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20 November 2009
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Introduction
In this essay, I'm going to try and provide an explanation about what
this is all about, in the simplest terms, because politicians and the media
have made such a complete mess that Joe Public has become totally
misinformed or, at least, confused about what these terms really mean. To
make this as clear as possible, I'll separate the related theories of global
warming and climate change but, first, let me give you a few definitions.
What is weather?
Weather is the effect of a number of variables at a given place and
at a given time. These variables include temperature, dew point or
relative humidity, precipitation, wind speed and direction, clouds, sun,
dust and other aerosols etc. Note that this has got nothing to do with
climate; one can have a cool day in a place with a hot climate, such as
when it drops to under a maximum of 25°C in a Nicosia July, for
example.
What is climate?
Climate is the effect of the average weather over a period of time
(commonly 30 years) and over a large area. For example, the
Mediterranean climate, which we enjoy in Cyprus, is categorised by hot dry summers and cool wet
winters, such as may be experienced along the whole north coast of
Africa and the whole south coast of Europe. This has nothing to do with
the weather of the region. Global climate is averaged over the whole
earth.
What is a theory?
The average man, if asked this question, will reply that it is something
for which there may be circumstantial evidence but for which there is no
proof. This definition does not satisfy the scientist; he would call this a
hypothesis. No, a theory is more than a hypothesis and requires substantial
observations and calculations to confirm the known facts (and to try and find out
more!)
There are many theories based on scientific fact, such as relativity
and gravity. To take the latter, legend has it that Newton watched an
apple fall from a tree. That is an observation. From this, he deduced
that every mass attracts every other mass by a force pointing along the
line intersecting the mass centres of both masses. The force is
proportional to the product of the two masses and inversely proportional
to the square of the distance between the point masses and can thus be
expressed as an equation:
From
http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation
That Newton was able to discover this in the 17th
century is rather amazing and is a tribute to his genius, but no one has
yet been able to discover why gravity exists, what causes the
actual force. For this reason, gravitational force is a theory. We can
observe that the apple does not fly to the moon and we can mathematically
prove it, provided we know the values of the variables.
Theory of global warming
We owe this theory to another great scientist, the Swede Svante
Arrhenius. In 1896, yes, 113 years ago, he published a paper entitled
On the Influence of Carbonic Acid in the Air upon the Temperature of
the Ground. Initially, this was a hypothesis but he later published
his work as a theory. His calculations were later refined but were
remarkably accurate. What he did not foresee was that man would extract
billions of tonnes of fossil fuels and burn them willy-nilly, so his
predictions of climate change (see below) were not accurate.
So what did Arrhenius say? Essentially, that the sun heated the
earth's surface because some of the energy in solar short-wave
electromagnetic radiation, peaking in the light spectrum, was absorbed.
The resultant rise in temperature caused the earth to re-radiate this
energy, at a much longer wavelength, in the far infra-red part of the
spectrum, out towards space. If the earth were to have an atmosphere of
only transparent gases like oxygen and nitrogen, then the energy
radiated outwards would be substantially equal to the incoming energy.
The temperature of the earth's atmosphere would be very cold, averaging
about -18°C, over the globe. So, why is the global average temperature
about +15°C, what caused this global warming of 30-odd degrees?
Various scientists had previously suggested that small amounts of
various gases could absorb black body radiation at 15°C. Arrhenius
measured the absorption spectra of gases, notably water vapour and
carbon dioxide (carbonic acid) and calculated that the radiation was
absorbed sufficiently to explain the global warming. One particular
point that he made and he disputed with Ångstrom
was that absorption spectrum of carbon dioxide covered most of the
radiation spectrum, so that small quantities of the gas were as
important as the comparatively large quantities of water vapour present
in the atmosphere. This absorbed only a comparatively narrow band
towards the edge of the radiation spectrum. This compelled him to state
that small variations of carbon dioxide levels would have important
effects on global warming. His calculations showed that a decrease of
carbon dioxide levels by a third would produce an average temperature
drop of a little more than -3°C, creating an ice age, while a 50%
increase would cause a temperature rise of about +3.5°C with latitudinal
variations. Apart from these theoretical carbon dioxide variations, he
also found that variations in water vapour concentrations would have a
smaller effect from practical observations.
