The evidence that global warming is a reality is compelling. Our
planet’s climate is increasingly prone to extreme weather events,
and there is a steady trend towards higher temperatures.
This is triggered by the excessive concentration of carbon dioxide
(CO2) in the atmosphere. CO2 is the by-product
of burning fossil fuels. Deforestation has also diminished the planet’s
capacity to recapture CO2 from the atmosphere.
The largest share of these emissions is from energy generation, especially
coal-fired power stations. Other greenhouse gases, such as methane and nitrous
oxide, also contribute to the overall effect.
Our planet has its own greenhouse balance. After all, water vapour is the most
abundant and important greenhouse gas. In the atmosphere it increases
the temperature by about 30 °C, without which there would be no life.
The cycle of water in our planet strikes a balance between the incoming
heat and that which is dissipated into outer space. The heat that is trapped
in our atmosphere generates weather patterns, ocean currents, and is absorbed
by vegetation.
The net balance in this heat equation is zero, otherwise our planet would
either warm up or cool down. Geological cycles of warmer or cooler
global temperatures correspond to periods of time when the Sun’s
variability altered this balance. Opacity due to volcanic eruptions is another
factor too. If the Sun is going through a warmer or cooler cycle,
so is our planet. As a result, there have been past periods
of global warming and global cooling as part of a recurring
cycle and in sync with the Sun’s temperature variations.
The excess of CO2 disturbs this balance in a way
that is not cyclical. The excess heat that is trapped by CO2 is not
dissipated or consumed by weather systems in the atmosphere, so it leads
to a rise in global temperatures (see “predictions”). The heat excess simply accrues
indefinitely.
Global warming caused by CO2 is eventually a runaway
process, with warm phases are followed by warmer ones.
There are secondary effects in the process. CO2 emitted by human
activity stays at a low altitude in the atmosphere for a long
time. Therefore the greenhouse effect is most active at lower altitudes.
The immediate consequence of this is that the atmosphere is becoming
warmer at lower altitudes and cooler at higher. This gradient is the
cause of a greater occurrence of extreme weather patterns.
Global warming also suffers from “feedback” effects, i.e. self-enhancing
processes that reinforce it still further. One such effect is that with
rising temperatures the planet’s vegetation absorbs less CO2 and
therefore the greenhouse effect becomes still stronger, in turn accelerating
temperature rises.
mitigation
Global warming is a fast and dramatic process, as discussed above.
Our best chance to mitigate its effects, and ultimately to reverse them,
is by curbing our CO2 emissions drastically. In any
case, global warming will continue its course for some time, for as long as the
concentration of CO2 and other greenhouse gases remain too
high. The reason is because, even if we were able to bring all
our CO2 emissions to zero immediately, the excess concentration
of CO2 already present in the atmosphere will continue
to trap large amounts of heat from the Sun. This heat has a cumulative
effect and it is what ultimately leads to an increase in global
temperature. Unless we had a means to dissipate the extra heat, the
increase in temperature remains with us permanently.
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Emission cuts
The reality of modern life is that reducing CO2 emissions
in any significant way is an immense challenge. In the next
decades, it will be possible, with international goodwill and consensus
of the highest CO2-emitting nations to head towards a low-carbon
economy and cut CO2 emissions as much as possible. However,
the key question is whether emissions can be ultimately brought down to such
a low level as to reach a sustainable phase. In actual
terms, this would entail cutting down emissions to below 80% their actual
level. The goal would be that ultimately our planet’s capacity for carbon
capture should be able to absorb all our annual emissions. If such
a phase can be achieved, CO2 concentrations in the
atmosphere would reach a plateau and remain constant.
