D. Earth and Space Science: Climate Change · Climate ChangeThe Natural Greenhouse Effect: Physics, Importance and a Model You Design
D3.3 D2.2 D2.1 A1.1 A1.4 A1.5 A1.8About 75 minutes
Learning goals- I can explain the natural greenhouse effect using Earth's energy budget and the difference between shortwave and longwave radiation.
- I can explain why some gases absorb infrared radiation and others don't.
- I can explain why the natural greenhouse effect is essential for life.
- I can distinguish the natural greenhouse effect from the anthropogenic (enhanced) greenhouse effect.
- I can design, build and evaluate a model of the greenhouse effect and use it to explain the anthropogenic greenhouse effect.
Words to know
electromagnetic radiation
Energy that travels as waves, including radio waves, infrared, visible light and ultraviolet.
wavelength
The distance from one wave crest to the next; shorter wavelengths carry more energy per photon.
shortwave radiation
Mostly visible and near-infrared light, given off by the very hot Sun.
longwave (infrared) radiation
Infrared radiation given off by cooler objects such as Earth's surface and atmosphere.
greenhouse gas
A gas that absorbs and re-emits infrared radiation, such as water vapour, carbon dioxide, methane, nitrous oxide and ozone.
energy budget
The balance between the energy Earth absorbs from the Sun and the energy it sends back to space.
natural greenhouse effect
Warming of Earth's surface by naturally occurring greenhouse gases and clouds.
anthropogenic (enhanced) greenhouse effect
Additional warming caused by greenhouse gases added by human activities.
Let's start with the Moon
The Moon is the same distance from the Sun as Earth. Yet its surface swings from about 120 °C in sunlight to below −130 °C at night, while Earth's average stays near a comfortable 15 °C, with much smaller day-night swings.
The Moon has almost no atmosphere. Earth has one, and a few of the gases in it act like a partial blanket that holds in heat. In Grade 9 you learned the basic idea of the greenhouse effect. In this lesson you'll learn why it works (the physics of radiation and molecules), how big it is, and you'll design your own model to test it.
Two kinds of radiation
Everything gives off electromagnetic radiation, and the hotter an object is, the shorter the wavelengths it gives off.
- The Sun's surface is about 5500 °C, so it gives off mostly shortwave radiation: visible light, plus some ultraviolet and near-infrared.
- Earth's surface averages about 15 °C, so it gives off longwave radiation: infrared, which you can't see but can feel as heat (like the warmth from a sidewalk after sunset).
The atmosphere treats these two kinds of radiation very differently. Most sunlight passes straight through the clear air. But much of the infrared rising from the ground is absorbed by greenhouse gases and clouds before it can escape.
Why only some gases are greenhouse gases
Nitrogen (N₂) and oxygen (O₂) make up about 99% of dry air, but they are not greenhouse gases. Molecules made of two identical atoms can't vibrate in a way that interacts with infrared.
Molecules with three or more atoms, such as water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃), can bend and stretch in ways that shift their electric charges. When infrared radiation of the right wavelength hits them, they absorb it and vibrate more. They then pass the energy on by colliding with other air molecules (warming the air) or re-emit infrared in a random direction, including back down toward the surface.
That's why tiny amounts matter: CO₂ is only about 0.04% of the air, but almost all of the air is N₂ and O₂, which do nothing to infrared.
Earth's energy budget
On average, the top of Earth's atmosphere receives about 340 W/m² of sunlight (averaged over day and night and the whole globe).
- About 100 W/m² (roughly 30%) is reflected back to space by clouds, ice, snow and bright surfaces (Earth's albedo).
- About 240 W/m² is absorbed by the atmosphere and surface.
- To stay at a steady temperature, Earth must send about 240 W/m² back to space as infrared.
If Earth had no greenhouse gases, the surface itself would have to radiate the 240 W/m² directly to space. Physics tells us a surface radiating that much would average about −18 °C. Instead, greenhouse gases and clouds absorb most of the surface's infrared and send about 340 W/m² back down. The surface, receiving energy from both the Sun and the atmosphere, warms to about +15 °C, where it gives off about 398 W/m².
The difference, about 33 °C, is the natural greenhouse effect. Research estimates that about half of it comes from water vapour, about a quarter from clouds, about a fifth from CO₂, and the rest from other gases.