Greenhouse gases
Because carbon dioxide and water vapour absorb radiative energy from
the earth's surface, causing the air temperature to rise, they have been
likened to the glass in a greenhouse, which stops much of the heat
radiated from the plant beds from escaping outside the structure. They
have therefore been nicknamed greenhouse gases. Other important
greenhouse gases are methane, some halocarbon gases and nitrous oxide.
Theory of climate change
The theory of global climate change is a very complex subject. It is
not fully understood, but the major causes have been well documented in
scientific literature. The climate of the Earth is constantly changing
if looked at over long periods of time. Up to recently, the changes have
been natural and occasionally very important such as during the ice ages
and interglacial periods. There are a number of reasons why this happens
including solar cycles, cataclysmic events, changes in the atmospheric
composition etc.
Of course, during global climate change phenomena, regional climates
also change. It is important to note that some regional changes may not
be in the same sense as global changes. This can mean, for example, that
during a period of higher global temperature, some regions may have a
lower temperature.
One natural phenomenon that popularly draws a lot of attention is the
major volcanic eruption. For example, the 1991 Pinatubo eruption did
affect world weather. Vast amounts of dust and sulfur dioxide were
thrown into the lower stratosphere by the force of the explosion. This
blocked almost 20% of the solar energy from reaching the earth's surface
and air temperatures in the northern hemisphere dropped by an average of
about 0.6°C and in the southern hemisphere by about 0.4°C. The energy
that was blocked heated the lower stratosphere, of course. The aerosols
from major eruptions have a relatively short lifetime and the drop in
received radiation rises again in a very short time as the dust settles
and rains out. The radiation block then follows an exponential curve
back to normal levels in a 2 to 5 year time scale. The following graph
shows the effect of four large eruptions on light levels in Hawaii.
From:
http://www.cmdl.noaa.gov/albums/cmdl_overview/Slide18.sized.png
Of course, it is impossible to predict the frequency
and magnitude of volcanic eruptions, so no accurate estimations can be
included in the climate modelling. This is not terribly important
because the effects are over a relatively short time scale. It is only
if there were a continuous series of Volcanic Explosion Index 5 or 6
eruptions over decades that the global climate would be affected,
causing a drop in temperatures with consequent side-effects. This is a
similar scenario to the 1980s scaremongers' "nuclear winter", which
would be unlikely to happen as the explosions would be unlikely to
propel much dust into the stratosphere.
Another similar natural effect is that of massive
forest fires. I was a student in Edinburgh in September 1950 and, one
Wednesday afternoon, I noticed the sun was bright blue. That evening,
the three-quarters moon was also very blue (and I've only seen it that
colour once in my lifetime, so the adage may have some truth!). I could
hardly believe my eyes! I later learnt that the cause was soot particles
from massive forest fires in British Columbia carried by the jet stream
many thousands of kilometres. The next day, the sun and moon appeared
normal. I don't believe this phenomenon would have much climate
influence because it was ephemeral and there would be little
stratospheric mixing.
In very recent years, it is very likely that human activity has also
started to cause a climate change, as the media constantly report,
although they usually use the term global warming, even though climate
includes many phenomena other than temperature, such as precipitation,
wind speed and direction, cloud cover and so on. One of the popular
media themes in southern Europe, and Cyprus especially, is potential desertification. Even
though this includes a possible temperature change, it also means a
considerable reduction in precipitation and humidity.