At present, the CO2 concentration in the atmosphere is 430 ppm
(in CO2 equivalent, if we take into account the combined
effect of all greenhouse gases). As we discussed in the earlier
sections, this is very close to the threshold of 450 ppm, where
global warming enters a more serious and possibly irreversible phase. We are
therefore at the threshold of this situation. The annual increase in concentration
is about 2.5 ppm, which corresponds to our annual emissions of about
42 giga-tonnes of CO2 (1 giga-tonne equals 1 billion
tonnes). In order to achieve a stable concentration of CO2 at around
the level of 450 ppm, we must within the next 10 years take
steps to decrease our annual emissions by about 1.5 giga-tonnes each
year, for about 30 years until 2050, when the total emissions should be below
12 giga-tonnes per annum, or in other words, less than 70% the
present level.
This is an immensely challenging undertaking. Unless we turned to nuclear
energy to generate 100% of our electricity and drastically restructured
our economies to turn away from fossil fuels in the shortest time possible,
we would have no hope of meeting such a target. Even a more
realistic target of cutting down CO2 emissions by 50% (with
respect to the current levels) by 2050, as suggested in the
G8 summit in July 2008, will prove very elusive without very specific
year-by-year targets and an immediate reduction of high-emitting sources
such as coal-fired power stations, that are currently proliferating in developing
countries.
In reality, supposing that a reduction of 70% of our emissions
were feasible by 2050, we would be only half-way to our objective.
Having stabilised the concentration of CO2 at 450 ppm,
this will continue to create a year-by-year steady trend to global
warming. Between the present time and 2050, this concentration is likely to have
increased the average global temperature by as much as 2-5°C,
and if it remained at that level thereafter, the temperature increase
by the end of the century may exceed 10 °C. With this increase
of temperatures, the feedback effects kick in and the planet’s natural
ability to capture carbon is also weakened. As a result, the
stability of CO2 concentration in the atmosphere will
be undermined by these effects. With increased warming, the release of large
amounts of CO2 stored in peat deposits will in all
likelihood make it impossible to maintain stability in the concentration
of CO2.
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Carbon sinks
Therefore, all good efforts to cut emissions drastically in the next few
decades will not achieve the desired result if CO2 concentrations
in the atmosphere remain high. The additional ingredient that is required
is carbon capture from the atmosphere in order to decrease the concentration
of CO2. This in itself will be at least a difficult
a challenge as implementing drastic emission cuts. The excess concentration
must be removed over the next few decades and returned to a safe
level, somewhere between pre-industrial concentrations (280 ppm —
the ideal level) and approximately 350 ppm (a more realistic objective).
The sooner this is done the better, because the excess heat accrued every year
will keep pushing temperatures up. The ideal timescale to readjust the
CO2 concentration to prevent dramatic global warming is about
100 years.
Mitigation is indeed possible in theory but it can only be effective
if the reduction of our annual CO2 emissions runs in parallel
with CO2 capture on quite a colossal and completely unprecedented
scale. We need to put these general principles in numbers to see
if this is realistic. The general principle is that reforestation
on a very large scale will increase the planet’s natural capacity
to absorb carbon, and will therefore offset part of our CO2 emissions.
Over time, as CO2 emissions decrease and the capacity to absorb
them increases, there will be a stage where both will equal and this is when
the CO2 concentration in the atmosphere peaks. Beyond that,
the excess capacity to absorb carbon will decrease the CO2 concentration,
and eventually restore it to the chosen level. This has to be fine-tuned
to finally reach an equilibrium, because if one was to overshoot
and increase the CO2 capture excessively, then we would transform
this problem into its diametrical opposite, creating a situation of CO2 depletion.
Mitigation model
The magnitude of the CO2 sequestration from the atmosphere
must be such that it will make a significant impact on the total
CO2 concentration on an annual basis, and the decrease
must be rapid enough to avoid irreversible global warming. Let us take
for example the scenario of aiming to reduce CO2 emissions
by 70% in 2050. The present emissions are 42 giga-tonnes of CO2 per
annum, and if one brings this down gradually over the next 40 years to 12 giga-tonnes
in 2050 we reach stability at 450 ppm. At the same
time, we offset 2.5 giga-tonnes per annum, by planting of the
order of 3 billion fast-growing trees at tropical latitudes. This
is an annual rate of reforestation equivalent to one tenth of the
surface area of the United Kingdom. The scale of this undertaking is a significant
challenge, but by no means beyond our technological abilities. This level
of offset allows us to reach stability in the CO2 concentration
at 450 ppm 10 years earlier than before, at around 2040. From
then on, the CO2 concentration will decrease every year by just
under 1 ppm. At this rate of reforestation, it will take of the
order of 100 years to restore a CO2 concentration
of about 350 ppm, having completed a reforestation of an area
equal to ten times that of the UK (or approximately one quarter
of that of the US).
