Ausgeology. Senior secondary lessons

SACE Stage 1 Earth and Environmental Science · Topic 4: The Earth’s atmosphere · Topic 5: Importance of the hydrosphere

Air and water.

The oxygen you breathe was made by microbes, and the water you drink may have come from space. Read eight short texts as a group, taking turns to lead the talk, and work out how air and water shape the planet.

How the lesson works

A · Air

Readings 1 to 5. Where the air came from, its layers, ozone and albedo, the greenhouse effect, and the winds.

B · Water

Readings 6 to 8. Where the water came from, why it is unusual, and how the oceans move heat and change Australia’s rain.

Each reading

Predict, then read a part. Clarify and question as you go, and summarise at the end.

Show what you know

Screen 10. Eleven questions, one for each I can statement.

How reciprocal teaching works

You read in a group of four, one part at a time. Each of you leads one strategy: predicting, clarifying, questioning or summarising.

Every question is labelled with its strategy. Predict before each part, clarify and question as you read, and summarise at the end. Type the group’s answer before you open the model answer, and swap roles for each reading.

Choose your role in the group

What we are learning

How the atmosphere formed and changed, how its layers, ozone, albedo and greenhouse gases control the energy reaching the surface, how moving air spreads heat, where Earth’s water came from, why water is unusual, and how the oceans shape climate.

I can

  • 1discuss how volcanic outgassing formed the early atmosphere, how its gases compare with today’s, and why nitrogen is essential for life.
  • 2describe how photosynthesis changed the amount of oxygen in the air over geological time.
  • 3describe the four layers of the atmosphere and how temperature changes with height.
  • 4explain how the ozone layer protects living things and how albedo affects the climate.
  • 5explain how greenhouse gases absorb and reradiate thermal radiation from the surface to warm the atmosphere.
  • 6explain how convection cells and the Coriolis effect set wind direction, and how moving air masses affect ecosystems.
  • 7discuss theories of the origin of Earth’s water and the evidence for each.
  • 8describe the three phases of water in Earth’s spheres, the hydrological cycle and where Earth’s water is.
  • 9explain how water’s unusual properties sustain life and shape Earth processes.
  • 10explain how temperature and salinity drive the ocean conveyor belt, and its role in Europe’s climate and in nutrient and carbon dioxide cycles.
  • 11discuss the causes and effects of ENSO, and how changes to the water cycle affect ecosystems and people’s use of water.

SACE Stage 1 Earth and Environmental Science, Topic 4: The Earth’s atmosphere and Topic 5: Importance of the hydrosphere. This lesson covers all ten of the two topics’ Science Understanding statements. The teacher notes at the end quote them in full.

My notebook

Printable activity

Reading record

Three pages: the four roles with question starters, a record for each reading with space for the group’s prediction, question and summary, and space to draw and label the energy budget and the Pacific in a neutral year and an El Niño.

Reading record (PDF) Editable (Word)

AAir

Reading 1: the oxygen story.

For half of Earth’s history there was almost no oxygen in the air. Where did it come from?

The air you are breathing is about 21% oxygen. Almost all of it was made by living things, starting with microbes in shallow seas billions of years ago.

This reading · about 40 minutes

  1. Predict from the title and the chart before you read.
  2. Read part 1. Clarify the words you are unsure of and ask questions about it.
  3. Read part 2, then summarise the whole reading.

Look at the title and the chart below. What do you predict this reading will say about how the air has changed? Predict

The air today
Show the numbers as a table

Part 1 · An atmosphere without oxygen

Earth’s first gases did not last. “Lighter gases such as hydrogen and helium dissipated into space” (Webb). A second atmosphere came from inside the planet: as magma rises, “the gases are released through a process called outgassing”, among them “water vapor, carbon dioxide (CO2), sulfur dioxide (SO2), carbon monoxide (CO)” (Webb). Volcanoes still do this. In an average year they release “between about 180 and 440 million tonnes of carbon dioxide”, but “By far the most abundant volcanic gas is water vapor” (USGS).

That early atmosphere was very different from today’s. Free oxygen “was absent on the early Earth” (Olson and others, 2018). Carbon dioxide was perhaps 10 to 50 times its level before the industrial era, while nitrogen may have been about as abundant as now, between 0.52 and 1.1 atmospheres of pressure 3.5 to 3.0 billion years ago (Olson and others, 2018).

Nitrogen stayed because it hardly reacts: “N2 gas is a very stable compound due to the strength of the triple bond between the nitrogen atoms” (Bernhard, 2010). Carbon dioxide dissolved in the oceans and was locked into rocks, and water vapour condensed into the oceans, but nitrogen built up in the air. It is now about 78% of dry air (NOAA). Life cannot do without it. Nitrogen “is a necessary component of many biomolecules, including proteins, DNA, and chlorophyll” (Bernhard, 2010), but most living things cannot use N2 directly. They depend on nitrogen fixation, “The process of converting N2 into biologically available nitrogen” (Bernhard, 2010), done mainly by microbes.

In part 1, what does outgassing mean? Clarify

Write one question about part 1 that the text answers, and one that it makes you wonder about. Start with “Why…”, “How…” or “What would happen if…”. Question

Why is nitrogen the main gas in the air today? Question

Part 2 · Microbes change the air

Cyanobacteria were “the first micro-organisms that could produce oxygen by photosynthesis” (Australian Museum). Some of them build stromatolites, mounds of microbes and trapped sediment. Living stromatolites still grow at Hamelin Pool in Shark Bay, Western Australia, which has “the most diverse and abundant examples of stromatolites” in the world; such structures “dominated marine ecosystems on Earth for more than 3,000 million years” (UNESCO). The oldest known stromatolites, in the Pilbara of Western Australia, are 3.48 billion years old (ScienceDaily, 2022), although scientists still debate whether the microbes that built them made oxygen.

At first the oxygen did not reach the air. It reacted with iron dissolved in seawater: “The dissolved iron and silicon combined with oxygen in the water to make insoluble iron oxides and silica”, which settled as banded iron formations, like those of the Hamersley Province in the Pilbara (Australian Museum). Only later could oxygen build up. “The first time oxygen was significantly present in the atmosphere was about 2.43 billion years ago”, and for about 200 million years the levels were “highly unstable” (University of Leeds, on Poulton and others, 2021). This is the Great Oxidation Event.

Even then, oxygen stayed low for a long time. After the event “oxygen plummeted to a tiny fraction of today’s level” for a period “spanning from roughly 2 to less than 1 billion years ago” (ScienceDaily, on Lyons and others, 2014). A second rise, from about 800 million years ago, may have brought oxygen up to the levels animals need, with the rise culminating “around c. 550 Ma” as the first large animals appeared (Och and Shields-Zhou, 2012). Since then oxygen has gone up and down, possibly reaching about 35% of the atmosphere around 300 million years ago (Olson and others, 2018), and it is now about 21%.

Oxygen through time
WhenWhat happenedOxygen in the air
4.0 to 2.5 billion years agoArchean: outgassed atmosphere, cyanobacteria appearEffectively none: below about one hundred-thousandth of today’s level
About 2.43 to 2.2 billion years agoGreat Oxidation EventRises for the first time, but unstable
About 2 to 0.8 billion years agoA long low periodDisputed: from less than 0.1% to about 40% of today’s level
About 0.8 to 0.55 billion years agoNeoproterozoic oxygenation; first large animalsRises towards modern levels
About 300 million years agoCarboniferous forestsPossibly about 35% of the air
Today20.95% of dry air

From Olson and others (2018), Poulton and others (2021) and Lyons and others (2014). The ranges are wide because the evidence is indirect.

