Concept 04 / 05Environmental Chemistry
Greenhouse Effect and Global Warming
How the greenhouse effect keeps the Earth warm, the main greenhouse gases and their sources, the enhanced greenhouse effect and global warming with its consequences, and measures to limit it.
About of the solar energy reaching the Earth is absorbed by the surface, warming it; the surface then radiates energy as infrared (longer wavelength). Some gases in the atmosphere absorb this infrared and re-emit it in all directions, partly back to the surface (see figure). This trapping of heat is the greenhouse effect, named after the glass greenhouse that lets sunlight in and keeps heat from escaping.
Natural greenhouse effect. Without it, the Earth's average surface temperature would be about instead of about , and life as we know it would not exist. Water vapour and carbon dioxide are the main natural greenhouse gases.
Greenhouse gases and sources. Carbon dioxide (burning of fossil fuels, deforestation, cement production), methane (paddy fields, cattle, landfills, coal mines, natural gas leaks), nitrous oxide (fertilisers, combustion), chlorofluorocarbons (refrigerants, aerosols), ozone and water vapour. Per molecule, methane is about to times and CFCs thousands of times more effective than , but contributes most overall because of its large concentration. Molecules with a changing dipole during vibration (asymmetric stretching and bending of , all modes of , ) absorb infrared; and do not.
Enhanced greenhouse effect and global warming. Atmospheric has risen from about before the industrial revolution to over today. Increased concentrations of greenhouse gases trap more heat, raising the average global temperature. Consequences: melting of polar ice caps and glaciers, rising sea levels (flooding of coastal areas), changes in rainfall patterns and more extreme weather, shifting crop zones, and spread of diseases such as dengue and malaria (sleeping sickness, yellow fever) to new regions.
Measures. Reduce the use of fossil fuels (switch to renewable energy), improve energy efficiency, reduce deforestation and plant trees (carbon sinks), cut methane emissions, and phase out CFCs.
Key Takeaways:
- Greenhouse gases absorb the infrared radiated by the Earth and re-emit part of it back.
- Natural greenhouse effect: average temperature about instead of .
- Main gases: , , , CFCs, , water vapour.
- Global warming results from the enhanced greenhouse effect caused by human activity.
- Pitfall: the greenhouse effect itself is natural and essential; the problem is its enhancement.
Brain-Teaser 1: Why do and , the main constituents of air, not contribute to the greenhouse effect while does?
Solution: A molecule absorbs infrared only if a vibration changes its dipole moment. Homonuclear diatomics (, ) have no dipole moment in any vibration. , though non-polar at rest, develops an oscillating dipole in its asymmetric stretching and bending modes, so it absorbs infrared.
Brain-Teaser 2: Burning of octane () releases how much ?
Solution: . octane gives , that is . Burning fuel produces about three times its own mass of , because oxygen from the air is added.
Brain-Teaser 3: Name two greenhouse gases that also damage the ozone layer, and one sink that removes naturally.
Solution: CFCs and nitrous oxide (whose breakdown gives NO in the stratosphere) both trap heat and destroy ozone. Natural sinks of include photosynthesis by plants and dissolution in the oceans.
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