Download What effect is human activity really having on our climate ?

What effect is human activity really having on our climate ?
PSB320 Land Development Practice 1
Richard Stanaway
School of Planning, Landscape Architecture and Surveying
Queensland University of Technology
25th October 1998
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INTRODUCTION
The development of the silicon chip has increased computational power exponentially
since the seventies allowing climatoligists to input a wide variety of climate data into
computer based modelling systems called Global Circulation Models (GCMs). GCMs
extrapolate current climate trends into the future and examine their effect. GCM
predictions in the early eighties predicted alarming sea-level rises during the next
century and were exploited by the world’s media at the time, influencing popular
public opinion. It has become apparent that many of these earlier GCMs omitted
essential components of many aspects of the climate and overgeneralised others. The
earth’s climate is an extremely complex system and many interactions between
components of the system are not yet fully understood, therefore introducing a level of
uncertainty into the modelling process. Added to this is the unreliability of much of
the climate data that is input into a GCM to enable an accurate analysis. Minor
alterations to the weighting of certain components of the climate model can also lead
to large differences in future predictions.
The climate is a very chaotic system and an unusual combination of unforseen
“surprise” minor events have been known to alter the global system drastically.
Paradoxically, major events such as a volcanic eruptions and solar activity often
appear to only have a limited influence. Climatoligists are currently ascertaining
whether human (anthropogenic) activity is influential on the future climate, or whether
anthropogenic emissions are being absorbed by the background “noise” of much
larger climate change.
This essay attempts to make sense of the conflicting conclusions made by many
climatologists. The causes of global warming and the anthropogenic inputs are
discussed, followed by a brief history of climate change during the last 100,000 years.
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THE GREENHOUSE EFFECT - An overview
Certain compounds exist in the atmosphere which absorb long wavelength energy
emitted from the earth’s surface and re-emit it back, thus increasing existing surface
temperatures. This process is referred to as the ‘greenhouse’ effect. The ‘greenhouse’
effect maintains the earth’s mean surface temperature at a pleasant 15°. The absence
of these trace ‘greenhouse’ gases would render much of the earth uninhabitable to
most life forms with a rather chilly mean temperature of -18° (Bryant, 1997)!
Naturally occurring greenhouse gases (GHGs) include water vapour which accounts
for 75% of the net ‘greenhouse’ effect, Carbon Dioxide, Methane, Nitrous Oxide and
Ozone. Atmospheric levels of these latter GHGs have been enhanced by
anthropogenic activity, particularly in the last century and has resulted in what is
referred to as the enhanced ‘greenhouse’ effect. Further, certain trace compounds such
as halocarbons (CFCs) and polyflourinated carbons (PFCs) which have accumulated
in the atmosphere since the 1930s have a surprisingly potent greenhouse potential. In
addition, many CFCs have depleted ozone levels, though fortunately the Montreal
protocol of 1987 has subsequently capped certain harmful CFC levels. Ozone
depletion is not a major influence on global warming and is not discussed further.
Enhanced global warming by the addition of GHGs is offset to a certain extent by the
build up of sulphate aerosols and other airborne particles resulting from industry and
burning. Natural processes, such as volcanic activity also enhance these inputs. These
substances prevent short-wave solar radiation from reaching the earth surface and thus
exert a cooling effect, but do not remain in the atmosphere for very long and generally
only have a local effect. The collapse of Eastern European heavy industry has already
resulted in large increases in surface temperatures in that region. Table 1 describes
quantitatively the anthropogenic inputs in the atmosphere.
Compound
Formula
Pre-industrial
concentration
3
trend
% of the
Lifetime
in
concentration
ppm/vol
in 1994
Carbon Dioxide
CO2
280
356
per
year
(%)
+0.46
total
warming
effect
63.8
atmosphere
(years)
Methane
CH4
0.8
1.7
+0.7
19.2
12-17
Halocarbons
various
nil
0.0013
+1.2
12
5-50,000
Nitrous Oxide
N2O
0.28
0.31
+0.25
5.7
120
Ozone
O3
0.007-0.08
0.01-0.1
varies
4-30
0.1
Sulphur Dioxide
SO2
0.003-0.006
0.024-0.09
varies
-22
0.02
30-100
Table 1. A summary of anthropogenic inputs that influence the ‘greenhouse’ effect (AATSC, 1995)
There appears to be a consensus that pre-industrial CO2 levels will double by 2100
resulting in global warming in the order of 1.5°-5.5° (IPCC, 1995). The global mean
surface temperature has already increased by 0.4° in the twentieth century. The most
conservative estimate of 1.5° still exceeds the rate of warming of any other period in
the last 7,000 years.
THE GREENHOUSE GASES
Carbon Dioxide
Carbon Dioxide levels have increased by 29% since the dawn of the industrial age,
currently increasing at rate of 0.46% per year at a rate that has been increasing over
the last 20 years (Bryant, 1997). 75% of this excess Carbon dioxide is produced by the
combustion of fossil fuels (coal, petroleum and gas). Before 1840 Carbon dioxide
levels were fairly constant.
