1. Greenhouse Effect
It is said that the temperature of the Earth's surface averages about 15 degrees C, but how do we arrive at this figure ?
The only energy the Earth receives from outside sources are the light energy radiated by the Sun. Every day, the Earth is irradiated with huge amounts of solar energy. If this energy had been simply accumulated on our planet, the Earth would just become warmer and warmer each day.
The Earth, however, releases the same amount of energy to the space, in other words, the incoming and outgoing energy remains in equilibrium. As a result, the ground surface temperature remains at an average of 15 degrees C over many years.
It is worth noting that most of the energy received from the Sun is in the form of visible light, that are, of course, easy to see. The energy radiated to space from the Earth, however, is not visible; because the Earth's temperature is relatively low, this energy is emitted in the form of infrared light.
The Earth's atmosphere contains various gases such as water vapor, carbon dioxide (CO2), and ozone in very small amounts. The components of these gases have two characteristics relevant to this discussion: they are transparent to visible light, yet they absorb infrared light to some extent.
As a result of this process, the energy from the Sun reaches the surface of the Earth mainly in the form of visible light. The energy released to space from the surface of the Earth is given off in the form of infrared light, and, part of this energy is to be absorbed by the various gases in the atmosphere.
If the Earth's atmosphere did not contain gas components capable of absorbing infrared light, the temperature of the surface of the ground would be minus 18 degrees C. In reality, however, the surface of the ground is 15 degrees C, a full 33 degrees C higher. This is because the atmosphere serves to prevent the cooling of the planet's surface by absorbing infrared light. The function of the atmosphere is called as "greenhouse effect" and gasses which has this effect are called as "greenhouse gas".
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Water vapor which has also strong greenhouse effect, is not usually called as "greenhouse gas", because its concentration Can$ be controlled by human being
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Many years before the Industrial Revolution, the concentration of carbon
dioxide in the atmosphere (measured as a volume ratio compared to dry air)
was about 0.028 percent (280 ppmv). Recently, however, the concentration
of carbon dioxide has rapidly increased due to massive depletion of forests
and massive consumption of fossil fuels: it now reached about 0.036 percent
(360 ppmv).
According to the results of observations, the average temperature of
the Earth became about 0.6 degrees C higher during the past one hundred years,
and it is said that a part of this increment has been a result of the growing
amount of carbon dioxide in the atmosphere.
If the concentration of carbon dioxide continues to increase, the greenhouse
effect will become more and more serious, increasing our concern of an acceleration
of global warming.
As
mentioned previously, carbon dioxide is not the only greenhouse gas. Methane,
nitrous oxide, CFCs/HCFCs/HFCs, ozone, and water vapor also contribute to the
greenhouse effect. A comparison of the relative greenhouse effect of each gas
reveals that nearly two-thirds of the total greenhouse effect can be attributed
to carbon dioxide. Therefore, as a temporary step and as a most relevant
approach to prevent global warming, regulating carbon dioxide emissions is
planned.
To discuss from another point of view, influence of the greenhouse gases
on the Earth's surface temperature depends on to which extent the gas in
question absorbs the infrared radiation from the surface of the Earth.
Methane, for example, absorbs comparatively larger degree of infrared radiation
than an equivalent amount of carbon dioxide. Still, its concentration in
the atmosphere is less than that of carbon dioxide, so it contributes less
to the greenhouse effect than carbon dioxide. Concerning CFCs and HCFCs,
their contribution to the greenhouse effect is rather significant because
their infrared-radiation-absorption-rate is quite large, although the actual
concentration of CFCs and HCFCs is quite small.
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On the other hand, there is a specific life time for each gas. Therefore,
it is necessary to take this life time into consideration, when analyzing
the potential effect on the Earth's surface temperature in the future entailed
by the increase of the here concerned greenhouse gases. For this purpose,
a concept called as GWP (global warming potential) was introduced. The GWP
is an index used for monitoring the influence on the temperature of the
Earth surface due to the increase of the concerned greenhouse gas, compared
to the effect due to same amount of carbon dioxide.
The production of CFCs has been prohibited by the Montreal protocol in order
to prevent the destruction of the ozone layer. However the reduction of
CFCs, HCFCs and HFCs is also an important measure concerning the global
warming (cf. the table).
