P. E. HODGSON is Senior Research Fellow Emeritus
in Physics at Corpus Christi College, Oxford.
When I first became interested in the
applications of nuclear physics I was
most concerned by the coming shortage of
energy. Since then it has become clear that
this is not the main problem. There is plenty
of energy for the next few hundred years:
enormous deposits of coal, substantial amounts
of oil and natural gas, and the likelihood of
increasing contributions from nuclear power.
The main concern is now the effects on the
world’s climate from the pollution of the
atmosphere from fossil fuel power stations. It
will be many decades before fossil fuel power
stations can be replaced by non-polluting
sources such as nuclear and renewable energy,
and all that time the pollution will
increase. Detailed studies by the Intergovernmental
Panel for Climate Change show
that the amount of carbon dioxide in the
atmosphere is inexorably increasing and the
evidence for its effects on the climate is
steadily becoming more convincing. In addition,
the predicted rise in sea level will have
catastrophic effects on low-lying countries.
It is now becoming clearer that the principal
danger is not the effects of gradual
changes in the climate but the possibility of
rapid and irreversible changes. We tend to
think of the earth and its climate as a reliable
and generally stable system where the weather
remains more or less the same when averaged
over long periods. There is now increasing
evidence that this may not be true, that there
is a distinct possibility of large, unexpected,
and irreversible changes that quite rapidly
have catastrophic consequences.
The study of climate changes is fraught
with serious difficulties. Since time immemorial
the weather has fluctuated unpredictably,
with cold and hot periods, heavy rainfall
and droughts, hurricanes, and earthquakes.
How can the changes due to man’s
activities be distinguished from these natural
changes? It is notoriously difficult to establish
the presence of a new trend in a fluctuating
quantity, and the difficulty is compounded
when the fluctuations are on several different
timescales, as is the case for climate. There
are changes from year to year and ice ages on
a much longer timescale. If a trend over a few
decades is established, how do we know
whether it is soon to be reversed by a major
change acting on a longer timescale?
When definite evidence for climate change
has been found, it is important to understand
the underlying causes and to deduce what is
likely to happen in the future. The earth’s
seas and atmosphere form a highly complex
system, and although much research has been
done we still know very little compared with
what there is to know. This is an area of
highly speculative science, where hypotheses
are made to explain a few observations and
are then refuted by the discovery of new
facts. Scientists group themselves into schools
of thought and argue fiercely with scientists
of other schools. An agreed consensus may
arise one year and dissipate the next. There
is only one thing on which all agree, namely
that the more we know the more frightening
are the prospects for the future of the world.
Even if we understand the forces determining
the climate we still have the problem
of deciding what to do about it. Even if we
have decided on the optimum course of
action, we then have the problem of persuading
governments to do what has to be done.
This article is concerned with the evidence
for climate change and the possibility
of future catastrophic changes. The political
problems will be discussed in the next article.
The Evidence for Climate Change
By climate we mean the sum of the many
variables describing the condition of the
atmosphere: the temperature and humidity
of the air, the rainfall, the strength of the
winds, and the clouds. All these are constantly
changing, and we can take averages for a
local region or for the whole earth. Climate
is determined by many natural causes, and in
addition there is evidence that it is affected
by human actions. We cannot do anything
about the natural causes, but if there is a
causal link between human actions and climate
change we may have reason to expect
the present changes to continue, and furthermore,
we will have a strong incentive to take
action to mitigate their harmful effects.
Such a causal link has been proposed.
Extensive measurements have shown that the
concentrations of carbon dioxide, methane,
and some other gases in the atmosphere are
steadily increasing. In the 1780s the level of
atmospheric carbon dioxide was about 280
parts per million (ppm), as it had been for the
last six thousand years. Industrialisation increased
the level to 315 ppm by the 1930s,
330 ppm by the mid-1970s, and 360 ppm by
the mid-1990s. In the last ten years the level
has risen by a further 20 ppm. By the middle
of the present century it could rise to 500
ppm. The annual increase of carbon dioxide
is now 0.4 percent, that of methane 1.2
percent, of nitrous oxide 0.3 percent, of the
chlorofluorocarbons 6 percent, and of ozone
about 0.25 percent. In the European Union,
fossil fuels are the main source: oil 50 percent,
natural gas 20 percent and coal 28 percent.
