Climate Evidence, Greenhouse Gases & Responses — Print · Grade 6 Science

Climate Evidence, Greenhouse Gases & Responses

Learning goals

I can explain how a greenhouse gas warms the planet in terms of infrared.
I can describe the evidence that the recent rise in carbon dioxide comes from human activities.
I can separate a question evidence can settle from one that involves values and trade-offs.

Readings for this unit

A Rainy Day and a Longer Record

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

Sunlight arrives mostly as visible light, passes through the atmosphere, and warms the ground. The warmed ground radiates energy back upward, but because it is far cooler than the Sun it radiates in the infrared, and certain gases absorb infrared strongly and re-emit it in all directions, including back downward.

The effect is large and entirely natural. Without it Earth's average surface temperature would be around minus 18 degrees Celsius instead of about 15 degrees Celsius, a difference of roughly 33 degrees Celsius, and the planet would be frozen. Water vapour does most of the work, with carbon dioxide and methane next.

What has changed is the amount. Carbon dioxide has risen from about 280 parts per million before 1800 to over 420 parts per million now, a rise of about 50 per cent, and ice cores show nothing like that level for at least 800,000 years.

Build your understanding

Where this fits

This lesson closes the chapter on air and climate, and it uses nearly everything in the course: infrared from the light chapter, radiation from the heat chapter, the water cycle, and the statistical reasoning about weather and climate from the last lesson. It is also the clearest example in the course of a case where the measurements and what to do about them are separate questions.

Use three flexible 40-45-minute sessions: how greenhouse gases work; what has changed and how we know; where the evidence ends and choices begin, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · Why infrared is the key

Everything radiates, and what it radiates depends on how hot it is. The Sun at about 5500 degrees Celsius radiates mostly visible light, which passes straight through the atmosphere's main gases. Earth's surface at about 15 degrees Celsius radiates in the infrared instead, and this is where the asymmetry lies: nitrogen and oxygen, which make up 99 per cent of the air, are transparent to both. Carbon dioxide, methane and water vapour are transparent to visible light and absorb infrared strongly, so they let energy in and slow its escape.

The word greenhouse is a poor name, which is worth saying because it causes confusion. A real greenhouse works mainly by stopping warm air from being carried away by convection, which is the mechanism from chapter 3, while the atmosphere works by absorbing and re-emitting radiation. The physics is radiative, not convective. The absorption itself was measured in a laboratory by John Tyndall in 1859, who found that carbon dioxide and water vapour blocked infrared while nitrogen and oxygen did not, and that measurement has been repeated with far better instruments ever since. Satellites now measure the effect directly from above. Instruments looking down at Earth record the infrared leaving the planet across a range of wavelengths, and the spectrum shows distinct bites taken out of it at exactly the wavelengths carbon dioxide and methane absorb. Comparing measurements decades apart shows those bites deepening, which is the absorption predicted in a nineteenth-century laboratory being observed from orbit.

The effect is what makes the planet habitable, and the change is a matter of degree rather than of kind. Saying both keeps the explanation honest and avoids treating the gases themselves as the problem.

Pause and explain

Why do greenhouse gases let sunlight in but slow energy escaping?

Check your reasoning after answering

Because the incoming and outgoing radiation are different. The Sun is hot enough to radiate mostly visible light, which these gases do not absorb, so it passes through. Earth's much cooler surface radiates in the infrared, which carbon dioxide, methane and water vapour absorb strongly and re-emit in all directions, including downward, slowing the loss to space.

Part 2 · What the measurements show

Carbon dioxide has been measured continuously at Mauna Loa in Hawaii since 1958, chosen because it is far from any city or forest, and the record shows a steady climb from about 315 parts per million then to over 420 now, with an annual wiggle caused by northern hemisphere plants growing in summer and dying back in winter. For the period before direct measurement, air bubbles trapped in Antarctic ice provide actual samples of ancient atmosphere, dated by counting annual layers, going back about 800,000 years.

Those ice cores show carbon dioxide oscillating between roughly 180 and 300 parts per million through eight glacial cycles, never once exceeding 300, and then rising almost vertically in the last two centuries to a level outside the entire range. The rate matters as much as the level: the natural swings took thousands of years, while the recent rise of over 120 parts per million has happened in about 200. Global average surface temperature has risen roughly 1.1 degrees Celsius over the same period, measured independently by thousands of weather stations, ships, buoys and satellites. The independence of those instruments matters for the reason chapter 1 gave about surprising claims. Land stations, ships taking sea surface readings, drifting ocean buoys and satellites measuring from orbit use completely different methods with completely different sources of error, and several separate research groups compile them without sharing assumptions. They agree closely, which is far stronger evidence than any one record could provide.

