Env-107 - Lecture noteProf. Dr. Md. Anisur Rahman Khan (ARK)

Climate Change and Global Warming

Climate refers to the representative or characteristic atmospheric conditions for a region onEarth. The term climate refers to these conditions over long time periods, such as seasons, years, or decades, whereas weather refers to shorter periods of time, such as hours, days, or weeks.

When we say it's hot and humid in Dhaka today or raining in Chittgong, we are speaking of weather. When we say Rajsahi has cool, wet winters and warm, dry summers, we are referring to the Rajsahi climate. Climate depends in part on precipitation and temperature, both of which show tremendous variability on a global scale. Because the climate of a particular location may depend on extreme or infrequent conditions, however, climate is more than just the average temperature and precipitation of a region.

Important Terminologies

Atmospheric pressure: Pressure is force per unit area. Atmospheric pressure is the weight of overlying atmosphere (air) per unit area; it decreases as altitude increases because there is less weight from overlying air. At sea level atmospheric pressure is 14.7 lb/in 2.

Temperature refers to relative hotness or coldness of materials, such as air, water, soil, and living organisms. In a quantitative sense, temperature is a measure of the thermal energy (heat) of a substance and is measured with a thermometer.

Saturtion: When air holds the maximum amount of water that it can, given its particular temperature, it is said to be saturated, which means that no more water vapor may be added to the air.Humidity is the water vapor content of air. The relative humidity expressed as a percentage, is a measure of how close the air is to saturation. For example, a relative humidity of 100% means that the air is completely saturated.

Wind results from atmospheric (air) pressure differences, with wind speed and direction dependent on the extent of the horizontal variation in atmospheric pressure, the deflection effect due to the Earth’s rotation, and friction with the Earth’s surface.

Wind speeds are to a large degree a function of acceleration of air from regions of high pressure to regions of low pressure

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Global Air Circulation Pattern

The atmosphere is a dynamic system, changing continuously. Winds, the movement of clouds, and transitions from stormy to clear skies indicate that the atmosphere changes rapidly and continuously. This atmospheric circulation takes place on a variety of scales. On a global scale, atmospheric circulation results primarily from the Earth’s rotation and the differential heating of Earth’s surface and atmosphere. It is within the tropics that the surface receives the most intense insulation and, therefore, is heated most strongly. The excess heat energy in the equatorial regions causes the air to rise. As the hot equatorial moist air rises in the troposphere, it cools by expansion and loss of water, and then sinks again.

The air circulation patterns in which this occurs are called Hadley cells. There are three major groupings of these cells, which result in very distinct climatic regions on Earth’s surface. The air in the Hadley cells does not move straight north and south, but is deflected by Earth’s rotation and by contact with the rotating earth; this is the Coriolis effect, which results in spiral-shaped air circulation patterns called cyclonic or anti-cyclonic, depending upon the direction of rotation. These give rise to different directions of prevailing winds, depending on latitude. The air and wind circulation patterns shift massive amounts of energy over long distances on Earth. If it weren’t for this effect, the equatorial regions would be unbearably hot, and the regions closer to the poles intolerably cold. About half of the heat that is redistributed is carried as sensible heat by air circulation, almost 1/3 is carried by water vapor as latent heat, and the remaining approximately 20% by ocean currents.

Weather

At every moment at any spot on the earth, the troposphere (the inner layer of the atmosphere containing most of the earth's air) has a particular set of physical properties. Examples are (1) temperature, (2) pressure, (3) humidity, (4) precipitation, (5) sunshine, (6) cloud cover, and (7) wind direction and speed. These short-term properties of the troposphere at a particular place and time are weather. Meteorologists use weather balloons, aircraft, ships, radar, satellites, and other devices to obtain data on variables such as (1) atmospheric pressures, (2) precipitation, (3) temperatures, (4) wind speeds, and (5) locations of air masses and fronts.

Warm Fronts and Cold Fronts: Masses of air that are warm or cold, wet or dry, and contain air at high or low pressure constantly move across the land and sea. Weather changes as one air mass replaces or meets another. The most dramatic changes in weather occur along a front, the boundary between two air masses with different temperatures and densities.

A warm front is the boundary between an advancing warm air mass and the cooler one it is replacing Because warm air is less dense (weighs less per unit of volume) than cool air, an advancing warm front will rise up over a mass of cool air. As the warm front rises, its moisture begins condensing into droplets to form layers of clouds at different altitudes. High, wispy clouds are the first signs of an advancing warm front. Gradually the clouds thicken, descend to a lower altitude, and often release their moisture as rainfall. A moist warm front can bring days of cloudy skies and drizzle.

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A cold front is the leading edge of an advancing mass of cold air. Because cold air is denser than warm air, an advancing cold front stays close to the ground and wedges underneath less dense warmer air. An approaching cold front produces rapidly moving, towering clouds called thunderheads. As the overlying mass of warm air is pushed upward, it cools and its water vapor condenses to form large and heavy droplets that fall to the earth's surface as precipitation. As a cold front passes through, we often experience high surface winds and thunderstorms. After the front passes through, we usually have cooler temperatures and a clear sky.

