Global warming

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Topical Science

Issue No. 5

May 2017

TOPICAL SCIENCE : GLOBAL WARMING Much has been written in the media about global warming, but the physico-chemical mechanisms involved are seldom explained.

This issue of Topical Science attempts to explain these mechanisms.

May 2017 In this issue, I try to explain the way the gases in our air interact with electromagnetic radiation.

Carbon dioxide and water vapour are transparent to visible light, but absorb infrared radiation and therefore can trap heat energy.

The composition of our atmosphere, coupled with the way in which the different gases in the air interact with electromagnetic radiation, plays a crucial role in determining the temperature of our planet. The table on page 2 shows the main gases present in the air we breathe.

Carbon dioxide is essential to life.

It keeps our planet warm and is needed for photosynthesis.

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But too much CO2, in the air, causes global warming!

Our Atmosphere: The layer of gases enveloping our planet Earth.

The main constituent of our atmosphere is the gas

nitrogen, which makes up almost 80%, by volume, of the air around us. At normal ambient temperatures, this gas is chemically unreactive and serves to dilute the oxygen, which accounts for most of the remaining 20%(approx.) The third most abundant gas in the atmosphere is the inert gas argon, which accounts for about 0.9% of air.

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We need oxygen to sustain life. With every breath we take, oxygen is drawn into our lungs and goes into our bloodstream, where it is pumped, by the action of our heart, to every part of our body. We derive energy from our food when it combines with the oxygen we breathe. We exhale carbon dioxide (CO2) as a waste product, but green plants use this CO2.

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Topical Science

Issue 5

May 2017

THE COMPOSITION OF THE AIR WE BREATHE GASES. The gases in our atmosphere play a crucial role in controlling the temperature at the surface of our planet.

However, nitrogen and oxygen, the two gases that between them account for 99% by volume, of our air, do not have a warming effect. We need oxygen to survive and we draw this gas into our lungs with every breath.

Oxygen makes up 21%, of dry air. Nitrogen is the main component of air, accounting for 78% by volume. It serves to dilute the oxygen, which at higher levels, would be toxic and would give rise to more frequent wildfires.

WATER VAPOUR The amount of water vapour in air varies from place to place and from day to day and depends on the temperature and pressure. Warm air can hold more water vapour than cold air, so the greatest amount of water vapour occurs in tropical air over the oceans and the least amount in polar air and in desert regions.

Water vapour acts as a so-called ‘greenhouse’ gas, because it absorbs infrared (IR) radiation.

Atmospheric Gases Nitrogen

% 78

Oxygen

21

Argon

0.9

Carbon Dioxide ‘Greeenhouse’ gas Water Vapour

0.04

Inert Gases (e.g. neon, helium) Other Greenhouse Gases, e.g.

Methane, Ozone, Nitrogen oxides

Varies (0 to 4%) Trace Trace

Water Vapour and Carbon Dioxide can trap Heat.

Water vapour is invisible, as are the other main gases that make up our atmosphere. The clouds in this picture are made up of tiny droplets of liquid water that have condensed, because there is a limit to the amount of water vapour that the air can hold. The picture also shows solid water in the form of snow. Water vapour and carbon dioxide can trap heat in the atmosphere, because of the way their molecules vibrate. The molecular vibrations interact with Infra red radiation and keep our planet warm enough to sustain life.

Water, water everywhere, clouds above, snow below and presumably water vapour in between.

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Topical Science

Issue No. 5

May 2017

The Greenhouse Effect Some of the gases in our atmosphere (the socalled greenhouse gases) are capable of interacting with infrared (IR) radiation, which traps heat energy and helps to keep our planet warmer than it would otherwise be. To understand how this happens, we need to know a little about radiant energy.

Much of the harmful ultra violet rays are absorbed by the ozone layer in the upper atmosphere. The radiant energy that is absorbed by the earth’s surface is re-emitted as infrared radiation. The fact that the sun’s energy spectrum peaks in the visible region, while that of the earth peaks in the Infrared, is due to the fact that the sun has a much higher temperature than the earth.