Why did oxygen made by cyanobacteria not build up in the air straight away? Question

Which is the best summary of the whole reading? Summarise

Now write your own summary of the reading in two or three sentences, in your own words. Was your prediction right? Summarise

AAir

Reading 2: the layers of the air.

Why does it get colder as you climb a mountain, but warmer again higher up?

Up to about 600 km, the atmosphere is divided into four layers by how its temperature changes with height. NOAA also names a fifth, the exosphere, where the air fades into space. Each change of direction has a cause.

This reading · about 35 minutes

  1. Predict from the graph before you read.
  2. Read the text, clarify and question as you go.
  3. Summarise each layer in the table at the end.
Temperature with height
Show the numbers as a table

The line on the graph changes direction three times. Before you read, predict a reason for one of the changes. Predict

Four layers

The troposphere is the layer we live in. It is “11-12 miles (18-20 km) high at the equator” and “just under four miles (6 km) high at the poles”, and “almost all weather occurs in this region” (NOAA). The Sun mostly heats the ground, and the ground heats the air above it, so the air is warmest at the bottom: temperature falls “from an average around 62°F (17°C) to -60°F (-51°C) at the tropopause” (NOAA).

Above it, in the stratosphere, the temperature climbs again, “to a maximum of about 5°F (-15°C)” at about 50 km. The stratosphere “holds 19 percent of the atmosphere’s gases but very little water vapor”, and it holds the ozone layer: “Heat is produced in the process of the formation of ozone, and this heat is responsible for temperature increases” (NOAA). Ozone and oxygen absorb ultraviolet light from the Sun, and that energy warms the air.

In the mesosphere, from about 50 to 85 km, there is little ozone and the temperature falls again. The gases there “are now thick enough to slow down meteors hurtling into the atmosphere” (NOAA), which is why most meteors burn up at these heights.

Above about 85 km is the thermosphere, where “incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules”. Temperatures rise from “as low as -184°F (-120°C)” at the bottom to “as high as 3,600°F (2,000°C) near the top” (NOAA). The gas is so thin, though, that it would not feel hot.

What is the tropopause? Clarify

Why is the troposphere warmest at the bottom? Question

Why does the temperature rise through the stratosphere? Question

The thermosphere can reach 2,000 °C, yet it would not feel hot. Write a question you would ask to understand this, then try to answer it. Question

Summarise each layer in one line: where it is, what the temperature does, and why. Summarise

AAir

Reading 3: ozone and albedo.

Why can unprotected skin burn in 15 minutes in an Australian summer, and why does melting ice speed up warming?

Not all sunlight reaches the ground. The ozone layer absorbs most of the harmful ultraviolet, and bright surfaces reflect sunlight straight back to space.

This reading · about 40 minutes

  1. Predict, then read part 1 on ozone. Clarify and question.
  2. Read part 2 on albedo, using the chart. Clarify and question.
  3. Summarise both parts.

You will read about the “ozone hole”. Predict what it is, and whether it has got better or worse since it was found in 1985. Predict

Part 1 · The ozone layer

“The ozone layer is located in the stratosphere and surrounds the entire Earth” (NOAA). It acts as a filter for ultraviolet (UV) light. “UV-C radiation is entirely absorbed within the ozone layer”, and “Most UV-B radiation emitted by the Sun is absorbed by the ozone layer; the rest reaches Earth’s surface” (NOAA). That matters for living things: “increased exposure to UV-B radiation raises the risks of skin cancer and cataracts”, and too much UV-B “also can damage terrestrial plant life” (NOAA).

Australians know about UV. “Two in three Australians will be diagnosed with skin cancer in their lifetime”, and in January the UV Index is forecast “to be 11 or more across most of the country on clear sky days”, when “unprotected skin can be damaged in as little as 15 minutes” (ARPANSA and Cancer Council). UV is not heat: “clear skies or high temperatures cannot be relied on to determine when sun protection is needed” (ARPANSA and Cancer Council).

In 1985 scientists of the British Antarctic Survey reported “Large losses of total ozone in Antarctica”. They had measured ozone there since 1957, and found “during the springtime, a 40% or more decline” (Nature, 2019). The cause was human-made chemicals such as chlorofluorocarbons (CFCs). The Montreal Protocol to phase them out was adopted on 16 September 1987, and it “is widely regarded as the most successful ever environmental treaty” (WMO). At Kennaook/Cape Grim in Tasmania, air has been measured since 1976, and the records show a fall in ozone-depleting substances such as CFC-11 (Bureau of Meteorology). The 2025 ozone hole “was relatively small and short-lived”, and the ozone layer is expected to recover to 1980 values “by around 2066 over the Antarctic” (WMO).

Which best describes the ozone hole? Clarify

Why does it matter for living things that ozone absorbs UV-B? Question

Part 2 · Albedo

Albedo is the fraction of sunlight a surface reflects. Scientists estimate that “Earth’s average albedo” is about 0.30: about 30% of sunlight is reflected and about 70% is absorbed and warms the planet. “If Earth was completely covered in ice, its albedo would be about 0.84”, but “if Earth was covered by a dark green forest canopy, the albedo would be about 0.14” (NASA Earth Observatory).

Sea ice shows why albedo matters for climate. According to NSIDC, “thick sea ice covered with snow reflects as much as 90 percent of the incoming solar radiation”, but “A typical ocean albedo is approximately 0.06” (NSIDC). When ice melts, ponds form and then dark water is exposed, so “the albedo continues to decrease, leading to higher absorption of solar radiation and an increased rate of melting” (NSIDC). Melting makes more melting: a positive feedback.

How much sunlight surfaces reflect
Show the numbers as a table

From the chart, which surface absorbs the most sunlight? Question

Explain the ice feedback in part 2 as a chain of four or five steps, starting with “The Arctic warms”. Question

Summarise how the ozone layer and albedo each change the sunlight that reaches and warms the Earth’s surface. Summarise

AAir

Reading 4: the greenhouse effect.

Venus and Mars both have air that is mostly carbon dioxide. Why is one over 460 °C and the other about −60 °C?

Without its greenhouse gases the Earth would be frozen. With more of them, it warms. This reading explains how a few gases that, on average, make up less than 1% of the air can do that.

This reading · about 40 minutes

  1. Predict, then step through the energy model.
  2. Read part 1, clarify and question.
  3. Read part 2 with the graph and the planets table, then summarise.

Many people say greenhouse gases “trap heat”. Before you read, predict what the gases actually do to the heat. Predict

Where the Sun’s energy goes

Part 1 · Absorbing and radiating again

Sunlight arrives at the top of the atmosphere at an average of “approximately 340 watts per square meter”. “About 29 percent” is reflected back to space, “About 23 percent of incoming solar energy is absorbed in the atmosphere” and “48 percent passes through the atmosphere and is absorbed by the surface” (NASA). The warmed surface then gives off energy as infrared radiation, which we cannot see.

