Fig. 1. The oxygen-carbon dioxide cycle.

The carbohydrates formed by photosynthesis are complex compounds of carbon, hydrogen, and oxygen, and include sugar, starch, and cellulose – the first two familiar as important nutrients, the last the chief constituent of cell walls in plants (wood is mostly cellulose). Photosynthesis can be represented by the equation

CO₂ + H₂ O + sunlight → carbohydrates + O₂

The energy in sunlight is not taken up directly by the carbon dioxide and water but instead by the substance chlorophyll, which is part of the green coloring matter of leaves; the chlorophyll is not permanently changed but serves to pass on the energy it absorbs to the reacting molecules in a complicated way. About 70 billion tons of carbon dioxide are cycled through plants each year.

The reverse reaction, called respiration, is the process by which living things obtain the energy they require for growth, motion, and so forth. Like photosynthesis, respiration occurs in a series of complex steps, but its overall result is straightforward: Carbohydrates + O₂ → CO₂ + H₂O + energy

Photosynthesis not only maintains the oxygen content of the atmosphere but was apparently responsible for it in the first place. The early atmosphere of the earth, which is thought to have consisted of gases emitted during volcanic action, contained oxygen only in combination with other elements in such compounds as water (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂). Primitive organisms, which probably obtained their own energy by fermentation, eventually began to produce free oxygen by photosynthesis, and in time the oxygen content of the atmosphere increased to the point where more complex organisms could evolve. In addition to the oxygen now present in the atmosphere, photosynthesis is believed to account for the much larger quantity combined with other elements in the oxides, carbonates, and sulfates found in sediments and sedimentary rocks.

Ex. 1. Say if these statements are true or false.

The Ozone Layer

1. Many scientists prefer to regard the stratosphere and mesosphere as being different parts of the same layer, because apart from temperature their properties are similar. Sounding balloons filled with hydrogen or helium routinely penetrate the stratosphere with instruments of various kinds and send down data by radio to ground stations; some aircraft, too, are capable of exploring the stratosphere. The still higher elevations of the mesosphere require rocket-borne apparatus if direct measurements are to be made, but a number of experimental methods have been devised that enable observatories on the ground to determine some of the physical properties of this part of the atmosphere.

2. The most striking feature of the stratosphere and the lower mesosphere is the presence of ozone, a form of oxygen whose molecules contain three oxygen atoms instead of the usual two. The chemical symbol for ozone is accordingly O₃. Ozone is an excellent absorber of ultraviolet radiation; so excellent, in fact, that the relatively small amount of ozone in the upper atmosphere completely filters out the dangerous short-wavelength ultraviolet radiation emitted by the sun. Living things on earth would certainly have evolved very differently without the protection of the ozone layer, since few of today's organisms could survive exposure to solar ultraviolet rays at their full strength.

3. The ozone layer lies between 15 and 55 km. At its maximum density less than 1 molecule in 4 million is О₃ - hardly an impressive concentration for so efficient a filter. At sea-level temperature and pressure, all the ozone in the atmosphere would form a layer less than an inch thick. The elevated temperatures of the upper stratosphere and lower mesosphere are due to the heating effect of the solar ultraviolet energy absorbed at the top of the ozone layer.

4. Why is the ozone content of the atmosphere concentrated in a particular region instead of being more or less uniformly distributed? The first step in the formation of an ozone molecule is the breaking up of an O₂ molecule into two О atoms by solar ultraviolet light. The second step is the attachment of an О atom to an O₂ molecule to form O₃. The rate of ozone production thus depends upon both the O₂ concentration and the intensity of solar ultraviolet light. At extremely high altitudes there are not enough O₂ molecules for an appreciable amount of О₃ to be formed. Between 15 and 55 km above the ground, however, the atmosphere is dense enough for the production of О₃ but not so dense that the unstable ozone molecules break up into O₂ + О too often in collisions with other molecules. At lower altitudes the ultraviolet light has already been absorbed so no ozone can come into being there except as a result of lightning strokes in the lower atmosphere.

Ex. 1. Put the sentences in the logical order.

Greenhouse gases

An increase in the greenhouse effect (caused by an increase in the concentration of greenhouse gases in the atmosphere) may lead to global warming, with disastrous consequences.

The higher average temperatures produced by global warming could cause dramatic changes in the weather. Less rain might fall over large land masses. Central Africa, south Asia and some of the United States could risk severe drought and famine. More rain might fall in coastal areas and over the oceans; there might be more storms and hurricanes in the Pacific. A rise in the earth’s average temperature of only one or two degrees would probably melt large expanses of ice in the Arctic and the Antarctic (the polar ice caps) and raise sea levels. Sea levels throughout the world are already rising by about two millimeters a year. If the polar ice caps melt, sea levels could rise by more than a meter over a few decades. Many heavily populated regions, such as Bangladesh, the Nile delta, the Netherlands and Indonesia, would be permanently flooded. Cities are often found on the coast where a river meets the sea, so many of the world’s major population centers could become uninhabitable. About one billion people would lose their homes and become environmental refugees. Some islands, such as the Maldives in the Pacific, might disappear completely.