Highs and Lows: Weather is also affected by changes in atmospheric pressure. Air pres-sure results from the zillions of tiny molecules of the gases (mostly nitrogen and oxygen) in the atmosphere zipping around at incredible speeds and hitting and bouncing off of anything they encounter.

Gravity affects atmospheric pressure. Pressure is greater near the earth's surface because the molecules in the atmosphere are squeezed together under the weight of the air above.An air mass with high pressure, called a high, contains cool, dense air that descends toward the earth's surface and becomes warmer. Fair weather follows as long as the high-pressure air mass remains over an area.

In contrast, a low-pressure air mass, called a low, produces cloudy and sometimes stormy weather. This happens because less dense warm air spirals inward toward the center of a low-pressure air mass. Because of its low pressure and low density, the center of the low rises, and its warm air expands and cools. When the temperature drops below the dew point, the moisture in the air condenses and forms clouds. If the droplets in the clouds coalesce into large and heavy drops, precipitation occurs.

Tornadoes and Tropical Cyclone: In addition to normal weather, we sometimes experience weather extremes. Two examples are violent storms called tornadoes (which form over land) and tropical cyclones (which form over warm ocean waters and sometimes pass over coastal land).

Strong tornadoes often have two or more smaller vortexes or funnel-shaped clouds that move around the center of a larger vortex. These funnel-shaped clouds often are black or red because of the dust and dirt sucked up from the ground.

Tropical cyclones that form in the Atlantic Ocean are called hurricanes; those forming in the Pacific Ocean are called typhoons. Tropical cyclones take a long time to form and gain strength. As a result, meteorologists can (1) track their path and wind speeds and (2) warn people in areas likely to be hit by these violent storms.

Hurricanes and typhoons can kill and injure people and damage property and agricultural production. In some cases, however, a tropical cyclone can have long-term ecological and economic benefits that can exceed its short-term negative effects.

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Climate

Climate is a region's general pattern of atmospheric or weather conditions over a long period. Average temperature and average precipitation are the two main factors determining a region's climate and its effects on people is a generalized map of the earth's major climate zones.The simplest classification of climate is by latitude-tropical, subtropical, midaltitudinal (continental), subarctic (continental), and arctic. Several other categories are used as well.

Different climatic zones

A German botanist and climatologist, Wladimir Koppen classified world climate into six zones in the early part of the 20th century as:

Humid equatorial, Dry, humid Temperate, Humid cold, Cold polar, and Highland climate—

and each is subdivided into climate types. But several other categories are necessary, including humid continental, Mediterranean, monsoon, desert, and tropical wet-dry, among others.

Humid Equatorial Climate: No dry season, Short dry season, Dry winter;Dry Climate: Semiarid, Arid; Humid Temperate Climates: No dry season, Dry winter, and Dry summer;Humid Cold Climate: No dry season, dry winter;Cold Polar Climate: Tundra and ice;Highland Climate: Unclassified highlands.

Humid equatorial climate: Every month is warm; mean temperature is over 18 C.Dry climate: Evaporation exceeds precipitation in most months.Humid temperate climate: Distinct winter and summer seasons; winters are mild; mean temperature in coldest month is above 3 0C.Humid cold climate: Distinct winter and summer seasons; winters are cold; mean temperature in coldest month is above 3 0C.Cold polar climate: Distinct winter and summer seasons; winters are long and extremely cold; mean temperature in the warmest month is below 10 0C.Highland climate: Climate changes as elevation increases and from one side of the mountain to the other; characteristic of high plateaus and mountains.

Similar climates produce similar kinds of ecosystems. This concept is important and useful for environmental science. Knowing the climate, we can predict a great deal about what kinds of life we will find in an area and what kinds could survive there if introduced.

Microclimates: Various topographic features of the earth's surface create local climatic conditions, or microclimates, that differ from the general climate of a region. For example, mountains interrupt the flow of prevailing surface winds and the movement of storms. When moist air blowing

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inland from an ocean reaches a mountain range, it cools as it is forced to rise and expand. This causes the air to lose most of its moisture as rain and snow on the windward (wind-facing) slopes. As the drier air mass flows down the leeward (away from the wind) slopes, it draws moisture out of the plants and soil over which it passes. The lower precipitation and the resulting semiarid or arid conditions on the leeward side of high mountains are called the rain shadow effect. Cities also create distinct microclimates. Bricks, concrete, asphalt, and other building materials absorb and hold heat, and buildings block wind flow. Motor vehicles and the climate control systems of buildings release large quantities of heat and pollutants. As a result, cities tend to have more haze and smog, higher temperatures, and lower wind speeds than the sur rounding countryside.

Why Do Different Organisms Live in Different Places? Why is one area of the earth's land surface a desert, another a grassland, and another a forest? Why do different types of deserts, grasslands, and forests exist?

The general answer to these questions is differences in climate caused mostly by differences in average temperature and precipitation caused by global air circulation. Different climates promote different communities of organisms.