Worked exampleExample 1: Reading the energy budget
Use the numbers in the diagram. (a) What percentage of incoming sunlight is reflected? (b) How much is absorbed? (c) By how many degrees does the natural greenhouse effect warm the surface?
- (a) 100 ÷ 340 × 100% ≈ 29% (often rounded to 30%).
- (b) 340 − 100 = 240 W/m² absorbed.
- (c) Actual average surface temperature 15 °C minus no-greenhouse temperature −18 °C: 15 − (−18) = 33 °C.
About 29% is reflected, 240 W/m² is absorbed, and the natural greenhouse effect adds about 33 °C.
Why the natural greenhouse effect matters for life
- Liquid water: at −18 °C, most of Earth's surface water would be frozen. Liquid water is essential for every known form of life.
- Stable temperatures: the atmosphere keeps nights from getting as cold as the Moon's and days from getting as hot, so living things aren't exposed to extreme daily swings.
- A habitable range: most of Earth's ecosystems, from boreal forests to coral reefs, depend on the temperature range the natural greenhouse effect provides.
Compare our neighbours: Venus has a very thick CO₂ atmosphere and a surface hot enough to melt lead (about 465 °C), even though it reflects most sunlight. Mars has a very thin atmosphere (less than 1% of Earth's surface pressure), so its greenhouse effect is weak and it is far colder. Earth's greenhouse effect is "just right" for life as we know it.
Natural versus anthropogenic greenhouse effect
The natural greenhouse effect comes from greenhouse gases at their natural levels (for CO₂, about 280 parts per million before the Industrial Revolution) and clouds. It is essential and has kept Earth habitable for billions of years.
The anthropogenic (enhanced) greenhouse effect is the extra warming from greenhouse gases that humans add, mainly by burning fossil fuels, clearing forests and farming. The physics is exactly the same; the difference is the amount and the speed of change. CO₂ is now above 420 ppm, about 50% higher than before industrialization, and has been rising about 2 to 4 ppm per year in recent years.
Adding greenhouse gases means infrared escapes to space from higher, colder levels of the atmosphere. Colder layers radiate less, so less energy leaves than arrives. Earth gains energy (an energy imbalance) and warms until it is once again sending out as much as it receives.
Grade 9 model versus Grade 10 model
In Grade 9 you compared an open jar with a jar covered by plastic wrap. That model mostly trapped warm air (stopped convection), which isn't how greenhouse gases work. In this lesson's investigation you'll change the gas itself, which makes a better model of the real process, and you'll judge its strengths and limits.
Worked exampleExample 2: Designing a fair test of the model
A student wants to test whether air with extra CO₂ warms more under a lamp than ordinary air. Identify the variables and two ways to make the test fair.
- Question: Does a bottle containing extra CO₂ warm more under a lamp than a bottle of ordinary air?
- Independent variable: the gas in the bottle (ordinary air or air with extra CO₂).
- Dependent variable: the temperature change after a set time.
- Controlled variables: identical bottles, same thermometers (checked beforehand to read the same in the same place), same distance from the lamp, same starting temperature, same lighting time, same dark paper on the bottle bottoms.
- Fairness: swap the bottles' positions between trials (in case one side of the lamp is hotter), and repeat each test at least three times, then average.
Independent: type of gas. Dependent: temperature change. Control everything else, swap positions and repeat.
Worked exampleExample 3: Analysing model data (A1.8)
A student's results (temperature rise in 15 minutes): Air bottle: 6.2 °C, 5.9 °C, 6.4 °C. CO₂ bottle: 7.8 °C, 7.1 °C, 7.6 °C. Does the evidence support the hypothesis that extra CO₂ makes the bottle warm more?
- Average for air: (6.2 + 5.9 + 6.4) ÷ 3 = 18.5 ÷ 3 ≈ 6.2 °C.
- Average for CO₂: (7.8 + 7.1 + 7.6) ÷ 3 = 22.5 ÷ 3 = 7.5 °C.
- Difference: 7.5 − 6.2 = 1.3 °C. In every trial, the CO₂ bottle warmed more, and the ranges don't overlap (air 5.9 to 6.4; CO₂ 7.1 to 7.8).
- The evidence supports the hypothesis, but the model has limits: a bottle is tiny compared with the atmosphere, the lamp also heats the plastic directly, and CO₂ is denser than air, so it can also affect convection inside the bottle. Real atmospheric CO₂ is about 0.04%, while the bottle may have much more.