Of this, electricity generation accounts for
37 percent, transport 28 percent, industry 16
percent, households 14 percent, and the
service sector for 5 percent. These are established
facts, and in addition there is a strong
correlation between carbon dioxide concentrations
and temperature changes. It is then
suggested that these increased concentrations
are responsible for global warming and that
global warming is responsible for other climate
changes and predicted effects such as a
worldwide rise in the sea level. The evidence
for anthropogenic climate change has increased
in recent years, and its reality is now
generally accepted.
The connection between the increase in
carbon dioxide and global warming is known
as the greenhouse effect. The carbon dioxide
in the atmosphere acts like the glass in the
greenhouse; it lets the heat of the sun reach
the earth, and some of this heat is emitted
with a different wavelength that is stopped by
the carbon dioxide. The heat that is trapped
in this way warms the earth so that the
average temperature is 14 C. Without the
greenhouse gases it would be –18 C.
There is impressive evidence for the reality
of climate change during the last few
decades. Some of this has been described in
a book by Sir Ghillean Prance, former Director
of the Royal Botanic Gardens in Kew,
London.1 He recalls that there were devastating
floods in Mozambique and Venezuela
and quite serious ones in England. In other
countries there has been drought that in the
Midwest of the United States in 1988-9
caused losses estimated at $39 billion. Hurricane
Mitch killed ten thousand people in
Central America. The average temperatures
are rising in many countries: of the five
warmest years ever recorded in the United
Kingdom, four have been in the last decade.
The heatwave in 2003 killed about 20,000
people in Italy and 15,000 in France as the
temperatures topped 40 degrees celsius during
the day and 30 degrees celsius at night.
Crops failed, forests burned, and rivers reached
an all-time low. It was almost certainly the
direct result of global warming. On a longer
timescale, one result of these increasing temperatures
is that in some regions the growing
season for plants is increasing, with earlier
development in spring, and autumn events
being delayed. Birds and animals are also
affected, and some species, unable to cope
with the climate change, have become extinct.
There are however many examples of
climate change occurring long before large
amounts of carbon dioxide were emitted into
the atmosphere by human beings. Some communities
that have flourished for centuries
have been destroyed by climate change. One
example is the Anasazi who lived in Colorado
and were finally forced by major droughts in
1130-1180 and 1275-1299 to abandon their
cities and to move away to areas with a climate
that allowed them to continue their way of life.
Greenland, as its name implies, was at one
time a fertile land and supported a colony of
Vikings until colder weather forced them to
move elsewhere.
Other changes have not been quite so
catastrophic. The same cold spell around the
fourteenth century caused parts of the Baltic
Sea to freeze, and also the river Thames. In
the 1930s the reduction in rainfall on the
Great Plains in the USA, followed by winds
that removed the topsoil and created a
dustbowl, forced farmers to pack up and
move away. In other parts of the world
weather patterns are subject to violent changes.
The normally regular monsoons in India, for
example, can sometimes fail, causing catastrophic
famines. The warm ocean current
called El Niño can have disastrous effects on
the eastern Pacific shores. It is now known to
be part of a vast global water circulation, and
satellite observations and computer modelling
now enable some predictions of its effects
to be made2. Thus for example severe floods
and storms were predicted to occur in California
in 1997-1998. During autumn and
over Christmas the weather was fine, but in
January and February hurricane-force winds
battered San Francisco, floods rose, and
mudslides swept houses away. Floodwaters
submerged the freeway to Los Angeles and
swept away the Southern Pacific railroad
bridge. Fourteen years earlier another El
Niño caused floods and landslides that caused
a billion dollars worth of damage. In other
tropical regions, the 1997-1998 El Niño
caused over $10 billion in damage. There
were severe droughts in Australia and Southeast
Asia, vast forest fires in Indonesia and
Mexico, and famine in Brazil.
The weather cycles in Peru and Bolivia
are quite regular, unless they are interrupted
by El Niño, which is unpredictably variable.
Mostly the result is torrential rainstorms,
warmer seas, and changes in fish populations.
