Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.11

The Greenhouse Effect.

In a cold country there is ice outside, yet the plants inside a glass house stay green. The air above our heads works like a glass house too. Without that glass the earth would be a ball of ice at −30°C. So what is there to fear? The glass is getting thicker.

1. The glass house and the earth's air

In cold countries green vegetables are grown inside a glass-roofed house called a greenhouse. However cold it is outside, the temperature inside stays between 38°C and 39°C.

The reason lies in how glass behaves. Every wave of visible light (VIBGYOR) passes through the glass and heats the ground. The hot ground then radiates long-wavelength infrared (IR) rays. These cannot get back out through the glass, so the heat stays inside.

The earth has a greenhouse of its own. In the homosphere near the ground, N₂, O₂ and O₃ share the air with CO₂ and water vapour (H₂O). These two gases soak up the heat of the IR rays the ground radiates. That keeps the air at an average of about 15°C, which suits living things.

Without CO₂ and H₂O, that is without the greenhouse effect, the average temperature of the surface and air would be −30°C, and there would be no life at all. So the greenhouse effect is not the villain. Its growth is.

2. Which gases trap heat

A gas that absorbs the IR rays radiated by the ground and raises the air temperature is a greenhouse gas. The main one is CO₂. The others are water vapour, CH₄, N₂O, CFC and O₃.

But not every gas absorbs IR. A molecule has to meet three conditions.

• Condition 1: the molecule has three or four atoms, as in H₂O, CO₂ and CH₄.
• Condition 2: the vibration frequency of the bonded atoms equals the frequency of the IR rays the ground radiates.
• Condition 3: the molecule has an easily changing dipole moment. The C=O of CO₂, O-H of H₂O, N-O of N₂O and NO, and C-Cl and C-F of CFC are polar. N₂, H₂ and O₂ have no greenhouse effect.

3. How the warming began

In 1850 the CO₂ in the air was steady at about 0.029%. Since the industrial revolution it has been climbing. The average temperature rose 0.4°C between 1880 and 1900, and 0.78°C between 1880 and 2018. On the 20th-century average, the present value is taken as 15°C.

Global warming is the continuing rise of temperature all over the world. Its cause is the steady growth, in the air, of greenhouse gases from industry and motor vehicles.

Each gas has its own share of the warming (Table 1.7): CO₂ 50%, CH₄ 19%, CFC 16%, O₃ 8%, N₂O 5%, water vapour 2%.

A bigger share does not mean a stronger molecule. Heat-holding power per molecule compared with CO₂: CH₄ 25 times, CFC 15000 times, O₃ 10 times, N₂O 270 times, water vapour 0.2 times (5 times less). The share comes from the power and the number of molecules in the air, taken together. Match each gas to its share and power.

4. CO₂, the biggest share

CO₂ is the most abundant greenhouse gas in the air, with a role of about 50%. Animals release CO₂ in respiration. Green plants make carbohydrate and O₂ from CO₂ and H₂O by photosynthesis. CO₂ also dissolves in sea water. Plants and the sea are the two main sinks of CO₂.

Yet CO₂ keeps rising, for three reasons. (1) Unrestricted use of coal in industry and petrol in vehicles. (2) Forests are being destroyed, so plants absorb less CO₂. (3) Warmer sea water dissolves less CO₂. The sources grow while both sinks weaken.

From 0.029% (290 ppm) in 1850, CO₂ rose to more than 0.0385% (385 ppm) in 2008, which equals a 33% increase. NASA data in 2011 gave an increase of 40%.

Where does 33% come from? Rate of increase = (final − start)/start × 100 = (385 − 290)/290 × 100 = 32.76% ≈ 33%. And ppm = percent × 10000, so 0.029% = 290 ppm and 0.036% = 360 ppm. One rule: both values must be in the same unit. Try it yourself.

Raise the CO₂ in the air and see how much of the ground's IR is held back, and where the thermometer goes.

At the present rate, CO₂ in the air may reach 500 to 700 ppm by 2050 and the global average temperature will rise by 3°C. Put 500 and 700 against 290 in the calculator and see how big the jump is.

5. The other four gases

Methane (CH₄) comes right after CO₂, with 19%. Its concentration in the air is 1.7 ppm, growing 1.2% a year, and its heat-holding power is about 25 times that of CO₂.

There are three sources. (1) After rice-field and wetland plants die, bacteria decay the cellulose without air: 2(C₆H₁₀O₅)n + 2nH₂O → 6nCH₄ + 6nCO₂ (in the presence of bacteria). (2) The dung of cattle and the decay of dead animals. (3) Organic waste, and gas and oil mines.

CFC is third, with a 16% share. Its concentration in the air is low, but one CFC molecule holds 15,000 to 20,000 times the heat of CO₂. That is why it makes a super green house effect.

CFC is used as the coolant liquid in AC and refrigerators, in cosmetic sprays, in the foam industry and in dry cleaners. It stays in the atmosphere for a long time. So under the Montreal Protocol, signed in Montreal, Canada, in 1987, the production and use of CFC in developing countries was banned completely by 2010.

Ozone (O₃) is fourth, with 8%. It is useful in the stratosphere but a harmful pollutant in the troposphere. Its yearly average growth in the troposphere is 0.4 to 1.0%, and its heat-holding power is about 10 times that of CO₂.

Why does O₃ rise in the troposphere? Coal and petrol burn in vehicles, coal power stations and industry. That makes oxidising NOx, which reacts with the hydrocarbons and O₂ of the air to form O₃.

Nitrous oxide (N₂O) has a share of about 5%, but its heat-holding power is about 270 times that of CO₂. It rises from burning coal and petrol in vehicles, power stations and industry, with a yearly average growth of about 0.2 to 0.3%.

6. What the warming does

In the last 50 years the global temperature has risen by 0.6°C. Of the extra heat absorbed because of the CO₂ rise, 20% went into warming the air and 80% into evaporating sea water.

The damage has already shown. 1997 to 1999 were the hottest three years. The 1987-1997 decade had 10 times the sea storms and floods of the one before. Global snow area fell 10%, the Arctic sea ice layer fell 40%, ice freezing fell 20%, and northern hemisphere rainfall rose 10%.

Computer models say that if CO₂ keeps rising at the present rate, the average temperature will rise by 3°C, from 15°C to 18°C, by 2050. About 17% of Bangladesh's land will be submerged by the sea, with food shortage and climate change to follow.

By the conclusion of Dennis Murphy of Stanford, 44% of mammals, 23% of butterflies and a few percent of birds will be lost.

A last tally: where the sunlight goes. About 34% of the light reaching the earth is reflected back to space. 42% is absorbed by the air, land and sea. 23% runs the water cycle. A little under 1% drives wind and sea currents. Only 0.023% goes into photosynthesis.

7. Check yourself

Three questions in the exam's style. Start with the first.

In the second, recall the values.

Use your reasoning on the last.

The whole note on seven cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.11.