3. The observation network of Carbon Dioxide
A precise measurement of the carbon dioxide concentration started by Dr.
C. D. Keeling of the Scripps Institution of Oceanography in 1958, at the
NOAA's (National Oceanic and Atmospheric Administration) observation station
located on Mt. Mauna-Loa in Hawaii island, and NOAA also started same measurement
from 1974(See below).
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After that, various countries started the measurement of the carbon dioxide. In
Japan, measurements program shown below is taking place. (But these are just a
few examples and more observations are being operated.)
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These observations proved that the concentration of the carbon dioxide in the
atmosphere is steadily increasing. Now, how can we estimate the concentration
of the carbon dioxide in the past?
It can be measured by air accumulated in the “ice core” drilled from the
ice bed on the Antarctic and on the Greenland.
In the following figure, both CO2 concentration measured by the ice core and observation of Mt. Mouna Loa
are presented.
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An analysis of these study revealed that, (1) Many years before the Industrial
Revolution, the concentration of carbon dioxide in the atmosphere (measured
as a volume ratio compared to dry air) was about 0.028 percent (280 ppmv),
and, (2) Recently, the concentration of carbon dioxide has rapidly increased
due to massive depletion of forests and consumption of fossil fuels: it
now measures about 0.036 percent (360 ppmv).
As shown already, the average temperature of the Earth has increased about
0.6 degrees C during last one hundred years, according to the results of
various observations, and it is said that a part of this increment has been
a result of the growing amount of carbon dioxide in the atmosphere.
If the concentration of carbon dioxide continues to increase in this manner,
the greenhouse effect will become more and more conspicuous, increasing
our concern on the acceleration of global warming.
4. The Influence of Global Warming
To establish effective countermeasures against global warming, it is
essential to predict the future global warming by using model calculation,
etc.
The WMO and the United Nations Environment Program (UNEP) established together
the Intergovernmental Panel on Climate Change (IPCC) in 1988, and are seeking
the participation of scientists from many countries to undertake a scientific
and technical evaluation of global warming. IPCC members include many Japanese
researchers from related official agencies and universities.
The first assessment report issued by IPCC in 1990 provided the scientific basis for the UN Framework Convention on Climate Change, the signing of which took place at the United Nations Conference on Environment and Development (Earth Summit) in Rio de Janeiro in 1992.
IPCC released its second assessment report(SAR) in 1995, which forecasts
that the average surface temperature of the Earth will rise about 2 degrees
C with a corresponding 50 cm rise of the sea water level by the year 2100,
as a most probable case when we do not make effective countermeasures. (Range
of the forecast for rise in temperature and sea water level is 1.0-3.5 degrees
C and 15-95cm)
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Projections of IPCC SAR
IPCC released its third assessment report(TAR) in Apr/2001, which forecasts
that the average surface temperature of the Earth will rise 1.4-5.8 degrees
C with a corresponding 9-88cm rise of the sea water level by the year 2100.
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Projections of IPCC TAR
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Projected Pattern by Models
These estimations were based on the results of climate model calculations
carried out by major research organizations in various countries. These values,
however, are less than certain, so it is necessary to make efforts to improve
their accuracy in the future.
Although the average temperature rise is
estimated as only 2 degrees C, this value should be considered as very serious,
because the temperature difference between ice age and now is only 5 degrees
C.
Also, the IPCC concluded that the temperature rise in local areas
seriously diverges from the averaged temperature rise of 2 degrees C. In other
words, the temperature rise in polar areas would be higher than average, and
less in the tropic areas.
If this estimation is correct, granary areas spread in the middle latitude
will be critically affected, and the desertification will be accelerated
in the areas having a desertification tendency. The forest distribution
will also be affected, because it is impossible to relocate their territory
to more comfortable areas in such a short period.
Furthermore, it is concerned that some island countries and some big cities
developed near the shoreline could be below the sea level, because of the
sea level rise following the global warming.
5. Predicting Global Warming
As discussed already, there is no doubt scientifically that increasing emissions
of greenhouse gas are contributing to global warming. Therefore, majority
of the scientists are considering that the average temperature of the Earth
continues its gradual rise in the future.
However, it is quite difficult to forecast how much the temperature will
rise by year 2XXX, how much the temperature will rise on a certain part
of the Earth, or how and where the climate will change.