Occasionally however an El Niño event
causes significant changes to the climate and
brings ruin to fishermen and farmers. Thus in
1925 the sea temperature off northern Peru
rose over six degrees in ten days. Millions of
seabirds perished as the anchovies on which
they fed moved to cooler nutrient-rich waters.
Cloudbursts turned dry ravines into
raging torrents and the city of Trujillo received
396 mm of rain instead of the normal
1.7 mm. Farmlands and irrigation systems
were destroyed by a sea of mud, and hundreds
of people starved. Many other examples
could be given of the devastating
effects of El Niño.
It is conjectured that El Niño events
played an important role in the decline and
fall of ancient civilizations when they were
already seriously stressed by other economic
and political factors. Particular examples are
the Maya civilisation in Yucatan and the
Moche civilisation in Peru. In these cases, the
devastation caused by El Niño was the final
event that caused the once-flourishing societies
to collapse.
The rapid changes in climate associated
with El Niño events took place long before
the temperature increase associated with global
warming and still continue. They are not
yet fully understood and make it more difficult
to ascribe climate changes to anthropogenic
emissions.
Another source of global warming is the
variation in the brightness of the sun, already
observed over several centuries. The sunlight
intensity fell by 4 to 6 percent from the 1950s
to the 1980s. Now it is rising again and there
has been a 4 percent rise since 1990; it is
estimated that this could account for a rise in
temperature of 0.4 C by 2100. There are also
daily variations of around 0.2 percent, and
these could produce significant changes in
the climate. In addition, there is a correlation
between the temperature and the length of
the sunspot cycle. The physical basis for this
is suggested by another correlation, namely
that between the cosmic ray intensity and the
low cloud cover. The miniature Ice Age in
the later part of the seventeenth century is
known as the Maunder Minimum, which is
correlated with the sunspot minimum between
1645 and 1715. The sunspot cycle is a
measure of the solar activity, and this in turn
affects the cosmic ray intensity. The cosmic
rays produce ions in the atmosphere, and
these can form condensation nuclei for clouds,
which have a strong influence on the earth’s
temperature. Detailed studies suggest that
solar effects may be responsible for 30 to 57
percent of the observed global warming.
Since this varies with the sunspot cycle, it
sometimes reinforces and sometimes weakens
the effects of global warming. Some
fluctuations have been observed and these
could be due to varying amounts of aerosols
in the atmosphere.3
There is evidence that the mini Ice Age in
the seventeenth century and the medieval
warm period are part of a cycle that occurs
over a period of about 1500 years. Supporting
evidence is provided by the bands of rock
in cores from the Northern Atlantic. These
rocks came from Northern Canada, and
must have been carried to where they were
found by glaciers, indicating periods of cooling
and warming. More evidence came from
Greenland ice cores which revealed a series
of temperature changes, again with a period
of about 1500 years. There were also large
increases in the dust particles in sediments off
the coast of West Africa suggesting dust
storms inland, also with the same period. It
has been suggested that this cycle is ultimately
due to periodic changes in the sun. The
resulting changes in solar radiation changes
the temperature and this may be detected by
changes in the ratios of cosmogenic isotopes
in ice cores. All this shows the extreme
sensitivity of the climate to very small changes
in the intensity of the solar radiation. The
same must also be the case for man-made
changes in the atmosphere.4
A recent study5 has shown greatly increased
frequencies for the more devastating
hurricanes like Katrina, which struck New
Orleans and the surrounding states in 2005.
This increase has been attributed by some
scientists to the rising temperatures of the
oceans due to global warming. If this is the
case, some regions of the earth will be liable
to more devastating hurricanes in the future.
The cost of the damage due to the hurricane
Katrina has been estimated to be around
$100 billion.
On a much longer timescale, the mathematician
Milutan Milankovitch identified
cold and warm periods alternating every
100,000 years, with smaller cycles every
41,000 and 10,000 years. These cycles are
attributed to perturbations of the sun’s orbit
by the moon that cause the precession of the
equinoxes and by other small effects due to
the planets and are confirmed by world-wide
measurements of glaciers, coral reefs, peat
bogs, and polar ice caps.
