CLASS 7TH | GEOGRAPHY | GREENHOUSE EFFECT AND GLOBAL WARMING | REVISION NOTES

What is Global Warming?

Earth absorbs about 75 % of the total solar energy reaching its surface thereby increasing its temperature. Some of this energy is radiated back into the atmosphere. The gases present in the atmosphere for example ozone, methane, carbon dioxide, water vapour and chlorofluorocarbons are called greenhouse gases, they absorb some heat thereby restricting the heat to escape our atmosphere. These gases add to the heating of the atmosphere and result in global warming.

In places where the temperature is low, we use glass covered areas known as a greenhouse to grow flowers, fruits, and vegetables. It is very interesting to know that even we live in a greenhouse, but the difference is that we are not covered by the glass but by the blanket of air called the atmosphere. It is this atmosphere which has kept the earth’s temperature constant for centuries and helped in the survival of life. Atmosphere traps the heat around the earth and keeps it warm. This is called as natural greenhouse effect because it maintains the temperature and sustains life.

In a greenhouse, the solar energy enters through the glass, warms the soil and atmosphere and helps in the growth of plants. In return, the soil and plants emit infrared radiation, which is partly absorbed and partly reflected by the glass. This mechanism traps the sun’s energy in the greenhouse. Similarly, we have carbon dioxide which absorbs heat (as they are transparent to sunlight but not to infrared heat radiation) and is the major contributor to the global warming.

Other than carbon dioxide we have methane, ozone, CFC’s and nitrous oxides forming a major part of greenhouse gases. These chemicals either occur naturally or are man-made. The use of these compounds should be reduced; otherwise, the average temperature of the earth will rise. This will result in melting of polar ice caps and flooding of the coastal areas. Increase in the global temperature also increases the incidence of diseases like dengue, malaria, yellow fever etc.

What is the Greenhouse Effect?

To understand the nature of the greenhouse effect on climate change which leads to global warming, we must first know what the greenhouse effect is.

When you enter a car that has been in the sun for quite some time, what is the first thing you notice? It is a lot hotter inside than it is outside. The sun’s rays (UV radiation, thermal radiation, visible light) enter the car through the glass panes and all the rays entering the car do not leave owing to the build up of gases inside the car and the refractive properties of the glass itself. The weaker thermal radiation does not completely leave the car. This eventually heats up the inside of the car.

The same thing happens in a greenhouse. The plants in a greenhouse require a warm temperature to grow. Have you ever seen a greenhouse? It is made almost entirely out of glass. The heat is retained and the plants thrive. The temperature inside a greenhouse is always higher than the temperature outside.

A typical greenhouse used to regulate the climate for plant growth

This is called the greenhouse effect. What happens when the greenhouse effect occurs on a large scale in the world itself? This is where greenhouse gases come in.

Let’s have a little back-story here. Other planets in our solar system are either extremely cold or really hot. It’s the only planet earth that has a climate which is mild enough to support life. This is because of the presence of a thin layer of naturally occurring greenhouse gases like carbon dioxide, methane gas, water vapour and nitrous oxide. These gases are part of our atmosphere. The atmosphere here plays the role of the glass pane like in the greenhouse. They let the sun’s rays inside but not all of it is reflected back. The greenhouse gases even facilitate the absorption of the heat thereby warming up the earth and not letting it become extremely cold like it otherwise should have been.

The Greenhouse Effect

Since the beginning of the 18th century, the concentration of these gases in the atmosphere has started rising gradually. Since then, CO₂ levels have risen around 40%. And why is this happening? Different human activities, mainly industrial, have led to the production of these gases; the most common being carbon dioxide. The unnatural presence of increased greenhouse gases has led to a more pronounced greenhouse effect which has altered the temperature of the earth, leading to a phenomenon termed ‘Global Warming’.

Climate Change Due To Global Warming

A universal consensus of climate scientists is that there has definitely been a rise in the global temperature over the past century. The phenomenon of Global Warming if continued unchecked will have profound implications.

One of the main effects of global warming will be the rise in sea and ocean levels. Currently, this is already occurring around us. Melting of glaciers and polar ice caps will contribute to the rise in water levels all over the world. Apart from this, fresh water sources will also reduce. According to scientific bodies like NASA, other consequences of global warming are ocean acidification, extreme weather events and other natural and societal impacts.

Projected change in annual average precipitation for the 21st century based on the SRES A1B emissions scenario, and simulated by the GFDL CM 2.1 model

So can we control or check this from happening? There are some like Josef Werne, an associate professor at the department of geology & planetary science at the University of Pittsburgh, who believes that we have already crossed the point of no return. All we can do now is to adapt to the changing environment and the rising sea and ocean levels. We can still lessen the severity of climate change by aggressively enforcing policies that require different bodies to lessen CO₂ levels in the atmosphere. Still, others are even more optimistic believing that strong international agreements and actions can save the planet and it’s changing atmosphere.

Projected impact of climate change on agricultural yields by the 2080s, compared to 2003 levels (Cline,2007)

Greenhouse Gases

Many chemical compounds in the atmosphere act as greenhouse gases. These gases allow sunlight (short wave radiation) to freely pass through the Earth’s atmosphere and heat the land and oceans. The warmed Earth releases this heat in the form of infrared light (long wave radiation), invisible to human eyes. Some infrared light released by the Earth passes through the atmosphere back into space. However, greenhouse gases will not let all the infrared light pass through the atmosphere. They absorb some and radiate it back down to the Earth. This phenomenon, called the greenhouse effect, is naturally occurring and keeps the Earth’s surface warm. It is vital to our survival on Earth. Without the greenhouse effect, the Earth’s average surface temperature would be about 60° Fahrenheit colder, and our current way of life would be impossible.

We know that several gases in the atmosphere can absorb heat. These greenhouse gases are produced both by natural processes and by human activities. The primary ones are:

• Carbon dioxide (CO₂)
• Methane (CH₄)
• Nitrous oxide (N₂O)

Industrial Gases, including hydrofluorocarbons, per fluorocarbons, and sulphur hexafluoride.

Greenhouse Gas Conc. Chart in ppm

Greenhouse Gases List

Greenhouse gas	How it’s produced	100-year global warming potential	Average lifetime in the atmosphere
Methane	Released during the production and transport of natural gas, coal, and oil. It also results from agricultural practices, livestock and decay of organic waste in municipal solid waste landfills.	21	12 years
Carbon Dioxide	Released primarily when fossil fuels like natural gas, coal, and oil are burnt. Burning of trees, wood, and waste also release carbon dioxide. Changes in land use also play a role. Soil degradation and deforestation add carbon dioxide to the atmosphere, while forest regrowth takes it out of the atmosphere.	1
Nitrous oxide	Emitted during industrial and agricultural activities as well as during combustion of solid waste and fossil fuels	310	114 years
Fluorinated gases	A group of gases that includes per fluorocarbons, hydrofluorocarbons, and sulphur hexafluoride, among other chemicals. These gases are released from a variety of commercial and industrial processes and household uses and do not occur naturally. Sometimes used as substitutes for ozone-depleting substances such as chlorofluorocarbons (CFCs).	Varies	Few weeks to thousands of years