The source of the radiant energy that sustains all life on earth is the sun. It lights and warms the earth and provides energy used by green plants to synthesise the compounds that keep them alive and serve as food for almost all other organisms. Solar energy also powers the bio-geochemical cycles and drives the climate and weather systems that distribute heat and fresh water over the earth's surface.
The sun is a gigantic gaseous fireball composed mostly of hydrogen and helium gases. Temperatures in its inner core reach 30 million degrees Fahrenheit, and pressures there are so enormous that the hydrogen nuclei are compressed and fused to form helium gas. This thermonuclear or nuclear fusion, reaction taking place at the center of the sun continually releases massive amounts of energy, which pass through a thick zone of hot gases surrounding the inner core and eventually reach the surface. There the energy is radiated into space as a spectrum of heat, light, and other forms of radiant energy that travel outward in all directions through space at a speed of 300,000 kilometers (186,000 miles) per hour.
Each type of radiant or electromagnetic radiation can be viewed as a wave with different wavelengths: the distance between the crests of one wave and the next. The longer the wavelength, the lower the energy content of a wave of radiant energy. This explains why the lower-energy, longer-wavelength types of radiant energy are not harmful to most living organisms, whereas the higher-energy, shorter-wavelength types are forms of ionizing radiation harmful to most organisms. Fortunately, most of these harmful forms of radiant energy from the sun are absorbed by molecules of ozone in the upper atmosphere and water vapour in the lower atmosphere. Without this screening effect, most life on earth could not exist.
About 34% of incoming solar radiation is reflected back to space by clouds, chemicals, and dust in the atmosphere and by the earth's surface. Most of the remaining 66% warms the atmosphere and land, evapourates water and cycles it through the biosphere, and generates winds; a tiny fraction (0.023%) is captured by green plants and used to make glucose essential to life.
Most of the incoming radiation not reflected away is degraded into longer-wavelength heat, or far-infrared radiation, in accordance with the second law of energy, and flows into space. The amount of energy returning to space as heat is affected by the presence of molecules such as water, carbon dioxide, methane, and ozone and by some forms of solid particulate matter in the atmosphere. These substances, acting as gatekeepers, allow short-wavelength radiant energy from the sun to pass through the atmosphere and back into space, but they absorb and reradiate some of the resulting longer-wavelength heat (far-infrared radiant energy) back toward the earth's surface.
Concern is growing that human activities affect global climate patterns by disrupting the rate at which incoming solar energy flows through the biosphere and returns to space as longer-wavelength heat. For example, according to some scientists, increases in the average levels of carbon dioxide in the earth's atmosphere, due primarily to the burning of fossils fuels and land clearing, may trap increasing amounts of far-infrared radiation that otherwise would escape into space, thus raising the average temperature of the atmosphere.
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gigantic, spectrum, gaseous, helium, solar, reaction, biosphere, distance, massive, zone, kilometer, portion, substance, electromagnetic, energy, ozone, effect.
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|3. A lack of vitamins can have wide-ranging effects.||The ... of potential diseases is considerable.||Lack of vitamins causes illnesses which ... from goitre to anaemia|
|4. Cotton is a textile with short fibres.||It is cheap because of the ... of the fibres.||Genetically modified cotton can ... the growing season.|
|5. There is a weak attractive force between the molecules.||Because of the ... of attraction the molecules can be separated.||Raising the temperature ... the molecular attraction.|
|6. The deluxe model is well-equipped.||The car has got first class ...||It has been ... with the latest gadgets.|
|7. When the eye becomes red, It is a symptom of bacterial infection.||... Of the eye is a symptom of conjunctivitis.||The eye ... as the bacteria spread.|
|8. How ... is a horse?||The ... of a horse can attain roughly 1,000 kg.||A horse can weigh almost 1,000 kg.|
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The oceans and the global carbon cycle | The oceans and atmospheric regulation | Snow and Ice | Human Influences | Hydrology | Vegetation | Deforestation | Carbon and Oxygen Cycles | Biogeochemical Cycles | Phosphorus Cycle |