Yes, the data support it (average 7.5 °C vs 6.2 °C, consistent in all trials), with important limitations to discuss.
Just for funThe name "greenhouse effect" is a bit like calling a whale a fish: catchy, but not quite right. A garden greenhouse stays warm mainly by stopping warm air from blowing away, while greenhouse gases absorb and re-emit infrared. Scientists kept the name anyway, because "infrared-absorbing-molecule effect" never caught on.
Hands-on activityDesign, build and evaluate a greenhouse model (D2.2, A1.1, A1.4, A1.5)
You need: 2 identical clear plastic bottles (1 or 2 L) with caps, 2 thermometers or digital probes that fit through a hole in the cap (a grown-up can make the hole), modelling clay or tape to seal around them, black paper, a desk lamp with an incandescent or halogen bulb if possible (they give off more infrared than LEDs), a separate jar, baking soda, white vinegar, a timer.
- Question and hypothesis: write your own testable question and a hypothesis with a reason.
- Plan: list your independent, dependent and controlled variables. Write numbered steps. Include how you'll keep it safe.
- Build: put a circle of black paper in the bottom of each bottle (the "ground"). Fit a thermometer through each cap so the tip is in the middle of the bottle, and seal around it.
- Make CO₂: in the separate jar, add 2 tablespoons of baking soda, then pour in about 100 mL of vinegar. It fizzes and releases CO₂. CO₂ is denser than air, so after the fizzing slows, gently "pour" the gas (not the liquid!) from the jar into one bottle, then cap it quickly.
- Let both bottles sit for 5 minutes so they start at the same temperature. Record.
- Place both bottles the same distance (about 25 cm) from the lamp. Record temperatures every minute for 15 minutes.
- Repeat with the bottles' positions swapped. Do at least three trials, refilling the CO₂ each time.
- Analyse: graph the results and calculate the average temperature change for each.
- Use the model: explain how the result models the anthropogenic greenhouse effect (adding more greenhouse gas to the air).
- Evaluate: list three ways your model is like the real atmosphere and three ways it is different. Suggest one improvement.
Optional: try a free online simulation such as the PhET "Greenhouse Effect" simulation from the University of Colorado. Increase the greenhouse gas concentration and watch what happens to the infrared photons and the temperature.
What to look for: a clear, testable question, a controlled design, repeated trials, averages, and an honest evaluation. The CO₂ bottle often warms a little more, but differences can be small and results can vary; a student who explains why results vary is doing real science.
Safety: a grown-up should make the holes in the caps. Desk lamps get hot: don't touch the bulb. Vinegar can sting eyes, so wear glasses or goggles and don't seal the jar while it's fizzing (pressure can build up). Don't breathe the gas directly from the jar.
Your turn
1Why does the Sun give off mostly visible light, while Earth gives off mostly infrared?
The hotter an object is, the shorter the wavelengths it gives off. The Sun's surface is about 5500 °C, so it emits mostly visible light; Earth is much cooler (about 15 °C on average), so it emits infrared.
2Name four greenhouse gases.
Any four of: water vapour, carbon dioxide, methane, nitrous oxide, ozone, halocarbons (such as CFCs and HFCs).
3Why aren't nitrogen and oxygen greenhouse gases?
They are made of two identical atoms, so they can't vibrate in a way that absorbs or emits infrared radiation.
4What would Earth's average surface temperature be without a greenhouse effect? What is it now?
About −18 °C without; about +15 °C now.
5Give two reasons the natural greenhouse effect is essential for life.
It keeps most of Earth's water liquid, and it keeps temperatures in a range (and with day-night swings) that ecosystems depend on.
6In one or two sentences, distinguish the natural greenhouse effect from the anthropogenic greenhouse effect.
The natural greenhouse effect is the warming from naturally occurring levels of greenhouse gases and clouds; it keeps Earth habitable. The anthropogenic greenhouse effect is the extra warming caused by greenhouse gases humans add, mainly from fossil fuels, deforestation and agriculture.
7In the model investigation, why should the bottles' positions be swapped between trials?
In case one side of the lamp or room is warmer than the other. Swapping makes sure any difference is caused by the gas, not the position.
More practice: warm-up
1About what fraction of incoming sunlight is reflected by Earth?
2What type of radiation do greenhouse gases absorb?
Infrared (longwave) radiation.
3Which gas causes the largest share of the natural greenhouse effect?
