Climate Change
Senior 6 · Geography
Climate Change
Lesson 2 of 4
25 min
If the Earth had no atmosphere, the energy it receives from the Sun would balance at an average surface temperature of roughly −18 °C: every lake, including Lake Kivu, would be ice. The actual average is about 15 °C. The difference of some 33 °C is the natural greenhouse effect, and without it there would be no farming, no rivers and no life as we know it. The problem today is not the greenhouse effect itself but its enhancement by the gases people add to the air.
Definition
Greenhouse effect: the warming of the Earth's surface because gases in the atmosphere absorb heat (infrared radiation) given off by the surface and send part of it back down.
Greenhouse gas (GHG): a gas that absorbs and emits infrared radiation, such as water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O).
Enhanced greenhouse effect: the extra warming caused by human additions of greenhouse gases.
Global warming: the rise in the Earth's average surface temperature since the pre-industrial period (1850–1900), caused mainly by the enhanced greenhouse effect.
Figure 1: the greenhouse effect. The grey dots stand for greenhouse-gas molecules.
| Gas | Main human sources | Concentration in 1750 and 2019 | Warming per tonne compared with CO₂ over 100 years (GWP₁₀₀) |
|---|---|---|---|
| Carbon dioxide (CO₂) | Burning coal, oil and gas; cement making; clearing and burning forests | 278.3 ppm → 410 ppm | 1 (part stays in the air for centuries) |
| Methane (CH₄) | Cattle and other ruminants, rice paddies, landfills, leaks from gas wells and coal mines | 729.2 ppb → 1,866 ppb | About 27 (biological sources) to 30 (fossil sources); lasts about 12 years |
| Nitrous oxide (N₂O) | Nitrogen fertilisers, manure, some industry | 270.1 ppb → 332 ppb | 273; lasts about 109 years |
| Fluorinated gases (HFCs and others) | Refrigerators, air conditioners, some industrial processes | None before industry | Hundreds to thousands |
| Water vapour (H₂O) | Not controlled directly by people: a warmer atmosphere holds more of it | Varies from day to day | Acts as a feedback, not a driver |
All values from IPCC AR6 Working Group I (2021); ppm = parts per million, ppb = parts per billion. Note that water vapour is the most abundant greenhouse gas, but its amount is set by temperature, so it amplifies a warming started by other gases.
Definition
Global warming potential (GWP₁₀₀): how much heat one tonne of a gas traps over 100 years compared with one tonne of CO₂.
CO₂-equivalent (CO₂e): the mass of CO₂ that would cause the same warming as a given mass of another gas: .
Albedo: the fraction of incoming sunlight a surface reflects, from 0 (absorbs all) to 1 (reflects all). Fresh snow is high; dark ocean and forest are low.
Feedback: a process in which a change causes effects that increase it (positive feedback) or reduce it (negative feedback).
Worked example 1: a farm's emissions in CO₂-equivalent
Problem. In one year a dairy farm releases 2.0 t of methane from its cattle (GWP₁₀₀ = 27.0), 0.10 t of nitrous oxide from fertiliser (GWP₁₀₀ = 273) and 15 t of CO₂ from diesel. Find its total emissions in CO₂e and the share due to methane. (Illustrative farm; GWP values from IPCC AR6.)
Solution.
Although the farm released only 2 t of methane, it is the largest part of its climate impact. This is why livestock and rice farming matter in climate policy.
Worked example 2: how much have the gases increased?
Problem. Using the table, find the percentage increase in CO₂ and in CH₄ from 1750 to 2019.
Solution.
Methane has risen proportionally more than CO₂, but CO₂ still causes most of the warming because so much more of it is emitted and it lasts far longer.
Figure 2: the ice–albedo feedback, a positive feedback that makes the Arctic warm faster than the global average.
Other feedbacks work the same way. A warmer atmosphere holds more water vapour, which traps more heat. Thawing permafrost in the Arctic releases CO₂ and methane from frozen soil. Some feedbacks are negative: plants grow faster with more CO₂ and take some of it back. Clouds can act either way and remain the largest uncertainty in climate projections.
The IPCC projects warming for several shared socio-economic pathways (SSPs), each a story of how emissions might develop. Warming in 2081–2100 compared with 1850–1900:
| Pathway | Emissions | Best estimate | Very likely range |
|---|---|---|---|
| SSP1-1.9 | Very low: net zero CO₂ around 2050 | 1.4 °C | 1.0–1.8 °C |
| SSP1-2.6 | Low | 1.8 °C | 1.3–2.4 °C |
| SSP2-4.5 | Intermediate | 2.7 °C | 2.1–3.5 °C |
| SSP3-7.0 | High | 3.6 °C | 2.8–4.6 °C |
| SSP5-8.5 | Very high | 4.4 °C | 3.3–5.7 °C |
Source: IPCC AR6 WGI (2021). The Paris Agreement (2015) aims to hold warming "well below 2 °C" and to pursue 1.5 °C; only the lowest pathways meet these goals.
Warning
Common mistakes
Tip
Memory hook. "Short in, long out": sunlight comes in as short-wave radiation; the Earth sends heat out as long-wave radiation, and greenhouse gases catch the long waves. In an exam, draw the diagram: a labelled sketch of the greenhouse effect can earn as much as a paragraph.
TED-Ed: "Climate change: Earth's giant game of Tetris" by Joss Fong. Watch how the carbon cycle is pictured as falling blocks, and note what happens when CO₂ is added faster than it can be removed.
Practice
Summary
Sources and credits
Photo: Mauna Loa Observatory north flank of Mauna Loa (Brian Vasel, NOAA, public domain), via Wikimedia Commons.
Video: "Climate change: Earth's giant game of Tetris", TED-Ed, embedded from YouTube.
Figures 1 and 2, the worked examples and the exercises were drawn and written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
