The Greenhouse Effect, the Carbon Footprint and Sustainable Solutions

Solar radiation reaches the Earth mainly as visible and near-infrared short waves, which pass through the atmosphere almost unabsorbed. About 30% is reflected back to space by clouds, dust and bright surfaces (the albedo) and about 70% is absorbed by the surface and warms it. The warmed body then radiates according to its own temperature — at about 288 K the peak is near 10 µm, in the long-wave infrared.

• Greenhouse molecules absorb particular infrared bands because their vibrations change the dipole moment of the molecule; they then re-emit the energy in random directions, so part of it returns to the surface.
• The result is that the surface and the lower atmosphere are warmer than they would be in radiative balance with space: about 33 °C warmer than a bare blackbody Earth.
• The enhanced greenhouse effect is the additional warming produced when human activity raises the concentration of these gases; between 1750 and today CO₂ has gone from 280 to more than 420 ppm and the mean surface temperature has risen about 1.1 °C.
• Windows of transparency: the atmosphere lets visible light in and lets some infrared out, but absorption bands of CO₂, H₂O and CH₄ close part of the infrared window — that closure is the physical cause of the effect.

• The carbon footprint is the total amount of greenhouse gases released directly or indirectly by an individual, an organisation, an event or a product, expressed in tonnes of CO₂-equivalent (t CO₂e) per year so that all gases are comparable.
• Primary (direct) footprint: fuel burnt on site — petrol in a car, gas for heating and cooking, diesel in machinery.
• Secondary (indirect) footprint: emissions of the power stations that supply the electricity, plus everything embedded in the goods and services consumed, the transport of food and the treatment of waste.
• Typical factors: petrol about 2.3 kg CO₂ per litre; diesel 2.7; natural gas about 0.19 kg per kWh of heat; grid electricity from 0.1 to 0.9 kg CO₂ per kWh depending on the fuel mix; a return flight London–Cairo roughly 0.5–0.7 t CO₂ per passenger.
• Global average footprint ≈ 4.7 t CO₂e per person; high-income countries 8–15 t; Egypt about 2.3–2.5 t per person, well below the world average, which is the ethical core of the climate-justice argument.
• Carbon trading and carbon tax: a cap is set and permits are bought and sold, or a price is placed on each tonne, so that reducing emissions becomes the cheaper option for a firm.

Worked calculation

A household burns 1,200 L of petrol a year in a car, uses 3,000 kWh of electricity from a grid emitting 0.45 kg CO₂/kWh, and 150 m³ of natural gas (about 0.2 kg CO₂/m³ × 10.5 kWh/m³, i.e. roughly 32 kg per m³). Direct and indirect emissions: 1,200 × 2.3 = 2,760 kg; 3,000 × 0.45 = 1,350 kg; 150 × 2.1 ≈ 315 kg (as heat) — total ≈ 4.4 t CO₂e a year.

• Sea-level rise of 0.15–0.20 m since 1900, from thermal expansion and melting ice, threatening low-lying coasts and deltas, including the Nile Delta, and pushing salt water into the aquifer.
• Changes in the water cycle: wet regions wetter, dry regions drier, more evaporation from reservoirs, and a higher probability of both flash floods and drought.
• Agriculture: heat stress shortens the grain-filling period of wheat and maize, and higher CO₂ raises yield only where water and nutrients are not limiting.
• Ecosystems: coral bleaching above 1–2 °C of warming, shifting of species ranges, and loss of the least mobile ones; the more diverse an ecosystem, the better it resists change.
• Health: heat stroke, wider distribution of vector-borne disease, and reduced air quality as warm nights increase ozone formation.
• Cryosphere: glaciers retreat first, reducing dry-season river flow, before they disappear altogether.

Mitigation — attacking the cause

• Energy efficiency in buildings, industry and appliances; district heating; waste-heat recovery.
• Renewable electricity and heat, with storage and grid interconnection (see the companion lesson on renewable energy and green hydrogen).
• Transport: mass transit, electric vehicles charged from clean power, cycling and walking, rail freight, and efficient shipping and aviation fuels.
• Industry: green steel and cement chemistry, capture at process sources, circular economy and recycling, which reduces the emission embedded in new material.
• Agriculture and land use: water management in rice, better manure handling, reduced food waste, and afforestation and protection of existing forests and wetlands as sinks.

Adaptation — living with the effect

• Coastal defence with sea walls, breakwaters and beach nourishment; controlled retreat from the most exposed land.
• Water: canal lining, modern irrigation, treated-waste-water re-use, desalination and rainwater harvesting.
• Crops: heat- and salt-tolerant varieties, shifted planting dates, resilient seed banks and insurance for smallholders.
• Cities: cool roofs, green corridors, drainage sized for extreme rain, and early-warning systems for heat waves.

1. Why does CO₂ absorb infrared while N₂ does not? — CO₂ is a linear molecule whose asymmetric stretch and bending change its dipole moment, so it interacts with the radiation; N₂ is symmetric homonuclear and its vibration produces no dipole change.
2. A country produces 12 million tonnes of cement. With about 0.8 t CO₂ per tonne of clinker, estimate the process emission. — ≈ 9.6 Mt CO₂, mostly from calcination, not from fuel.
3. Rank by GWP over 100 years: CO₂, CH₄, N₂O, CFC-12. — CO₂ = 1 < CH₄ ≈ 28 < N₂O ≈ 265 < CFC-12 ≈ 10,000.
4. A family replaces 40 halogen bulbs of 60 W by 9 W LEDs running 5 h a day. Find the annual cut if the grid emits 0.45 kg CO₂/kWh. — Saving = 40 × 51 W × 5 h × 365 = 372 kWh ⟹ ≈ 167 kg CO₂ a year.
5. Explain how melting Arctic sea ice accelerates warming. — Bright ice reflects most sunlight; when it is replaced by dark ocean, absorption rises, the water warms, more ice melts — a positive albedo feedback.