Notes

Ozone (O3) is a naturally occurring gas in Earth’s stratosphere. It is a pale blue colored oxygen. Ozone is poisonous and explosive in nature. It consists of three oxygen atoms, making it a highly reactive molecule.

It forms a part of the upper atmosphere known as the ozone layer. The ozone layer spans roughly 15 to 30 kilometres above Earth. It protects Earth from the Sun’s ultraviolet radiation (UV rays). UV rays can be classified as UV-A, UV-B, and UV-C.
The upper atmosphere blocks the UV-C rays. The ozone layer protects all forms of life on Earth by largely blocking the Sun’s harmful UV-B rays. However, it does not strongly absorb UV-A radiation. Exposure to UV-B rays increases human risk of skin cancer. It also weakens the immune system, and results in genetic damage and cataracts. UV-B rays can damage other forms of life on Earth, such as terrestrial plant life, aquatic ecosystems, and single-celled organisms.

Natural processes ensure that ozone is constantly created and destroyed in the atmosphere. The ozone layer naturally depletes and recovers at the same pace. However, some pollutants deteriorate the ozone layer. The destruction of ozone molecules allows UV-B rays to penetrate through the ozone layer and reach Earth’s surface. Let us learn about these pollutants.

Chlorofluorocarbons (CFCs): CFC gases are organic compounds
that consist of carbon, fluorine, and chlorine. CFC gases are widely found in aerosol sprays. Before 1995, refrigerators and air conditioners used CFCs as refrigerants. The chlorine in CFCs destroys ozone molecules.

Halon: Halon is a type of compressed gas that prevents fire from spreading. Therefore, fire extinguishers use halon. Halon is relatively unreactive and stable, but can damage the ozone layer.

Methyl chloroform: It is an organic compound that is mainly used as an industrial solvent.

Carbon tetrachloride: It is a manufactured chemical that is used in fire extinguishers and as industrial solvents.

Hydrofluorocarbons (HCFCs): HCFC gases are replacing CFC gases. They perform the same function as CFCs, but are less harmful. However, HCFCs still have ozone-depleting properties.

We are observing a general thinning of the ozone layer all over the globe. Since the 1960s, Australia has witnessed about five to nine percent of ozone layer depletion in the stratosphere. The continent hence faces the threat of overexposure to UV radiation.

We have also witnessed a phenomenon called the ozone hole. This has been taking place every year since the 1980s in Antarctica. The ones recorded in 2000 and 2006 are by far the largest. Ozone-depleting substances reaching the stratosphere and climatic conditions of a region affect the ozone layer.

Developed countries banned unnecessary use of CFCs and other ozone-depleting substances in 1996. In 2010, developing countries also banned these substances. Since then, observations show that the atmosphere’s chlorine concentration has gradually decreased. If countries maintain such caution, the ozone layer can recover to its pre-1980 levels between 2050 and 2065.

Acid rain is atmospheric moisture or precipitation mixed with gases and elements. This causes it to be more acidic than normal. The precipitation can be in the form of rain, snow, hail, dew, or fog. Natural rainwater carries dissolved carbon dioxide in the form of carbonic acid, which is a weak acid. Thus, natural rainwater is mildly acidic in nature.

However, acid rains contain high levels of sulfuric and nitric acid. They can cause a lot of damage to life on Earth. Unpolluted rainwater has a pH level of 5.6. However, the first acid rain sample from North America at Hubbard Brook, New Hampshire in 1963 had an alarming pH level of 3.7.

Most problems associated with acid rain are due to artificial causes. However, natural events such as volcanic eruptions and emissions from vegetation also cause acid rain.

Fossil fuel combustion releases sulfur dioxide (SO2) and nitrogen oxide (NOx) into the atmosphere. These gases are the key artificial causes of acid rain. Fossil fuel combustion in power plants and factories releases sulfur. Sulfur combines with the oxygen in the air to form sulfur dioxide. Automobiles release nitrogen oxide into the atmosphere. The gases released in the air react with tiny water droplets in the atmosphere to form sulfuric and nitric acids.

Acid rain was first discovered in North America in 1963. The Clean Air Act imposed regulations of SO2 and NOx emissions in 1970 in the United States. Emissions of SO2 and NOx, and concentrations of sulfuric and nitric acid in precipitation have gradually decreased since then.

Acid rain has many adverse effects that can even cause permanent damage.

Acid rain endangers mollusks such as snails and mussels. Their shells are made of calcium carbonate (CaCO3), which dissolves in acid.
A study conducted to measure the effects of acid rain showed a 30 percent increase in the number of birds laying defective eggs. The eggshells were thinner and highly porous. They failed to hatch properly. The shell would break. The shells were thin because of lack of calcium in the birds’ diet. Snails normally provide this calcium, but acid rain damaged their shells.
Insects and crustaceans cannot survive in water having pH level less than 5.
Crayfish too is rare in water that has pH values less than 5. Depleted populations of crayfish are a matter of concern, as they are food to several other species of marine life.


The eggs of amphibians may be sensitive to acidic water.
Diversity of plankton decreases in acidic water. An acidic environment affects the gills of fish. This may ultimately result in death. Other effects include reproductive failure, decreased growth rate, skeletal deformities, and increased exposure to heavy metals.
Because of acidic rainwater, soil particles release increased amounts of aluminum. Increased concentrations of aluminum in soil harm trees, fish, and other organisms.
Acid rain also damages stone buildings and monuments.