By the end of that period, global warming will have increased the average
temperature by a few degrees, but continuation of this process for
a long time will eventually permit our atmosphere to dissipate this excess
heat. One must bear in mind also that the process of reforestation must
be managed to be effective, replacing trees after their natural life
cycle of about 50 years, in order to sustain the capacity of carbon
capture. After an additional 100 years, the CO2 concentration
will in all likelihood fall below pre-industrial levels, and our planet will
be able to undergo a phase of mild global cooling and dissipate
the extra heat accumulated during the period of global warming.
This illustration summarises a mitigation model where the CO2 emission
cuts and the increase of capacity of carbon capture are probably optimistic.
But one must bear in mind that both factors are needed in the equation
to combat global warming and restore the planet’s natural balance. If emissions
are cut by 50% by the middle of the century, instead of 70% as discussed
in the example, then the rate of reforestation will have to exceed
the rate we have discussed above, otherwise the CO2 concentration
in the atmosphere will not decrease at a quick enough rate to prevent
excessive global warming. On the other hand, if a reforestation rate
higher than what is described above can be achieved, then it will
allow some leeway to decrease emissions at a slower rate. Mitigation
relies on the right balance between these two competing processes.
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myths about global warming
The belief that global warming is a myth or somehow open to debate
has been discredited by all the available scientific data. The evidence for
climate change and global warming is everywhere and it comes from many
sources. Global warming is a reality that will remain with us for
as long as the concentration of CO2 in the atmosphere
is high. Our actions to mitigate global warming will not have an effect
for at least a few decades, so global warming will continue for the
best part of this century.
Research shows that the public perception is that climate change is a confusing
subject, and people don’t know how it relates to them. One reason
could be that climate change awareness has not been part of the mainstream
debate for long. People want to know how this is going to affect
them personally and whether this is just a problem for the future and
not so much for the present.
A commonly held view is that the problem is so huge that there
is nothing an individual can do to change it. This is obviously
not true, we can all do our bit to combat climate change. An individual’s
lifestyle choices can contribute to decrease our net CO2 emissions,
whether it is through small things such as using low-energy light
bulbs or with more significant decisions like fitting a wind-turbine to generate
the household’s electricity.
The problem is big but so is the force of public opinion. One
individual cannot on his own change the way electricity is generated by big
companies that supply to entire countries. If these companies generate
electricity by burning fossil fuels, then an individual’s choices
alone will have little impact in reducing CO2 emissions. But
the collective awareness and the force of public opinion can change the way
power companies operate, not only in one country but globally. This is after
all a global issue, and a single country’s mitigation policies would
go nowhere if we don’t engage in an international
effort to combat climate change.
There are many myths surrounding the causes of global warming, and here is a discussion
of two common misconceptions.
1. “Global warming is caused by the Sun”
The Sun’s output of heat and light is not exactly constant, it has
a small variability. It experiences cycles of higher and lower intensity
(“warmer” and “cooler” cycles). These variations can have
an impact on our planet’s climate but on their own they cannot
explain the level of warming that our planet has experienced in the last
decades.
Four billion years ago, the Sun emitted about one third less heat and radiation
than it does today, and has been steadily getting hotter and brighter since.
However, our planet has experienced higher global temperatures in the past
than it does today. The reason for this is that during our planet’s
early years there was an abundance of greenhouse gases that trapped much
of the Sun’s heat.
