaged.
These oscillations happen over periods of a few thousand years
at most, and are called Dansgaard-Oeschger cycles if they
represent rapid warming (with 23 such episodes recognized
between 110,000 and 23,000 years ago), and Heinrich cycles
if they represent cooling (with 6 such episodes identified).
During D-O cycles, air temperatures rose rapidly, up to 8°C
in 40 years, returning to glacial conditions more slowly,
over hundreds of years. A Heinrich cycle occurs when vast
quantities of icebergs flow into the North Atlantic disrupting
the ocean conveyer belt. ‘The emergence of this new data
during the 1990s made it clear to scientists that warm and cold
swings during the ice age were far more widespread and could
unfold far more rapidly than they had thought before.’*
Following the end of the Ice Age 15,000 years ago, when the
orbital cycles of the Earth’s tilt and changes in the position of
the Earth’s axis synchronised, there have been less extreme
climatic changes. However, this does not mean that the
climate has been static. The Younger Dryas period (named
after an alpine flower, Dryas octopetala, that flourished during
conditions too cold for most other life) started about 13,000
years ago, lasting for just over 1,000 years.
Evidence of ice sheet evolution: ∂18 O
A North Atlantic Ocean sediment core holds a 3-Myr
∂18 O record of ice volume and deep-water temperature
change. After a pre-glaciation phase with no major ice
sheets before 2.75 Myr ago, small ice sheets grew and
melted at cycles of 41,000 and 23,000 years until 0.9
Myr ago, and then large ice sheets grew and melted at
a cycle of 100,000 years. The diagonal white line shows
a gradual long-term ∂18 O trend toward more ice and
colder temperature.
The marine geologist John Imbrie has produced a model
which hypothesizes that the size of the ice sheets gradually
became larger and larger, passing a threshold which made them
resistant to melting on the warm summer side of the 41,000
year cycle. The size of the ice sheets increased the flow of deep
water in the oceans, providing positive feedback to their own
growth and melting. The change in ice sheet volume mostly
affected the Northern Hemisphere because there is little room
for expansion of ice sheets in Antarctica.
Dryas octopetala (Mountain avens)
Dansgaard-Oeschger and Heinrich cycles
The Pleistocene Ice Age has produced many different glacial
and interglacial periods. But how quickly can the climate flip
7
* Adapted from page 218 in The Rough Guide to Climate Change,
Robert Henson, January 2008, by kind permission of Penguin
Group UK
The Geography Magazine
According to ice core information from Greenland, it took
only 10 years or so for it to start and to finish. Average
temperatures in Britain during this period were as low as
-5°C. Many climatologists believe that the massive Lake
Agassiz, made up of meltwater from the Laurentide Ice Sheet
across Northern Canada, sent large pulses of fresh water into
the North Atlantic, disrupting the global circulation of water
(fresh water is less dense than salt water, therefore it does not
sink so easily).
Gradual warming and peaks in summertime sunshine, caused
by changes in orbital cycles, produced intense monsoons across
North Africa and South Asia. Much of the Sahara was green.
But from 5,000 years ago, gradual cooling and drying created
the conditions we see in the Sahara today.
Sea ice on the coast of Iceland
The frequency of sea ice along the coast of Iceland
increased into the nineteenth century, then declined
precipitately in the twentieth century.
Sunspot history from
telescopes
Measurements made with
telescopes over the last
several hundred years show
an 11-year sunspot cycle, as
well as earlier intervals such
as the Maunder minimum,
when sunspots were absent
for several decades. The
longer-term average number
of sunspots resembles
observed temperature
changes during the
twentieth century (top).
8
More recent climatic change
The last 1,000 years has also seen changes. From 1000 to 1300,
a period of warm conditions called the Medieval Climatic
Optimum existed. This period was warm enough for people
from Scandinavia to settle on the coast of Greenland and farm
the land. Subsequent cooling affected the period from 1400
to 1850. This period is known as the Little Ice Age. Glaciers
expanded in the Alps, the Thames froze over and frost fairs
were held on the river, and sea ice became more common off
the coast of Iceland.
When considering the causes of natural variation in the
climate over the last 1,000 years, one further cause of climatic
change can be established. Satellite measurements of the Sun’s
radiation have shown variations of 0.15% over a cycle 11 years
in length. Changes over this length of time do not allow the
Earth’s climate to respond fully before the cycle is reversed.
This corresponds to changes in the number of sunspots (with
greater radiation when sunspot numbers are high because of
increased emission in the bright areas surrounding sunspots).
But there have been periods when sunspot activity has virtually
ceased (the Maunder minimum from 1645 – 1715 and the
Sporer minimum from 1460 – 1550). Some astronomers
estimate that the Sun’s radiative output was 0.25% weaker
during these periods, which could produce temperatures
0.5°C below late 20th century temperatures.
The Geography Magazine
The past is the key to the future?
So what does all this information
about the past tell us about what
might happen in the future? This
depends upon decisions being
made at present. Optimistic
projections consider a doubling
of CO2 concentrations in the
atmosphere when compared
with pre-industrial levels. A
projection of current trends
estimates a larger rise in emissions
to a fourfold concentration
of CO2. The CO2 emitted
by humans will eventually
be mixed into the subsurface
ocean. Certainly, the excess
CO2 will last in the atmosphere
for several hundred years.
