International A Level Biology / Environment and climate change
The greenhouse effect and anthropogenic climate change
Edexcel International A Level Biology (YBI11), chapter 16: Environment and climate change, specification point 5.17. Free lesson.
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The natural greenhouse effect keeps Earth warm enough for life. Human activities add greenhouse gases to the atmosphere, enhancing the effect and causing anthropogenic climate change.
The natural greenhouse effect keeps Earth warm
The greenhouse effect is natural, not something caused by people. Without greenhouse gases, Earth's mean surface temperature would be about −18 °C rather than about 15 °C. The effect happens because incoming and outgoing energy have different wavelengths. Short-wave radiation from the Sun, mostly visible light, passes through the atmosphere. Some is reflected back to space. Land and sea absorb much of the short-wave radiation, so Earth's surface warms. The warm surface re-emits energy as long-wave infrared radiation. Greenhouse gases in the atmosphere absorb this infrared radiation…
More greenhouse gases enhance the effect
The enhanced greenhouse effect uses the same mechanism as the natural effect. The difference is that human activity increases the concentration of greenhouse gases. More long-wave radiation is then absorbed and re-emitted downwards, so the atmosphere and surface warm further. This human-caused change is anthropogenic climate change. The amount of short-wave radiation coming from the Sun is unchanged. Burning fossil fuels adds carbon dioxide to the atmosphere. A higher concentration of greenhouse gases means more long-wave radiation is re-emitted towards Earth's surface and less escapes to…
The gases absorb long-wave, not short-wave, radiation
A common error is to say that greenhouse gases trap the Sun's short-wave radiation. Most short-wave radiation passes through them. Greenhouse gases absorb the long-wave infrared radiation re-emitted by Earth's warmed surface. In an explanation, state that the gases build up in the atmosphere. State that they absorb long-wave infrared radiation and re-emit some towards Earth. Explain that less energy escapes to space, so both the atmosphere and the surface warm.
Natural and human sources of greenhouse gases
The main gases to know are carbon dioxide, methane and water vapour. Each has natural sources, while human activities increase some of them. Carbon dioxide is released naturally by respiration, decomposition and volcanoes. Burning fossil fuels and deforestation add it through human activity. Methane is released naturally when material decays without oxygen in wetlands. Cattle, rice paddies and landfill sites are human-related sources. Water vapour comes from evaporation and is the most abundant greenhouse gas. People add little directly, but warmer air holds more water vapour.…
How human activities release greenhouse gases
The process matters when you name a source. In particular, it is the burning of fossil fuels that releases carbon dioxide, not simply using them. Burning fossil fuels: combustion releases carbon dioxide from carbon stored underground for millions of years. Deforestation: cleared trees release carbon dioxide when burnt or decomposed. With fewer trees, less carbon dioxide is removed by photosynthesis. Cattle: microorganisms in the gut break down cellulose in grass and produce methane. Rice paddies: flooded, waterlogged soil has little oxygen. Microorganisms decomposing organic matter…
More lessons in Environment and climate change
- Energy transfer between trophic levels
- Gross and net primary productivity
- The carbon cycle
- Using the carbon cycle to reduce atmospheric CO₂
- Evidence for climate change: CO2 and temperature records
- Evidence for climate change: pollen in peat bogs and dendrochronology
- The climate debate: correlation, causation and who concludes
- Extrapolating data: climate models and their limitations
- How climate change affects plants and animals
- Temperature, enzymes and Q10
- Core practical 12: temperature and the development of organisms
- Evolution: a change in allele frequency
- Allopatric and sympatric speciation