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|NatureInterface > No.01 > P040-043||[Japanese]|
Global warming problem is getting more serious these days. It is said that Japanese rice might not crop anymore in Kyushu in the 21st century. The editorial staff reports the present condition of the global warming, its mechanism, and what is the greenhouse gas.
The temperature would rise by 6 degrees centigrade by the maximum in the 21st century.
How much is a rise of the temperature in the 21st century? Global warming problem is getting more serious. The "Intergovernmental Panel on Climate Change" (IPCC) <http://www.ipcc.ch/> which is organized by the United Nations Environment Programme (UNEP) <http://www.unep.ch/> and the World Meteorological Organization (WMO) <http://www.wmo.ch/> is completing the 3rd evaluation report. In this report, it is predicted that the mean temperature of the earth would rise around 1.5 degrees centigrade to 5.8 degrees centigrade by 2100. The prediction value is doubled comparing with the report in 1995, which predicted that the temperature would rise around 1.0 degree centigrade to 3.5 degrees centigrade.
The temperature rose by 0.6 degrees centigrade in the 20th century
According to the IPCC, the mean temperature rise in the 20th century was about 0.6 degrees. But the temperature did not continue rising constantly during the century. The temperature fell considerably around 1910, and it had tended to fall from the 1940s to the 70s. In the 70s, there were the opinions that a glacier time came again. However, warming progressed quickly from the 1980s (Fig. 1).
The main reason of the warming is an increase of the carbon dioxide gas due to increased consumption of fossil fuels. Although the carbon dioxide gas concentration in the atmosphere had been steady at about 280 ppm (0.028%) before the 19th century when people began to use large quantities of coal, it increased rapidly thereafter and is 356 ppm now. The carbon dioxide gas concentration has been measured in the meteorological station in Mauna Loa in Hawaii from 1958. The concentration before 1958 was investigated by analyses of the air in the ice of Greenland or elsewhere (Fig. 2).
The blue line in Fig. 2 shows the amount of consumption of fossil fuels, and it is consistent with the increase of the carbon dioxide gas concentration in the atmosphere. Fig. 3 shows the carbon dioxide emission rate in each region.
This figure shows the emission mass (Gigaton: 1 billion ton) of the carbon dioxide by fossil fuel consumption in each region. Although it was little in 1860, it reached 1 gigaton around 1930 and now it is about 6 gigatons. While developed countries such as the United States, European countries, and ex-Soviet Union take a large proportion, the ratio of Asia and Latin America are also increasing.
Not only carbon dioxide but also methane and chlorofluorocarbon gases have greenhouse effects. The rate of contribution of carbon dioxide to the warming is estimated to be at about 60 percent. Methane and chlorofluorocarbon show more powerful greenhouse effects, although only a quantity far slighter than carbon dioxide gas exists in the atmosphere. In order that chlorofluorocarbon may destroy an ozone layer, hydro chlorofluorocarbon is used as a substitute for it, but it shows further strong greenhouse effect.
Mechanism of global warming
Why does increase of greenhouse gases lead to global warming? The energy source of the earth is solar. The sun pours down electromagnetic waves, such as ultraviolet rays, visible rays, and infrared rays, upon the earth. An electromagnetic wave transmits energy in the form of a wave through space, and a solar ray is one of the electromagnetic waves with a short wavelength from 0.15 micrometers to 6 micrometers. Solar rays are called short-wave radiation, since their wavelength is short. Among the electromagnetic wave poured upon the earth, visible rays centering on the blue light have the strongest radiation. Most of ultraviolet rays are absorbed by the ozone layer and they do not reach the earth surface. Infrared rays do not arrive at the surface of the earth directly, since the rays with certain wavelengths are absorbed by water vapor (H2O) and carbon dioxide (CO2) in the atmosphere. Near-infrared rays, which are used in the everyday life equipments, such as a remote control of television and a security camera, are not visible to the naked eye. A certain portion of visible light and near-infrared rays are reflected by the clouds and dust, and return to space. Twenty one percent of solar energy arrives at the surface of the earth directly as visible rays. Some of the visible rays and infrared rays that reached the earth surface are reflected there and return to space. The rate of reflection is called albedo. A mirror and a white object have a high reflectance, or albedo. The rays that are not reflected are absorbed by the surface of the earth, and change to heat. This will be simply understood from the fact that you would feel hot in a black dress under the strong summer sunshine. The albedo of the whole earth is estimated to be 0.3, that is, about 30 percent of the rays that reached the surface of the earth are reflected.