There are thus many ways the climate can
be changed in addition to the effects of the
greenhouse gases. Careful scientific analysis
is therefore needed before their contribution
can be established. Many scientists worldwide
are making detailed calculations using
increasingly sophisticated models of the atmosphere.
This is obviously a very complicated
task. What, for example, do we mean
by the temperature of the atmosphere? We
can measure the temperature at a particular
place and height, but this needs to be done
over the whole surface of the earth and for
heights up to several miles. The best we can
do is to establish a grid of points and measure
the temperatures at these points as a function
of the time. Even a coarse grid contains
millions of points and the calculations are
very time-consuming even on a fast modern
computer. The more accurate we want our
calculations to be the longer they will take. In
addition, the results may be very sensitive to
the initial conditions; this is called the butterfly
effect. The main uncertainty at present
seems to be the effects of water vapour,
which are greater than those of all the other
gases combined. These are sensitively affected
by changes in the cloud cover, which
in turn changes the amount of solar energy
absorbed or reflected.
The results of such calculations are published
periodically by the Intergovernmental
Panel on Climate Change, consisting of
about two thousand of the world’s leading
climate scientists, under the chairmanship of
Sir John Houghton.6 With many qualifications,
the conclusion of their assessment is
that there is good evidence that world temperature
is increasing, and it is predicted that
the average temperature will rise by about
four degrees centigrade by the year 2100. In
the same period the sea level will rise by about
60 cm, or by 40 cm if the carbon dioxide
emissions are controlled. Such rises will eliminate
many islands such as the Maldives in the
Indian Ocean and will inundate much of
Bangladesh and some of Holland. Already
the sea level has risen by 0.1 to 0.2 cm per
year during the twentieth century.
The connection between the rise in temperature
and the rise in sea level has been
attributed to the melting of the polar ice caps.
However, the ice immediately around the
North Pole and in the ice shelves around
Antarctica is floating, and so when it melts, it
has very little effect on the sea level, as
Archimedes knew very well. There may
however be some small effects due to differences
in salinity between the ice and the sea.
There are other effects of melting ice in
Antarctica that are discussed below.
Another uncertainty is the effects of soot on
global warming. This soot comes from the
incomplete burning of coal, biomass, and
diesel, and also from forest fires, domestic
heating, and factories. It has been said to
“mask” global warming and also that it “generates”
global warming. It has been called “a
cooling agent” and also “the biggest cause of
global warming after carbon dioxide.” What
seems to happen is that the soot shields the
earth from the sun’s rays, thus making it
cooler. It also absorbs some of the heat and reradiates
it into the surrounding air. Thus soot
heats the air and cools the ground. Some
scientists think that soot is the third most
important contributor to global warming,
after carbon dioxide and methane (4).
Catastrophic Events
In some respects the earth is a self-regulating
mechanism so that any change initiates secondary
changes that restore it to its original
state. This idea has been developed by James
Lovelock into the concept called Gaia (the
Mother Earth).7 The mechanism works for
small changes, but he recognises that there
may be changes that irrevocably flip it into
another state. We are familiar with such
changes, as for example when a bridge collapses
under a particularly heavy load. If this
happens to the climate, the results could be
devastating for large numbers of people.
One way this could happen is if even a
small change initiates a series of events that
reinforce the original change; this is called
positive feedback. It is a runaway process
that continues until the system is destroyed.
Some of these processes are already happening
to our climate, and others are serious
possibilities.
One example is the melting of the polar
icecaps, which is already having devastating
effects on people living in the Arctic. In
several places their traditional method of
hunting seals is no longer possible because the
ice has melted. As the ice melts the albedo, a
measure of the fraction of sunlight that is
reflected, falls rapidly from 0.8 – 0.9 to less
than 0.1 The result is that more sunlight is
absorbed, melting more ice in a continuing
feedback effect. This is one of the main
reasons why the Arctic ice is melting so
rapidly, thus providing a sensitive indicator
of global warming. Some computer models
indicate that by 2080 the Arctic will be icefree
in summer, making it impossible for the
polar bears to survive.
