Download The greenhouse effect and global warming

Topic 6: Global Warming
The greenhouse effect
and global warming
In this topic we will consider:
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The natural greenhouse effect and greenhouse gases
The recent increase in greenhouse gases and the enhanced greenhouse effect.
Potential effects of global warming
Feedback mechanisms associated with global warming
Pollution management strategies to address global warming; global and local
Arguments surrounding global warming (for and against)
Conflicting perceptions about global warming/climate change
Topic 6: Global Warming
The greenhouse effect
(6.1.1)
A garden greenhouse keeps plants warmer than they would be outside. It does this because the
glass traps some of the Sun’s radiation energy. The atmosphere keeps the Earth warm in a similar
way. This is because the energy entering our atmosphere is mostly short wave, which includes
light energy and energy leaving the planet is mostly long wave, which includes heat energy. Some
long wave radiation is absorbed by greenhouse gases before it escapes from the atmosphere and
is reflected back to Earth. Without the greenhouse effect the Earth would be about 33°C cooler
than today’s pleasant average of 15°C. The 'greenhouse effect' is a normal and necessary
condition for life on Earth.
Look at the diagram on the front page. Neatly write the following labels onto the diagram in the
correct places.
Natural greenhouse effect: http://sciencebitz.com/?page_id=582
Simulation: http://www.sumanasinc.com/webcontent/animations/content/greenhouse.html
P135 – ESS, Rutherford
If there was no atmosphere and the greenhouse effect did not exist, what would the normal
temperature of the Earth be?
Greenhouse gases
Historically, the main greenhouse
gases were water vapour, carbon
dioxide, nitrous oxide and methane.
Evidence for the link between the
concentration of carbon dioxide in
the atmosphere and global mean
temperatures over the last quarter
of a million years is shown in the
graphs. Similar data exists going
back hundreds of millions of years.
How was this data collected?
What conclusion can you draw from the two graphs?
Topic 6: Global Warming
The effect of greenhouse gas increases is called the enhanced greenhouse effect or simply
global warming.
Recent evidence for global warming
The following graph shows data collected
at one of the most unpolluted places on
Earth, Mauna Loa, Hawaii in the middle of
the Pacific Ocean.
What is the increase in mean CO2 concentration in the last 40 years?
Look at the more detailed inset.
Why do CO2 levels decrease between May and September every year?
Why do the CO2 levels climb higher each year from October until May?
Topic 6: Global Warming
Global mean temperatures since the Industrial Revolution
The graph below shows the Central England Temperatures, or CET data, which go back to 1660
when thermometers were first used.
The 20th century has clearly had the warmest in recent history. In the last 100 years the average
temperature has increased by 0.6°C, but this warming has speeded up, jumping 0.5°C in the last
25 years. The 10 hottest years since records began have been since 1990. Computer modelling is
now used to predict future temperature increases, which currently are between 1.4 – 5.8°C.
What evidence is there that methane, nitrous oxide and CFC-11 may also be contributing to global
warming? Look at the graph on p137 of ESS Rutherford.
What do we mean by the term ‘’Radiative forcing’’?
What does the term albedo mean? Why is it so important?
Topic 6: Global Warming
Anthropogenic greenhouse gas emissions
(6.1.2)
The list of greenhouse gases now includes carbon dioxide, water vapour, nitrous oxide, methane
and chlorofluorocarbons (CFCs). Complete the table from the information on p137 of ESS
Rutherford:
Greenhouse
gas
Pre-industrial
concentration
Present
concentration
Greenhouse
effect
% greenhouse
effect
CO2
Methane, CH4
Nitrous oxide
CFC-11
Why is water vapour seldom mentioned as a greenhouse gas?
Anthropogenic sources of greenhouse gases
Complete the table with sources due to human activities.
Greenhouse gas
Sources due to human activities
Carbon dioxide
Methane
Ozone
Nitrous Oxide
CFC’s (chloroflurocarbons)
Water vapour
Atmospheric life
times / years
Topic 6: Global Warming
Use your Rutherford book (p133 onwards) and the information pack to complete these questions.
a) Approximately what proportion of world greenhouse gas emissions come from energy?
b) Which part of the energy sector emits most greenhouse gases (by volume)?
c) What are the main greenhouse gases emitted? What is their relative % contribution in world
emissions?
d) Apart from the energy sector, what are the main sources of greenhouse gases?
e) What are the main sources of
(i) methane?
(ii) nitrous oxides?