Our planet’s cooling during the ice ages was far greater than can be explained
by the small variations in the Sun’s activity. In the same way,
the warmer interglacial periods cannot be explained by the Sun alone.
All this shows that the correlation between changes in solar activity and our
planet’s climate during its lifetime is far from simple and it cannot
be understood without taking into account other effects.
During its warmer periods, the Sun shows a high incidence of sunspots
on its surface. This indicates periods of greater activity, when the Sun
emits more light and heat. In the early part of the 20th century, there
was direct link between solar activity and rising global temperatures, and satellite
measurements show that during the second half of the century, solar activity
went actually into a decline whilst global temperatures have been rising steadily.
Therefore, during the last decades other effects have played a more active
role than the solar activity, which on its own, would have indicated a decrease
in global temperatures.
It is likely that during the first half of the 20th century the warmer
Sun cycle compounded the human contribution to CO2 emissions
and both effects contributed to rising temperatures. In the later part
of the second half of the century, the cooler cycle of the Sun in all
likelihood offset the rising temperatures caused by escalating CO2 emissions
and the resulting global temperature increase is likely to have been mitigated
with respect to what it would have otherwise been the case if the
Sun continued in a warm cycle.
In addition to the human effect, there are other natural effects that
contribute to complicate the relationship between the Sun’s cycles and
our planet’s climate.
Volcanic eruptions have a significant cooling effect in our planet’s
atmosphere by releasing significant amounts of ash, which has the effect
of increasing the opacity and preventing that the Sun’s radiation from
reaching the surface. Another cooling effect is the presence of sulphate
aerosols, a by-product of burning of fossil fuels. Oddly, the contributions
of fossil fuels to global warming is made up of two competing
forces: on the one hand it produces CO2 emissions, and
on the other hand sulphate aerosols.
The figures for global temperatures prior to 1940 show that rising temperatures
can be explained by an increase in CO2 concentrations,
enhanced by an increase of solar activity. Between 1940 and
1970, global temperatures declined slightly, which is explained by strong
volcanic activity and an abundance of sulphate aerosols from a sudden
increase in the use of fossil fuels. However, from 1970 onwards,
in spite of a decline in the solar activity, global temperatures
rose above the pre-1940 levels, which can only be explained by the
fact that the concentration of CO2 in the atmosphere has
reached such a high level that it is now the dominant factor in determining
the global temperature changes.
2. “Global warming is not caused by humans: the Earth has experienced
global warming in the past”
It is true that our planet’s climate is constantly changing.
It has experienced warmer and cooler periods in the past, so our
planet is not a stranger to “natural” forms of global
warming and global cooling. It is only very recently in the Earth’s
long life that humans have the ability to make a global impact on the
environment. Even within human history, the post-industrial age that is responsible
for escalating CO2 concentrations in the atmosphere is a tiny
fraction of time. It is therefore difficult to believe that
our planet’s long-spanning natural cycles do not prevail and that they
would be disturbed by such a brief recent episode in our history.
Different parts of the world also experienced regional cycles of warmer
or colder weather. The medieval warm weather in the UK, when vines
were widely cultivated in the south of England is one such example,
or the “little ice age” during which the Thames and other rivers
in the UK regularly froze during winter.
Having said that, there are significant differences between the kind of rapidly
evolving global warming that we are seeing now and the cycles of warm
and cold global temperatures in the past. Some of these phases of global
warming and cooling were a combination of factors: changes in our
planet’s orbit in relation to the Sun, variability of the intensity
of heat emitted from the Sun, and periods of increased volcanic activity.
The combination of all three is responsible for the global cooling that
led to the ice ages. Global warming of the magnitude that has been detected
during the last century cannot be explained by any combination of these
natural effects.
In the last century, the global temperature has risen by about 0.75°C,
a rather abrupt increase compared to the very slow increases over very
long periods of time that has been the characteristic of past warm cycles.
