D. Earth and Space Science: Climate Change · Climate Change

Greenhouse Gases: Sources, Sinks and Heat-Trapping Power

D3.5 D3.6 D2.1 A1.13About 70 minutes

Learning goals

I can describe the main natural and anthropogenic sources and sinks of carbon dioxide, methane, nitrous oxide, halocarbons and water vapour.
I can explain how a gas's concentration, ability to absorb infrared and atmospheric lifetime determine its effect on heat retention.
I can use global warming potential to calculate CO₂-equivalent emissions.
I can explain the bathtub model and the global carbon budget.
I can explain why sinks may weaken as the climate warms.

Words to know

source
A process or place that releases a greenhouse gas into the atmosphere.

sink
A process or place that removes a greenhouse gas from the atmosphere and stores or destroys it.

reservoir
A place where a substance is stored, such as carbon in forests, soils, oceans or fossil fuels.

atmospheric lifetime
The typical time a gas stays in the atmosphere before it is removed.

global warming potential (GWP)
How much heat a mass of a gas traps over a time period (usually 100 years) compared with the same mass of CO₂.

CO₂-equivalent (CO₂e)
The mass of CO₂ that would cause the same warming as a given mass of another gas: mass × GWP.

halocarbons
Human-made compounds containing carbon and halogens (chlorine, fluorine, bromine), such as CFCs and HFCs.

ppm / ppb
Parts per million / parts per billion: how many molecules of a gas there are for every million / billion molecules of air.

Let's start with a bathtub

Imagine a bathtub with the tap running and the drain open. If water flows in exactly as fast as it drains, the level stays the same. Open the tap a bit more, and the level slowly rises, even if the drain is still working.

That's exactly what's happening to greenhouse gases. Natural sources (the tap) and sinks (the drain) were roughly in balance for thousands of years. Human activities opened the tap wider. The sinks are still working, even a little harder than before, but they can't keep up, so the level, the concentration in the atmosphere, keeps rising.

The bathtub model. The amount of a greenhouse gas in the air depends on the balance between sources (the tap) and sinks (the drain). Human emissions have opened the tap wider than the natural sinks can drain, so concentrations keep rising.

Remember from Grade 9

In Grade 9 you learned the carbon and nitrogen cycles, the main greenhouse gases, global warming potential, and Canada's emissions by sector. This lesson looks at both sides of the budget, sources and sinks, for each gas, and at why some gases trap more heat than others.

Sources and sinks, gas by gas

Gas	Natural sources	Human (anthropogenic) sources	Sinks	Approximate lifetime
Water vapour (H₂O)	Evaporation from oceans, lakes, soil; transpiration from plants	Very little directly; mainly increases because warmer air holds more	Condensation and precipitation	About 9 days
Carbon dioxide (CO₂)	Respiration, decomposition, wildfires, volcanoes, release from ocean	Burning coal, oil and natural gas; cement making; deforestation and land clearing	Photosynthesis (forests, plankton), dissolving in ocean, rock weathering over very long times	No single value: much removed in decades to centuries, but some stays for thousands of years
Methane (CH₄)	Wetlands and peatlands, termites, natural seeps	Livestock digestion, landfills, oil and gas leaks and venting, coal mines, rice paddies, manure	Reacts with hydroxyl (OH) in the air; some soil bacteria consume it	About 12 years
Nitrous oxide (N₂O)	Soil bacteria (nitrogen cycle), oceans	Nitrogen fertilizers, manure, some industrial processes, fuel burning	Broken apart by ultraviolet light in the stratosphere	About 110 years
Halocarbons (CFCs, HCFCs, HFCs)	Essentially none for these compounds	Refrigerators, air conditioners, foam insulation, old aerosol sprays	Broken apart by ultraviolet light in the stratosphere or by reactions in the air	Years to thousands of years, depending on the compound

A few points to notice:

Water vapour is the biggest greenhouse gas, but humans don't control it directly. Its amount is set by temperature: warm the air with other gases, and water vapour increases, which adds more warming (a feedback). It also leaves the air in about 9 days, so any extra quickly rains out unless the air stays warmer.
CO₂ is the most important gas that humans add. It has no fast chemical sink. The ocean and land take up about half of what we emit, but some of the added CO₂ stays in the air for thousands of years.
Methane is broken down in about a decade, so cutting methane emissions slows warming quickly.
Halocarbons are almost entirely human-made. Some (CFCs) also destroy the ozone layer (Lesson 12).