Topic 6: Global Warming
Effects of global warming
(6.1.3)
The potential effects of global warming will affect human societies and biodiversity. Shifting
climate belts will move biomes, affecting local weather, biodiversity, agriculture and human
health. Melting ice caps and glaciers combined with thermal expansion of the seas will lead to
coastal inundation and loss of some island states. Explain the above effects with examples and
data where possible.
Shifting climatic belts and biomes
What are the possible trends? Complete the diagrams.
Effect 1: Biodiversity
Effect 2: Agriculture
Effect 3: Human health
Melting ice caps and glaciers combined with thermal expansion of the seas
Read Annex B ‘The big melt’. Highlight examples of these two issues.
Topic 6: Global Warming
Coastal inundation
Loss of nation states
Topic 6: Global Warming
Feedback mechanisms and global warming
(1.1.6 & 6.1.4)
Review of feedback
The changes associated with global warming are long-term and are most easily understood in
terms of feedback.
Feedback is the return of part of the output from a system as input, so as to affect
succeeding outputs.
There are two kinds of feedback:
Negative feedback is feedback that tends to reduce or counteract any deviation from an
equilibrium, and promotes stability.
For example, increased evaporation in tropical latitudes leads to increased snowfall on the polar
ice caps, which reduces the mean global temperature.
Positive feedback is feedback that amplifies or increases change; it leads to exponential
deviation away from an equilibrium.
For example, increased thawing of permafrost leading to an increase in methane levels, which
increases the mean global temperature.
Read the following article and highlight instances of positive feedback. Draw a diagram to
summarise the links.
Feedbacks that could be paybacks
Particularly alarming are the possibilities of indirect
effects of global warming that could further
accelerate climatic changes. These are known as
‘positive feedbacks’ and so far they have not been
adequately accounted for in the climate models.
With the warming of the oceans and the surface air
above them, evaporation would increase, increasing
the amount of water vapour in the air. Water vapour
is in fact the most potent natural greenhouse gas,
and any increase, caused indirectly by warming due
to increases in other greenhouse gas concentrations,
would further trap heat. Frank Wentz, a physicist at
Remote Sensing Systems of Santa Rosa, California,
analysed data from three NASA satellites to come
up with the alarming conclusion that this feedback
has already begun. During the 1990s he found the
amount of water vapour in the atmosphere had gone
up by two per cent.
Once positive feedbacks are triggered, they could go
on to trigger others, leading to runaway warming —
the models may not predict it but that it is possible
cannot be denied. Here is a hypothetical but not
entirely improbable doomsday scenario. As
greenhouse gases build up in the atmosphere,
temperatures rise. Forests begin to dry and die back
or burn. Felling continues apace, diminishing forests’
ability to fix atmospheric carbon dioxide. Areas
under ice melt to expose the Earth below, which
begins to soak up the sun’s heat instead of reflecting
it. Long-frozen dead tundra vegetation begins to
decompose releasing more carbon dioxide and
methane. The seas, swollen by rising temperatures
and melting polar ice, swallow densely populated
coastal regions. With warming seas also begin to lose
their ability to absorb carbon dioxide and could
start releasing the gas already dissolved in them —
estimated at 50 times the amount contained in the
atmosphere... and so on. As vicious circles go this one
is hard to beat.
As the 20th century closed, the warming speeded
up, with average global temperatures jumping 0.5°C
in the last 25 years. This would be the equivalent of
2°C per century. However the amount of change to
which ecosystems can adapt is estimated at a
maximum of 1°C over a century. And that is if no
further changes are expected. But as Thomas Karl,
director of the National Climate Data Centre put it,
‘We are already experiencing the rate of warming
predicted right through this coming century. And
there could be worse to come, as today’s effects are
believed to be mainly the work of carbon dioxide
emitted half a century ago. The much higher levels
of emissions today are damage we are storing up for
the future. Also to be factored into the equation are
the effects of sulphate aerosols, by-products of
industrial pollution which have masked the
greenhouse effect by their cooling properties. But
they have a short atmospheric lifetime and as
cleaner production processes become more desirable
in our increasingly polluted world, their role could
well decline, revealing the true extent of warming.
Topic 6: Global Warming
For more examples of positive feedback read the rest of this article, Annex B The big
melt and Annex C Carbon store.
Draw a Diagram of the potential feedback processes that you have highlighted.
Topic 6: Global Warming
Pollution management strategies
(6.1.5)
Creating international commitments stimulate local action and to change personal lifestyles
are three ways to tackle global warming.
