The Same Gases That Keep Earth Warm Could Overheat It if They Keep Rising

Smoke from a chimney seems to vanish, but the gases spread through the atmosphere, where they trap heat, return as acid rain or thin the ozone layer.

This part covers the greenhouse effect and global warming, acid rain and ozone depletion, the wider effects of pollution, and the measures used to control it.

The greenhouse effect is the trapping of heat by atmospheric gases that keeps Earth warm enough for life; global warming is the rise in average temperature as these gases increase.

How it works:

- Sunlight passes through the atmosphere and warms Earth's surface
- The warm surface gives off heat as infrared radiation
- Greenhouse gases absorb much of this heat and send part of it back towards the surface

Main greenhouse gases and sources:

- Carbon dioxide — burning coal, petrol and wood; cutting forests
- Methane — flooded paddy fields, cattle, rotting garbage
- Nitrous oxide — nitrogen fertilisers
- CFCs — old refrigerators and air conditioners
- Water vapour

Consequences of global warming:

- Melting glaciers and polar ice, and rising sea levels that threaten low coastal land
- Changes in rainfall, with more intense heat waves, floods and droughts
- Spread of mosquito-borne diseases to new areas
- Loss of species, such as corals that die in warmer seas

An everyday example. A closed car parked in the sun gets far hotter inside than outside — sunlight enters, but heat cannot easily escape.

The misconception. The greenhouse effect itself is not harmful; without it Earth would be too cold. The problem is the extra heat trapped as the gases increase.

Acid rain forms when sulphur and nitrogen oxides from burning fuels dissolve in rain as acids; the ozone layer thins when chlorine from CFCs breaks down ozone.

Acid rain — causes:

$$2{\text{SO}}_2+{\text{O}}_2\to 2{\text{SO}}_3{\text{SO}}_3+{\text{H}}_2\text{O}\to {\text{H}}_2{\text{SO}}_4$$
$$4{\text{NO}}_2+2{\text{H}}_2\text{O}+{\text{O}}_2\to 4{\text{HNO}}_3$$

Acid rain — effects:

- Corrodes marble and limestone buildings, statues and metal structures
- Makes lakes acidic, killing fish
- Damages leaves and washes nutrients out of soil

Worked example. Rain of pH $4$ compared with pH $6$: each pH unit is a tenfold change in acidity, so

$$6-4=210\times 10=100\text{ times more acidic}$$

Ozone depletion — cause:

- The ozone layer in the upper atmosphere absorbs harmful ultraviolet (UV) rays
- CFCs from old refrigerators, air conditioners and aerosol sprays release chlorine atoms
- Chlorine breaks ozone apart and is set free again to repeat the process:

$$\text{Cl}+{\text{O}}_3\to \text{ClO}+{\text{O}}_2\text{ClO}+\text{O}\to \text{Cl}+{\text{O}}_2$$

Ozone depletion — effects of extra UV: skin cancer, cataract, weaker immunity, and damage to crops and ocean plankton.

An everyday example. Newer refrigerators are labelled CFC-free, using gases that do not attack ozone.

The boundary case. Ozone high above protects us, but ozone at ground level in smog is a harmful pollutant that irritates the lungs.

Pollution warms the climate and changes weather patterns, causes lung, water-borne and poisoning diseases in people, and kills or weakens plants and animals.

Effects on climate

- Warming from greenhouse gases
- Smog and haze that reduce sunlight and visibility

Effects on human health

- Air pollution — asthma, bronchitis, lung cancer, eye irritation; carbon monoxide reduces oxygen carried by blood
- Water pollution — cholera, typhoid, jaundice and diarrhoea
- Heavy metals — lead harms the brain, mercury damages nerves
- Excess fluoride or arsenic in groundwater — damage to teeth and bones, or skin disease

Effects on other organisms

- Acid rain damages forests and kills fish
- Oil spills coat seabirds so they cannot fly or stay warm
- Algal blooms starve water of oxygen

An everyday example. On winter days of severe smog, some cities advise schools to close and people with asthma to stay indoors.

The substance. Children, the elderly and people with lung or heart disease suffer most.

Pollution is controlled through cleaner fuels, vehicle standards, fuel saving, tree planting, sewage treatment, less plastic, organic farming and cleanliness.

- Unleaded petrol — removes lead, a poisonous metal, from exhaust
- CNG — burns more cleanly than petrol or diesel, with far less smoke; many city buses and autorickshaws run on it
- Switching off engines at traffic signals — saves fuel and cuts exhaust
- Social forestry — planting trees on roadsides, canal banks and common land, which absorb carbon dioxide and dust
- Sewage treatment plants — remove solids and use microbes to break down wastes
- Ban on polythene bags — reduces plastic waste in drains and fields
- Organic farming — compost and natural pest control instead of chemical fertilisers and pesticides
- Bharat Stage emission standards, based on Euro norms — limit what new vehicles may emit
- Swachh Bharat Abhiyan — a national cleanliness mission for toilets, waste collection and clean surroundings

Worked example. Suppose a car waits $3$ minutes at each of $10$ signals a day, and burns $0.6$ litres of fuel per hour while idling.

$$3\times 10=30\text{ min}=0.5\text{ h}0.5\times 0.6=0.3\text{ L a day}0.3\times 300=90\text{ L in }300\text{ days}$$

An everyday example. Traffic police in several cities run campaigns asking drivers to switch off at red lights.

The substance. Each measure targets a specific source of pollution.