This is where greenhouse gases come in. “When greenhouse gas molecules absorb thermal infrared energy, their temperature rises”, and they “then radiate an increased amount of thermal infrared energy” in all directions, some of it back towards the ground (NASA). The surface is warmed both by the Sun and by this radiation from the air above. The result: the natural greenhouse effect makes the surface “more than 30 degrees warmer than it would be if it didn’t have an atmosphere” (NASA). The Earth averages about 15 °C (NASA Earth Fact Sheet); without greenhouse gases it would be about 33 °C colder (NASA GISS).

Water vapour is “responsible for about half of Earth’s greenhouse effect”, but it is a response to warming, not the cause: “Increased water vapor in the atmosphere amplifies the warming caused by other greenhouse gases” (NASA). Of the absorption, clouds cause about 25% and carbon dioxide about 20%; ozone, methane and other minor gases, with a little from particles in the air, make up the rest (NASA GISS).

What kind of radiation do greenhouse gases absorb? Clarify

Which is the best description of what greenhouse gases do? Question

Write a question about water vapour that part 1 answers, and answer it. Question

Part 2 · More gas, more warming

Before industry, carbon dioxide was “typically quoted as 278 ppm” (CSIRO). In 2024 the global average was 423.9 ppm (WMO), about 52% higher. In 2019 carbon dioxide “concentrations were higher than at any time in at least 2 million years”, and the increases “are unequivocally caused by human activities” (IPCC, 2021). Methane is more than two and a half times its pre-industrial level (WMO). At Kennaook/Cape Grim in Tasmania, CSIRO and the Bureau of Meteorology have measured some of the world’s cleanest air since 1976 and recorded “an ongoing increase in CO₂ over the last 50 years from human-induced activities” (CSIRO).

The planets next door show what the amount of gas does. Venus’s atmosphere is 96.5% carbon dioxide at a surface pressure of 92 bars, and its surface averages 464 °C (NASA): its “thick atmosphere traps heat in a runaway greenhouse effect” (NASA). Mars’s air is 95% carbon dioxide too, but its surface pressure is only 6.36 millibars, less than 1% of Earth’s, and its average temperature is about −59 °C (NASA). On Mars, “because the atmosphere is so thin, heat from the Sun easily escapes” (NASA), so its greenhouse effect is very weak.

Carbon dioxide at Mauna Loa, Hawai‘i
Show the numbers as a table
VenusEarthMars
Carbon dioxide in the air96.5%About 0.04%95.1%
Surface pressure92 barsAbout 1 bar0.0064 bars
Average temperature464 °C15 °CAbout −59 °C

NASA planetary fact sheets. Earth’s carbon dioxide from NOAA.

Venus and Mars both have air that is about 95% carbon dioxide. What explains the huge difference in their temperatures? Question

Summarise the greenhouse effect in three sentences: what the gases do, what difference they make, and why adding more matters. Summarise

AAir

Reading 5: moving air.

Why does Adelaide get most of its rain in winter and Darwin in summer, and why is central Australia a desert?

The tropics get more sunlight than the poles. The atmosphere evens this out by moving air, in a pattern that sets where the world’s deserts, rainforests and winter rains are.

This reading · about 40 minutes

  1. Predict, then read part 1 and step through the model.
  2. Clarify and question.
  3. Read part 2 about Australia, then summarise.

Perth, Adelaide, Cape Town and Santiago in Chile are all at about 32° to 35° south. Predict what their climates might have in common. Predict

Part 1 · Three cells and a spinning Earth

Warm air near the equator rises, flows towards the poles high up, cools and sinks. If the Earth did not spin, one big loop might reach each pole. Instead, “there are three circulations in each” hemisphere (NOAA): the Hadley cell near the tropics, the Ferrel cell in the middle latitudes and the polar cell. Air sinking in the Hadley cell makes “high-pressure regions over the subtropical oceans and the world’s hot deserts” (Met Office), and NOAA points to “the number of deserts located along the 30°N/S latitude.” Sinking air warms and dries, so clouds rarely form.

The Earth’s spin also turns the winds. Because of the Coriolis effect, moving air is deflected “toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere” (NOAA). Air flowing back towards the equator is turned into the trade winds, which in the southern hemisphere “are east to south-easterly” (Bureau of Meteorology). In the middle latitudes the surface winds blow from the west: the westerlies.

All this moving air carries heat. “The net effect is to transfer energy from the tropics towards the poles in a gigantic conveyor belt” (Met Office). At 35° latitude the atmosphere carries 78% of the heat moved towards the poles in the Northern Hemisphere and 92% in the Southern Hemisphere; the ocean carries more only in the tropics between 0° and 17° north (Trenberth and Caron, 2001, via NCAR).

The circulation, step by step

In part 1, what is the Coriolis effect? Clarify

Why are many of the world’s deserts near 30° north and south? Question

Part 2 · Australia’s winds

In the southern hemisphere the sinking air of the Hadley cell makes “A band of high pressure systems” across the middle latitudes: the subtropical ridge (Bureau of Meteorology). It brings “stable and dry conditions”, which is one reason so much of the continent is dry. The ridge moves with the seasons. “From November to April, the subtropical ridge is generally south of Australia”, and there it “blocks rain-bearing cold fronts”. “In autumn the subtropical ridge moves north, over the continent”, and while it is north, “it’s easier for cold fronts to reach southern Australia”, bringing “colder south-westerly winds and showery conditions” (Bureau of Meteorology). That is why Adelaide and Perth get most of their rain in winter.

The same pattern gives dry-summer, wet-winter climates elsewhere on the west sides of continents between 30° and 40°: the Mediterranean, “the Cape Town area of South Africa, central Chile, and southwestern Australia” (Ritter).

Darwin’s rain comes at the opposite time of year. In the tropical north the monsoon takes over, “a seasonal reversal of winds”, in which a low-pressure trough “draws in moist air from the Indian Ocean and southern Asian waters”, mostly in summer (Bureau of Meteorology). The Bureau dates the monsoon’s arrival by the change of wind at Darwin, “with dry southeasterly trade winds shifting to moist north-westerly winds”, and the average date for all wet seasons since 1957–58 is 29 December (Bureau of Meteorology). Darwin Airport averages 432 mm of rain in January and 1 mm in July (Bureau of Meteorology). Across the country, “the majority of rainfall in northern Australia occurs during the wet season” (October to April), while “in southern Australia, most of the rainfall tends to fall in the winter months” (Bureau of Meteorology). Ecosystems follow the air. Much of the northern vegetation “grows rapidly during summer and senesces during winter”, then burns in the dry season (Harris and others, 2008).

In the south-west, cool-season rainfall has fallen by “around 16%” since 1970, as rain-bearing systems “have become less common over southern Australia” (CSIRO and Bureau of Meteorology).

Why does Adelaide get most of its rain in winter? Question

Adelaide gets most of its rain in winter and Darwin in summer. Which explanation fits both cities? Question

Write a “What would happen if…” question about the subtropical ridge, and answer it using the reading. Question

Summarise how heating, cooling and the Earth’s spin make the global winds, and what this means for Australia. Summarise

BWater

Reading 6: where the water came from.

Did Earth’s oceans come out of its own rocks, or were they delivered from space?

Scientists still argue about this. The evidence comes from tiny crystals in Western Australia, from craters on the Moon and from spacecraft that have sampled comets and asteroids.

This reading · about 40 minutes

  1. Predict, then read part 1. Clarify and question.
  2. Read part 2 with the chart. Clarify and question.
  3. Summarise both theories and the evidence for each.