International action: a timeline of agreements and commitments for action
1979
1988
1990
1992
1995
1997
2001
2004
First World Climate Conference. Climate change officially recognized as a serious problem
needing an international response when evidence of increasing carbon dioxide levels
established.
Intergovernmental Panel on Climate Change, IPCC established by United Nations
Environment Programme (UNEP) and the World Meteorological Organization. The IPCC is a
collaborative activity comprising over 2000 climate scientists worldwide. Its main activity to
provide in regular intervals an assessment of the state of knowledge on climate change
First IPCC Report on Climate Change. The Report confirmed that climate change was a reality
and was supported by scientific data.
Rio Earth Summit (United Nations Conference on Environment and Development). United
Nations Framework Convention on Climate Change (UNFCCC) signed by 154 governments.
The objective of the Convention is to stabilize greenhouse gas concentrations. The
governments of developed or annex I nations were voluntarily committed to developing national
strategies for reducing greenhouse gas emissions to 1990 levels by the year 2000.
First UNFCCC conference. Governments recognized that voluntary commitments were
inadequate and work started to draft a protocol for adoption at the third Conference of
Parties in 1997. Second IPCC report concludes that the balance of evidence suggests a
discernible human influence on the global climate.
The Kyoto Protocol signed by some 160 nations at third UNFCCC conference. The Protocol
calls for the first ever legally binding commitments to reduce carbon dioxide and five other
greenhouse gas emissions to 5.2 per cent below 1990 levels before 2012.
Third IPCC Report states that anthropogenic emissions will raise global mean temperature by
5.8°C by 2050. The fourth IPCC Report is due in 2007.
The Kyoto protocol is still ineffective! For the Kyoto Protocol to be effective at least 55
countries have to ratify (fully adopt the commitments) and there must be enough annex I
(developed) countries who together are accountable for ore than 55% of the emissions
according to the 1990 levels. However the percentage of annex I countries is only 37.5%.
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Why is the protocol not on track?
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What three ways can Annex I countries have to meet their Protocol commitments
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Local action: Agenda 21, the blueprint for sustainable development
(6.1.5)
As adopted at the 1992 Rio Earth summit urges countries to
 promote national energy efficiency and emissions standards
 tax industries in ways that encourage, clean, safe technologies
 transfer clean technologies to developing countries
 integrate energy, environment and economic policies in a sustainable manner
 develop efficient, cost-effective, less polluting and safe rural and urban mass
transport systems.
Topic 6: Global Warming
National policies
List as many ways as possible that countries could reduce greenhouse gas emissions
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What alternative energy sources could be developed?
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What is meant by a carbon tax? What would it be used for?
Personal lifestyles
List as many ways as you can that you could help reduce local greenhouse emissions:
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Evaluation of management strategies
Which of the above strategies would be the most effective in reducing greenhouse gases?
Consider whether people would cooperate, technology availability, etc.
Which potential solutions are ecocentric or technocentric?
Topic 6: Global Warming
Arguments surrounding global warming
(6.1.6)
Read the article Annex A Reaping the whirlwind.
Do you need any more evidence that climate is changing?
Contrasting human perceptions of the issue of global warming
Individual and group perceptions
(6.1.7)
Topic 6: Global Warming
Global warming review questions
1.
Explain what is meant by the green house effect.
2.
Why is the greenhouse effect normally of benefit to the Earth and its organisms?
3.
List 4 greenhouse gases in order of importance.
4.
Explain the sources of these four greenhouse gases.
5.
Describe the increase in the levels of carbon dioxide i.e. how much has it increased,
when did the increase begin and what are the main causes of the increase in carbon
dioxide level?
6.
What are the main sources of methane, CFC’s and water vapour, the other greenhouse
gases?
7.
Have their levels been increasing and if so, why?
8.
Explain how at least four human activities such as deforestation, burning fossil fuels,
rice and cattle farming and use of CFC’s add to greenhouse gases.
9.
Discuss four ways in which global emission of greenhouse gases can be reduced
(include conservation of energy (including carbon tax) and use of alternative energy
sources).
10.
What is global warming and how is it related to the greenhouse effect?
11.
Why might the following processes occur due to global warming: (j) thermal expansion
of the oceans, (ii) melting of the polar ice caps, (iii) increased evaporation in tropical
latitudes leading to increased snow fall on the polar ice caps, triggering a new ice age,
(iv) the effect of air pollutants (aerosols) in reflecting radiation, thus offsetting the
warming trends.