Satellite observations have shown that the
perennial Arctic sea ice covers about seventeen
billion acres. This area varies from year
to year, but in recent years the overall trend
has been strongly downward, particularly in
the Beaufort and Chukchi seas and also to a
lesser extent in the Siberian and Laptev seas.
The shrinkage now amounts to about 250
million acres.
Another example is the melting of the
permafrost, a thick layer of frozen soil in the
Arctic. This contains moss and lichen that has
accumulated for thousands of years and frozen
before it could rot. The dry weather has
caused widespread forest fires in Alaska, and
the temperature of the permafrost has risen
by two or three degrees. It has been predicted
that most of the top three metres of permafrost
across the Arctic will melt during the
present century. As the thawed vegetation
finally rots, it will release ten of billion tonnes
of carbon dioxide if there is oxygen present,
and the far more damaging gas methane if it
is not. This methane will increase the rate of
global warming still further, melting the
permafrost by a positive feedback loop (4).
Antarctica occupies 13.2 million sq.km.,
about 1.3 times the area of Europe, and the
ice cap is up to 4 km. thick. No less than 90
percent of the world’s ice is in Antarctica,
and if this were all to melt the world sea level
would rise by 70 to 90 meters. However the
ice in central Antarctica is at a temperature
from –40 to –60 degrees and so is unaffected
by a rise in temperature of a few degrees. The
same applies to central Greenland which
occupies an area about one-sixth of Antarctica.
The ice shelves surrounding Antarctica
are somewhat warmer, but are floating like
the Arctic ice and so melting them has little
effect on the sea level. However the sea level
is affected by warmer coastal glaciers flowing
into the sea from the ice caps of Antarctica,
Greenland, and Northern Canada. The glaciers
are very thick, and the pressure on the
ice where it rides over the ground is enough
to liquefy it, and this makes it easier for it to
slide down into the sea. An additional effect
has been suggested: when the surface of the
ice melts, lakes are formed and this water
flows down through cracks in the ice until it
reaches the bedrock. There it spreads and
reduces the friction between the ice and the
bedrock, further increasing the rate of travel
of the ice towards the sea. When this happens
it causes the sea level to rise, but by an
amount that is difficult to estimate. There is
increasing evidence that the ice shelves are
breaking up, and this reduces the pressure on
the glaciers so that their rate of flow increases.
The possibility of another type of irreversible
change is provided by the Gulf
Stream, which warms northwestern Europe.
Without it, the climate would be like that of
Labrador, at the same latitude on the western
Atlantic. At present the Gulf Stream brings
warm water from the tropics toward Europe
by what is called the thermohaline circulation.
This is due to the freezing of the Arctic
water, which causes the salt water to drain
out of the ice. This salty water is heavier than
fresh water and so it sinks, thus drawing
warmer water northwards from the tropics.
As this water cools it becomes denser and also
sinks, thus attracting more warm water. If
the oceans are heated by global warming and
more freshwater enters the polar seas it could
slow and even stop the Gulf Stream. This
could cause the temperature to fall by six to
eight degrees celsius and so it would then be
frozen for much of the year, and London
would become like Siberia.
Further evidence of major climate changes
is provided by the melting of glaciers in the
tropics. In many countries in the Andes and
the Himalayas, the Arctic, Alaska, and East
Africa, studies of ice cores and the glaciers
themselves provide massive evidence of permanent
changes. Isotopic analyses of the ice
cores show the evolution of climate from the
start of the El Niños about 5,500 years ago,
the drought that terminated the Moche empire,
and the current effects of global warming.
The glaciers began to form in South
America about 25,000 years ago and were
followed by glaciers in other tropical countries.
The growth of the glaciers depends on
the latitude, and this is linked with the slow
precession of the earth’s axis. During this
period the latitude at which the sun was
directly overhead moved steadily from the
Tropic of Cancer to the Tropic of Capricorn.