Part 2 is called “A fingerprint in the water”. Predict how scientists could test where water came from, billions of years later. Predict

Part 1 · Water from inside the Earth

In Reading 1 you met outgassing: “Volcanic activity releases many different gases, including water vapor” (Webb). Early in Earth’s history this happened on a huge scale. Britannica calls the water and other gases released from inside the Earth “excess volatiles”, and says “major degassing occurred early in Earth’s history”. While the surface was very hot the water stayed as vapour, but “Below 100 °C (212 °F) all of the water would have condensed” (Britannica), falling as rain to fill the first oceans. Today the Earth adds very little new water this way: about 0.3 cubic kilometres a year (Britannica).

How early were there oceans? The best evidence is Australian. In the Jack Hills of Western Australia, geologists found a zircon crystal “as old as 4,404 +/- 8Myr”. Its oxygen isotopes show it formed from material that had “undergone low-temperature interaction with a liquid hydrosphere”, making it “the earliest evidence for continental crust and oceans on the Earth” (Wilde and others, 2001). Those early oceans may not have lasted. NASA reports that “asteroids large enough to boil away the ocean probably hit Earth several times” in the planet’s first few hundred million years.

According to part 1, how could outgassing make an ocean? Clarify

How can a zircon crystal be evidence for an ocean? Question

Part 2 · A fingerprint in the water

The other theory is that water was delivered by icy bodies from further out in the solar system. The Moon keeps the record of these impacts, because its craters have had “little erosion over billions of years” (NASA). Its cratered surface shows that “the terrestrial planets were battered”, though “the timing, sources, and dynamical implications of these impacts are controversial” (Bottke and Norman, 2017).

To test where water came from, scientists compare its D/H ratio: the amount of heavy hydrogen (deuterium) compared with ordinary hydrogen. A source with the same ratio as the oceans could have supplied them. In 2014 the Rosetta spacecraft measured comet 67P and found a ratio “more than three times greater than for Earth’s oceans” (ESA). Earlier, six comets from the distant Oort cloud averaged about twice Earth’s value, but comet Hartley 2 matched, which “expands the reservoir of Earth ocean-like water to include some comets” (Hartogh and others, 2011). Meteorites from asteroids “also match the composition of Earth’s water” (ESA), and Japan’s Hayabusa2 brought back grains from asteroid Ryugu in 2020; in 2022 scientists reported that they held “carbonated water containing salt and organic matter” (Nakamura, via phys.org).

A close match does not settle it. Ryugu’s ratio is close to Earth’s, yet a 2023 study of its minerals estimates that this kind of material supplied only “∼3%” of the hydrogen at Earth’s surface (Piani and others, 2023). Another study found that rocks like Earth’s own building blocks contain “sufficient hydrogen to deliver at least three times the amount of water contained in the Earth’s oceans” (Piani, via ScienceDaily, 2020). ESA’s summary: “the origin of Earth’s water is still an open question”.

Heavy hydrogen compared with Earth’s oceans
Show the numbers as a table

What does the chart suggest about comets like 67P? Question

Ryugu’s water matches Earth’s, yet it supplied only about 3% of Earth’s surface hydrogen. Write a question that helps explain this, and try to answer it. Question

Summarise the two theories for the origin of Earth’s water, with one piece of evidence for each. Summarise

BWater

Reading 7: an unusual liquid.

Almost all of Earth’s water is in the sea. Why does so little of it count as fresh, and what makes water so good at supporting life?

Water is found naturally on Earth as a solid, a liquid and a gas. It moves between all four spheres, and its unusual properties shape both living things and the land.

This reading · about 45 minutes

  1. Predict, then read part 1 with the chart. Sort the examples of water’s phases.
  2. Read part 2 and the properties table. Clarify and question.
  3. Summarise.

Predict where most of Earth’s fresh water is: rivers and lakes, underground, or frozen in ice. Predict

Part 1 · Where the water is, and how it moves

Of the world’s water, “over 96 percent is saline” (USGS). Oceans, seas and bays hold 96.5%. Fresh water is only about 2.5% of the total, and most of that is out of easy reach: 68.7% of fresh water is in ice caps, glaciers and permanent snow, and 30.1% is groundwater. Rivers hold just 0.006% of the fresh water (USGS, from Shiklomanov, 1993).

Water keeps moving through the hydrological cycle. “Most of the moisture in the atmosphere (about 90%) came from water evaporating from oceans, seas, lakes, and rivers”, and “The rest of the moisture in the atmosphere came from plant transpiration and (a very small amount) from sublimation” (USGS). About 495,000 cubic kilometres pass through the air each year, “as if the entire amount of water in the air were removed and replenished nearly 40 times a year” (NASA). “Once evaporated, a water molecule spends about 10 days in the air” (USGS), while “The oceans have a water residence time of 3,000 to 3,230 years” (Britannica). “Over the oceans, evaporation exceeds precipitation”, and the extra vapour is carried over land (Britannica), where it falls as the rain that fills rivers, soils and groundwater.

Where Earth’s fresh water is
Show the numbers as a table

Solid, liquid or gas? Sort water in each sphere

Why is evaporation from the oceans so important for rain on land? Question

What does “residence time” mean in part 1? Clarify

Part 2 · Why water is unusual

A water molecule has a slightly negative oxygen end and slightly positive hydrogen ends, so molecules attract one another. Without this, water “would be expected to have a boiling point nearly 200 °C (360 °F) lower than its observed boiling point” (Britannica): hydrogen sulfide, a heavier molecule, boils at about −60 °C (NIST). The attraction between molecules “is called a hydrogen bond” (University of Wisconsin–Madison), and it explains the properties in the table.

Most substances shrink when they freeze. Water does the opposite: “Upon freezing, the density of ice decreases by about 9 percent”, and “ice is less dense than liquid water, which causes it to float” (USGS). Water also “has a high specific heat capacity”: “it absorbs a lot of heat before it begins to get hot” (USGS), so “Cities near the oceans will tend to have less change and less extreme temperatures than inland cities” (USGS). Compare Adelaide’s coast with Oodnadatta. Turning water into vapour takes even more energy, 540 calories for each gram (Penn State), and that energy is released again when the vapour condenses in clouds.

Water’s properties at work
PropertyFor living thingsFor Earth processes
Ice is less dense than waterIce floats and covers lakes, so life survives underneath in winterWater expands as it freezes in cracks, prising rocks apart (frost wedging)
High heat capacity: 4,184 joules to warm 1 kg by 1 °CSteady temperatures in seas, lakes and bodiesOceans store heat and soften the climate of coasts
Large energy to evaporateSweating and transpiration cool animals and plantsHeat released in clouds powers storms and moves heat towards the poles
Dissolves many substancesCarries nutrients in sap and bloodChemical weathering; salt in the sea
High surface tensionInsects such as water striders stand on waterWater is drawn into soil pores and small spaces

USGS Water Science School; Britannica. The links to Earth processes are standard textbook examples.

Why does Adelaide have milder summers and winters than Oodnadatta? Question

Write a “What would happen if…” question about one of water’s properties, and answer it. Question

Summarise where Earth’s water is, how it moves, and two properties that make it important. Summarise

BWater

Reading 8: oceans and climate.

How can water sinking near Greenland and Antarctica, or warm water drifting across the Pacific, change the weather in Europe and Australia?

The oceans store and move enormous amounts of heat. Slow changes in their currents, and faster swings between the ocean and the air, decide where rain falls.

