CLIMATE CHANGE  

One of the major reason for extreme events such as intense storms, frequent floods, severe drought, melting and receding glaciers etc.  is CLIMATE CHANGE For several years, global warming or climate change was considered as ‘sceptics’ argument. But today it has been accepted as a reality. According to the Inter-Governmental Panel on Climate Change (IPCC), climate change refers to

Any change in climate over time, whether due to natural variability or as a result of human activity”

The United Nations Framework Convention on Climate Change (UNFCC) in Article 1 defined it as: “climate change refers to a change in climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time period.

The causes for climate change could be grouped into two:

• natural and anthropogenic 

Natural Sources of Climate Change:

• Solar Variability:

This happens due to variation in solar energy output and is measured by the data obtained through satellite. The variation in the total output has been observed both throughout the day as well as during the solar cycle. These variations can directly affect Earth’s climate. But the exact role that solar variability plays is still a controversial and not unanimously accepted by the scientific community.

• Changes in Earth’s Orbit and Tilt:

. Earth generally changes its orbit over a 100,000-year cycle. It normally shifts from almost a circular to elliptical orbit. This has led to changes in the tilt of Earth’s axis between 21.8 and 24.4 degrees. As a result of which there has been change in the planet’s solar energy. The northern hemisphere is now closer to the sun in winter and farther away in summer. As a result of which it receives five percent less sunlight in summer than 12,000 years ago. More tilt leads to greater seasonal variations. This is known as Milankovitch cycle

• Plate Tectonics:

Millions of years ago most of the Earth’s land mass was a single continent known as Pangea and surrounded by one huge water bodies known as Panthalassa. Over time, portions of it drifted apart, gradually forming the continents and ocean basins existing today. The redistribution of land mass and ocean area has had a major effect on global climate.

• Biological Evolution:

This natural process took place mainly during the evolution of photosynthetic organism. several billion years ago Earth’s pre-life atmosphere was mainly covered by carbon dioxide and methane. The gradual shift in the atmospheric concentrations from carbon dioxide and methane to nitrogen and oxygen took place because photosynthetic organisms consumed carbon dioxide and generated oxygen

Climate Change due to Anthropogenic Activities

Different anthropogenic activities lead to emissions of four principal greenhouse gases: carbon dioxide, methane, nitrous oxide and the halocarbons (a group of gases containing fluorine, chlorine and bromine).

Carbon dioxide has been increasing from the utilization of fossil fuels in transportation, building heating/ cooling and in the manufacture of cement and other goods. Deforestation releases CO2 and reduces its uptake by plants. Carbon dioxide is also released in natural processes such as the decay of plant matter.

Methane has increased as a result of human activities related to agriculture, natural gas distribution and landfills. Methane is also released from natural processes that occur, for example, in wetlands.

Nitrous oxide is emitted by human activities such as fertilizer use and fossil fuel burning. Natural processes in soils and the oceans also release N2O.

Halocarbon gas concentrations have increased primarily due to human activities. Principal halocarbons include the chlorofluorocarbons (e.g., CFC-11 and CFC-12), which were used extensively as refrigeration agents and in other industrial processes before their presence in the atmosphere was found to cause stratospheric ozone depletion.

AS per the Observations of Changes in Climate at Global Level by the IPCC

Some of the highlights of the 5th Assessment Report are as follows:

1. Each of the last three decades has been successively warmer at the earth’s surface than any preceding decade since 1850.
2. Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010. Further uptake of carbon by the ocean will increase ocean acidification.
3. Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and arctic sea ice and northern Hemisphere spring snow cover have continued to decrease in extent.
4. The rate of sea-level rise since the mid-nineteenth century has been larger than the mean rate during the previous two millennia. Over the period 1901–2010, global mean sea level rose by 0.19 m (0.6 ft).

GREEN HOUSE EFFECT

The greenhouse effect is a natural process responsible for keeping the earth at the temperature needed to sustain life. Acting just like the glass walls of a greenhouse, gases like carbon dioxide, methane, and nitrous oxide trap the sun’s heat in the atmosphere and prevent it from escaping into space.

About half of the sun’s radiation that travels toward the earth never makes it to the earth’s surface. Clouds and the atmosphere reflect about one-third of the radiation back toward the sun, and they also absorb another 20 percent. The rest of the radiation—about 50 percent—reaches the earth, where it is absorbed by oceans and land. This keeps the earth warm and sustains plant, animal, and human life. The earth also releases heat back toward space. Some of this heat passes through the atmosphere, but most of it is captured and retained by greenhouse gases before it can escape. This is the mechanism that keeps the earth warm.

Normally, the greenhouse effect keeps the earth just warm enough to sustain life. Scientists say that without the greenhouse effect, the average temperature of the earth would drop from 14˚C (57˚F) to as low as −18˚C (−0.4˚F).

Increase in the emission of greenhouse gases is mainly due to urbanization and industrialization, thus causing increase in the atmospheric temperature

Greenhouse Gases: Their sources and Causes

OZONE LAYER DEPLETION

Formation of ozone layer

Ozone (O3) is a highly reactive molecule containing three oxygen atoms. The upper part of the earth’s atmosphere, between 10 and 50 km above the earth surface called stratosphere contains a thin layer of ozone. This ozone layer serves as a natural filter for blocking deadly incoming UV radiation from the sun. Since, the early 1970’s levels of the stratospheric ozone have thinned markedly over certain regions of the earth, particularly over the Antarctic region. The thinning of stratospheric ozone layer is termed ‘“ozone hole”.

5.4.1Causes of ozone layer depletion:

Ozone (O3) layer can be destroyed both by natural and man-made causes-

(i) Natural causes: A number of naturally occurring substances destroy stratospheric ozone. Most important of these compounds are Hydrogen oxide (HOx), Methane (CH4), Hydrogen gas (H2), Nitrogen oxides (NOx). Chlorine monoxide (ClO); during volcanic eruptions, significant amount of chlorine may be released in the stratosphere. Tiny particulate matter in the stratosphere, known

as stratospheric aerosols, may also lead to ozone destruction.

(ii) Human activity related causes: Any event, which release chlorine atoms into the atmospheric, can cause severe ozone destruction, because chlorine atoms in the stratosphere can destroy ozone very efficiently. Most damaging among such agents are human made chlorofluorocarbons (CFCs), which is widely used as refrigerants and to pressurize sprays cans. In stratosphere, chlorine atoms from CFCs react with ozone to form chlorine monoxide and oxygen molecule. 

Effect of O3-layer depletion:

without the ozone-shield the deadly UV radiation shall pass through the atmosphere and reach the earth surface. A small amount of UV -radiation is necessary for well-being of human beings and other organisms, such as UV -B promote synthesis of vitamin-D. UV-radiation also act as a germicide to control microorganisms. However, increased UV dose is highly dangerous to living organisms.