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Lesson 4 of 8Climate Change
Unit 3 · Lesson 4 of 8

Greenhouse Gases: Sources, Sinks and the Carbon Cycle

Where carbon dioxide, methane, nitrous oxide, halocarbons and water vapour come from and where they go, how their warming power is compared with global warming potential, how the carbon cycle and the global carbon budget work, and Canada's share of emissions.

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Key things to remember

  1. 1A source releases a greenhouse gas; a sink removes and stores it. Concentration rises when sources exceed sinks.
  2. 2Main human sources: fossil fuels and cement (CO₂), livestock, landfills and fossil fuel leaks (CH₄), fertilizers (N₂O), refrigerants (halocarbons).
  3. 3GWP compares warming per tonne with CO₂ over 100 years: CO₂ = 1, CH₄ ≈ 28, N₂O ≈ 270, some halocarbons thousands.
  4. 4
  5. 5About half of human CO₂ emissions stay in the air; the rest is taken up roughly equally by oceans and land.
  6. 6Canada: about 694 Mt CO₂e in 2023, about 1.5% of world emissions, but among the highest per person.

Every time you ride in a gas-powered car, eat a hamburger or toss food scraps in the garbage, greenhouse gases end up in the air. But plants, soils and oceans are pulling some of those gases back out at the same time. Whether the amount in the atmosphere rises or falls depends on the balance between sources, which release a gas, and sinks, which remove and store it. Right now, human activity has tipped the balance so that sources outrun sinks.

The main greenhouse gases

GasNatural sourcesHuman sourcesSinks and removal
Carbon dioxide, CO₂Respiration, decomposition, volcanoes, wildfires, release from warm ocean waterBurning coal, oil and natural gas; cement making; clearing and burning forestsPhotosynthesis (forests, peatlands, soils); dissolving in the ocean; very slow rock weathering. Part of any added CO₂ stays in the air for centuries or longer
Methane, CH₄Wetlands, termites, thawing permafrostLivestock digestion (especially cattle), landfills, rice paddies, leaks from natural gas, oil and coal productionBreaks down by chemical reactions in the atmosphere in about a decade
Nitrous oxide, N₂OBacteria in soils and oceansNitrogen fertilizers and manure, some industrial processes, burning fuelsBroken down slowly by ultraviolet light in the stratosphere, over about a century
Halocarbons (CFCs, HCFCs, HFCs)Almost none: nearly all are human-madeRefrigerants in fridges and air conditioners, aerosol propellants, foams, solventsBroken down very slowly by ultraviolet light in the stratosphere; some last for centuries
Water vapour, H₂OEvaporation from oceans and lakes, transpiration from plantsVery little directlyCondensation and precipitation, within about a week or two
Sources and sinks of the main greenhouse gases. Ground-level ozone, a greenhouse gas that is also a pollutant, is covered later in the unit.
Sources (add gas)AtmosphereSinks (remove CO₂)burning fossil fuelsCO₂ (largest source)cement makingCO₂deforestationCO₂ (trees cut, burned)livestock and landfillsCH₄ (methane)fertilizer useN₂O (nitrous oxide)melting permafrostCO₂ and CH₄CO₂CH₄N₂Owhen sourcesexceed sinks,these gasesbuild upforestsphotosynthesis stores CoceansCO₂ dissolves in watersoilsstore dead plant carbonpeatlandslong-term carbon store
Fig 1Sources and sinks of greenhouse gases: arrows into the atmosphere from fossil-fuel combustion, cement, deforestation, livestock and landfills (methane), fertilizer (nitrous oxide) and melting permafrost, and arrows out of the atmosphere into forests, oceans, soils and peatlands.
Water vapour is a feedback, not a control knob
Water vapour is the most abundant greenhouse gas and causes the largest share of the natural greenhouse effect. But the amount of water vapour in the air is set by temperature: extra vapour rains out within days, and warmer air holds more of it (about 7% more per degree Celsius). So people cannot directly change it much. Instead, when CO₂ warms the air, the air holds more water vapour, which warms it further. Water vapour amplifies a change started by the long-lived gases.

Comparing gases: global warming potential

Tonne for tonne, some gases trap far more heat than CO₂. Global warming potential (GWP) compares the warming caused by 1 tonne of a gas over 100 years with the warming caused by 1 tonne of CO₂, which is defined as GWP = 1. Methane's GWP is about 28, nitrous oxide's about 270, and some halocarbons have GWPs in the thousands. To add up different gases, scientists convert each one to carbon dioxide equivalent (CO₂e). CO₂ still causes most of the human-caused warming, because we release so much more of it and much of it stays in the air for a very long time.

Carbon dioxide equivalent: the mass of CO₂ that would cause the same warming over 100 years.