This reading · about 50 minutes

  1. Predict, then read part 1 and step through the conveyor belt.
  2. Read part 2 and step through El Niño and La Niña.
  3. Read part 3 about Australia’s water. Clarify, question and summarise.

Cold water is denser than warm water, and salty water is denser than fresh water. Predict where in the oceans water might sink to the sea floor. Predict

Part 1 · The global conveyor belt

Winds drive the surface currents, but deep currents are driven by differences in density. Cold, salty water is dense and sinks. This thermohaline circulation (“thermo” for heat, “haline” for salt) “drives a global-scale system of currents called the ‘global conveyor belt.’” It “begins on the surface of the ocean near the pole in the North Atlantic”, where water cools, gets saltier and sinks. Deep water flows south, and “travels around the edge of Antarctica, where the water cools and sinks again”. Eventually the deep waters warm and become less dense, “so that they rise to the surface (upwelling)”. A cubic metre of water “takes about 1,000 years to complete the journey” (NOAA).

Australia is next to one of the sinking regions. Off the Adélie Land coast directly south of Tasmania, CSIRO’s Steve Rintoul found that “about a quarter of the total volume of bottom water in the world ocean is formed there” (CSIRO, 1998). When sea ice forms it leaves salt behind, making very cold, very salty water that sinks to the sea floor.

The conveyor matters for climate and life. Its Atlantic branch, the AMOC, carries “warm water near the surface” towards the pole, “such as the Gulf Stream in the North Atlantic” (NOAA), and the Gulf Stream helps keep British winters “milder than those experienced at similar latitudes elsewhere in the world” (Met Office). Some scientists argue that the air does more of this work: “it is the atmosphere, not the ocean, that does the lion’s share” (Seager, Columbia University). The conveyor also carries nutrients and carbon dioxide: “Warm surface waters are depleted of nutrients and carbon dioxide”, and upwelling deep water brings them back (NOAA). The ocean takes up “about 30% of the carbon dioxide” released into the atmosphere (NOAA).

Climate models keep being tested against new measurements. One study found the AMOC has weakened by “around 15 per cent” since the mid-twentieth century (Caesar and others, 2018). The IPCC says it “is very likely to weaken over the 21st century”, but with “medium confidence that there will not be an abrupt collapse before 2100” (IPCC, 2021). South of Australia, the deep overturning slowed “by almost a third (30%)” between 1992 and 2017, earlier than predicted (UNSW and CSIRO, 2023).

The conveyor belt, step by step

What makes water sink in the North Atlantic and near Antarctica? Clarify

Why is upwelling important for life in the ocean? Question

Part 2 · El Niño and La Niña

In the tropical Pacific, “during the neutral phase, steady trade winds blow across the tropical Pacific from the east to west”. “These winds pile up warm water in the western Pacific”, and “the western tropical Pacific is 8–10° C warmer than the eastern tropical Pacific” (Bureau of Meteorology). Moist air rises over the warm water north of Australia, bringing clouds and rain, and sinks again in the east: “a huge connected cycle called the Walker Circulation”. The ocean and air change together, “because ENSO is a coupled climate phenomenon” (NOAA). ENSO stands for El Niño–Southern Oscillation.

In an El Niño, “Trade winds weaken or reverse, and warmer surface water builds up in the central and/or eastern Pacific”. The clouds shift east, bringing, usually, “below average winter–spring rainfall for eastern parts of the country” and “a greater risk of drought for northern and eastern Australia, higher temperatures and more heatwaves”. In a La Niña, “Trade winds strengthen and warm waters build up north of Australia”, bringing “a higher risk of widespread flooding” (Bureau of Meteorology). The Southern Oscillation Index measures the pressure difference between Tahiti and Darwin: sustained values below −7 “often indicate El Niño episodes” (Bureau of Meteorology). Each event differs: “widespread drought does not occur with every event” (Bureau of Meteorology). The Indian Ocean Dipole matters too: a positive phase brings “less rain and higher than normal temperatures over parts of Australia during winter and spring” (Bureau of Meteorology).

In the Top End, ENSO shifts the wet season. The monsoon starts in tropical Australia “generally two weeks earlier during La Niña years than in El Niño years”, so “rainfall in the northern tropics is typically above average during the early part of the wet season in La Niña years”. La Niña years typically bring more tropical cyclones too: across the Australian region, “On average, twice as many make landfall during La Niña than during El Niño years” (Bureau of Meteorology).

The Pacific in three states

During an El Niño, why does eastern Australia often get less rain? Question

Part 3 · Australia’s water

Changes in the water cycle reach people and ecosystems. In the south-east, April to October rainfall has fallen by “around 9%” since 1994, and “There has been a decrease in streamflow at most gauges across Australia since 1970” (CSIRO and Bureau of Meteorology, 2024). In the Millennium Drought, “the Murray-Darling Basin has experienced below average rainfall since 2002” (Bureau of Meteorology, 2009). At the river’s mouth, the Lower Lakes fell below sea level, and “more than 75 per cent of the Coorong has salinities greater than double that of sea water” (Senate committee report). Then the 2010–11 La Niña, “one of the strongest observed”, brought “widespread flooding in many regions between September 2010 and February 2011” (Bureau of Meteorology).

People changed how they use water. The River Murray “supplies around 50% of our drinking water” in South Australia, and the state’s yearly share of the river “may be reduced during dry conditions” (SA EPA). Adelaide’s desalination plant was built “primarily in response to the millennium drought that was experienced in South Australia from 2001 to 2009” (SA EPA). It can make 100 gigalitres a year and is “Adelaide’s only climate-independent source of drinking water” (SA Water).

In the Northern Territory, “Most towns use groundwater except for Darwin, Katherine and some remote communities” (Power and Water). About 85% of the Darwin region’s water comes from Darwin River Dam and the rest from groundwater, and demand “is expected to more than double by 2050” (NT Government). A full dam does not guarantee supply. In March 2026, after flash flooding and further rain pushed the dam “to more than 110% capacity”, flooding of its pump station stopped it operating. Power and Water said its borefields “cannot meet the full shortfall”, and the dam was restored to normal operations later that week (Power and Water).

Alice Springs has no permanent surface water, so groundwater is its “only reliable source of water”. Most of it comes from the Mereenie aquifer, “ancient water within the Amadeus Basin that receives very little modern recharge” (NT Government). Power and Water estimates that “for every 100 litres of water we consume, each year only 20 litres is renewed from rainfall”.

In June 2026 the Bureau of Meteorology declared an El Niño, and on 27 October it reported that “El Niño continues to strengthen” (Bureau of Meteorology).

Why is pumping groundwater in Alice Springs a long-term concern? Question

Write a question that links the conveyor belt or ENSO to people’s use of water in South Australia or the Northern Territory, and answer it from the reading. Question

Summarise how the oceans move heat, and one way changes in ocean and air together affect Australia. Summarise

5Show what you know

Show what you know.

Eleven questions, one for each of the I can statements on screen 1. You get one try at each, so think before you tap.

Which statement best describes the early atmosphere formed by volcanic outgassing?

Where did almost all the oxygen in today’s air come from?

In which layer does temperature rise with height because ozone absorbs ultraviolet light?

Arctic sea ice is shrinking. What happens to the albedo of the region, and why does it matter?

Which best explains how greenhouse gases warm the Earth’s surface?