4About how much CO₂ was in the air before the Industrial Revolution?
5Which planet has a runaway greenhouse effect with a surface around 465 °C?
More practice: on your own
1If Earth's albedo increased so that 110 W/m² of the 340 W/m² were reflected, how much would be absorbed? Would Earth warm or cool, all else equal?
340 − 110 = 230 W/m² absorbed, less than before, so Earth would cool.
2Explain, step by step, what happens to a packet of infrared energy given off by the ground on a clear night.
Some passes straight to space. Much is absorbed by water vapour, CO₂ and other greenhouse gases. The molecules either pass the energy to other air molecules by collisions (warming the air) or re-emit infrared in a random direction; some of this goes back down to the ground, keeping it warmer than it would be otherwise.
3Cloudy winter nights in Ontario are usually warmer than clear ones. Explain using the greenhouse effect.
Clouds absorb infrared from the ground and send much of it back down, like greenhouse gases. On clear nights, more infrared escapes directly to space, so the ground and air cool faster.
4A student says, "The greenhouse effect is bad and we need to stop it." How would you correct this statement?
The natural greenhouse effect is essential; without it Earth would average about −18 °C. The concern is the anthropogenic (enhanced) greenhouse effect: extra warming from greenhouse gases humans add. We need to reduce those emissions, not eliminate the greenhouse effect.
5Give two limitations of the bottle model compared with the real atmosphere.
Any two: the real atmosphere is kilometres thick and open to space, while the bottle is tiny and closed; the lamp is not like the Sun (it gives off a different mix of wavelengths and heats the bottle directly); the CO₂ concentration in the bottle is far higher than in the air; convection inside the bottle is not like weather; the bottle walls absorb and trap heat themselves.
Challenge
1Earth emits about 240 W/m² to space, but the surface emits about 398 W/m². How can both be true at the same time?
The surface emits 398 W/m² upward, but most of it is absorbed by greenhouse gases and clouds in the atmosphere. The atmosphere, in turn, emits infrared both up and down. What finally escapes to space (about 240 W/m²) mostly comes from the colder upper atmosphere, not the surface. The 340 W/m² sent back down is why the surface can emit much more than escapes.
2Explain why adding CO₂ makes Earth emit to space from a higher, colder level, and why this causes warming.
With more CO₂, infrared is absorbed more times on its way up, so the level from which it finally escapes to space moves higher. In the troposphere, higher means colder, and colder gas emits less infrared. So less energy leaves Earth than arrives from the Sun, and the extra energy warms the planet until the escaping radiation again equals 240 W/m².
3Mars's atmosphere is about 95% CO₂, much more CO₂ by percentage than Earth's, yet Mars has only a weak greenhouse effect. Suggest an explanation.
Mars's atmosphere is extremely thin (less than 1% of Earth's surface pressure), so the total amount of CO₂ above each square metre is still small, and there is almost no water vapour. Percentage alone doesn't determine the greenhouse effect; the total number of greenhouse gas molecules the infrared must pass through matters.
Quiz
1. What kind of radiation does Earth's surface mainly give off?
2. Which gas is not a greenhouse gas?
3. Without its natural greenhouse effect, Earth's average surface temperature would be about...
4. What is the anthropogenic greenhouse effect?
5. Of about 340 W/m² of sunlight reaching the top of the atmosphere, how much is absorbed?
6. In a model experiment, why repeat the trials?
7. Greenhouse gases warm Earth's surface mainly because they...
Big idea: Sunlight (shortwave) passes through the air; Earth gives off infrared (longwave), which greenhouse gases absorb and partly send back down. This natural greenhouse effect keeps Earth about 33 °C warmer than it would be and is essential for life; the anthropogenic (enhanced) effect is the extra warming from gases humans add. This lesson covers D3.3, and the activity meets D2.2 (design and build a model).
How to help: Help with the bottle model; let your teen plan it first, then check the plan for safety and fairness. If results are unclear, that's normal; ask them to explain what might be causing the variation. The PhET simulation is a good free alternative or addition.
Common difficulties:
- Saying greenhouse gases "trap sunlight". They mostly let sunlight through and absorb infrared from the ground.
- Thinking the greenhouse effect itself is bad.
- Confusing the greenhouse effect with the ozone hole (Lesson 12).
Safety: you make the holes in the caps; supervise the lamp, and use eye protection with vinegar. Never seal a container while the baking soda and vinegar are fizzing.