8.26 - Today's atmosphere and climate effects
Describe the main gases in today's atmosphere, connect human activities to carbon dioxide and methane emissions, and compare ways to reduce climate risks with attention to scale and environmental effects.
The composition of today's atmosphere
Today's atmosphere is mostly nitrogen and oxygen. The other gases are present in much smaller amounts, but small amounts can still matter chemically and climatically.
For GCSE Chemistry, learn these rounded dry-air figures:
| Gas in today's atmosphere | Approximate percentage by volume | What to remember |
|---|---|---|
| nitrogen, N2 | 78% | the most abundant gas |
| oxygen, O2 | 21% | needed for respiration and combustion |
| argon, Ar | about 0.9% | the main gas in the remaining 1% |
| carbon dioxide, CO2 | about 0.04% | a trace greenhouse gas |
| other trace gases, including methane, CH4 | very small amounts | can still have important effects |
Water vapour is excluded from dry air. In ordinary air its proportion varies with temperature and humidity. The CO₂ figure is a rounded GCSE value; its actual concentration changes over time.
Trace gas
A trace gas is a gas present in a very small proportion of the atmosphere.
The word trace does not mean unimportant. Carbon dioxide and methane are trace gases by percentage, but they absorb infrared radiation and contribute to the greenhouse effect. A common mistake is to argue that carbon dioxide cannot matter because it is only about 0.04% of the atmosphere. That is not a valid chemistry argument: gases can have important effects even at low concentration if their molecules interact strongly with radiation.
Human sources of carbon dioxide and methane
Carbon dioxide and methane occur naturally in the atmosphere, but this lesson is about increased levels generated by human activity. The specification gives two examples that must be secure:
| Human activity | Main gas linked to the activity | Chemistry link |
|---|---|---|
| burning fossil fuels such as coal, oil products and natural gas | carbon dioxide, CO2 | carbon in the fuel is oxidised during combustion |
| livestock farming | methane, CH4 | methane is released by digestive processes in some animals and from animal waste |
Burning fossil fuels releases energy, but it also converts carbon in the fuel into carbon dioxide. For a hydrocarbon fuel, complete combustion produces carbon dioxide and water. Earlier fuel lessons teach combustion in more detail; here, the important link is:
fossil fuel burned -> carbon in fuel reacts with oxygen -> carbon dioxide released
Methane is linked to livestock farming because animals such as cattle and sheep produce methane during digestion, and animal waste can also release methane as it decomposes. You do not need detailed biology for this point. The GCSE chemistry answer is that livestock farming can increase methane emissions, and methane is a greenhouse gas.
Greenhouse gas
A greenhouse gas absorbs and emits infrared radiation, contributing to the warming of the surface and lower atmosphere.
Potential effects on climate
Climate means the pattern of weather conditions over a long period of time. Weather is what happens on a particular day; climate is about long-term averages, ranges and patterns.
Climate
Climate is the long-term pattern of weather conditions in an area, including temperature, rainfall and seasonal changes.
Increased levels of carbon dioxide and methane can enhance the greenhouse effect. More infrared radiation is absorbed and re-emitted by greenhouse gases, so less energy escapes directly to space. This causes warming and changes to the climate system; the precise regional effects depend on place, timescale and future emissions.
They do not need to claim that one named storm, flood or heatwave was caused only by climate change. A better description is that climate change can alter the risk, frequency, intensity or duration of types of events.
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