Why do the trade winds over northern Australia blow from the south-east rather than straight from the south?

Rosetta found comet 67P’s water has more than three times as much heavy hydrogen as Earth’s oceans. What does this suggest?

About what share of Earth’s fresh water is frozen in ice caps, glaciers and permanent snow?

Why is it important for life in lakes that ice is less dense than liquid water?

What drives the deep part of the global ocean conveyor belt?

During a strong La Niña, what usually happens in the Pacific and in eastern Australia?

Printable activity

Reading record

Three pages: the four roles with question starters, a record for each reading with space for the group’s prediction, question and summary, and space to draw and label the energy budget and the Pacific in a neutral year and an El Niño.

Reading record (PDF) Editable (Word)
For teachers

Curriculum

SACE Board of South Australia, Stage 1 Earth and Environmental Science Subject Outline (reissued for 2025), Topic 4: The Earth’s atmosphere and Topic 5: Importance of the hydrosphere. NT students take the subject as part of the NTCET. The two topics open:

Topic 4: The Earth’s atmosphere

The composition of gases in the Earth’s atmosphere has varied over time. Students examine different theories to explain how the atmosphere was derived, and explore reasons for its changing composition. They examine effects of atmospheric ozone in protecting the environment and albedo on the climate, and use the mechanisms that cause atmospheric circulation to explain wind direction.

Students investigate how the movement of atmospheric air masses influences local ecosystems and weather patterns. They recognise how climate models are continually evolving as new evidence becomes available.

The topic’s Science Understanding statements and dot points, all of which this lesson covers:

Different theories exist that describe how the Earth’s atmosphere was derived.

  • Discuss theories, such as volcanic outgassing, about the derivation of the Earth’s atmosphere.
  • Compare the approximate proportion of nitrogen, oxygen, argon, and carbon dioxide in the Earth’s atmosphere at different times in its history.
  • Explain why nitrogen is essential for life.

The composition of the Earth’s atmosphere has been significantly modified by photosynthesising organisms.

  • Describe fluctuations in the proportion of oxygen in the Earth’s atmosphere over geological time.

The modern atmosphere has a layered structure: the troposphere, mesosphere, stratosphere, and thermosphere.

  • Discuss the key features that characterise the four main layers of the atmosphere.
  • Describe the variation of temperature with altitude in the layers of the atmosphere.

The net transfer of solar energy through the atmosphere to Earth’s surface is influenced by the interaction of ultraviolet radiation with atmospheric ozone, and albedo.

  • Explain the importance of the ozone layer in protecting living organisms from damaging UV radiation.
  • Describe how albedo affects the Earth’s climate.

Certain gases in the Earth’s atmosphere (known as ‘greenhouse gases’) produce a phenomenon known as the ‘greenhouse effect’.

  • Explain how greenhouse gases absorb and reradiate some of the thermal radiation emitted from Earth’s surface to warm the atmosphere.

The movement of atmospheric air masses, due to heating, cooling, and the Earth’s rotation, causes systematic atmospheric circulation; this is the dominant mechanism for the transfer of thermal energy around the Earth’s surface.

  • Explain how convection currents promote atmospheric circulation.
  • Explain how convection cells promote atmospheric circulation in each hemisphere.
  • Explain how the Coriolis effect influences wind direction around the Earth.
  • Explain how the movement of atmospheric air masses influences local ecosystems.

The interaction between the Earth’s atmosphere and oceans changes over time and can result in anomalous global weather patterns.

  • Discuss the causes and effects of ENSO (El Niño–Southern Oscillation).

Topic 5: Importance of the hydrosphere

Water exists in three phases on the Earth and plays an important part in the interactions between the Earth’s systems. Students investigate evidence that gives rise to different theories about the origins of water on the Earth. They study the properties of water and how these affect its role in biotic and abiotic environments.

Students explore how the action of oceans, as heat sinks, leads to formation of ocean currents and how the global-ocean conveyer belt influences climate in regions around the world. They use specific terminology in discussions of interactions between the hydrosphere and the other Earth systems.

The topic’s Science Understanding statements and dot points, all of which this lesson covers:

Water is present on the Earth as a result of volcanic outgassing and the impact of icy extraterrestrial bodies.

  • Discuss theories of the origin of the Earth’s water.

Water occurs in three phases on the Earth, solid, liquid, and gas.

  • Describe examples of the occurrence of the three phases of water in each of the Earth’s spheres.
  • Describe the hydrological cycle.
  • Explain why evaporation from large bodies of water, e.g. the ocean and rivers, is important for atmospheric moisture.
  • Explain how changes to the hydrological cycle impact on ecosystems and people’s use of resources.
  • Discuss global water distribution.
  • Explain the importance of water’s unique properties in sustaining life on Earth and in shaping Earth processes.

The global-ocean conveyor belt is a constantly moving system of deep-ocean circulation caused by a combination of thermohaline currents in the deep ocean and wind-driven currents on the surface.

  • Explain how differences in temperature and salinity produce movement of water deep in the ocean.
  • Explain the role of the global-ocean conveyor belt in regulating temperatures in Europe.
  • Explain the role of the global-ocean conveyor belt in global-ocean nutrient and carbon dioxide cycles.

Where the lesson covers them:

  • Theories of how the atmosphere was derived, the proportions of its gases through time, and why nitrogen is essential for life: Reading 1, part 1 and the chart of today’s air; x1.
  • Changes in oxygen over geological time: Reading 1, part 2 and the oxygen table; x2.
  • The four layers and temperature with height: Reading 2 and the temperature graph; x3.
  • Ozone and UV, and albedo and climate: Reading 3 and the albedo chart; x4.
  • Greenhouse gases absorbing and reradiating thermal radiation: Reading 4, the energy model, the Mauna Loa graph, and Venus and Mars; x5.
  • Convection, convection cells, the Coriolis effect and air masses affecting ecosystems: Reading 5 (Adelaide’s winter rain and Darwin’s monsoon) and the circulation model; x6.
  • ENSO: Reading 8, part 2 (including the Top End) and the Pacific model; x11.
  • Theories of the origin of Earth’s water: Reading 6 and the D/H chart; x7.
  • Three phases in each sphere, the hydrological cycle, evaporation and atmospheric moisture, and global water distribution: Reading 7, part 1, the fresh-water chart and the phase sort; x8.
  • Water’s unique properties: Reading 7, part 2 and the properties table; x9.
  • Changes to the hydrological cycle and their effects on ecosystems and people: Reading 5, part 2 (south-west rainfall) and Reading 8, part 3 (the Murray, Darwin and Alice Springs); x11.
  • The conveyor belt: temperature and salinity, Europe, nutrients and carbon dioxide: Reading 8, part 1 and the conveyor model; x10.
  • Climate models evolving with new evidence: Reading 8, part 1 (the AMOC and the Antarctic overturning).

Stage 1 is assessed through an investigations folio and skills and applications tasks. The Mauna Loa data, the albedo chart and the fresh-water chart suit a skills and applications task. A practical on water’s heat capacity (heating equal masses of water and sand) or on density currents (coloured salty and fresh water) fits the investigations folio.