Pause and explain

How can scientists know the composition of the atmosphere 100,000 years ago?

Check your reasoning after answering

From air trapped in ice. Snow falling on Antarctica traps bubbles of the atmosphere as it compacts into ice, and because the ice builds in annual layers that can be counted, each bubble can be dated. Those bubbles are actual samples of ancient air rather than an inference, and they extend the record back about 800,000 years.

Part 3 · How we know where the extra carbon came from

Three independent lines of evidence point to human activities rather than to a natural source, and the fact that they are independent is what makes the case strong. First, the arithmetic: the quantity of coal, oil and gas burned each year is recorded for tax and trade purposes, and the carbon dioxide it produces is roughly twice the measured rise in the atmosphere, the remainder being absorbed by oceans and plants. The books balance in the right direction and by the right amount.

Second, carbon from fossil fuels has a distinctive isotopic signature. Carbon from ancient plants contains almost none of the radioactive form carbon-14, which decays away over tens of thousands of years, and is slightly enriched in the lighter stable form carbon-12. Both signatures are seen diluting the atmosphere exactly as burning fossil fuels would predict. Third, the oxygen concentration in the atmosphere is falling by precisely the amount that burning that quantity of carbon would consume, which rules out sources such as volcanoes that release carbon dioxide without using oxygen. Volcanic emissions have also been measured directly and are around one per cent of human emissions. It is worth noting what would count against this conclusion, since chapter 1 insisted that a claim worth anything must be refutable. If the recorded quantity of fuel burned were far smaller than the atmospheric rise, if the isotopic signature were unchanged or moving the wrong way, or if oxygen were holding steady while carbon dioxide climbed, the explanation would be in serious trouble. Each is a measurement anyone can take, and each has been taken repeatedly.

Pause and explain

Why does falling atmospheric oxygen help show that the extra carbon dioxide comes from burning fuel?

Check your reasoning after answering

Because burning consumes oxygen while other sources do not. Volcanic carbon dioxide is released without using atmospheric oxygen, so a volcanic explanation predicts no oxygen decline. The measured oxygen loss matches the amount that burning the recorded quantity of fossil fuel would consume, which fits combustion and rules the alternative out.

Part 4 · Where the evidence ends

Everything above is a measurement or a deduction from one, and the right way to challenge any of it is with better measurements. How much gas is in the air, how fast it is rising, where it came from, how much infrared it absorbs and how much the surface has warmed are all questions evidence can settle, and scientists disagree about details within them in the ordinary way, by publishing data.

What to do about it is a different kind of question and this course is careful not to blur the two. Deciding how fast to change energy supplies, who should bear the cost, how to weigh present expense against future risk, and what to do about countries at very different stages of development all involve judgements about fairness, priorities and acceptable risk. Those are value judgements, and adopting one is taking a political position rather than reporting a result. A student should be able to state what the measurements show, state clearly which questions the measurements cannot answer, and recognise when a confident claim has quietly crossed from the first into the second. The distinction is not a way of avoiding the subject, and it cuts in every direction. Someone who dismisses the measurements because they dislike the policies that might follow has made the same error as someone who treats a particular policy as settled by physics. The measurements do not tell anyone what to value, and no amount of disliking a policy changes what an instrument reads. Keeping the two apart is what allows a disagreement about what to do to be an honest one.

Pause and explain

Which climate questions can measurement settle, and which cannot?

Check your reasoning after answering

Measurement can settle how much carbon dioxide is in the air, how fast it is rising, where it came from, how much infrared it absorbs and how much the surface has warmed. It cannot settle how quickly to change energy supplies, who should pay, or how to weigh costs now against risks later, because those require judgements about fairness and priorities rather than readings.

Apply the whole unit · Sort the claims

Four statements: (a) carbon dioxide absorbs infrared radiation; (b) atmospheric carbon dioxide has risen about 50 per cent since 1800; (c) the extra carbon dioxide comes mainly from burning fossil fuels; (d) this country should close its coal power stations within ten years.

Your task

Decide for each statement what kind of question it is and what would be needed to settle it.

What your response should show

Correctly separates the three empirical claims from the policy claim
Names specific evidence for each empirical claim
Identifies the value judgements the policy claim requires
States that the policy claim does not follow automatically from the evidence

Response guidance — after your attempt

For each statement, say whether measurement alone could settle it.
For the measurable ones, name the evidence that does settle them.
Explain what the last statement additionally requires.