12.
How may global warming affect the planetary distribution of biomes?
13.
If the distribution of biomes changes what impacts might this have on global
agriculture.
14.
What international measures have been taken to combat global warming. How
successful have these measures been’
15.
List five ways in which emissions of greenhouse gases can be reduced in your local
community.
16.
Discuss the complexities and uncertainties surrounding the issue of climate change.
Topic 6: Global Warming
Syllabus
6.1.1 Describe the role of greenhouse gases in maintaining mean global temperature. 2
The greenhouse effect is a normal and necessary condition for life on Earth. Consider carbon
dioxide (CO2) levels in geological times.
6.1.2 Describe how human activities add to greenhouse gases.
2
Water, CO2, methane and CFCs are the main greenhouse gases. Human activities are increasing
levels of CO2, methane and CFCs in the atmosphere which may lead to global warming.
6.1.3 Discuss qualitatively the potential effects of increased mean global temperature.
3
Consider the potential effects on the distribution of biomes, global agriculture and human
societies. Students should appreciate that effects might be adverse or beneficial, e.g. biomes
shifting
change in location of crop growing areas
changed weather patterns
coastal inundation (due to thermal expansion of the oceans and melting of the polar ice
caps)
human health (spread of tropical diseases).
6.1.4 Discuss the feedback mechanisms that would be associated with an increase in mean global
temperature.
3
6.1.5 Describe and evaluate pollution management strategies to address the issue of global
warming.
2, 3
Include: global—intergovernmental and international agreements, carbon tax, alternative
energy sources
local—allow students to explore their own lifestyle in the context of local greenhouse gas
emissions
preventive and reactive.
6.1.6 Outline the arguments that are surrounding global warming.
2
Students should appreciate the variety of sometimes conflicting arguments surrounding this
issue. Note the complexity of the problem and the uncertainty of global climate models (see
1.1.10).
6.1.7 Evaluate contrasting human perceptions of the issue of global warming.
3
Students should explore individual and group perceptions.
1.1.6 Define and explain the principles of positive feedback and negative feedback.
1, 3
The self-regulation of natural systems is achieved by the attainment of equilibrium through
feedback systems. Negative feedback is a self-regulating method of control leading to the
maintenance of a steady-state equilibrium—it counteracts deviation, eg El Niño and La Niña.
Positive feedback leads to increasing change in a system—it accelerates deviation. Feedback links
involve time lags.
1.1.10 Evaluate the strengths and limitations of models.
3
A model is a simplified description designed to show the structure or workings of an object,
system or concept. In practice, some models require approximation techniques to be used.
For example, predictive models of climate change may give very different results. In
contrast, an aquarium may be a relatively simple ecosystem but demonstrates many ecological
concepts.
Topic 6: Global Warming
Annex A Reaping the whirlwind
Extreme weather prompts unprecedented global warming alert
The Independent 03 July, 2003
In an astonishing announcement on global warming and
extreme weather, the World Meteorological
Organisation signaled last night that the world's
weather is going haywire. In a startling report, the
WMO, which normally produces detailed scientific
reports and staid statistics at the year's end,
highlighted record extremes in weather and climate
occurring all over the world in recent weeks, from
Switzerland's hottest-ever June to a record month
for tornadoes in the United States - and linked them
to climate change. The unprecedented warning takes
its force and significance from the fact that it is not
coming from Greenpeace or Friends of the Earth, but
from an impeccably respected UN organisation that is
not given to hyperbole (though environmentalists will
seize on it to claim that the direst warnings of
climate change are being borne out). The Genevabased body, to which the weather services of 185
countries contribute, takes the view that events this
year in Europe, America and Asia are so remarkable
that the world needs to be made aware of it
immediately. The extreme weather it documents, such
as record high and low temperatures, record rainfall
and record storms in different parts of the world, is
consistent with predictions of global warming.