How the lesson is taught

The lesson uses reciprocal teaching, developed by Annemarie Palincsar and Ann Brown. In their 1984 study, “The four study activities were summarizing (self-review), questioning, clarifying, and predicting”, and “the tutor and students took turns leading a dialogue centered on pertinent features of the text” (Palincsar and Brown, 1984). The adult models each strategy at first and then “gradually cedes her greater responsibility” to the students. The original students were “seventh grade poor comprehenders”, and the classroom transfer tests used science and social studies passages. Each reading here is split into parts, and every question is labelled with its strategy: predict before a part, clarify and question during it, summarise after it. A group of four takes the roles of predictor, clarifier, questioner and summariser, and swaps roles for each reading.

The evidence is positive, but the size of the effect depends on how it is measured and who runs it. Rosenshine and Meister’s review of sixteen studies found a median effect size of .32 on standardised tests and .88 on tests the researchers wrote (Rosenshine and Meister, 1994). A meta-analysis of whole-class reading-strategy programmes found “a very small effect on reading comprehension (Cohen’s d = .186)” on standardised tests, with larger effects when researchers rather than teachers delivered them (Okkinga and others, 2018). The Education Endowment Foundation rates reading comprehension strategies as “high impact on average (+7 months)” and notes they “can be integrated into literacy, humanities and science lessons”, but its primary trial of Reciprocal Reading found gains for the targeted version and no evidence of impact for the whole-class version (EEF). The only randomised trial in secondary schools we found, in England, found no significant effect on passage comprehension (+0.13) (Thurston and others, 2020). The What Works Clearinghouse rated reciprocal teaching as having “mixed effects for adolescent learners”. We found no controlled trial with senior secondary Earth science texts.

Teachers report that the hard parts are students’ questions and handing over too early. One teacher-researcher wrote “I too early transferred the responsibility to the pupils who weren’t able to take over that responsibility yet” (Jansen). Tes warns that reciprocal reading can turn into “whole-class, teacher-led didactic lessons where students are marched through the steps of a process” (Dixon, 2020).

  • Timing. About five and a half hours of reading (eight readings of 35 to 50 minutes), plus screens 1 and 10. Use the start of the first lesson for screen 1 and the end of the last for screen 10. Readings 1 to 5 cover Topic 4 and Readings 6 to 8 Topic 5, except ENSO, which is in Reading 8 because it joins ocean and air.
  • Model it first. Before groups work alone, lead the first part of Reading 1 yourself, thinking aloud through each strategy. Then let groups lead part 2 while you listen. Return to modelling whenever a group’s questions stay literal.
  • Running it. Groups of four on one device. Each role leads the talk at its point in the reading, and the whole group answers. Stop the class after part 1 of Readings 1, 4 and 8 and ask a few questioners to share their questions.
  • The model answers. A model answer opens only after the group has typed something. The prediction answers say what the reading shows; a wrong prediction is still a good one if it was reasoned.
  • Saved work. Answers save in the browser on the device being used. A group that changes devices should keep the printable record.

Handle with care

  • Climate change and drought can be personal for students from farming families or from areas hit by drought, fire or flood. The readings report what the Bureau of Meteorology, CSIRO and the IPCC say, and the questions ask students to explain mechanisms, not to take a position.
  • The El Niño of 2026 is current. Reading 8 quotes the Bureau of Meteorology’s update of 27 October 2026. Check the Bureau’s ENSO page before teaching and update the example.
  • The lesson uses the dual name Kennaook/Cape Grim, as the Bureau of Meteorology and CSIRO do. The Coorong and the River Murray matter to their Traditional Owners; the lesson does not tell Aboriginal knowledge of them, because we did not have a source approved by the Traditional Owners.

Misconceptions the readings target

  • The greenhouse effect is caused by the ozone hole (Reading 4, questions 2 and 3; x5). Dawson and Carson (2013) found among 438 Western Australian Year 10 students that “The most common misunderstanding we found was confusing the ozone layer with the greenhouse effect.”
  • Greenhouse gases reflect heat like a mirror, or form a layer (Reading 4, prediction and question 3; x5). Harris and Gold (2018) list “greenhouse gasses form a layer that reflects solar energy” among students’ models, and found that teaching how molecules absorb and re-emit radiation helps.
  • Carbon dioxide is the only greenhouse gas (Reading 4, part 1). Dawson and Carson found “up to 15% of students thought carbon dioxide was the only greenhouse gas”.
  • The early air was like today’s, or plants made the first oxygen (Reading 1; x1 and x2).
  • It gets steadily colder all the way up (Reading 2; x3).
  • The ozone hole is a hole with no ozone (Reading 3, prediction and question 2).
  • Water spins the other way down drains in Australia because of the Coriolis effect (Reading 5, question 2). Britannica: “the volume of water in the toilet is too small to be affected by Earth’s rotation (and the Coriolis force) in any appreciable way.”
  • Trade winds blow towards the east because they are called easterlies. Winds are named for where they come from (Reading 5; x6).
  • El Niño always means drought (Reading 8, part 2). The Bureau: “widespread drought does not occur with every event”.
  • Most fresh water is in rivers and lakes (Reading 7, prediction; x8).

About the data and the models

  • The chart of today’s air (Reading 1) uses NOAA for nitrogen, oxygen and argon, and the Mauna Loa monthly mean for September 2026 (426.07 ppm) for carbon dioxide. September is near the yearly low at Mauna Loa.
  • The temperature graph (Reading 2) is the US Standard Atmosphere, 1976, a mid-latitude average. Its tropopause at 11 km lies between NOAA’s 6 km at the poles and 18 to 20 km at the equator, and above 86 km its heights are geometric. The layer bands use NOAA’s round boundaries.
  • The albedo chart (Reading 3) combines NSIDC, Penn State and NASA Earth Observatory. Forest combines Penn State’s coniferous (0.09 to 0.15) and deciduous (0.15 to 0.18) ranges.
  • The energy model (Reading 4) uses NASA’s rounded percentages. The Mauna Loa graph uses NOAA annual means for each decade, on an axis that starts at zero.
  • The circulation, conveyor and Pacific models (Readings 5 and 8) are schematic and not to scale.
  • Darwin’s rainfall (Reading 5) is the Bureau of Meteorology’s long-term monthly mean for Darwin Airport (station 014015, records from 1941), rounded to the nearest millimetre.
  • The Top End ENSO figures (Reading 8) compare La Niña years with El Niño years, not with an average year, and the cyclone figure is for the whole Australian region.
  • The D/H chart (Reading 6) gives Ryugu’s value converted from 165 × 10⁻⁶ in Piani and others (2023).
  • The fresh-water chart (Reading 7) uses the USGS table, from Shiklomanov (1993). Fresh water as about 2.5% of all water is our sum of the table’s fresh stores.
  • The links between water’s properties and Earth processes in the table (Reading 7) are standard textbook examples rather than quotes from one source.
  • Quotations in the screens are short excerpts checked against the source pages.