Answering such questions and accurately predicting the future warming trends,
and to estimate the details of the global warming, it is required to understand
the chemical and physical processes involved in the atmosphere. In other
word, to predict the future climate change it is necessary to make mathematical
models including these mechanisms, to express in numerical formula, and
to run it in the super-computers.
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In the climate model, as the first step, space near the surface of the
Earth is divided into the tremendous number of pieces. Surface of the Earth
is divided into many plates, in which each plate has size of 100 km × 100
km for example, then, vertically sliced into 10 layers for the atmosphere
and 5 layers for the ocean, resulting a lot of grids on the Earth.
Second step is to input virtual climate data or actually observed climate
data on some day/month/year at each grid, and to input many equations governing
the climate at each grid.
The final step is to calculate future climate, i.e., the super-computer
can estimate the climate of next day using the equations in it, then, continuing
these calculation for a period of 50 year for example (this process is call
as integration), it would be possible to estimate future climate change.
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The following figure shows the results of the model calculation done by the CCSR, the NIES and the FRCGC. For this calculation, has used the “Earth Simulator”, one of the best super-computers having tremendous capability at this moment.
CCSR: Center for Climate System Research, University of Tokyo
NIES:
The National Institute for Environmental Studies
FRCGC: Frontier
Observational Research System for Global Change
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There are lots of climate models developed by many institutes in many countries, however, these models are not enough to predict the future climate change precisely at this moment, and it is required continuous improvements.
Then, it is important to obtain enough number of observed data for the
atmosphere and the ocean, because the ocean does accumulate more than 50
times more the carbon compared to the air in a form of non-organic carbon,
various rocks and plankton.
In other words, it is essential to improve the observation system both in/on the ocean and in the atmosphere, to accumulate the observed data, and to clarify CO2 flux between atmosphere and ocean, and finally, to reflect these results to the climate model, to improve the forecast accuracy.
For this purpose, enhancing the observation network would be important for
us. The carbon, which consists carbon dioxide, changes its appearance to
many kinds of chemical compound and circulates between atmosphere, ocean,
biosphere. Just taking a focus on “carbon cycle”, which is one of the most
basic factors for the forecast of the climate change, still remained unclear
in many points.
6. Halocarbon
CFCs, HCFCs and HFCs are familiar for us because they are used as refrigerant
for air conditioning systems and refrigerators.
As shown in following figure the CFCs have the similar construction as the
hydrocarbon (in this case, methane), except all hydrogen atoms are replaced
by halogen atoms such as chlorine and fluorine.
Thus, CFC (Chloro-Fluoro-Carbon) means a carbonic compound with chlorine
and fluorine.
There are several kinds of CFCs having different characteristics, and these
differences are due to the different combinations of carbon, chlorine and
fluorine. The structure of CFC-11 is shown on the figure.
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CFCs have been used as refrigerants for air conditioning systems and
refrigerators, because they are chemically stable and non-toxic, and also have
very practical boiling points enabling their use as a refrigerant.
On the other hand, CFCs are so stable, that once emitted to the atmosphere,
they are not resolved in the troposphere and continue climbing to the stratosphere,
at which they will be resolved by UV radiation from the Sun. This process
is known as “photo-dissociation” and CFCs will release the chlorine atoms
by this reaction.
The chlorine atoms released from CFCs will form chlorine monoxide (ClO),
and the ClOx (Cl ＋ ClO) will act as catalyst and break down the ozone in
the stratosphere.
Depleting ozone by this process, might cause skin
cancer patients going to rise, and affect agricultural products. Therefore, the
production of CFCs was prohibited by the Montreal protocol to prevent the
destruction of the ozone layer.
In order to solve this problem, it is
necessary to decrease the chemical stability of CFCs, because if there are some
components having less stability, they will be destroyed in the troposphere and
they won't be able to reach to the stratosphere.
The HCFCs are developed for
this purpose. In the HCFCs, a part of hydrogen atoms is retained instead of
complete replacement by chlorine atoms or fluorine atoms, resulting to a less
stable compound in the troposphere. The structure of HCFC-21 is shown on
following figure.
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To prevent the release of the chlorine atoms completely, it is necessary to remove all of the chlorine atoms from the compound. This kind of chemical compound is called as the HFCs. Shown in following figure is the structure of HFC-23.
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