The temperature increase is in fact getting bigger, and during the next
4 decades the increase could be up to 7 times what has been
experienced in the past century. An immediate consequence of this
is that the incidence of the years with the highest temperatures on record
will be higher in the most recent past. This pattern has been observed
already. These indicators suggest that global warming is in fact accelerating.
This can only be explained by the high concentration of CO2 in the
atmosphere, and it is consistent with the fact that the excess heat trapped
has a cumulative effect in increasing the global temperature. It is the
equivalent of compound interest of your money in the bank: the interest
adds to the capital and the increased capital generates yet more interest.
The growth is exponential. The temperature increase behaves in an identical
way: it weakens the Earth’s ability to capture carbon, releases more
CO2 from peat deposits and on the whole contributes to increase
the CO2 concentration which in turn leads to a yet
higher temperature increase.
The excess of CO2 in the atmosphere is mainly man-made
and very recent in our planet’s history. It has reached and crossed
a level at which it has a key role in driving climate change
over all other factors. Pre-industrial CO2 concentrations were about
60% of present levels and the excess is almost entirely due to the
burning of fossil fuels. Another important cause is deforestation. By diminishing
the planet’s natural ability to capture carbon, we have increased
its concentration in the atmosphere. Natural contributors such as volcanic
activity contribute less than a few percent of our industrial emissions,
so their relative impact is negligible.
The scientific evidence shows that global warming today is a different
process entirely and not comparable to past cycles of warm weather. It is exclusively
man-made and not part of a cycle. Global warming will continue for as long
as CO2 concentrations are high. By contrast, natural warm
and cold cycles in our planet have spanned long geological periods and have
been caused by natural variations in physical processes. It is very
possible that the decline of solar activity indicates the slow onset of a cooler
period in our planet’s life, which has probably offset part of the
man-made global warming during the 20th century. However, the present global warming
is a more dominant process and it will prevail over any of the
planet’s natural cycles in the future.
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predictions
Our planet will warm up considerably during this century. The increase in the
average global temperature will depend on the amount of CO2 emissions
during the next decades. Most conservative estimates predict that if our emissions
remain at the present level, the world’s average temperature will increase
by about 2-5°C in 2050 and by 5-10°C by the end
of the century.
This dramatic increase in temperature is unprecedented, and it is hard
to envisage the enormity of its impact on our planet’s ecosystems.
Several studies point out that there is a 20% chance that global
warming could still be greater, exceeding 10°C, and in addition compounded
by feedback effects which enhance the process by a further 2°C,
by the end of the century. A 20% likelihood of having a temperature
increase greater than 12°C by the end of this century is a not
a negligible probability.
The Intergovernmental Panel on Climate Change (IPCC) published in 2007 its
fourth assessment of the science of global warming. This remains the most
influential study on the subject, and we derive all our data from this
report. The Stern Review on the economic impact of global warming (also
derived from the IPCC data) is another influential and important work.
The reality of the scientific basis is that weather models are very uncertain,
owing to the chaotic processes in climatology. Even if we knew
the exact amount of CO2 that will be emitted in future,
the key question that needs to be successfully addressed is how sensitive
is the average global temperature to the increase of CO2 concentrations.
This is subject of a lot of debate but the answers are still
unclear and the models can only give us predictions on the basis of likelihoods
and confidence levels.
The IPCC assessment on global warming is the work of a large
number of scientists, and a work of consensus. Given that there is so much
uncertainty in the predictions, what is the accepted paradigm by the
IPCC is a conservative view of the extent of global warming
within the highest confidence levels; these are the predictions that we can
assume as a bottom-line of the changes that can be expected
with almost unfailing certainty. These already make a grim reading, but beyond
this, more dramatic increases are permitted by the models with significant
likelihood, because, it is perfectly possible, within a high probability,
that our planet is more sensitive to the impact of greenhouse gases
than previously thought.