Methane
Methane is 56 times more potent than CO2 in greenhouse (radiative forcing) potential
(Bryant, 1997). There has been a 115% increase in Methane levels since the preindustrial area, equating to an increase of 0.7% per year (Bryant, 1997). This rate is
now reducing slightly. Sources of atmospheric methane include; rice cultivation 22%,
flatulence 15% (increased by 40 times since WWII), decomposition of swamps, coal
mining & natural gas production 16%. Methane is a fairly chemically reactive gas
remaining in the atmosphere for only 17 years (Bryant, 1997).
Halocarbons
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Halocarbons are carbon gases that contain any halogen (fluorine, chlorine, bromine,
iodine). Halocarbons also known as CFCs are up to 8000 times more potent as a
greenhouse gas (Bryant, 1997). They are chemically stable lasting for up to 400 years
in the atmosphere. Until the Montreal Protocol of 1987 CFCs levels were rising at a
rate of 4% per year (Bryant, 1997). The increase in atmospheric levels of CFC-11
stopped in 1995 CFCs are eventually absorbed by soils and the ocean. CFCs are being
replaced by PFCs which do not deplete ozone layer but are still nonetheless potent
greenhouse gases. They have a lifetime of 50,000 years, increasing at 7% per annum
(Bryant,1997).
Nitrous oxide
Nitrous oxide is 280 times more potent than Carbon dioxide in its ‘greenhouse’
potential and levels have increased by 8% since the dawn of the industrial era. Levels
are currently increasing at a rate of 0.25% per year (Bryant, 1997). The major sources
of Nitrous oxide are; nitrate/ammonium fertilisers, followed by burning. Nitrous oxide
has a 150 year residence in the atmosphere and accounts for 10% of the greenhouse
effect.
WHAT ARE UNDISPUTED FACTS ?
All climate studies have concluded that global warming is occurring at a rate of about
0.45° degrees per century at present levels. Satellite measurements of the lower
troposphere had indicated that surface temperatures were remaining constant,
contradicting surface measurements. Sceptics had attributed increasing temperatures
to the urban ‘heat island’ effect. The world is become increasingly urbanised, and as a
consequence temperatures in cities have increased by up to 3° in the last century. The
majority of weather stations are not in urban areas however, and the bias resulting
from inclusion of urban temperature data would not account for such a large rise in the
global mean temperature. Lately, the anomaly of the satellite record has been
attributed to orbital decay, which if corrected for brings satellite measurements into
closer concurrence with the surface data record (Gaffen, 1998).
There appears to be a strong correlation between CO2 levels and global surface
temperatures, despite other influences. This correlation, surprisingly has not been
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emphasised by many climatologists but appears fairly consistent over a wide timespan. Alterations of 100 ppm (by vol) in CO2 levels have resulted consistently in
changes in the mean surface temperature of 10°, with occasional exceptions (Warrick,
et.al. 1993)
To most inhabitants on this planet, the climate does appear to be changing, noticeably.
Extreme climatic events appear to have increased in frequency during the latter part of
the twentieth century, though statistics on such events are very difficult to collate
quantitatively. Whether this variability can be attributed to anthropogenic activity is
still a matter of dispute. The earth has had a relatively stable and warm climate for the
last 6,000 years with annual variations of the mean global surface temperature rarely
exceeding 1°; for example, the so called mini ice-age and medieval warm epochs. The
last ice age (glacial maximum) where global mean temperatures were some 9° cooler
than today’s ended abruptly 11,500 years ago when the mean temperature jumped 7°
in the space of fifty years, triggering the rapid melting of the ice-caps which covered
large tracts of the Northern Hemisphere. This melting resulted in a 125 metre rise the
world’s sea level resulting in the submergence of the land bridges between continents
that had enabled the nomadic migrations to occur. At its peak 7,000 years ago, the sea
level was rising by 4cm per year, ten times current levels. Biblical and mythological
accounts of the great flood almost certainly relate to this period of human history.
WHAT HAVE BEEN CAUSES OF CLIMATE CHANGE IN THE
PAST?
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This historical fluctuation in the world’s climate has been attributed to minor changes
in the earth’s rotation, orbit and axial tilt, as well as variations in solar output. The day
of the equinox is changing by 50.256 seconds a year due to the precession of the
earth’s rotation. Again, as mentioned previously, it is almost impossible to correlate
all these factors together in order to isolate a singular dominant cause effecting
climate change.
ENVISAGED EFFECTS OF INCREASED GLOBAL TEMPERATURES
All GCMs appear to indicate the following changes;
Winters will warm more quickly than summers. The winter temperature increase will
be most evident (up to 8°) in the Northern Hemisphere continental cool temperate
regions. Central Asian and central North American summers will be 6-8° warmer.