Over the past 150 years, we have extracted many billions of tonnes of
coal, oil and gas which we have burnt without any thought about what it
may entail. Every tonne of any of these fossil fuels that is combusted
will produce between 2 3/4 and 4 1/2 tonnes of carbon dioxide. We are
allowing this to escape into the atmosphere at the rate of about 26 billion tonnes per year. As it happens, nature is able to remove
about half of this, mostly by absorption into the upper levels of the
oceans, acidifying the water. The other half is simply accumulating,
year by year, in an ever increasing quantity. For many centuries, the
natural level of carbon dioxide in the atmosphere has been relatively
constant at about 280 ppm. This has been an essential part of global
warming enabling life as we know it. The current level of carbon dioxide
in the atmosphere is about 385 ppm with projected trends up to 400 ppm
within a decade or two. This is an increase of 43%. You will remember
that Arrhenius predicted in 1896 that a 50% increase in carbon dioxide
concentration would result in a global increase in temperature of 3.5°C.
We are nearly there. This is therefore a man-made or anthropogenic
manifestation of one type of climate change.
 The
graphs on the left, taken from the report of Working Group I of the
Intergovernmental Panel on Climate Change, Summary for Policymakers, of
the AR4, 2007, show how the three major greenhouse gases have had levels
substantially constant for 10,000 years, until the last 100 or so years.
Can there be any real doubt that it is too much of a coincidence for the
increase in carbon dioxide to be anything other than largely due to the
increase from burning fossil fuels in the same time frame? Of course,
there are other contributory factors, such as the reduction of sinks by
deforestation and urbanisation.
On the right hand side of these graphs, there is a
scale of radiative forcing. Please let me digress and explain what this
means. The radiative forcing of the surface-troposphere system due to
the perturbation in or the introduction of an agent (say, a change in
greenhouse gas concentrations) is the change in net (down minus up)
irradiance (solar plus long-wave; in Wm-2) at the tropopause AFTER
allowing for stratospheric temperatures to readjust to radiative
equilibrium, but with surface and tropospheric temperatures and state
held fixed at the unperturbed values. This definition sounds a little
difficult to understand; more simply, it is the change in the difference
between incoming and outgoing radiated energy at the tropopause since
1750 due to a given cause. For carbon dioxide, this amounts to an
increase of about 1.5 Wm-2 in the last 259 years (mostly in the last 100
years). This change in energy levels is, in fact, what is driving the
atmospheric temperature up, creating the conditions for climate change.
The second graph is for methane and it can be seen
that the concentrations have more than doubled in the last 100 years or
so, even though the radiative forcing is less than that of carbon
dioxide. If this increase continues, it could catch up on the latter. It
is important to realise where this methane comes from, because there are
both natural and man-made sources. Amongst the natural ones, there is
the anaerobic decomposition of organic matter in wetlands (the true
natural gas), decomposition due to termites and the enteric fermentation
in herbivores, notably in Africa. The sudden increase over the last 100
or so years is due largely to the increased human population requiring
increased areas of rice paddies, more cattle, more landfills and, above
all, an insatiable greed for fossil natural gas. Most methane reacts,
sooner or later, with free hydroxyl radicals, from atmospheric humidity,
releasing a molecule of carbon dioxide for each molecule of methane, a
kind of double whammy!
The third graph shows a similar progression for
nitrous oxide. The natural sources are mostly from microbes in the soil
and in the ocean, from lightning strikes and from forest fires. The main
man-made ones are from nitrogen fertilisers, fossil fuel combustion and
biomass burning. It has a long lifetime because it decomposes mainly
only after it has reached the stratospheric ozone layer, where it
contributes to ozone depletion, another double whammy!
Other man-made greenhouse gases are from halocarbons.
