CHAPTER ONE
- INTRODUCTION
The earth’s climate is a dynamic system undergoing continuous change on seasonal, annual, decadal and longer timescales. Scientific evidence suggests that a complex interplay of natural and human-related forces may explain such climate variability and change. Some consequences of this variability are natural hazards such as earthquakes, volcanoes, landslides, floods, wildfires, extreme weather, coastal hazards, space weather plus major pollution events. Better preparation for any impacts due to climate variability and change requires better understanding of its causes and effects. Improved global observation is a fundamental need for filling knowledge gaps in climate science. Furthermore, a better understanding of greenhouse gas effects will greatly facilitate decision-making related to sustainable development of terrestrial, oceanic and atmospheric resources.
1.2 THE EARTH’S ATMOSPHERE
The earth’s atmosphere is a layer of gases surrounding the planet earth that is retained by earth’s gravity. It is the life giving blanket of the earth. It protects life on earth by absorbing ultra violet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.
1.2.1 Composition
Dry air contains approximately (by volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.03% carbon dioxide, 1% water vapor and small amount of other gases.
1.2.2 Structure of the Atmosphere
The primary indices for stratifying the earth’s atmosphere are the variation in pressure, density, temperature and composition. Hence considering the thermal variation in the atmosphere, the earth’s atmosphere is structured into four layers, which are the: troposphere, stratosphere, mesosphere and thermosphere.
Troposphere: It extends from the surface of the earth to between 7Km at the poles and 17Km at the equator. It contains approximately 80% of the mass of the atmosphere. The tropopause is the boundary between the troposphere and the stratosphere. It is in the troposphere that life exists. It is characterized by a decrease in temperature with altitude, intense convectional heat current and turbulent motions. This temperature pattern in the troposphere stems from absorption of the energy from the sun by earth’s surface and its remittance upward as infrared heat waves.
Stratosphere: The stratosphere extends from the tropopause to about 51Km up the atmosphere. Temperature increase with height and this hinders turbulent motions. The stratopause is the boundary between the stratosphere and the mesosphere. Because temperature rises with height in the stratosphere, the condition of warmer air above colder air exists. Such a condition is convectively stable. Vertical motions are therefore suppressed, leading to vertical stratification of the air masses it contains; hence the name stratosphere. This increase in temperature with height—the definition of an inversion–acts as a global cap on the weather. Convective motions are limited to the height of the tropopause. Air parcels rising up from the surface through the troposphere hit the tropopause and flatten out almost as if it were a rigid lid
Mesosphere: The mesosphere extends from the stratopause to 80-85Km up the atmosphere. Temperature decreases with height in the mesosphere. The mesopause is the boundary between the mesosphere and the thermosphere and is noted to be the coldest region in the atmosphere.
Thermosphere: The thermosphere extends from the mesopause to about 350-800Km up the atmosphere. Temperature increases with height from the mesopause up to the thermopause in the thermosphere and after then remains constant.
Other layers within these four thermally stratified layers include:
The Ozone layer: It is contained within the stratosphere, precisely from about 15 to 30Km altitude.
The Ionosphere: It is the part of the atmosphere that is ionised by solar radiation and stretches from 50 to 1000Km.
1.3 OZONE LAYER
The ozone layer is a belt of naturally occurring ozone gas that is found from 15 to 30 Kilometers above Earth and serves as a shield from the harmful ultraviolet radiation emitted by the sun. Ozone is a highly reactive molecule that contains three oxygen atoms. It is constantly being formed and broken down in the high atmosphere, 6.2 to31 miles (10 to 50 kilometers) above Earth, in the region called the stratosphere.
The ozone layer of the atmosphere protects life on Earth by absorbing harmful ultraviolet radiation from the Sun. If all the ultraviolet radiation given off by the sun were allowed to reach the surface of Earth, most of the life on Earth’s surface would probably be destroyed. Short wavelengths of ultraviolet radiation, such as UV-A and B are damaging to the cell structure of living organisms. Fortunately, the ozone layer absorbs almost all of the short wavelength ultraviolet radiation and much of the long wavelength ultraviolet radiation given off by the sun.
1.3.1 OZONE VARIATION
The interplay of ozone photochemistry and ozone transport processes gives rise to ozone variation at different latitudes and altitude. Generally, atmospheric variations could be classified into four time-scales namely: short-term, seasonal, interannual, and long term.