It might seem strange that glacier
formation takes place predominantly when
the temperature is highest, but this is because
maximum sunshine brings maximum rainfall
and at the altitudes where glaciers form the
temperature is always low enough to freeze
the rain, forming glaciers. The melting of
glaciers right across the tropics is quite unprecedented
and seems to be an irreversible
effect of global warming.4
About three billion people, half the people
on the earth, depend on the monsoon rains to
grow their food. There have been several
failures in the monsoon rains in the last two
centuries, and the resulting droughts and
food shortages have killed tens of millions of
people. The reasons for this variability are
not entirely clear, but a connection with the
El Nino current seems very likely. Famines in
India occur during large fluctuations in the
Pacific climate. A study of the strength of the
monsoons over ten thousand years based on
the amount of plankton in marine sediments
showed that that weak summer monsoons are
correlated with colder periods in the North
Atlantic whereas strong monsoons occur when
the Northern Atlantic seas are warm. It is not
clear just why these effects occur or how they
interact with each other. The effects may
prove to be benign or catastrophic.
Can These Changes
Be Stopped and Reversed?
Whatever we do, the amount of carbon
dioxide in the atmosphere will inevitably
increase. The danger of catastrophic climate
change can be mitigated, however, if resolute
action is taken to reduce carbon dioxide
emissions. The first essential step is to replace
fossil fuels power stations with sources that
emit nothing or only minuscule amounts.
Already, several countries have substantially
reduced their emissions by building nuclear
power stations. Thus France (80 percent
nuclear) has halved their emissions, Japan (35
percent nuclear) by 20 percent, and the USA
(20 percent nuclear) by 6 percent.
Several international conferences have
been held to encourage nations to promise
reduction. At the Kyoto Conference in 1997
Britain promised to reduce emissions by 20
percent by 2020. Already there has been a
reduction by 6 percent due to the improved
efficiency of nuclear power stations. As most
of these are scheduled to close in the next few
years, emissions are bound to rise. Both
Britain and the USA have no hope of reaching
these very modest targets.
The importance of the atmospheric halflife
of greenhouse gases in the atmosphere is
seldom recognised. Carbon dioxide lasts about
a hundred years, whereas methane is mostly
used up in a decade. A molecule of methane
causes a hundred times as much global warming
as one of carbon dioxide so methane is
much more important in the few years after
emission. Averaged over a longer period, the
effect of carbon dioxide increases, so that
over a hundred years the ratio of effectiveness
falls to about ten. It is therefore of great
importance to reduce methane emissions as
soon as possible, although this was not recognized
by the targets set at Kyoto. The reduction
of methane emission from landfill sites,
gas pipelines, coal mines, and many other
sources would have an important effect in
reducing global warming on the short term.
Soot lasts only a few days in the atmosphere
but has a large effect on global warming and
so its emission should also be reduced (4).
During the next forty years about two
thousand fossil fuel power stations must be
replaced. This can be done in several ways.
One is to build 4000 windmills occupying
500 square km each week. Or we can cover
ten square km of desert with solar panels each
week. Perhaps we can find more Severn
estuaries and build barrages costing £9 billion
every five weeks. Or finally, we can
build fifty new nuclear power stations each
year. This figure may be compared with the
43 that were built in 1983, the peak year for
nuclear construction. This is the choice faced
by world governments.
The next article will be devoted to an
account of how governments have reacted to
the world-wide threat to their very existence.
NOTES
- Sir Ghillian Prance, The Earth under
Threat: a Christian Perspective (Wild Goose Publications:
St Andrew’s Press, 1996). - Brian Fagan, Floods, Famines
and Emperors; El Niño and the Fate of Civilisations (New
York: Basic Books, 1999). Brian Fagan, The Long Summer:
How Climate Changed Civilisations. - Bjorn Lomborg,
The Skeptical Environmentalist (Cambridge: Cambridge
University Press, 2004). - Fred Pearce, The Last Generation
(London: Eden Project Books, 2006). - Richard A.
Ker, “Is Katrina a Harbinger of still more Powerful
Hurricanes?” Science 309.1807. 2005. P. J. Webster, G. J.
Holland, J. A. Curry, and H-R Chang, “Changes in
tropical cyclone number, duration and intensity in a
warming environment,” Science 309.1844. 2005. - John
Houghton, Global Warming: The Complete Briefing (Oxford:
Lion Publishing, 1994). - James Lovelock, Gaia:
A New Look at Life on Earth. 1979–2003. James Lovelock,
The Ages of Gaia: A Biography of our Living Earth.