The carbon cycle and the global carbon budget

Carbon moves among reservoirs: the atmosphere (as CO₂ and CH₄), living things, soils and peat, the oceans (as dissolved CO₂ and carbonate), and rocks and fossil fuels. In the fast carbon cycle, carbon moves between air, living things and the surface ocean over years to centuries through photosynthesis, respiration, decomposition and gas exchange. In the slow carbon cycle, carbon moves into and out of rocks over millions of years, through volcanoes and the weathering of rock. Fossil fuels are carbon that plants and plankton removed from the air millions of years ago. Burning them moves carbon from the slow cycle into the fast cycle in decades.

  • Fossil fuels released a record of about 38 billion tonnes of CO₂ in 2025, according to the Global Carbon Project; land-use change (mostly deforestation) added several billion tonnes more.
  • Over the past decade, roughly half of human CO₂ emissions stayed in the atmosphere, about a quarter to a third dissolved into the oceans, and about a fifth to a quarter was taken up by plants and soils on land.
  • The ocean sink has a cost: dissolved CO₂ forms carbonic acid, so ocean water is becoming more acidic (ocean acidification), which harms shell-building organisms such as corals, oysters and some plankton.
  • Sinks can weaken: hot, dry years and wildfires reduce how much carbon forests absorb, and thawing permafrost can turn a carbon store into a source.

Human factors and Canada's contribution

The main human factors that add greenhouse gases are burning fossil fuels for transportation, heating and electricity; industrial emissions from making cement, steel, chemicals and fertilizer and from extracting oil and gas; deforestation, which both releases stored carbon and removes a sink; and forest fires, which release CO₂ quickly (fires have natural causes too, but people start many of them, and a warmer climate makes large fires more likely). Agriculture adds methane and nitrous oxide.

Canada's emissions (2023 national inventory)Value
Total greenhouse gas emissionsAbout 694 million tonnes CO₂e
Change since 2005About 8.5% lower
Share of world emissionsAbout 1.5%
Per personAbout 17 tonnes CO₂e, well over twice the world average
Largest sectorOil and gas production, about 30% (208 Mt)
Second-largest sectorTransportation, about a fifth
Canada's greenhouse gas emissions as reported in its 2025 National Inventory Report (data for 2023).
Small share, large footprint
Canada produces a small share of the world's emissions, but it is among the highest emitters per person, because of its cold climate, long travel distances, large energy-producing industries and high consumption. Canada also has huge carbon sinks to protect: the boreal forest and the Hudson Bay Lowlands peatlands in northern Ontario store enormous amounts of carbon.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A landfill releases 50 tonnes of methane in a year. Using a GWP of 28, what is that in tonnes of CO₂ equivalent? If the methane were captured and burned to CO₂ instead (about 2.75 t of CO₂ per tonne of CH₄), what would the emissions be?
Example 2
In one year, people emit about 40 billion tonnes of CO₂. If about 50% stays in the atmosphere, 28% is absorbed by the ocean and the rest by land, how much goes to each?
Example 3
Classify each as a source or a sink of atmospheric carbon, and name the gas: (a) a growing spruce forest, (b) a herd of dairy cattle, (c) clearing a forest and burning the wood, (d) cold ocean water in the North Atlantic, (e) thawing permafrost.

Where marks get lost

  • Thinking methane is the main cause of warming because its GWP is high. Per tonne it is stronger, but CO₂ is emitted in far larger amounts and lasts much longer, so CO₂ causes most human-caused warming.
  • Saying volcanoes emit more CO₂ than people. Human activities emit roughly 100 times more CO₂ each year than all the world's volcanoes.
  • Treating water vapour as something people directly control. Its amount is set by temperature, so it acts as a feedback that amplifies warming caused by other gases.
  • Assuming a sink removes carbon forever. Forests can burn or be cut, and warming oceans absorb less CO₂, so stored carbon can return to the air.
  • Mixing up GWP and concentration. GWP compares warming per tonne; it does not tell you how much of a gas is in the air.

Flashcards 8

Source (of a greenhouse gas)
Sink (of a greenhouse gas)
Global warming potential (GWP)
CO₂ equivalent (CO₂e)
+ 4 more

Practice

1
Match each greenhouse gas to a major human source.
Carbon dioxide
Methane
Nitrous oxide
Halocarbons
2
A farm releases 3.0 tonnes of methane per year. Using a GWP of 28, how many tonnes of CO₂ equivalent is that?
t CO₂e
3
Canada emitted about 694 million tonnes of CO₂e in 2023 and had about 40 million people. Estimate the emissions per person, in tonnes, to the nearest whole number.
t
4
Order these gases from lowest to highest global warming potential (per tonne, over 100 years).
  1. 1Methane
  2. 2A typical CFC refrigerant
  3. 3Nitrous oxide
  4. 4Carbon dioxide
5
Which of these is a carbon sink?
6
Why does water vapour act mainly as a feedback rather than a driver of climate change?
7
About what fraction of the CO₂ released by human activity stays in the atmosphere?
8
What is the name for the drop in seawater pH caused by the ocean absorbing CO₂?
9
Which statement about Canada's greenhouse gas emissions is accurate?

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