Effect Global Air Circulation on Regional Climates

The temperature and precipitation patterns that lead to different climates are caused primarily by the way air circulates over the earth's surface. The following factors determine global air circulation patterns:

Uneven heating of the earth's surface because air is heated much more at the equator (where the sun's rays strike directly throughout the year) than at the poles (where sunlight strikes at an angle and thus is spread out over a much greater area). These differences in incoming solar energy help explain why tropical regions near the equator are hot, (2) polar regions are cold, and (3) temperate regions in between generally have intermediate average temperatures

Seasonal changes in temperature and precipitation because the earth's axis (an imaginary line connecting the north and south poles) is tilted. As a result, various regions are tipped toward or away from the sun as the earth makes its yearlong revolution around the sun

Rotation of the earth on its axis, which prevents air currents from moving due north and south from the equator. Forces in the atmosphere created by this rotation deflect winds (moving air masses) to the right in the northern hemisphere and to the left in the southern hemisphere. This results in the formation of six huge convection cells of swirling air masses (three north and three south of the equator) that transfer heat and water from one area to another

Long-term variations in the amount of solar energy striking the earth. These are caused by occasional changes in solar output and slight planetary shifts in

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which the earth's axis wobbles (22,000-year cycle) and tilts (44,000-year cycle) as it revolves around the sun.

Properties of air and water. Heat from the sun evaporates ocean water and transfers heat from the oceans to the atmosphere, especially near the hot equator. This creates cyclical convection cells that transport heat and water from one area to another (Figure 6-10). The resulting convection cells circulate air, heat, and moisture both vertically and from place to place in the troposphere. This leads to different climates and patterns of vegetation.

How Do Ocean Currents Form, and How Do They Affect Regional Climates?

The factors just listed, plus differences in water density, create warm and cold ocean currents. These currents, driven by winds and the earth's rotation (1) redistribute heat received from the sun and (2) thus influence climate and vegetation, especially near coastal areas.

For example, without the warm Gulf Stream, which transports 25 times more water than all the world's rivers, the climate of northwestern Europe would be subarctic. If the ocean's currents suddenly stopped flowing, there would be deserts in the tropics and thick ice sheets over northern Europe, Siberia, and Canada. Currents also help mix ocean waters and distribute nutrients and dissolved oxygen needed by aquatic organisms.

Upwellings: The winds blowing along some steep western coasts of continents towards the equator create an effect (called the Ekman spiral) that pushes surface water at right angles from the wind flow away from the land. This outgoing surface water is replaced by an upwelling of cold, nutrient rich bottom water. Upwellings, whether far from shore or near shore (1) bring plant nutrients from the deeper parts of the ocean to the surface and (2) support large populations of phytoplankton, zooplankton, fish, and fish-eating seabirds.

El Nino-Southern Oscillation (El Nino): Every few years in the Pacific Ocean, normal shore upwellings are affected by changes in climate patterns called the El Nino-Southern Oscillation, or ENSO. In an ENSO, often called El Nino, (1) the prevailing westerly winds weaken or cease, (2) surface water along the South and North American coasts becomes warmer, and (3) the normal upwellings of cold, nutrient-rich water are suppressed, which reduces primary productivity and causes a sharp decline in the populations of some fish species.A strong ENSO can trigger extreme weather changes over at least two-thirds of the globe, especially in lands along the Pacific and Indian Oceans Figure shows the occurrence of ENSOs .

La Nina: Sometimes an El Nino is followed by its cooling counterpart, La Nina . Typically a La Nina means (1) more Atlantic Ocean hurricanes, (2) colder winters in Canada and the Northeast, (3) warmer and drier winters in the southeastern and southwestern United States, (4) wetter winters in the Pacific Northwest, (5) torrential rains in Southeast Asia, (6) lower wheat yields in Argentina, and (7) more wildfires in Florida.

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The Natural and Anthropogenic Factors That Drive Climatic Change

What factors cause Earth's climate to change? The search for answers has been difficult becausethe climate system is very complicated, with many interacting subsystems. The factors that influence climate vary on time scales ranging from seasons, to centuries, to millions of years. Furthermore, modern human impacts on the climate system are making it increasingly difficult to separate natural from anthropogenic influences. We know that Earth's climate will change; the record of past climatic change is very clear. What we lack is a comprehensive understanding of how it will change and at what rate.

Several mechanisms cause natural climatic change. They involve the atmosphere, the lithosphere, the ocean, and the biosphere interacting in complex ways. Geographic changes resulting from tectonism-the shifting of continents, the uplift of continental crust and creation of large mountain chains, and the opening or closing of ocean basins-have a significant impact on oceanic and atmospheric circulation and therefore on global climate. Even Earth's internal processes affect climate. Large, explosive volcanic eruptions sometimes produce vast quantities of dust and tiny aerosol droplets of sulfuric acid, both of which scatter and block the Sun's incoming radiation and cause global cooling.