Night-time temperatures will increase more than daytime temperatures by a factor of
three. In other words the diurnal temperature range will decrease. An increase in
surface temperatures will increase humidity levels, leading to an increase in
cloudiness and precipitation. All GCMs support the historical relationship of a 3.2%
increase in precipitation with each degree of temperature increase. Cloudiness has
already increased 3.4-9.4% this century (Bryant, 1997). The increased cloudiness will
become more noticeable in cooler climates. Clouds moderate the diurnal temperature
range by trapping heat at night, and preventing solar radiation from increasing the
daytime temperature.
Current studies suggest that the mass balance of major ice-sheets is statistically close
to zero despite an increase in mean surface temps of 2° over the current century in
polar regions. It is anticipated that the Antarctic coast will warm by around 8° by
2100. Although such a rise in temperature will increase the melting of ice in these
fringing polar regions, the loss will be negated by increased precipitation feeding back
onto the ice-sheet as snow. Recent studies indicate that increased melting of Arctic sea
ice in and around Greenland could disrupt the North Atlantic ocean circulation,
particularly the Gulf Stream which will tend to move south. Should this occur,
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Northern Europe, especially inland regions will cool substantially and return to
another mini ice age there.
The projected increase in sea-levels is mainly attributable to thermal expansion and
will rise by approximately 50cm next century if the current best estimates of enhanced
greenhouse warming are continued.
The greater increase of temperatures over the poles will result in a smaller temperature
difference between the poles and the equator. The monsoon belt will widen as a result
of the weakening effect of the trade winds. Desertification and drought will increase
at more temperate latitudes with more severe and longer lasting droughts. The
increased atmospheric dust and other particles resulting from forest fires in these
regions will provide a temporary cooling effect, by blocking out solar radiation. The
moderation of the pressure gradient will eventually lead to more stable climate, but
until the ocean temperatures “catch-up” with surface temperatures the differential will
initially lead to more atmospheric instability and storm activity in coastal regions.
Urban temperatures have increased 0.12° per decade worldwide and up to 0.45° per
decade in Australia (Bryant, 1997) . Cities in industrialised third world countries do
not show this effect, possibly due to air pollution.
Australia can expect an increase of 4° in the southern states, inland and in the dry
season, with a 1° increase in the tropics. Queensland will become wetter with a 1020% increase in precipitation during the wet season. Precipitation will decrease by
10% in the SW Australia winter.
CONCLUSIONS
The evidence presented suggests that mankind is contributing to global warming and
climate change. The unabated increase in greenhouse gases is undisputed and should
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be a matter of serious global concern. Subtle changes in global temperatures will
result in much larger changes in regional climates as has been purported. The slight
rise in sea-levels will increase the incidence of flooding resulting from a combination
of storm and tidal surges in low lying coastal areas. The increase in precipitation will
result in increased widespread flooding of flood plains around rivers. A combination
of flooding and temperature increase will almost certainly result in an increase in the
incidence of disease, especially in poorly developed countries. The intensity of
droughts and heat-waves will increase, resulting in economic depression for affected
rural populations in the developed world, and starvation for people in the third world.
The recent UN treaties and protocols on control of greenhouse gas emissions appear to
only have influence in the developed countries. Yet, even in the developed world there
appears to be no sense of urgency in making active efforts to tackle the greenhouse
problem. The militancy and extreme views held by many activists in the Green
movement seem only to encourage scepticism and scorn amongst many people, which
is unfortunate. The irony of democratic consensus is that difficult environmental
decisions are not being made quickly enough by governments. The industrialisation
of the third world, assisted by first world economic aid is really only exacerbating the
greenhouse problem. Ironically, in an effort for the vast majority of the world’s
population to increase their standard of living to approach first world standards, they
are in fact accelerating destructive natural climate cycles that will not support the
world’s population as it stands.
REFERENCES AND SELECTED READING
1.
Australian Academy of Technological Sciences and Engineering (1995) Climate Change
Science: Current Understanding and Uncertainties;
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2.
Bryant E (1997) Climate Processes and Change Cambridge University Press
3.
Gaffen DJ (1998) “Falling satellites, rising temperatures?” Nature, Vol 394, 13 August
1998
4.
Jones P.D & Hegerl, G.C. (1998) “Anthropogenic Signal Removal” Journal of Geophysical
Research Vol. 103 D12 June 27th 1998
5.
Karl T.L et.al. (1997) “The Coming Climate”, Scientific American, May 1997,
6.
Rind D. (1998) “Latitudinal Temperature Gradients and Climate Change”
Geophysical Research Vol 103 D6 27th March 1998
7.
Warrick, R.A. Barrow E M & Wigley TML (1993) Climate and sea level change:
observations, projections and implications, Cambridge University Press
Journal of
INTERNET REFERENCES
http://www.dar.csiro.au/pub/programs/climod/impacts/data.htm
CSIRO climate model site
http://www.ianr.unl.edu/pubs/generalag/g1311.htm
http://www.ipcc.ch
( UNEP’s Intergovernmental Panel on Climate Change )
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