The earth receives energy from the sun in the form of electromagnetic radiation, which consists of oscillating electromagnetic waves.

These waves travel in a vacuum with the enormous speed of almost 300 000 kilometres per second. This is the speed of light, which is familiar to many people. But visible light, to which our eyes are sensitive, forms only a very small portion of the electromagnetic spectrum.

As well as visible light, the sun also emits ultra violet radiation, which is more energetic than visible radiation, as well as infrared radiation, which has lower energy. All of the gases listed in the table on page 2 are transparent to visible radiation, which passes through the air and is absorbed by the surface of the earth.

If our atmosphere consisted only of oxygen and nitrogen and the inert gases (helium neon, argon krypton and xenon), the earth’s IR radiation would escape into space. But molecules such as carbon dioxide, water vapour and nitrogen oxides can capture that energy.

This is because of the way their molecules vibrate. For example, the CO2 molecule consists of a carbon atom in the middle, (represented by the symbol ‘C’) bonded to two oxygen atoms, (symbol ‘O’) one on either side. It is a linear molecule and may be represented like this: O = C = O (where the ‘=’ signs represent the chemical bonds).

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Topical Science

Issue 5

May 2017

The Greenhouse Effect (Continued from page 3) Chemical bonds consist of pairs of electrons, which are shared between the two bonding atoms. Each of the two bonds in the CO2 molecule contains TWO shared electron pairs. They are described as ‘double bonds’ and this is why it is conventional to represent the bonds with a double line (=). These double bonds are stronger than single bonds (though not twice as strong) and they are not rigid, but can stretch and contract, like springs, as the molecules vibrate.

Global Warming

The rule is that a molecule can absorb infrared radiation if its vibrations result in a change in the molecule‘s dipole moment.

A dipole is the electrical equivalent of a magnet. It occurs when the centres of positive and negative charge in a molecule do not coincide.

The individual bonds in the CO2 molecule are polar, because the oxygen atoms have a greater pull on the shared electron pair than the carbon atom. However, as it is a linear molecule and the two bonds have the same length and are symmetrically arranged, the individual bond moments cancel out and consequently the molecule as a whole is non-polar (i.e. it does not possess a dipole moment). Similarly, when the two bonds stretch in phase, both extending to the same extent, the bond moments continue to cancel and therefore this ‘symmetric stretch’ in not infrared active.

However, the molecule can also vibrate in such a way that one of the bonds is extending, while the other is shortening. This is known as the ‘Asymmetric Stretch’ and this vibrational mode is IR active. There are also two ‘bending’ modes of vibration, where the molecule literally bends out of its linear arrangement, making a bond angle less than 180 degrees. The two bending vibrations occur at right angles to each other but they absorb the same frequency of IR radiation and so register as a single peak in the IR spectrum of the molecule.

The asymmetric stretching vibration of CO2, as well as the bending vibrations, absorb IR radiation because these vibration involve a change in the overall dipole moment of the molecule. It is this property of Carbon dioxide that helps to keep our planet warm.

The mechanism whereby certain gases, such as CO2 and methane, can trap energy in the atmosphere is called ‘The Greenhouse Effect’, as it is similar to the way in which the glass of a greenhouse is transparent to visible light, but absorbs IR radiation.

This natural greenhouse effect is good, because it keeps the Earth’s average temperature within the range that allows water to exist in the liquid state and so sustain life. Carbon dioxide also allows plants to make food through PHOTOSYNTHESIS, which uses the sun’s energy to combine water and carbon dioxide to form glucose.

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The natural greenhouse effect is good, as it keeps our planet warm enough to sustain life. We also need carbon dioxide for photosynthesis, which is the basis of all our food.

But too much global warming, caused by buildup of atmospheric greenhouse gases, due to burning of fossil fuels and excessive cattle farming ,is causing climate change, which is bad for the planet

Author:

Margaret Franklin Margaret is a retired chemistry lecturer, who cares about the environment and who tries to explain the science behind topical issues, in a way that can be understood by the general public.