Supercomputer models show that, as the atmosphere
warms, the climate not only becomes hotter but much
more unstable. "Recent scientific assessments
indicate that, as the global temperatures continue to
warm due to climate change, the number and intensity
of extreme events might increase," the WMO said,
giving a striking series of examples. In southern
France, record temperatures were recorded in June,
rising above 40C in places - temperatures of 5C to 7C
above the average. In Switzerland, it was the hottest
June in at least 250 years, environmental historians
said. In Geneva, since 29 May, daytime temperatures
have not fallen below 25C, making it the hottest June
recorded. In the United States, there were 562 May
tornadoes, which caused 41 deaths. This set a record
for any month. The previous record was 399 in June
1992. In India, this year's pre-monsoon heatwave
brought peak temperatures of 45C - 2C to 5C above
the norm. At least 1,400 people died in India due to
the hot weather. In Sri Lanka, heavy rainfall from
Tropical Cyclone 01B exacerbated wet conditions,
resulting in flooding and landslides and killing at least
300 people. The infrastructure and economy of
southwest Sri Lanka was heavily damaged. A reduction
of 20-30 per cent is expected in the output of lowgrown tea in the next three months. Last month was
also the hottest in England and Wales since 1976,
with average temperatures of 16C. The WMO said:
"These record extreme events (high temperatures,
low temperatures and high rainfall amounts and
droughts) all go into calculating the monthly and
annual averages, which, for temperatures, have been
gradually increasing over the past 100 years. "New
record extreme events occur every year somewhere
in the globe, but in recent years the number of such
extremes have been increasing. "According to recent
climate-change scientific assessment reports of the
joint WMO/United Nations Environmental Programme
Intergovernmental Panel on Climate Change, the global
average surface temperature has increased since
1861. Over the 20th century the increase has been
around 0.6C. "New analyses of proxy data for the
northern hemisphere indicate that the increase in
temperature in the 20th century is likely to have been
the largest in any century during the past 1,000
years." While the trend towards warmer
temperatures has been uneven over the past century,
the trend since 1976 is roughly three times that for
the whole period. Global average land and sea surface
temperatures in May 2003 were the second highest
since records began in 1880. Considering land
temperatures only, last May was the warmest on
record. It is possible that 2003 will be the hottest
year ever recorded. The 10 hottest years in the 143year-old global temperature record have now all been
since 1990, with the three hottest being 1998, 2002
and 2001. The unstable world of climate change has
long been a prediction. Now, the WMO says, it is a
reality.
Topic 6: Global Warming
Annex B The big melt
In March 2000 the World Watch Institute in
Washington sent out an urgent message that the
Earth’s ice cover was melting at a far speedier rate
than previously predicted: a clear signal that greenhouse gases were heating up the planet.
As this big melt becomes a reality, thoughts turn to
the fears of rising sea levels that have beset many
small island nations and low-lying coastal regions. The
British Meteorological Office’s Hadley Centre for
Climate Prediction and Research, a leading establishment in the field of climate modelling, forecasts an
almost 16-inch (40 centimetres) rise in sea waters by
2080 if nothing is done about greenhouse gas emissions. This would mean that annual floods could
threaten an estimated 94 million people — up from the
13 million at present. The coastal regions of southern
and Southeast Asia would be the worst affected, as
storm surges could push sea water deep inland.
For about 40 low-lying island nations worldwide the
combination of fiercer storms and sea-level rise could
spell complete disaster. Already the United Nations
Environment Program (UNEP) has recommended the
evacuation of Tarawa atoll, part of the Pacific island
nation of Kiribati. Some small islands fringing Kiribati
have disappeared under the waters. Roads have had to
be moved inland on the main island as the ocean gnaws
into the shore. In June 2000 New Zealand (Aotearoa)
made a promise of sanctuary to inhabitants of Tuvalu if
their coral-atoll sank under the sea. These are the
canaries in global warming’s coalmine. Yet the sea-level
rise that threatens them and which figures in the
Hadley Center’s predictions is due by and large to the
thermal expansion of the warmer sea-waters. Now
account also needs to be taken of the vast volumes of
water that could be released from the world’s large ice
masses.
The Antarctic Peninsula has reported a sustained
warming as high as 2.5° C. In places scientists have
observed that rocks that have been covered by ice for
millennia have begun to poke through. In the mid 1990s
a roughly 8,000 square kilometre ice-shelf, Larsen A,
broke away into the sea, looking — as an observing
scientist put it - ‘like bits of polystyrene foam smashed
by a child.’
According to the World Watch Institute report, the
ice sheet that covers the Arctic Ocean has lost 40 per
cent of its volume over the last 30 years and could be
completely gone in a matter of decades. This wouldn’t
make sea levels rise as the ice lies upon water to begin
with. But the loss of land-based ice is another story.
The Arctic’s Greenland ice sheet has had more than 1
metre shaved off it each year since 1993 along its
southern and eastern edge.