Earth's orbit and rotation play an important role in controlling the timing and cyclicity of variations in climate. The eccentricity (departure from circularity) of Earth's orbit; the tilt of the planet's axis of rotation; the precession (wobbling) of the axis; and the timing of perihelion (closest approach to the Sun) all have an impact on insolation, the amount of solar radiation reaching the surface at any given time. Periodic changes in climate caused by fluctuations in insolation that result front variations in Earth's orbital and rotational characteristics are called Milankovitch cycles, after the Yugoslavian mathematician who first recognized this phenomenon and its impact on the timing of glacial-interglacial cycles.

When processes like plate tectonics, volcanism, or orbital variations drive or profoundly influence the global climate system, it is referred to as climate forcing. The external forcing influences on climate are modified by the filtering action of the atmosphere-that is, the greenhouse effect (introduced in chapter 3). Acting somewhat like an actual greenhouse, radiatively active (greenhouse) gases in the atmosphere absorb outgoing infrared radiation, trapping heat and raising the surface temperature.

Global Warming: Earth's Energy Balance and the Greenhouse Effect

Global warming is defined as a natural or human-induced increase in the average global temperature of the atmosphere near the Earth's surface. The temperature at or near the surface of the Earth is determined by four main factors:

The amount of sunlight Earth receives. The amount of sunlight Earth reflects. Retention of heat by the atmosphere. Evaporation and condensation of water vapor.

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The Greenhouse EffectSunlight that reaches Earth warms both the atmosphere and the surface. Earth's surface and

atmospheric system then re-radiate heat as infrared radiation. Certain gases in Earth's atmosphere absorb and re-emit this radiation. Some of it returns to the Earth's surface, making Earth warmer than it otherwise would be. In trapping heat, the gases act a little like the panes of glass in a greenhouse (although the process by which the heat is trapped is not the same as in a greenhouse). Accordingly, the effect is called the greenhouse effect, and the gases-which include water vapor, carbon dioxide, methane, and chlorofluorocarbons (CFCs)-are called greenhouse gases.

Chemical Makeup of the Atmosphere that Lead to the Greenhouse EffectSmall amounts of certain gases play a key role in determining the earth's average temperatures and thus its climates. These gases include (1) water vapor (H20), (2) carbon dioxide (C02), (3) methane (CH4), (4) nitrous oxide (N20), and (5) synthetic chlorofluorocarbons (CFCs).

Together, these gases, known as greenhouse gases, act somewhat like the glass panes of a greenhouse: They allow mostly visible light and some infrared radiation and ultraviolet (UV) radiation from the sun to pass through the troposphere. The earth's surface absorbs much of this solar energy. This transforms it to longer wavelength infrared radiation (heat), which then rises into the troposphere.

Some of this heat escapes into space, and some is absorbed by molecules of greenhouse gases and emitted into the troposphere as even longer wavelength infrared radiation, which warms the air. This natural warming effect of the troposphere is called the green-house effect.

The basic principle behind the natural greenhouse effect is well established. Indeed, without its current greenhouse gases (especially water vapor, which is found in the largest concentration), the earth would be a cold and mostly lifeless planet.

The anthropogenic greenhouse effect is related to the following three factors:

The burning of fossil fuels, which in recent years has added about 5.5 GtC (gigatons, or billions of metric tons, of carbon) per year to the atmosphere. The carbon combines with oxygen to form C02. The Intergovernmental Panel on Climate Change (IPCC) reports present emissions of carbon to be 13% higher than this average, at about 6.3 GtC per year.

Deforestation, by burning trees, increases the concentration of atmospheric C02,

adding 1.6 GtC per year. Burning trees releases carbon stored in the wood that combines with oxygen to form C02

Human activities that emit other greenhouse gases, such as CFCs, ozone, methane, and nitrous oxides.

Human activities such as burning fossil fuels, clearing forests, and growing crops release carbon dioxide, methane, and nitrous oxide into the atmosphere. There is concern that large inputs of these greenhouse gases into the troposphere can enhance the earth's natural greenhouse effect and lead to global warming. If correct, this could (1) alter precipitation

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patterns, (2) shift areas where we can grow crops, (3) raise average sea levels, and (4) shift areas where some types of plants and animals can live.

Table: Some Important Greenhouse Gases, Common Sources, and Relative Contributions to the Enhanced (Anthropogenic) Greenhouse Effect

Greenhouse Gas Sources Contribution to EnhancedGreenhouse Effect

Carbon dioxide (COz) Fossil fuel burning, landconversion, deforestation

-55%

Methane (CH4) Rice culture, cattle, biomassburning, landfills, peatlands

-16%

Nitrous oxide (NA Fertilizers, land conversion,transportation

-5%

Chlorofluorocarbons(CFCs)

Refrigerants, aerosols,industrial solvents

-10%

Tropospheric ozone (03) Secondary pollutant (fromphotochemical reactions)

-14%

Water vapor (H20) Irrigation, industrialprocesses, enhancedevaporation

Unknown

Carbon Dioxide (CO2)Carbon dioxide is the most important anthropogenic greenhouse gas. Carbon dioxide is released from the interior of the Earth (volcanoes), and produced by respiration of biota, soil processes, oceanic evaporation, as well as human activities, such as deforestation (accounts for approx. 20% of the annual increase in this gas) and combustion of fossil fuels. Deforestation could be sending