There are virtually no natural sources of these, so any found in the
atmosphere must be of human origin. A halocarbon is any organic
substance where one or more hydrogen atoms are replaced by atoms of
fluorine, chlorine, bromine or iodine (ignoring astatine) or
combinations of them. The more stable a halocarbon is, the greater is
its effect on trapping the radiation from the earth, simply because it
can do so for longer periods of time before it decomposes. As a rule,
substances containing fluorine last longer than those containing
chlorine, in turn lasting longer than those containing bromine and so
on. Some gases containing only carbon and fluorine may last 10,000 years
before they break down, so you can imagine the damage they would cause
for tens of future generations if they were released in quantity
(fortunately they aren't). However, large quantities of substances known
as HFCs and HCFCs are used for refrigeration, aerosol propellants,
solvents etc. and these are much worse than carbon dioxide or even
methane per unit weight emitted.
The above chart, also taken from
the report of Working Group I of the Intergovernmental Panel on Climate
Change, Summary for Policymakers, of the AR4, 2007, shows the positive
and negative effects on radiative forcing (i.e., the potential climate
change) from each of the major man-made sources and the major natural
one of variations in the solar cycle. On the graphic bars, there are
extreme maximum and minimum limits shown by H-shaped bars, with the same
data between parentheses in the RF values column. The aggregate, at the
bottom obviously sums the limits, as well as average values, and it
shows that we are responsible, up to 2005, for a radiative forcing of
0.6 to 2.4 W/m² with a likely value of 1.6 W/m². It is perhaps important
to note that the tolerances of the values for the major gases is quite
small.
One influence that is very difficult to model
accurately is the positive feedback loop. A couple of examples will
illustrate this. Some arctic regions have thick peat layers on or close
to the surface which are perennially frozen (permafrost). These contain
large quantities of gases resulting from centuries of peat formation
from sphagnum moss. Amongst these gases is methane, free, dissolved or
as clathrates. As the global climate increases in temperature, the
permafrost melts, allowing the methane to escape, causing more climate
change, causing more permafrost to melt, and so on. Another example is
the decrease of glacial cover due to ice melt, laying bare darker rocks,
reducing the albedo (reflectivity of the solar radiation as outgoing
short-wave radiation) causing larger areas absorbing more energy. These
positive feedback mechanisms may "snowball", almost unpredictably, until
a new equilibrium is reached.
Equally, some phenomena may induce negative feedback
mechanisms. These always tend towards increased stability, but the new
equilibrium state is difficult to predict. One example is that increased
ocean temperatures, causing more evaporation of water, creating more
clouds whose high albedo will reflect more solar energy back to space,
thus reducing warming of the earth and the oceans, in turn, creating
less evaporation. This is why the cloud albedo effect in the above chart
has the widest margin of error.
How can the masses of global data of what has happened
in the past be used to predict future climate change? Scientists have
little faith in crystal balls, so groups around the world used estimates
of the amplitude of known causes over the past century, including
episodic phenomena like El Niño, volcanic eruptions
as well as meteorological data and natural causes, such as variations in
the sunspot cycle, orbital changes etc. and they tried to model the
exact mathematical curve that would provide the best fit to the
observations of temperature. It would then be possible to extrapolate
the different models to continue the trends forward. So, how well have
these models performed?
The above graphs, again taken
from the report of Working Group I of the Intergovernmental Panel on
Climate Change, Summary for Policymakers, of the AR4, 2007, show
the observations (black line) of global temperature, the blue areas show
the modelled results using only natural phenomena on the input, while
the pink shows that the natural plus the man-made phenomena, combined,
provide the best fit, although there is an anomaly about 1940,
especially for ocean data. For simplification, these curves are
simplified to the averages for each decade, to avoid unnecessary
busy-ness on the small dimensions. I emphasise that the ranges for the
blue curves cover the stated limits of 19 different simulations with 5
different models while the pink ones use 58 simulations from 14 models.
The correlation between the models all seem remarkably close.
The IPCC used these models for predicting future
trends, using different scenarios. They are reasonably confident about
what may happen over the next decades, even up to the end of the
century. However, the purpose of this essay is to provide as simple a
demonstration as is possible that climate change has already started and
that it is largely, but not entirely, due to human activities. I do not
propose, therefore, to discuss what may or may not happen in the future.
Quod erat demonstrandum.
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