In Antarctica, three great ice sheets have gone completely, but whether land ice is also melting as rapidly
is a matter of dispute. Some studies have suggested
that the smaller of the continent’s two land ice sheets
(with a mass the size of Mexico) is melting faster than
normal. If the West Antarctic ice sheet were to collapse, the seas would surge by a catastrophic 6 meters.’
To put this into perspective, it is estimated that a 1metre rise in sea level could flood many of the world’s
major coastal cities, such as New York, London and
Bangkok and swallow up three per cent of total land
area. Significantly, 30 per cent of the world’s
croplands could be lost.”
In the Netherlands, the famous sea defences would no
longer be able to protect large areas that are under
sea level. In April 2000 the inhabitants of the coastal
village of Bergen on See were told that the authorities
could no longer guarantee their safety due to rises in
sea levels. The village, whose name in English can read
as ‘Bergen on Sea’, could become ‘Bergen in Sea’. Cities
like Amsterdam and Rotterdam could follow suit in the
event of larger rises.
And it’s not just sea ice that is melting — all over the
world glaciers are in retreat as well, because the summer melt is more than can be replenished. The
Quelccaya glacier in Peru is retreating 10 times faster
than a decade ago, threatening water supplies for
Lima’s 10 million people.
Himalayan glaciers are, according to a UN study,
‘receding faster than in any part of the world’. If the
Worldwatch report’s prediction of shrinkage by a fifth
in this region in the next 35 years appears alarmist to
some, the UN study is even starker, threatening the
complete disappearance of these glaciers in the same
time frame. As environmental journalist Fred Pearce
put it ‘Their eventual disappearance is a potential
catastrophe for the hundreds of millions of people in
southern Asia, who depend on the summer melt even
more than the monsoon rains to irrigate their crops and
provide drinking water.
For the 6,000 people whose lives are at risk from the
brimming Tsho Rolpa glacial lake in Nepal, the situation
is more urgent. Dozens of such lakes have formed high
in the Himalayas in recent history, with only the debris
left behind by retreating glaciers damming them in.
About every three years, one bursts sending a wall of
water rushing down the valleys.
Topic 6: Global Warming
Annex C Carbon store
With temperatures in the high latitudes rising the
most rapidly not only is the temperature record of
the various layers of ice being lost as newly fallen
snow often melts completely in the spring thaw, but
there is the possibility that vegetation covered by
permafrost in the northern hemisphere since the
time when dinosaurs roamed the much warmer Earth
could be laid bare. This decomposing matter forms a
carbon store estimated at as much as 450 billion
metric tonnes, which could release enormous
quantities of carbon dioxide and methane into the
atmosphere.
The attack on the world’s forests has left us with
just a third of the land functioning as an intact
forest ecosystem than was once under forest cover.
Trees store carbon and draw it down to produce
food, but dead, burnt or cleared forests turn from
being carbon sinks to carbon sources. One hectare
of tropical rainforest contains between 100 and 250
tonnes of carbon in the form of organic matter (a
figure which is much higher if one includes the
carbon stored in the soil), three-quarters of which
could be liberated by burning or decomposition.
With such forests disappearing at the rate of one
per cent a year throughout the 1990s and countries
often being forced to exploit the economic potential
of their timber to service their international debts
the outlook is less than bright. The feedback
mechanism from rainforest destruction would not
only directly impact on warming but could also result
in a decline in rainfall in these areas.
Another potential feedback is the release of
methane from methane hydrates. These resemble
ice in that they are solid, but they are actually an
unstable mixture of water and methane forming at
low temperatures under considerable water pressure
in the oceans. An essential factor for their
formation is the presence of a thick enough layer of
sediments to generate the methane in the first
place. In the Arctic with its colder water
temperatures, less pressure is required for the
hydrates to form and as a result they occur in much
shallower waters and could conceivably be
destabilised if the water warms enough.
It is not known for sure but there may be many tens
of if not hundreds of billions of tonnes. And since
there is a mere five billion tonnes of carbon in
today’s stock of atmospheric methane, only a little
methane hydrate would need to be melted to boost
the greenhouse effect significantly.
It is just such considerations that don’t figure in climate models, but the potential they have to start a
chain of runaway feedbacks, however distant it may
appear, needs to be acknowledged. And as long as
that potential remains we need to act, because once
the juggernaut starts rolling it is too late to stop it.
This is known as the ‘precautionary principle’ in the
climate debate and the time to apply it is now. For
with each bit of corroborative evidence to support
global warming, the outline becomes clearer. Waiting
for the picture to be complete would be waiting
helplessly for catastrophe.