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an annual 4 billion tonnes of carbon dioxide into the atmosphere that would otherwise be taken up by plants during their metabolic processes. Automobile exhausts account for 30% of carbon dioxide emissions in developed countries. The generation of carbon dioxide as a result of combustion of fossil fuels suggests that if present trends continue, then its concentration will double every 50 years. Methane (CH4)Methane is produced by methanogenic bacteria anytime organic matter decomposes anaerobically. These conditions are typically found in flooded soils, in the digestive systems of ruminant animals, and when organic wastes are stored or handled as liquids. Major natural contributors are termites as they process wood. Today, rice culture, domestic ruminant animals, landfills, and our own use of methane as a fuel source provide additional methane to the atmosphere. About 70% of current emissions of methane are anthropogenic in origin. Methane contributes 15 to 20% to the anthropogenic greenhouse effect. The short atmospheric lifetime of methane suggests that efforts to reduce methane emissions could be effective in slowing the increase in the rate of global warming.

Chlorofluorocarbons (CFCs)Chlorofluorocarbons (CF2Cl2 and CFCl3), better known as ‘freons’ F-11 and F-12, respectively, are entirely anthropogenically produced, are very persistent compounds used in spray cans as aerosol propellants and in refrigeration units, and were not present in the atmosphere until the 1930s. Due to their persistence, and to their high efficiency in absorbing thermal IR, each CFC molecule has the potential to cause the same amount of global warming, as do tens of thousands of carbon dioxide molecules. It has been estimated that approximately 15 to 25% of the anthropogenic greenhouse effect may be related to CFCs in the atmosphere. Because CFCs are highly stable compounds, their residence time in the atmosphere is long. Even if production of these chemicals is reduced drastically or eliminated within the next few years, their concentrations in the atmosphere will remain significant for many years, perhaps for as long as a century.

Nitrous Oxide (N2O)Nitrous oxide, which is relatively inert, originates primarily from microbial activity (also abiotic means) in soils and in the oceans, by industrial combustion, automobiles, aircraft, biomass burning, and as a result of the use of chemical fertilizers. Various microbial pathways produce N2O, including nitrification and denitrification. It is released from all soils regardless of cultivation or fertilization. Nitrous oxide emissions due to fertilizer use would be expected to increase worldwide since nitrogen fertilizer use is increasing, particularly in countries with high population densities and increasing food demands. Nitrous oxide is a more potent greenhouse gas than carbon dioxide or methane, although it contributes only 5% to the anthropogenic greenhouse effect. Per molecule, nitrous oxide is 206 times as effective as carbon dioxide in causing an immediate increase in global warming. However, this gas also has a long residence time; even if emissions were stabilized or reduced, elevated concentrations of oxide would persist for at least several decades.

Tropospheric (ground level) ozone (O3)

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Like methane and nitrous oxide, tropospheric ozone is a “natural” greenhouse gas, but one which has a short tropospheric residence time. Ozone is formed in the troposphere as a result of pollution from power plants and motor vehicles, and from forest fires and grass fires, as well as from natural processes (i.e., by light-induced reaction of pollutants and the phenomenon is called photochemical smog). As a result of these anthropogenic activities, the levels of ozone in the troposphere probably have increased since preindustrial times. Approximately 10% of the increased global warming potential of the atmosphere results from increases in tropospheric ozone, though the value is uncertain.

Water VaporIn fact, water vapor is the most important greenhouse gas in the Earth’s atmosphere, in the sense that it produces more greenhouse warming than does any other gas, though on a per molecule basis it is a less efficient absorber than is carbon dioxide. On a global scale water vapor is unaffected by anthropogenic sources and sinks. Consequently, water is not usually listed explicitly among gases whose increasing concentrations are enhancing the greenhouse effect. Hydrogenated halocarbons (HFCs and HCFCs) are also entirely anthropogenic gases. They have increased sharply in the atmosphere over the last few decades, following their use as substitutes for CFCs. They generally have lifetimes of a few years, but still have substantial greenhouse effect.

Global Warming and Its Possible Consequences

Climatic change has been used instead of the more popular global warming. Global warming is actually a misnomer because global climatic change will not result in uniform warming of the planet's surface. We can examine the predicted consequences of climatic change. Many of these predictions are based on the work of the Intergovernmental Panel on Climatic Change (IPCC), a working group of scientists and policy makers that was established with a mandate to keep the world community informed about the best and most timely research on the climate system and climatic change.

The most robust analyses of surface temperatures for the past 1 00 to 150 years suggest that average global surface temperatures have increased by about 0.6°C over the past century. This appears to be an actual warming trend distinct from shorter-term fluctuations in temperature that are also evident in the data set. The IP'CC predicts that average surface temperature will continue to increase, reaching about 3°C to 5°C warmer than the present by about the year 2050. It may not sound like much, but this rate of change is considerably faster than anything known to have occurred in the climate system as a result of natural phenomena.