The naturally occurring concentration of CO2 in the atmosphere
is 280 ppm (parts per million). This is often referred to as the
“pre-industrial” (or pre-1750) level. Today, the measured concentration
of CO2 stands at 385 ppm, so we have increased
the naturally occurring concentration by just under 40%. It is widely
accepted that a CO2 concentration of 350 ppm is the
area where the excess of industrially-generated CO2 in the
atmosphere begins to become problematic and cause measurable global warming.
We already crossed that threshold in the mid-80s. Furthermore, if we also
take into account the impact of the other main greenhouse gases, such as methane,
nitrous oxide, PFCs, HFCs, etc, their combined contribution to global warming
together with CO2 is equivalent to a total concentration
of CO2 of 430 ppm. The contribution of these greenhouse
gases adds 45 ppm to the net effect of CO2 in our
atmosphere. Therefore, if we consider that CO2 was the
only contribution to global warming, the weather models must take into account
that the reality is a CO2 concentration of 430 ppm,
or in other words, a 60.7% increase over pre-industrial levels (and
not merely 385 ppm).
It is acknowledged that a level of CO2 of 450
ppm marks a grey area where global warming enters a dramatic and perhaps
irreversible phase. This area is in an ill-defined boundary of unchartered
waters from the scientific point of view, and it is little known
when an irreversible process may begin. However, the compound effect of all
man-made contributions to global warming already adds to a balance
of 430 ppm in CO2 equivalent, and this places us on the
threshold of the 450 ppm mark. Therefore it is likely that if the
current level of emissions is maintained (even without projecting an increase),
the CO2 concentration in the atmosphere will cross the 450 ppm
mark within the next decades, reaching 550 ppm by as early as 2035.
If the projected growth in CO2 emissions takes place (rather
than curbing our emissions), we will reach a CO2 concentration
of 1,000 ppm by 2100. The increase of global temperatures will
be well above 10 °C, and the planet’s warming will be well
into an irreversible phase.
The immediate implication of the rise of CO2 concentration
in the atmosphere is a change in the energy balance in our
planet. As more heat from the Sun is trapped in our planet, the atmosphere’s
temperature rises, and so does the average temperature of the oceans.
Ultimately there is a gradual melting of ice sheets, sea ice and
glaciers, and in parallel a rise of sea levels. The cycle of water
will change completely in the atmosphere, with greater rainfall concentrations
at higher latitudes and a greater incidence of drought at subtropical
latitudes, and in particular the Mediterranean. It will also become more
extreme, with stronger hurricanes, more floods and drought events.
Global warming is an uneven phenomenon. It affects different regions
of the planet in very different ways. It is least pronounced
over oceans and coastal regions, and the temperature increases are more accentuated
at higher and mid-latitudes. Heat-waves will be a regular occurrence
in continental climates, and they will be amplified in cities, owing
to the “heat island” effect of urban areas. The predicted
collapse of the Gulf Stream and the North Atlantic drift (probably by the
end of the century) will make Europe cooler, but this effect will be offset
by global warming.
There are on the other hand “feedback effects” which worsen the
problem of global warming, but have not been scientifically quantified. It is well
known that global warming weakens the ability of vegetation to capture
carbon. With an additional projected deforestation of the tropical rainforests,
in particular in the Amazon, the ability of the planet’s “carbon
sinks” to capture CO2 from the atmosphere, will be diminished,
adding a further complication to the problem. The increase of CO2 concentration
in the atmosphere will also increase the acidity of oceans and damage
the CO2 absorbing organisms, thus weakening the “ocean sink”,
which is also another natural mechanism of CO2 capture
that the planet relies on to maintain its natural carbon cycle.
Perhaps the most significant contribution to global warming from “feedback
effects” will come from the warming of peat deposits on the Earth’s
surface, where large amounts of CO2 and methane are stored.
With rising temperatures, CO2 and methane in peat deposits,
wetlands and thawing permafrost will be gradually released into the atmosphere.
The estimated total release (over a period of time) may amount to as much
as double the cumulative emissions resulting from fossil fuels. This will add
further to the temperature increases that we have discussed above.
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