The light and infrared rays, absorbed by the clouds and the atmosphere are converted into heat, and the heat reach the ground. This heat is 28% of the solar energy. Adding it to the visible light, which is 21% of solar energy as previously mentioned, about half of the solar energy reaches to the surface of the earth.
The surface of the earth emits heat as much as the energy it received. Otherwise, the earth surface will get hot rapidly. The energy emitted from the earth surface as heat has the electromagnetic wave centering on the wavelength of about 15 micrometers. The wave is called far-infrared rays. Far-infrared rays are called long-wave radiation, since its wavelength is relatively longer than solar light's wavelength.
Far-infrared rays are easily absorbed by so-called greenhouse gases, such as water vapor, carbon dioxide, methane, and chlorofluorocarbon. Far-infrared rays heat the atmosphere and the atmosphere emits far-infrared rays again upward and downward. The upward rays are emitted to space and the downward ones are radiated upon the surface of the earth again.
Is a greenhouse gas harmful?
A greenhouse gas is not harmful under normal conditions. It stores the heat from the sun in the daytime and emits it at night. If there were no greenhouse gases, the morning temperature would be rather low. This is why it is very cold at night in deserts with less water vapor though it is hot in the daytime.
The temperature on a cloudy day does not fall a lot because there is much water vapor. Greenhouse gases make the temperature difference in a day little so that our life is comfortable. However, high concentration of greenhouse gas causes a problem. It leads to global warming since the heat absorbed by greenhouse gases increase and the re-radiation to the ground becomes larger.
Why the IPCC estimate was reexamined?
The reason why the IPCC estimated that global warming rate became higher was not due to unexpected increase of carbon dioxide concentration. Eruptions of volcanoes and artificial sulfurous acid gas prevent the direct solar energy to the earth. Much sulfurous acid gas is emitted from the United States, Europe, and China.
As shown in Fig. 5, the direct radiation from the sun is inhibited by the sulfurous acid gas discharged by industrial activities. In the central part of North America, Europe, and China, radiation of 1 W m-2, which accounts for about 2 % of direct radiation, is inhibited. Furthermore, direct radiation is inhibited by smoke and dust in urban cities.
However, if the emission rate of sulfurous acid gas decreases by the anti-pollution measures, direct radiation would increase and global warming would be promoted. The use of chlorofluorocarbon was forbidden because it destroys the ozone layer. But the substitute hydro chlorofluorocarbon has stronger greenhouse effect and it may promote global warming.
Figure 1 Change of the mean temperature of the earth (cited from the IPCC Second Assessment Report.)
Figure 2 Increase of carbon dioxide concentration (cited from the IPCC Scientific assessment workgroup report)
CO2 emission rate from fossil fuels
CO2 concentration of the atmosphere
CO2 emission rate
CO2 emission rate from fossil fuels in 100 years
Figure 3 Increase of carbon dioxide mass due to fossil fuel consumption (cited from the IPCC Scientific assessment workgroup report.)
Asian planned economy states (including China)
The rest of Asia
The Middle East and North Africa
Central Europe and East Europe
OECD countries in Pacific Ocean
Figure 4 The mechanism of global warming
A part of energy (short-wave radiation) from the sun (1) reflects at the surface and returns to space (2).
A part of solar energy absorbed by the earth surface heats the earth surface (3).
Far-infrared rays are emitted from the surface of the earth that was heated (4).
Greenhouse gases, such as carbon dioxide, in the atmosphere absorb far-infrared rays (5)
And the heated gases emit heat towards the surface again (6).
Radiation in a short wavelength range (visible rays, ultraviolet ray, etc.)
Direct rays to the earth surface
Surface of the earth
Reflection at the earth surface
Radiation in a long wavelength range (infrared rays)
Directly to the outside of the atmosphere
Absorption by atmosphere
Radiation to the surface of the earth
Radiation from the atmosphere
Heat transportation other than radiation
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