Extremes and the distribution of temperature are also expected to change. Some places will get warmer, but some may get cooler on average. Low-temperature extremes are expected to get colder, and high-temperature extremes will get hotter. These changes in surface temperature will cause changes in atmospheric and oceanic circulation. A source of considerable concern is

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the potential impact on the oceanic themohaline circulation. If water in the North Atlantic becomes warmer (and fresher from the melting of sea ice in the Arctic), it could fail to sink and thus fail to initiate the deep oceanic circulation system that drives the global climate system. This could launch a faster and more profound shift in global climate.Changes in oceanic circulation would also have a major impact on nutrient supplies for the world's fisheries.

Shifts in the major atmospheric and oceanic circulation systems are expected to lead to changes in precipitation (more in some places, less in others) and changes in the frequency and intensity of storms. Changes in the geographical patterns of temperature, precipitation, and wind systems will inevitably result in changes in the characteristics, size, and geographic distribution of biomes, ecosystems, habitats, and growing seasons. Will organisms be able to migrate quickly enough to keep up with the shifts in their habitat. Some may; others may not.We are likely to experience more precipitation overall as a result of increased rates of evaporation due to warming of the ocean. Some locations that are currently arid or semiarid, such as the Sahel region of Africa, may receive more precipitation, which could be a good thing. Agricultural growing seasons may become longer in some areas such as Siberia; this, too, would appear to be a potential benefit of climatic change. Furthermore, it is possible that the increased carbon dioxide in the atmosphere may have a fertilizing effect on some plant types. In some areas, though, such as northern Canada, a longer growing season will be of no use, as it would take hundreds or even thousands of years for arable soil horizons to form in this previ -ously glaciated terrain.As warming occurs, changes in the polar ice caps are anticipated; however, this is a source of some controversy among scientists. If the temperature becomes warmer, you might expect melting of the ice caps to occur. However, some predictions call for increased precipitation in polar regions, which would still experience below-freezing temperatures for much of the year. Glaciers grow by forming new ice from snowfall that exceeds melting in a given year. This suggests that polar ice caps might grow, rather than shrink, in response to global warming.

In either case, it is anticipated that sea level will rise. Some of the increase may come from the melting of land ice in Antarctica (the melting of sea ice, which is less voluminous, is unlikely to play a major role in sea level rise). But the major contributor to sea level rise will be the warming of ocean water. Water, like most other substances, expands when it is warmed. If the ocean warms by, say, 0.5°C, every water molecule in the ocean will expand just a tiny bit. Overall, the entire volume of the ocean will increase. Sea levels worldwide are expected to rise by as much as 1 m(3.3 ft). In areas where coastlines are cliffed, a 1-m rise in sea level would not make a significant difference. In areas where the coast rises gradually, however, a 1 m rise in sea level would cause a significant incursion of seawater, leading to flooding of coastal lands and saltwater contamination of coastal water supplies. For cities located on coasts and estuaries (Venice, New York, New Orleans, Shanghai, Bangkok) and low-lying regions (Florida, Netherlands, Bangladesh), such an increase in sea level would be devastating.

Some of the anticipated impacts of global warming may have self-reinforcing effects. For example, warming may cause an increased carbon flux to the atmosphere from soil and peat gases. This would contribute to further warming in a positive feedback cycle.

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Warming of ocean water could cause the release of methane gas hydrates from where they are currently frozen in seafloor sediments. Releasing the methane from these deposits would contribute a massive volume of highly effective greenhouse gas to the atmosphere, possibly causing further warming-another positive feedback effect.

Finally, after all the predictions have been made, we must consider the potential impacts of climatic change on people and socioeconomic systems. Even without predicting dire consequences like the inundation of major cities, it is clear that the kinds of changes outlined above would seriously affect our economy and wellbeing. Industries like fisheries, agriculture, and tourism would be hit hard by changes in temperature, precipitation, and sea level. The risk of damage from droughts, floods, and storms would increase. The impacts of warming on already scarce freshwater resources could be devastating. Human health would be at risk because of the increase in diseases like malaria and West Nile virus carried by insects that thrive in hot climates. Interestingly, the insurance industry has taken the potential economic impacts of global change very seriously, commissioning many scientific studies on the topic.

What are some of the potential human health-related impacts of global warming? Can you think of some other health impacts in addition to those listed in the text?

Answer: In the text: increased incidence of diseases associated with insects that thrive in hot climates, such as malaria and West Nile virus. Others: diseases related to lack of sanitation and lack of clean drinking water as freshwater resources become more scarce, such as diarrhea and other intestinal diseases; increase in hot-weather problems, such as heat stroke and dehydration.

According to computer projection made by IPCC (Intergovernmental Panel on Climate Control), if no additional steps are taken to reduce emissions of carbon dioxide, and other problematic gases, then by about the year 2035 the average global air temperature will be 1 0C higher than it was in 1990. By 2100 it will increase by more than another 2 0C, with significantly greater temperature change at the polar regions while sea level will increase by 18 cm to 59 cm.

So, The Effects of Global Warming can be summerised as follows:

A warmer global climate could have a number of harmful effects for humans, other species, and ecosystems depending mostly on location and how rapidly the climate changes.

