Global warming

Two different kinds of radiation are doing the work. Sunlight arrives as short-wavelength radiation (visible and ultraviolet), which the atmosphere is largely transparent to — it passes through and warms the ground and oceans. That warmed surface does not stay warm silently; every warm object radiates energy outward, and because the Earth's surface is much cooler than the Sun, its emitted radiation is long-wavelength infrared. Some atmospheric gases are essentially invisible to the incoming short-wavelength sunlight but strongly absorb this outgoing long-wavelength infrared. Having absorbed it, they re-emit part of that energy in all directions, including back down toward the surface — heat that would otherwise have escaped to space is instead recirculated in the lower atmosphere. This selective absorb-and-resend behaviour is the greenhouse effect, named after the glass of a greenhouse, which similarly lets sunlight in but slows the escape of heat.

A natural process, pushed too far. A moderate greenhouse effect is not a problem — it is why the Earth's surface averages a habitable temperature rather than the frozen state a bare rock at our distance from the Sun would settle to. Global warming is specifically the human-caused enhancement of this natural effect: burning fuel, clearing forests, farming and dumping waste have all raised the atmospheric concentration of greenhouse gases well above their pre-industrial levels, so more infrared gets trapped than the natural balance intended.

The four gases the syllabus names, and where they come from:

• Water vapour ($H_2O$) is the most abundant greenhouse gas by far, arising naturally from evaporation off oceans, lakes and rivers; warming itself increases evaporation, so its atmospheric level rises as a consequence of warming as much as a cause.
• Carbon dioxide ($CO_2$) comes from burning fossil fuels — coal, petrol, diesel, natural gas — in power plants, vehicles and factories, and also from deforestation, since fewer trees means less $CO_2$ removed by photosynthesis.
• Methane ($CH_4$) comes from waterlogged paddy fields, digestion in cattle and other livestock, decomposing organic waste in landfills, and leakage during natural-gas extraction.
• Oxides of nitrogen ($NO$, $NO_2$) form from high-temperature combustion in vehicle engines and industrial furnaces, through the same reaction pathway that produces acid-rain precursors:

where: $N_2$ and $O_2$ are atmospheric nitrogen and oxygen, which do not normally react at everyday temperatures; the high temperature of an engine cylinder or furnace supplies the energy to force the reaction, first to $NO$ (nitric oxide) and then, on contact with more atmospheric $O_2$, to $NO_2$ (nitrogen dioxide). The most familiar greenhouse-gas equation, though, is simple combustion of a carbon fuel — coal is largely carbon, so burning it in sufficient oxygen gives:

where: $C$ is the carbon in the fuel, $O_2$ is atmospheric oxygen supplied for combustion, and $CO_2$ is the greenhouse gas released to the atmosphere — this single reaction, repeated across billions of tonnes of fuel burned worldwide each year, is the largest single source of added atmospheric $CO_2$.

There is no single formula for global warming — it is a descriptive chemistry topic, so the exam-relevant reasoning is a short chain of cause and effect, not an equation to invert. Grasping why each step follows from the last is what separates a memorised answer from a genuinely understood one.

Step 1. Fossil-fuel combustion, deforestation, agriculture and waste decomposition all release one or more of the four named greenhouse gases into the atmosphere, each through a different chemical pathway — direct combustion for $CO_2$ and $NO_x$, anaerobic bacterial decomposition for $CH_4$, and simple evaporation for $H_2O$.

Step 2. These gases accumulate rather than escaping quickly, because natural removal processes — photosynthesis absorbing $CO_2$, ocean uptake, atmospheric breakdown of $CH_4$ — run slower than the current rate of release. Deforestation makes this worse from the removal side too: fewer trees means less $CO_2$ pulled back out of the air.

Step 3. Because these gases are selectively transparent to incoming sunlight but absorb outgoing infrared, a higher concentration means a larger fraction of the Earth's outgoing heat gets intercepted and partly re-emitted back down, rather than escaping to space.

Step 4. More heat retained in the lower atmosphere, sustained over years and decades, raises the Earth's average surface temperature — this cumulative, compounding rise is global warming. The chain only works in this order: source, accumulation, selective absorption, sustained trapping. Skipping straight from "more $CO_2$" to "hotter planet" without the infrared-trapping mechanism in between is exactly the kind of answer that loses marks for missing the reasoning, even when the conclusion is right.

Question. A village burns coal for electricity, keeps a small herd of cattle and grows paddy rice, and has recently cleared a patch of forest for farmland. Trace, gas by gas, how each activity feeds into global warming, and state which single gas is produced by two of the four activities.

Step 1 — Coal burning. Coal is largely carbon; complete combustion follows $C + O_2 \rightarrow CO_2$. The $CO_2$ escapes through the chimney and accumulates in the atmosphere, since natural removal (photosynthesis, ocean absorption) is slower than continuous release from a running power plant.

Step 2 — Cattle. Digestion in cattle and other ruminants produces methane, $CH_4$, released mainly through belching — a steady, ongoing source unrelated to combustion.

Step 3 — Paddy rice. Waterlogged rice fields create low-oxygen (anaerobic) conditions where bacteria decomposing organic matter also release methane, $CH_4$ — the same gas as Step 2, but from a completely different chemical route (bacterial decomposition, not digestion).

Step 4 — Deforestation. Clearing the forest does not itself release a new greenhouse gas in this scenario, but it removes trees that would have absorbed $CO_2$ through photosynthesis — so it amplifies the $CO_2$ accumulation already building up from Step 1, without a new emission source of its own.

Step 5 — Trapping and warming. All the $CO_2$ and $CH_4$ released, now less effectively removed because of the cleared forest, absorb outgoing infrared radiation from the surface and re-emit part of it downward, trapping extra heat that accumulates over years into a measurable temperature rise.

Answer: methane ($CH_4$) is the gas common to two of the four activities — cattle digestion and paddy-field decomposition — while coal burning adds $CO_2$ directly and deforestation removes the natural sink that would otherwise pull some of that $CO_2$ back out.

• Treating the greenhouse effect and global warming as the same thing — the greenhouse effect is the natural, life-sustaining mechanism; global warming is its human-caused enhancement beyond the natural level.
• Assuming greenhouse gases block sunlight from reaching Earth — they actually let incoming short-wavelength sunlight through and instead trap outgoing long-wavelength infrared.
• Naming only $CO_2$ and forgetting water vapour, methane and oxides of nitrogen, each with distinct sources the syllabus expects you to know.
• Missing that deforestation counts as a cause of rising $CO_2$ even though it releases no new gas directly — it removes the natural sink (photosynthesis) that would otherwise absorb $CO_2$.

Ask the tutor: Want to work through more real-life scenarios (a factory, a landfill, a highway) and identify which greenhouse gas each one releases and why?