Statement (a) is settled by laboratory measurement, and has been since Tyndall measured infrared absorption in 1859; shining infrared through a tube of gas and measuring what emerges answers it directly. Statement (b) is settled by observation: continuous measurements at Mauna Loa since 1958 and air bubbles in Antarctic ice before that give the figures, from about 280 parts per million to over 420. Statement (c) is settled by evidence too, though less directly, resting on three independent lines: the recorded quantity of fuel burned, the isotopic signature of fossil carbon diluting the atmosphere, and the matching decline in atmospheric oxygen that rules out volcanic sources. Statement (d) is different in kind. It depends on the first three being true, but it also requires judgements that no measurement supplies: how much cost to accept now against how much risk later, who bears that cost, what alternatives are available and how quickly, and how to treat people whose jobs depend on those stations. Taking a side on (d) is adopting a political position, and the honest way to argue it is to state the evidence and the values separately rather than presenting the second as though it followed automatically from the first.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

Certain gases let sunlight through and absorb the infrared Earth radiates back, which keeps the planet about 33 degrees Celsius warmer than it would otherwise be. Measuring how much of those gases there are, and where the extra has come from, is a question evidence can settle. What to do about it is a different kind of question.

Worked examples

Example 1 · Compare two response proposals

Explain each purpose, feasible choice under the stated budget, and why the outcomes cannot simply be added.

Distinguish heat exposure and energy-use measures.
Apply the budget without inventing funds.
Preserve units and limits of illustrative data.

Answer: The shade plan addresses exposure adaptation but exceeds the five-token limit. The redesign fits the cost condition and targets energy use; any emissions benefit would depend on the energy source and other information. A 3 °C change and 20 energy units are different quantities and cannot be added as one score. The records are illustrative, not a tested policy or a required preference.

Example 2 · Transfer to a different case

A sealed-jar classroom demonstration warms. Does that alone prove the full atmospheric greenhouse mechanism?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. A jar also changes convection and other conditions. It can illustrate selected energy ideas, but a full atmospheric explanation needs appropriate radiation evidence and explicit model limits.

Example 3 · Combine independent evidence · a focused example

Why is independent evidence useful?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: It tests a pattern through different measurements and reduces dependence on one instrument, site, or repeated source.

Avoid these mistakes

Misleading claim: The atmosphere is a perfectly sealed roof that blocks all outgoing energy.
Misleading claim: A classroom jar or one trend graph proves every cause and policy conclusion.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

Does the model say no energy can ever leave Earth?

Discuss the answer

No. Outgoing radiation remains essential; the gases affect transfer and the temperature needed for balance, not an absolute seal.

Why is independent evidence useful?

Discuss the answer

It tests a pattern through different measurements and reduces dependence on one instrument, site, or repeated source.

Why does a graph alone not settle attribution?

Discuss the answer

Different influences can produce changes. Explaining cause requires mechanisms, comparisons, and evidence beyond an unlabeled correlation.

Can a response be judged solely by agreement with its slogan?

Discuss the answer

No. Define the mechanism, desired outcome, constraints, and measurements, then evaluate a feasible alternative fairly.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Climate Evidence, Greenhouse Gases & Responses · Model and explain

Assigned reading and preparation

Read A Rainy Day and a Longer Record and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Climate Evidence, Greenhouse Gases & Responses, study Parts 3–4 and attempt the worked case: Explain each purpose, feasible choice under the stated budget, and why the outcomes cannot simply be added. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

The shade plan addresses exposure adaptation but exceeds the five-token limit. The redesign fits the cost condition and targets energy use; any emissions benefit would depend on the energy source and other information. A 3 °C change and 20 energy units are different quantities and cannot be added as one score. The records are illustrative, not a tested policy or a required preference.

Activity 2 · Climate Evidence, Greenhouse Gases & Responses · Practice, revise and exit

Assigned reading and preparation

Read A Rainy Day and a Longer Record and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Climate Evidence, Greenhouse Gases & Responses, complete practice questions 1–6 and both choice checks, then answer independently: A sealed-jar classroom demonstration warms. Does that alone prove the full atmospheric greenhouse mechanism? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. A jar also changes convection and other conditions. It can illustrate selected energy ideas, but a full atmospheric explanation needs appropriate radiation evidence and explicit model limits.

Words to know

mitigation: Reducing causes or drivers of a problem, such as greenhouse-gas emissions in a climate response.
adaptation: Reducing exposure or harm under changed conditions.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Exit check

A sealed-jar classroom demonstration warms. Does that alone prove the full atmospheric greenhouse mechanism?

Check your explanation after answering

No. A jar also changes convection and other conditions. It can illustrate selected energy ideas, but a full atmospheric explanation needs appropriate radiation evidence and explicit model limits.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

Does the model say no energy can ever leave Earth?

Answer and explanation

No. Outgoing radiation remains essential; the gases affect transfer and the temperature needed for balance, not an absolute seal.