How carbon and nitrogen compounds affect heat retention

Why does one tonne of methane trap much more heat than one tonne of CO₂? Three things decide how much a gas warms the planet:

How strongly each molecule absorbs infrared. Methane and nitrous oxide absorb infrared at wavelengths where little else in the air absorbs (a part of the spectrum called the "atmospheric window"), so each extra molecule makes a big difference. CO₂ already absorbs most of the infrared at its main wavelengths, so each extra CO₂ molecule adds less than an extra methane molecule. (It still adds warming, mainly at the edges of its absorption bands.)
How long it stays in the air. N₂O lasts about 110 years; methane about 12 years; some CO₂ lasts thousands of years.
How much of it there is. CO₂ concentrations are about 200 times those of methane, so CO₂ causes the largest share of human-caused warming.

Scientists combine the first two into the global warming potential (GWP): how much heat 1 kg of a gas traps over a time period compared with 1 kg of CO₂.

Gas	GWP over 100 years (approximate)	GWP over 20 years (approximate)
CO₂	1	1
CH₄ (methane)	about 28	about 80
N₂O (nitrous oxide)	about 270	about 270
HFCs and CFCs	hundreds to more than 10 000	similar or higher

Values are rounded from IPCC assessments; different reports give slightly different numbers. Methane's 20-year GWP is much higher because methane does most of its warming in its first decade or two.

Worked example

Example 1: CO₂-equivalent emissions from a farm

A farm releases 3.0 t of methane (from cattle and manure) and 0.40 t of nitrous oxide (from fertilizer) in a year. Use 100-year GWPs of 28 and 270 to find its total emissions in CO₂e.

Methane: 3.0 t × 28 = 84 t CO₂e.
Nitrous oxide: 0.40 t × 270 = 108 t CO₂e.
Total: 84 + 108 = 192 t CO₂e.
Notice that 0.40 t of N₂O causes more warming than 3.0 t of CH₄ over 100 years, because its GWP is so high.

192 t CO₂e (84 from methane, 108 from nitrous oxide).

Approximate increases from pre-industrial levels to 2024 (ppm = parts per million; ppb = parts per billion). Methane has risen the most by percentage, but CO₂ causes the most total warming because there is so much more of it.

Sinks: where greenhouse gases go

CO₂ sinks:

Land: plants take in CO₂ during photosynthesis and store carbon in wood, roots and soil. Forests, grasslands and peatlands are large carbon reservoirs. Ontario's Hudson Bay Lowlands are one of the largest peatland areas in the world, storing huge amounts of carbon built up over thousands of years.
Ocean: CO₂ dissolves in seawater. Cold water dissolves more than warm water. Plankton use carbon, and some sinks to the deep ocean when they die. The cost: dissolved CO₂ makes seawater more acidic, which harms shell-building organisms.
Rocks: over hundreds of thousands to millions of years, rock weathering and limestone formation lock away carbon (Lesson 9).

Methane sink: most methane reacts with hydroxyl radicals (OH), very reactive molecules in the air, and is turned into CO₂ and water. Nitrous oxide and halocarbons drift up to the stratosphere, where strong ultraviolet light breaks them apart.

The global carbon budget

Every year, an international team of scientists (the Global Carbon Project) publishes a global carbon budget: an accounting of how much CO₂ humans emitted and where it went. In recent years, humans have emitted roughly 40 billion tonnes (40 Gt) of CO₂ per year from fossil fuels, cement and land-use change. Of this, roughly half stays in the atmosphere, about a quarter dissolves in the ocean, and about a quarter is taken up by plants and soils on land.

Where human CO₂ goes each year, rounded from the Global Carbon Project's annual global carbon budget (Gt = gigatonnes, billions of tonnes). The shares vary from year to year; for example, the land sink is weaker in hot, dry years with many wildfires.

Worked example

Example 2: From emissions to ppm

Adding about 7.8 Gt of CO₂ to the atmosphere raises its concentration by about 1 ppm. If humans emit 41 Gt of CO₂ in a year and about 47% stays in the atmosphere, estimate the yearly rise in ppm.

CO₂ that stays: 41 × 0.47 ≈ 19.3 Gt.
Rise in concentration: 19.3 ÷ 7.8 ≈ 2.5 ppm.
This is close to the measured rise of about 2.5 ppm per year averaged over the last decade (single years vary; 2024 set a record of more than 3 ppm), a good check that the budget adds up.

About 2.5 ppm per year.

Will the sinks keep working?

Scientists are concerned that some sinks may weaken as the climate warms:

Warmer ocean water dissolves less CO₂, and a more stratified (layered) ocean mixes less carbon into the deep.
Forests under stress from heat, drought, insects and wildfires grow less and can release stored carbon. In years with huge wildfires, Canada's forests can release more carbon than they absorb.
Thawing permafrost and drying peatlands can turn carbon stores into sources of CO₂ and methane.

If sinks weaken, a larger share of each year's emissions stays in the air, which speeds up warming: another positive feedback. That's one reason protecting forests, wetlands and peatlands is part of climate policy.

