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.