Sea level rise: Flooding of low-lying islands and coastal cities, flooding of coastal estuaries, wetlands and coral reefs, beach erosion, disruption of coastal fisheries, contamination of coastal aquifers with salt water.

Food Production: Shift in food growing areas, changes in crop yield, increased irrigation demands, increased pests, crop diseases, and weeds in warmer areas.

Forestry: Changes in forest composition and locations, disappearance of some forests, increased fire from drying, loss of wildlife habitat and species.

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Water resources: Changes in water supply, decreased water quality, increased drought, increased water pollution from coastal flooding.

Weather Extremes: Prolonged heat waves and droughts, increased flooding from more frequent, intense, and heavy rainfall in some areas.

Human Population and health: Increased deaths from heat and diseases (malaria, dengue fever, yellow fever, cholera, etc.), more environmental refugees, increased migration, disruption of food and water supplies, spread of tropical diseases to temperate area, increased respiratory disease and pollen allergies.

Biodiversity: Extinction of some plant and animal species, loss of habitats, disruption of aquatic life.

Solutions dealing with the threat of Global Warming

Two basic adjustments to potential global warming

1. Mitigation (reducing the severity of the problem) through reduction of emissions of greenhouse gases, or

2. Do nothing to combat it and live with future global climate change.

If global warming occurs, our most likely adjustment will be learning to live with it. However, if we are prudent, we will plan to reduce emissions of carbon dioxide and other greenhouse gases. Doing so will require changes in land management and energy use. Burning forests to convert lands to agricultural purposes accounts for about 20% of anthropogenic carbon loading into the atmosphere. Management plans that seek to minimize burning and protect the world’s forests would help reduce the threat of global warming, as would plans to plant trees. Of particular importance will be energy conservation, using energy more efficiently, and emphasizing alternative energy sources. Essentially, we should reduce emissions of greenhouse gases as much as is feasible and have contingency plans for greater reductions in emissions should they become necessary.

Combustion of fossil fuels with its accompanying carbon dioxide emissions is largely a function of human population size and level of consumption. The larger the population and the greater the degree of affluence, with its accompanying higher levels of consumption, the larger the global emissions of greenhouse gases. Obviously, it will be easier to stabilize carbon dioxide emissions if population growth is slowed.

Thus, the recommended strategy is somewhere between full mitigation and learning to adapt to change.

The Ozone Hole ( Ozone Layer Depletion)

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The ozone layer in the stratosphere is often called the ozone shield, because it absorbs approximately 99% ultraviolet (UV) radiation that is potentially damaging to life. Approximately 90% of the ozone in the atmosphere is found in the stratosphere, As ozone absorbs ultraviolet radiation in the stratosphere, it breaks down to diatomic oxygen (O2) and monoatomic oxygen (O), with the release of heat. Natural conditions prevailing in the atmosphere result in a dynamic balance between the creation and destruction of ozone. The concentration of ozone in the stratosphere is a steady-state concentration resulting from the balance of ozone production and destruction by the above processes.

Ozone Layer Depletion (thinning)

The major compounds in ozone depletion consists of CFC compounds, commonly known as “Freons”. These chemically stable (nonreactive), odorless, nonflammable, nontoxic, and noncorrosive compounds seemed to be dream chemicals. Cheap to make, they are used in refrigerants, aerosol spray propellants, industrial solvents, cleaning fluids, Styrofoam, foam-blowing agents, fire retardants, and a variety of other applications. In 1974 it was convincingly suggested, in a classic work that earned Mario Molina and Sherwood Rowland a Nobel Prize in 1995, that CFCs could catalyze the destruction of stratospheric ozone. Much of the anthropogenic chlorine (CI) in the atmosphere derives from the group of chemicals known as chlorofluorocarbons, or CFCs. Because they are relatively stable, CFCs survive in the atmosphere long enough to migrate to the stratosphere. Once there, they are subjected to intense solar radiation, which causes the molecules of CFCs to break down, releasing an atom of chlorine. The chlorine acts as a chemical facilitator, or catalyst, in the breakdown of ozone molecules.

Although quite inert in the lower atmosphere, CFCs undergo photodecomposition by the action of high-energy ultraviolet (UV) radiation in the stratosphere, which is energetic enough to break their very strong C-Cl. That is, the chlorine combines with ozone to produce chlorine monoxide, which in the second reaction combines with monoatomic oxygen to produce chlorine again. Following this, the chlorine can enter another reaction with ozone and cause additional ozone depletion. This series of reactions is what is known as a catalytic chain reaction, because the chlorine is not removed but reappears as a product from the second reaction, so the process may be repeated over and over again. It has been estimated that each chlorine atom may destroy approximately 100,000 molecules of ozone over a period of 1 or 2 years before the chlorine is finally removed from the stratosphere through other chemical reactions and rain-out.The most prominent instance of ozone layer destruction is the so-called “Antarctic ozone hole” that has shown up in recent years.

CFCs are not the only ozone-depleting compounds. A variety of cleaning solvents, such as carbon tetrachloride and methyl chloroform, contain chlorine and thus destroy ozone, as does halon, which contains bromine (used as fire retardant). Methyl bromide, a pesticide and nitrous oxide, a greenhouse gas also destroy ozone.Although not as severe as the ozone depletion over the Antarctic, ozone depletion over the Arctic each winter is troublesome.