Projected temperature changes
with a doubling of CO2 vary
from study to study, but a
consensus indicates a rise of
2.5°C. CO2 levels of this magnitude have not existed for 7
million years. A fourfold increase might perhaps mean a 5°C
rise. CO2 levels of this order are unprecedented during the
last 40 million years of Earth history, and possibly not since
the Cretaceous greenhouse world of 100 mya. However, we
cannot use our records of past climate as the only predictors
for future events. Not all parts of the climate system will be in
equilibrium. For example, the large ice sheets will not have
enough time during the few hundred years of our man-made
CO2 pulse to be destroyed.
Even so, climatic modelling indicates that with a 2 x CO2
world, Arctic sea ice will disappear. Most of the permafrost and
tundra will also be absent. The Greenland and West Antarctic
ice sheets will be affected on their margins by increased glacial
flow and this will increase global sea level by 30 cm or so.
A 4 x CO2 world should see increased rates of melting,
perhaps of the order of 1 metre sea level rise. All mountain
glaciers should disappear, and conifer forests will be making
their way northwards, taking over from the tundra. Increased
rates of melt-water entering the North Atlantic, lowering its
salinity and density, might shut off the ocean conveyer belt
which brings abnormally warm water to Western Europe.
This would mean atmospheric CO2 levels rising even faster as
the North Atlantic is one of the major sinks for CO2 to leave
the atmosphere.
It can therefore be seen that our present human-induced
change of the climate is of unprecedented scale and suddenness,
perhaps even beyond the capabilities of scientists to predict its
full impact. Do we really want to live in a world experiencing
the same climate as the age of the dinosaurs?
9
Suggested questions for consideration:
1 Describe and explain the factors that may have influenced
long-term climate change.
2 Contrast the level and speed of climate change during the
last 200 years with that during the geological past.
Web sites for further investigation
http://www.climatechange.com.au/
http://muller.lbl.gov/pages/IceAgeBook/history_of_
climate.html
http://environment.newscientist.com/channel/earth/
climate-change/
http://gcmd.gsfc.nasa.gov/KeywordSearch/Keywords.
do?Portal=GCMD&KeywordPath=Parameters%7C
PALEOCLIMATE&MetadataType=0
The Geography Magazine
Acknowledgements
Thank you to the following for permission to
reproduce figures:
Penguin Group UK/Swanston Publishing Limited, from
The Historical Atlas of the Earth: A Visual Exploration of
the Earth’s Physical Past, eds. R. Osborne and Donald
Tarling, 1996: p.1(t), 3(b)
Penguin Group UK, from The Rough Guide to Climate
Change, Robert Henson, 2008: p.1(b)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman, ©
2001/adapted from Environmental Geology, D. Merritts
et al, W.H. Freeman and Company, ©1997: p. 2(l)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman, ©
2001 / adapted from Greenhouse Puzzles, W. Broecker
and T.-H. Peng, Eldigio Press, 1993, by permission of
Professor Wallace Broecker: p. 2(r)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman, ©
2001: p. 3(t), 4, 5(l)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman,
© 2001/adapted from Ice Ages: Solving the Mystery,
John Imbrie and Katherine Palmer Imbrie. Copyright
© 1979 by John Imbrie and Katherine Palmer Imbrie.
Published by Enslow Publishers, Inc., 40 Industrial
Road, Box 38, Berkeley Heights, NJ. All rights reserved.:
p. 5(t,r)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman,
© 2001/adapted from Orbital Geometry, C02 and
Pleistocene Climate, N. Pisias and J. Imbrie, in Oceanus
29 (1986-87), 43-49, by permission of Woods Hole
Oceanographic Institution: p. 5(b,r)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman,
© 2001/adapted from Ice Core record of Atmospheric
Methane over the Last 160,000 Years, J. Chapellaz et al,
in Nature 345 (1990), 127-131, by permission of Nature
Publishing Group: p. 6(methane)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman, ©
2001/adapted from Age Dating and the Orbital Theory
of the Ice Ages: Development of a High-Resolution 0 to
300,000 Year Chronostratigraphy, Douglas Martinson
et al, in Quaternary Research 27 (1987), 1-29, by
permission of Elsevier Limited: p. 6(Monsoon signal)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman,
© 2001/adapted from Climate and Atmospheric History
of the Past 420,000 Years from the Vostok Ice Core,
Antarctica, J.R. Petit et al, in Nature 399 (1999), 429-436,
by permission of Nature Publishing Group: p. 6(r)
10
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman,
© 2001 /adapted from The Initiation of Northern
Hemisphere Glaciation, M.E. Raymo, in Annual Reviews
of Earth and Planetary Sciences 22 (1994) 353-383, by
permission of Annual Reviews: p. 7(l)
Gwasg Dwyfor/Islwyn Williams, from Blodau’r Mynydd,
Drawings by Islwyn Williams, Text by Twm Elias, 1987:
p. 7(r)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman, ©
2001 /adapted from Climate – Past, Present and Future,
Volume 2, H.H. Lamb, Methuen, 1977, by permission of
Cengage Learning: p. 8(l)
W.H. Freeman and Company/Worth Publishers, from
Earth’s Climate: Past and Future, William F. Ruddiman,
© 2001/adapted from Solar Signals in Global Climatic
Change, C.D. Schoenwiese et al, in Climatic Change
27, 1994, 259-281, by permission of Christian-D.
Schoenwiese: p. 8(r)
Intergovernmental Panel on Climate Change: p. 9
Every effort has been made to trace and acknowledge
ownership of copyright. The publishers will be pleased
to make suitable arrangements with any copyright
holders who have not been contacted.
Editor: Gwenda Lloyd Wallace
Designer: Richard Huw Pritchard
Thank you to Davyth Fear, Glyn Owen, Mari Jackson,
Janet Cadogan, David Griffiths and Nicola Hawley (the
monitors) for their valuable guidance.