Greenhouse Effect
The Sun provides light and heat to Earth. Earth’s atmosphere is transparent to visible light. However, it absorbs some of the ultraviolet rays. You will notice that the roads are warm even after the Sun has set. On average, the energy that Earth receives from the Sun’s radiation is almost exactly equal to the energy Earth emits back into space through its own radiation. This exchange has allowed Earth’s climate to remain relatively stable over billions of years

The surface of Earth reflects heat in the form of infrared rays. Greenhouse gases in Earth’s atmosphere trap these rays, preventing them from escaping into space. They help keep Earth’s climate in balance.

Earth itself gives off radiation in the infrared range. This radiation carries heat away from Earth’s surface into space.The greenhouse effect is similar to the glass walls of a greenhouse. These walls let in the sun’s energy. However, they prevent some of the heat from escaping. Greenhouse gases consist of water vapor, ozone, carbon dioxide, nitrous oxide, and methane. If it weren’t for them, Earth would be very cold at night.



Let’s understand greenhouse gases a little more.

Gas molecules in Earth’s atmosphere that have three or more atoms are called “greenhouse gases.” These gases have the capability to absorb the infrared radiation that Earth releases.
Water vapor (H2O) is the most important greenhouse gas. It is naturally present in the atmosphere, as it is an integral part of the water cycle. Some regions have high water vapor content, such as the equatorial region. In these regions, the greenhouse effect is also very large.

Carbon dioxide (CO2) is the second most important greenhouse gas in the atmosphere. It is naturally present in the atmosphere. The carbon cycle naturally adds to and removes CO2 is from the atmosphere. Fossil fuel combustion increases CO2 in the atmosphere.



Methane is a naturally occurring, common fuel source. One of the major sources of methane is its release from the digestive system of cows. Oil and gas industries also release methane. Methane is a more potent greenhouse gas than carbon dioxide (CO2). This is because methane can warm up the environment 20 times faster than CO2. However, CO2 occurs in the atmosphere around 200 times more than methane.

Nitrous oxide (N2O) forms naturally. It is also a result of agricultural activities. One nitrous oxide molecule is equivalent to the greenhouse warming power of 300 CO2 molecules. A single N2O molecule can stay in the atmosphere for 100 years before it is destroyed naturally.

Ozone (O3) concentrations vary hugely from place to place. So, it is difficult to track them. CFCs that are responsible for ozone layer depletion are also responsible for an increase in the greenhouse effect. They absorb the heat that Earth generates. While the ozone is a greenhouse gas, the ozone layer depletion has no connection to the greenhouse effect.

Global warming is an outcome of increased greenhouse effect arising from increased concentrations of greenhouse gases in the atmosphere.



Global Warming
Human activities are largely responsible for increasing the greenhouse gas effect. There has been a 25 percent increase in the greenhouse gases in the atmosphere since the industrial revolution. During the last 20 years, fossil fuel combustion due to human activities and industrialization has increased. These activities have contributed to three quarters of the current levels of CO2 in the atmosphere. This is a matter of serious concern.

The rise in the concentrations of greenhouse gases in the atmosphere has modified Earth’s system balance. It has led to a slow warming of Earth’s climate. This warming trend is taking place at a much faster rate compared to any of the previously recorded shifts in Earth’s climate. (Earth has been through a series of ice ages as well as ages of higher temperatures.) This rise in the temperature of Earth’s atmosphere, its air, and its oceans is global warming. Global warming is due to an increase in the levels of greenhouse gases. These gases mainly include CO2, methane, and other pollutants.

Global warming is progressing more rapidly than living beings can adjust to it. Let us understand the effects of global warming.

As Earth’s temperature rise on an average, some places might become warmer, while others might become cooler. Global warming will increase evaporation and precipitation on Earth. Some places will become wetter, while some will become drier.

The frequency of storms and other extreme weather phenomena is bound to increase. In other words, coastal areas will be more prone to storms and floods.

On the other hand, as dry places will tend to become drier, there could be water shortages. Irrigation water will not be sufficient for agriculture. These shortages can lead to decreased agricultural production and eventually to droughts. We can also anticipate an increase in forest fires.

Glaciers and ice caps hold 75 percent of Earth’s water. Global warming is causing them to melt. The water from the melting glaciers enters the ocean. The volume of seawater increases, thereby increasing the sea level. The warm portions of the oceans get warmer and expand slightly, adding to the increase in sea levels. The warming of the oceans has already resulted in the loss of plankton and bleaching of coral reefs. The rise in sea levels will submerge low-lying areas, such as Florida, the Netherlands, and Bangladesh.

The Intergovernmental Panel on Climate Change (IPCC) estimates that the change in the greenhouse gas effect will continue over the next few decades as well. This rise will increase global temperatures by roughly 0.2°C every decade. Attempts to bring down greenhouse gas emission can reduce the increase in temperature by a mere 50 percent per decade.



Many countries in the world are working towards reducing their carbon footprint. The Kyoto Protocol is an international treaty to reduce the carbon footprint. It came into effect in 2005. By the end of 2009, 187 countries agreed to the terms of the treaty. The European Union and 37 other industrialized countries committed to reducing their levels of greenhouse gas emissions.

At the personal end, we can ensure fuel and energy efficiency in our vehicles and home utilities. Using alternate sources of energy, such as solar energy, will greatly reduce CO2 emissions.