Just for fun

Riddle: I'm released by cows, swamps and landfills, I trap heat about 28 times better than CO₂, but I'm gone in about a dozen years. What am I?

Methane, the sprinter of greenhouse gases: strong, but short-lived.

Hands-on activity

Calculate your household's carbon footprint (A1.13)

You need: a grown-up's help with recent household bills or estimates, a calculator, and a data table.

Use these approximate emission factors:

Gasoline: about 2.3 kg CO₂ per litre burned.
Natural gas (home heating, stoves, water heaters): about 1.9 kg CO₂ per cubic metre (m³).
Electricity: depends on how it is generated. Ontario's grid is low-emission compared with many places because most electricity comes from nuclear and hydroelectric power; a rough value is about 0.03 to 0.1 kg CO₂e per kWh, and it varies by year and hour (it has risen in recent years as more natural gas is burned, to roughly 0.07 kg/kWh in 2024). Use a value your grown-up finds from a reliable source, or 0.07 kg/kWh.

Estimate how many litres of gasoline your household uses in a typical month (or record it for a month).
Read the natural gas bill (in m³) and electricity bill (in kWh) for one month, if your home uses them.
Calculate monthly CO₂ for each source and the total. Then multiply by 12 for a yearly estimate. Round to a sensible number of significant digits (2 or 3), because the factors are approximate.
Which source is largest? Suggest two realistic ways to reduce it.
Reflect: these calculations include only CO₂ from energy use. What sources of greenhouse gases are missing (food, products, waste, flights)?

What to look for: correct calculations with units, a sensible number of significant digits, and specific, realistic suggestions (for example, "lower the thermostat by 2 °C at night" or "combine errands into one trip").

Your turn

Define source and sink, with one example of each for CO₂.

Name two natural and two human sources of methane.

What is the main sink for nitrous oxide?

Why is water vapour called a feedback rather than a direct cause of human-caused warming?

Calculate the CO₂e of 5.0 t of methane using a GWP of 28.

Give three factors that determine how strongly a gas affects heat retention.

In the global carbon budget, roughly what share of human CO₂ emissions stays in the atmosphere?

More practice: warm-up

What does GWP stand for?

What is the GWP of CO₂?

About how long does methane last in the atmosphere?

Name one human-made halocarbon use.

About what was the pre-industrial concentration of CO₂?

More practice: on your own

A landfill releases 120 t of methane per year. A project captures 75% of it and burns it to make electricity (which turns it into CO₂). Using a GWP of 28, by how many t CO₂e per year does the project reduce emissions? (Ignore the CO₂ from burning, which is small by comparison: about 2.75 t CO₂ per tonne of methane.)

CO₂ has risen from 280 ppm to about 424 ppm. Calculate the percentage increase.

Explain why cutting methane emissions slows warming faster than cutting the same CO₂e of carbon dioxide.

Why is the ocean CO₂ sink both helpful and harmful?

A household uses 150 L of gasoline and 200 m³ of natural gas in a month. Estimate its CO₂ emissions from these two sources.

Challenge

Using a 20-year GWP of 80 instead of the 100-year GWP of 28, recalculate the farm's methane emissions in Example 1 (3.0 t of methane). Which time period would a policymaker focused on slowing warming in the next two decades prefer, and why?

Explain how a hot summer with record wildfires in Canada could affect the global carbon budget that year in two ways.

Peatlands in the Hudson Bay Lowlands have stored carbon for thousands of years because waterlogged, cold conditions slow decomposition. Predict what could happen to this carbon if the region warms and dries, and why this matters globally.

Check your understanding

Quiz

Which is a sink for carbon dioxide?

What removes most methane from the atmosphere?

What is the CO₂e of 2.0 t of N₂O (GWP 270)?

Why does CO₂ cause the most human-caused warming even though methane's GWP is higher?

About how long does nitrous oxide stay in the atmosphere?

In the bathtub model, why do CO₂ concentrations keep rising even though sinks absorb about half of our emissions?

Which gas's amount in the air is controlled mainly by temperature rather than by direct human emissions?

For parents and teachers

Big idea: Every greenhouse gas has sources and sinks. Concentrations rise when sources exceed sinks. Gases differ in how strongly they absorb infrared and how long they last, which is summarized by global warming potential. This covers D3.5 (sources and sinks of CO₂, methane, nitrous oxide, halocarbons and water vapour) and D3.6 (how carbon and nitrogen compounds influence heat retention).

How to help: The household carbon footprint activity needs your help with bills; it is also a good conversation about family choices. Keep the focus on learning, not guilt: the point is to understand which sources are biggest.

Common difficulties:

Thinking sinks have stopped working. They still absorb about half of emissions; they just can't keep up.
Confusing GWP (per kilogram) with total effect (which also depends on how much is emitted).
Rounding: emission factors are approximate, so answers should be given to 2 or 3 significant digits.