The Future of Ozone Depletion

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A troubling aspect of ozone depletion is that if the manufacture, use, and emission of all ozone-depleting chemicals were to stop today, the problem would not go away, because millions of metric tons of those chemicals (long persistent) are now in the lower atmosphere, working their way up to the stratosphere.

Environmental Effects : Ozone depletion has several serious potential environmental effects, including damage to Earth’s food chains on land and in the oceans, and human health effects including increases in all types of skin cancers, cataracts, and suppression of immune systems.

Solution: In recognizing the potential problems associated with CFCs, industry started to produce CFC-free propellants for spray cans and assisted in supporting an intense research progam aimed at finding alternatives to the use of CFCs as refrigerants.

Collection and Reuse: One way to lower emissions of CFCs into the atmosphere is to develop ways to collect and reuse CFCs. Methods have been developed to liberate and collect these CFCs when refrigerators are recycled. The CFCs used as the coolant gas can also be collected. One company in Germany recycles approximately 6000 refrigerators per month (50 millions are discarded/yr worldwide). The same techniques can be used to recover the CFCs in the air conditioners used in automobiles and homes.

Substitutes for CFCs: Two substitutes for CFCs being experimented with are hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs). These chemicals are controversial but do have advantages. The advantage of HFCs is that they do not contain chlorine. Fluorine is 1000 times less efficient in causing ozone depletion. The fluorine they contain may be broken down in the lower atmosphere. Their atmospheric lifetime is considerably shorter than that of CFCs.

Injection of Chemicals : Work is just beginning on potential solutions to ozone depletion by injecting propane (C3H8), which will react with chlorine to form hydrochloric acid, tying up chlorine and not allowing it to enter into ozone-depleting reactions. Early studies suggest that approximately 50,000 tons of propane might do the job. The propane would be injected at an elevation of approximately 15 km, utilizing several hundred large aircraft.

Short-term Adaptation to Ozone Depletion: The major short-term adaptation by people will be learning to live with an increase in exposure to UV radiation. In the long-term, achievement of “sustainability” with respect to stratospheric ozone will require management of human-produced ozone-depleting chemicals.

What is the role of chlorofluorocarbons in stratospheric ozone depletion?

Answer: Chlorofluorocarbons break down in the presence of sunlight in the stratosphere, releasing chlorine; the chlorine acts as a facilitator, or catalyst, for ozonedepleting reactions. Once in the stratosphere, chlorine atoms can be recycled over and over, potentially causing thousands of ozone molecules to break down.

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What is the role of the polar vortex in stratospheric ozone depletion?

Answer: The polar vortex, which is particularly strong over the South Pole, causes very cold air to circulate and become isolated over the pole; this allows the formation of tiny ice crystals, which provide sites on which ozone-depleting reactions occur.

Thermal Inversion of Air

During daylight, the sun warms the air near the Earth’s surface. Normally, this warm air and most of the pollutants it contains rise to mix with the cooler air above it. This mixing of warm and cold air creates turbulence, which disperses the pollutants. When air movement ceases, stagnation can occur with a resultant buildup of atmospheric pollutants in localized regions. Although the temperature of air relatively near the Earth’s surface normally decreases with increasing altitude, certain atmospheric conditions can result in the opposite direction—increasing temperature with increasing altitude. Such conditions are characterized by high atmospheric stability and are known as temperature (thermal) inversions. Because they limit the vertical circulation of air, temperature or thermal inversions result in air stagnation and the trapping of air pollutants in localized areas.

Inversions can occur in several ways. In a sense the whole atmosphere is inverted by the warm stratosphere, which floats atop the troposphere, with relatively little mixing. An inversion can form from the collision of a warm air mass (warm front) with a cold air mass (cold front). The warm air mass overrides the cold air mass in the frontal area, producing the inversion.There are two types of thermal inversions: Radiation inversion and subsidence inversion.

Radiation inversions are likely to form in still air at night when the Earth is no longer receiving solar radiation. The air closest to the Earth cools faster than the air higher in the atmosphere, which remains warm, thus less dense. Furthermore, cooler surface air tends to flow into valleys at night, where it is overlain by warmer, less dense air.

Subsidence inversions, often accompanied by radiation inversions, can become very widespread. These inversions can form in the vicinity of a surface high-pressure area when the high-level air subsides to take the place of surface air blowing out of the high-pressure zone. The subsiding air is warmed as it compresses and can remain as a warm layer several hundred meters above ground level.

A marine inversion is produced during the summer months when cool air laden with moisture from the ocean blows onshore under warm, dry inland air. Thermal inversions prevent mixing of air pollutants, thus keeping the pollutants in one area. This not only prevents the pollutants from escaping, but also acts like a container in which additional pollutants accumulate. Furthermore, in the case of secondary pollutants formed by atmospheric chemical processes, such as photochemical smog, the pollutants may be kept together such that they react with each other and with sunlight to produce even more noxious products.

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