Short-Term Variability
Short-term variability refers to day-to-day and week-to-week variations. For example, the effects of the passage of a weather system are classified as short-term variability. A global map of ozone for a given day looks very much like a weather map with high and low ozone amounts corresponding to weather systems, though in the reverse (anti correlated) sense to high and low pressure systems. The map for the next day will show movement of both these weather systems and the ozone amounts. They are several types of short- term variability that affect ozone photochemical process rates in the stratosphere (especially the upper stratosphere). These include diurnal variations, variations in solar ultraviolet radiation, temperature driven fluctuations, and particle precipitation events that originate from electromagnetic storms on the Sun.
Seasonal Variability
The seasonal variability occurs on top of a general patternthat repeats every year. This repeating variation is called an annual cycle or a seasonal cycle. This is very much analogous to the annual cycle of temperature near the earth’s surface. It is known that it will be cold and dry during the dry seasons and hot and wet during the raining season. Ozone amounts vary in similar manner. In the upper stratosphere, the seasonal variability of ozone is driven by the seasonal variation in the temperature acting through temperature dependent photochemistry that governs ozone production and loss processes. In the lower stratosphere, photochemistry is much slower, owing to reduced UV flux, and transport of ozone by large-scale atmospheric motions (known as circulations) becomes important.
Interannual Variability
The shape and amplitude of the annual distribution will not precisely be the same from year to year. This year-to-year variability is referred to as interannual variability. Again an analogy could be made with temperature. It is known that some dry seasons are exceedingly cold on average while others are relatively warm. Likewise, some dry seasons will have large amounts of ozone while others have small amounts. Factors that produce this year-to-year variations are the Quasi- Biennial Oscillation (QBO), El Nino Southern Oscillation (ENSO), volcanic eruptions and other interannual planetary wave variations.
Long-Term Variability
Long-term variability is a ‘catch-all’ term for variations with time scales typically on the order of decades. Cyclic variability, with a time scale longer than our available measurement record, make up one class of long-term variability. One cause of long-term variability is a trend. For instance, long-term variability in the ozone loss process can result in a trend in ozone amounts. Such long-term variability can have other causes, such as the gradual build-up in the amount of ozone-destroying chlorine in the stratosphere from chlorofluorocarbons or CFCs.
1.3.2 OZONE DEPLETION
Over Earth’s lifetime, natural processes of ozone photochemistry and transport have regulated the balance of ozone in the stratosphere. It has been discovered that ozone levels vary periodically as part of regular natural cycles such as seasons, periods of solar activity, and changes in wind direction. Concentrations are also affected by isolated events that inject materials into the stratosphere, such as volcanic eruptions. Polar Regions reflect the greatest changes in ozone concentrations, especially the South Pole. The topography of Antarctica is such that a stagnant whirlpool of extremely cold stratospheric air forms over the region during the long polar night. The air stays within this polar vortex all winter, becoming cold enough to allow the formation of polar stratospheric clouds. Polar stratospheric clouds speed up the natural process of ozone destruction by providing ice crystal surfaces on which the destructive reactions take place. These reactions would free chlorine from “inactive” forms into “reactive” forms, where the chlorine could destroy ozone in the catalytic cycles. Originally proposed by Crutzen et al. (1986), McElroy et al. (1986), and Solomon et al. (1986), this theory proposed that reactions which normally do not occur in gas phase might be greatly enhanced if chlorine-containing compounds such as ClONO2 (chlorine nitrate) and HCl (hydrochloric acid) could collect on the surfaces of these particles and then react to release the chlorine into a reactive form that could cause large ozone losses. After the long polar winter, ozone within this extremely cold vortex is very vulnerable to the arrival of sunlight. As spring arrives, major ozone losses occur. In the southern hemisphere, the area of most severe ozone depletion is localized above Antarctica and is generally referred to as the ozone hole. The hole appears in the southern spring, following the continent’s coldest season and polar night. Ozone depletion over the Arctic is not as well defined as in Antarctica. The topography at the North Pole results in an air circulation pattern that is quite different from that of the South Pole, but expeditions have shown that the atmospheric chemistry of the two Polar Regions is very similar. In the Northern Hemisphere, the polar vortex is not as strong as it is in the Southern Hemisphere. It can break up and reform several times during the course of winter. One air mass after another enters the polar darkness and soon emerges back into low sunshine. Thus, a bit of ozone is lost from each parcel of air, rather than a large amount from one parcel as in the southern hemisphere. The end result is that they are loss of ozone in both hemispheres. Ozone levels in the atmosphere have been monitored from the ground since the 1950s and by satellite since the 1970s.