Carbon dioxide accounts for 55 percent of the greenhouse effect; CFCs account for 17 percent; methane for 15 percent and nitrous oxide for 5 percent. Carbon dioxide occurs naturally in the atmosphere. It is produced when animals and plants respire. But “natural” carbon dioxide forms only 0.03 percent of the atmosphere. Higher concentrations of carbon dioxide are not natural at all. Carbon dioxides are produced when living things burn, so it is a by-product of industrial processes which use fossil fuels (coal, gas or oil), and motor vehicles which burn gasoline or diesel fuel. It is also produced when volcanoes erupt and when tropical rainforests are cleared by burning. Methane is also a “natural” gas, produced when living things decompose in the absence of oxygen. Methane in the atmosphere comes from rotting vegetation, particularly rice fields, and from cattle (because bacteria in their intestines produce methane). It also comes from leaks in the extraction of natural gas. Methane in the atmosphere breaks down relatively quickly (in about 10 years, compared to over 100 years for carbon dioxide and CFCs), so it is a relatively minor environmental problem. However, some scientists believe that huge quantities of methane are trapped within the polar ice caps and will be released suddenly if the polar ice caps melt. This phenomenon would accelerate global warming. Nitrous oxide in the atmosphere comes from bacteria beneath the earth’s surface, which convert nitrates in the soil to the gases nitrogen and nitrous oxide. The increased use of artificial fertilizers in recent years has increased the production of nitrous oxide. Levels of nitrous oxide in the air will continue to increase for many years, because there is already a large reservoir of artificial nitrates within the soil.

The air we breathe

Air pollution comes in many forms, but four pollutants are particularly important: the sulphur oxides, emitted mainly by power stations and industry; nitrogen oxides, emitted by power stations, industry and vehicles; carbon monoxide, emitted mainly by vehicles; and soot and dust, known technically as suspended particulate matter (SPM), found everywhere where fuels are burnt.

The industrial countries have begun to clean up their air and have invested heavily in the technology to do so. As a result, emissions of sulphur oxides in OECD countries fell from 65 to 40 million tonnes a year over the past two decades. But they increased in the rest of the world, now slowly industrializing from 48 to 59 million tonnes.

GEMS monitors air pollution at 175 sites in 75 countries. In a recent assessment of sulphur dioxide pollution in 54 cities, GEMS reports that air quality was acceptable in 27 cities, marginal in 11 (including London, New York and Hong Kong) and unacceptable in 16 (including Madrid, Paris and Rio de Janeiro). Dust and soot levels were acceptable in 8 cities, marginal in 10 and unacceptable in 23 (including Bangkok, Rio de Janeiro and Tehran).

The effects of acid rain, which prompted Sweden to help set up the UN Conference on the Human Environment 20 years ago, have not diminished over the past two decades. In fact, measurements over large areas of North America and Europe have shown that rain is often 10 times more acid than normal. Thousands of lakes have been affected in Canada, Scandinavia, Scotland and the United States – and in many of them all fish has been killed. While acid rain used to be a problem only in developed countries, the issue is emerging, or will probably soon do so, in countries such as Brazil, China, India, Jamaica, Venezuela and Zambia.

Many countries have introduced tougher laws, cleaner fuels and installed pollution control equipment. In this way Bulgaria, for example, managed to reduce emissions of SPM by 1.6 million tonnes a year during 1976 – 80. A witness to all this effort is the sale of pollution control equipment which stood at US 12.7 billion in 1991 – more than double the expenditure of 10 years previously.

During the 1980s, two new international protocols were signed on limiting emissions of sulphur and nitrogen oxides. Several countries have already gone further than these conventions required. Nine countries had pledged themselves to reduce sulphur dioxide emissions to less than half of 1980 levels by 1995; and Austria, Germany and Sweden had committed themselves to a two-thirds reduction. Twelve west European countries had agreed to reduce nitrogen oxide emissions by 30 percent by 1998.

Climate

Climate is the aggregate of day-to-day weather conditions over a period of many years. It is the result of the interaction of many different elements, the most important of which are temperature and precipitation.

Climatic patterns are a result of the interaction of three geographic controls.¹ The first is latitude. The earth is tilted on its axis² with reference to³ the plane of its orbit around the sun. As it makes its annual revolution around the sun, first the Northern Hemisphereand then the Southern are exposed to the more direct rays of the sun. During the Northern Hemisphere’s summer, higher latitude locations have longer days, with far northern points experiencing a period of continuous daylight. Daylight periods during the winter months are shorter at higher latitudes, whereas more southerly locations have both longer days and exposure to more direct rays of the sun.

The second control is based on the relationship between land and water. Land tends to heat⁴ and cool more rapidly than water. In a tendency called continentality, places far from large bodies of water experience greater seasonal extremes of temperature than do coastal communities⁵. Parts of the northern Great Plains experience annual temperature ranges close to 65⁰C; annual differences of as much as 100⁰ C (from 50⁰C to – 50⁰C) have been recorded in some locations.

The converse effect occurs at maritime locations, especially on the western coast of continents in the mid-latitudes. These locations have smaller temperature ranges as a result of what is called a maritime influence. Summer and winter extremes are moderated by the movement onshore of prevailing westerly wind systems⁶ from the ocean. Horizontal and vertical ocean currents minimize seasonal variations in the surface temperature of the water. The moderated water temperature serves to curb⁷ temperature extremes in the air mass above the surface.

Proximity to large water bodies also tends to have a positive influence⁸ on precipitation levels, with coastal locations receiving generally higher amounts. The reason for this should be obvious; large water bodies provide greater levels of evaporation and thus increase the amount of moisture in the atmosphere.

The third prime geographic influence on climate is topography. Most obvious is the relationship between elevation and temperature, with higher elevations cooler than lower elevations. The influence of topography can be broader, however, because of its effect on wind flow. If a major mountain chain lies astride a normal wind direction, the mountains force the air to rise and cool. As the air mass cools, the amount of moisture that it can hold is reduced. Precipitation results if the cooling causes the relative humidity to reach 100 percent. Moisture falls on the windward side, and the lee is dry.