Where sources disagree

  • The Great Oxidation Event: about 2.3 billion years ago (ScienceDaily, on Lyons and others, 2014) or first significant oxygen about 2.43 billion years ago (Leeds, on Poulton and others, 2021).
  • Oxygen in the long low period, about 2 to 0.8 billion years ago: “~1-40% PAL” in the traditional view against “much lower than 1% PAL, and perhaps less than 0.1% PAL” (Olson and others, 2018).
  • Oxygen before cyanobacteria: Bush Heritage Australia says “Before cyanobacteria the air was only 1% oxygen”, far above the research values; we did not use it.
  • Layer boundaries: NOAA puts the stratosphere’s top at about 50 km and the mesosphere’s at 85 km; the 1976 standard puts them at 47 to 51 km and about 86 to 91 km. NOAA gives the thermosphere up to 2,000 °C; the standard assumes moderate solar activity and reaches about 726 °C at 500 km.
  • The Earth’s albedo: about 0.30 (NASA Earth Observatory) or 0.31 (Penn State).
  • Venus: 464 °C and 92 bars (NASA fact sheet) or about 467 °C and “93 times” Earth’s pressure (NASA). Mars carbon dioxide: 95.1% (NASA fact sheet); many textbooks give 95% to 96%.
  • The natural greenhouse effect: “more than 30 degrees” (NASA Earth Observatory) or about 33 °C (NASA).
  • Carbon dioxide in 2024: 423.9 ppm (WMO) or 422.8 ppm (NOAA), from different station networks.
  • The Hadley cell: sinking at about 30° (NOAA) or reaching “between 30 and 40 degrees” (Met Office).
  • How long water stays in the air: about 10 days (USGS; Britannica), 7 to 9 days (Science Learning Hub), or about 9 days if NASA’s “nearly 40 times a year” is divided out. Ocean residence time: 3,000 to 3,230 years (Britannica) or 3,000 to 3,500 years (Science Learning Hub). NOAA’s 1,000 years for the conveyor is the time for one trip, not a residence time.
  • Groundwater: 30.1% of fresh water (USGS table) or “approximately 22 to 30%” (NASA).
  • The origin of water: ESA says Rosetta favours asteroids; Hartogh and others (2011) found a comet that matches; Alexander and others (2012) favour CI chondrites; Piani and others (2020) argue Earth’s building blocks held enough; Piani and others (2023) put Ryugu-like material at about 3%. Britannica gives chemistry inside the planet as a rival view.
  • The Late Heavy Bombardment: the spike at 3.9 billion years from Apollo samples is now disputed; Bottke and Norman (2017) still see “a discrete episode” from about 4.2 to 3.5 billion years ago.
  • Europe’s mild winters: NOAA and the Met Office stress the Gulf Stream and the AMOC; Seager argues the atmosphere does most of the work and “The ocean heat transport contributes a small warming across the basin.”
  • The AMOC: about 15% weaker since the mid-twentieth century from a sea-surface fingerprint (Caesar and others, 2018); the IPCC is more confident about the projected decline this century than about the observed one.
  • The Millennium Drought: below-average rainfall in south-east Australia from 1997 and in the Murray-Darling Basin from 2002 (Bureau of Meteorology, 2009); 2001 to 2009 in South Australia (SA EPA).
  • Adelaide’s water: the SA EPA says the River Murray supplies “around 50%” of South Australia’s drinking water. The often-quoted “40% on average, up to 90% in dry years” could not be confirmed on an official page. The desalination plant’s “around 50%” of Adelaide’s supply is its capacity, not its usual output.
  • The northern wet season: October to April (Bureau of Meteorology) or November to April (NT Government). The monsoon is only part of it: the Bureau dates the onset by the change of wind at Darwin, on average 29 December.
  • Darwin’s water: 85% from Darwin River Dam (NT Government), but “Up to 95%”, and 83% in the 2025/2026 demand figures, in Power and Water’s August 2025 update. We use about 85%.
  • Alice Springs: Power and Water’s 20 litres renewed for every 100 used, against about 750 ML a year of recharge for about 8,500 ML a year of use at the Roe Creek borefield (Geoscience Australia, 2019); the areas and methods differ. The often-quoted age of 10,000 to 30,000 years for the town’s water comes from the Arid Lands Environment Centre; the only official age we found is the NT Government background report’s “recharged tens of thousands of years ago”, so the reading says “ancient”.
  • Reciprocal teaching: an effect size of 0.74 (Hattie) against .32 on standardised tests (Rosenshine and Meister, 1994) and .186 for whole-class reading-strategy programmes (Okkinga and others, 2018).

Answers at a glance

  • Reading 1: q2 the release of gases from rising magma; q4 it hardly reacts; q5 iron in seawater first; q6 the long summary.
  • Reading 2: q2 the top of the troposphere; q3 the ground heats the air; q4 ozone absorbs ultraviolet.
  • Reading 3: q2 a thinning over Antarctica each spring; q3 skin cancer, cataracts and plant damage; q4 open ocean.
  • Reading 4: q2 infrared from the surface; q3 absorb and radiate again; q5 Venus has far more gas.
  • Reading 5: q2 turning because the Earth spins; q3 dry air sinks at 30°; q4 the ridge moves north in winter; q5 the wind belts move with the seasons.
  • Reading 6: q2 vapour condensed as the surface cooled; q3 oxygen isotopes show contact with water; q4 too much heavy hydrogen.
  • Reading 7: q2 most moisture evaporated from open water; q3 the average time in one store; q4 the sea’s heat capacity. Phase sort: hail, sea ice and ice in a crack are solid; cloud droplets, groundwater and sap are liquid; humidity, volcanic water vapour and transpiration are gas.
  • Reading 8: q2 cold and salty water is dense; q3 nutrients return to the surface; q4 the trade winds weaken and the rain moves east; q5 ancient water, little renewed.
  • Exit check: x1 water vapour, carbon dioxide and nitrogen; x2 photosynthesis; x3 stratosphere; x4 albedo falls; x5 absorb and reradiate; x6 the Coriolis effect; x7 not the main source; x8 about 69%; x9 ice floats; x10 cold, salty water sinks; x11 stronger trades, flooding.

Saved progress

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References

  • Alexander, C. M. O’D., and others (2012). The provenances of asteroids, and their contributions to the volatile inventories of the terrestrial planets. Science, 337, 721–723.
  • Altwegg, K., and others (2015). 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science, 347, 1261952 (abstract via ESA).
  • Arid Lands Environment Centre (n.d.). Why we need a new water allocation plan for Alice Springs.
  • ARPANSA and Cancer Council (n.d.). Cancer Council and ARPANSA stress importance of improved understanding as UV reaches extreme levels this summer (media release).
  • Australian Museum (n.d.). Banded iron formation.
  • Bernhard, A. (2010). The nitrogen cycle: processes, players, and human impact. Nature Education Knowledge, 3(10), 25.
  • Bottke, W. F., & Norman, M. D. (2017). The Late Heavy Bombardment. Annual Review of Earth and Planetary Sciences, 45, 619–647.
  • Britannica (n.d.). Hydrosphere: origin and evolution of the hydrosphere; the water cycle. Water. Do toilets in different hemispheres flush in different directions?
  • Bureau of Meteorology (n.d.). Climate Knowledge Centre: trade winds; subtropical ridge; monsoon; El Niño and La Niña; Indian Ocean Dipole. About the Southern Oscillation Index. El Niño and Australia. The 2010–11 La Niña. ENSO update, issued 27 October 2026. Drought statement, 7 September 2009. Fifty years measuring the world’s cleanest air (2026). Monsoon arrives at Darwin (26 December 2025). Australia’s 2025–26 northern wet season (14 May 2026). Climate statistics for Australian locations: Darwin Airport. Rainfall percentage of annual rainfall maps. Explainer: what is a La Niña? (blog, 29 September 2020).
  • Bush Heritage Australia (n.d.). Stromatolites.
  • Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556, 191–196.
  • CSIRO (1998). Antarctic bottom water (media release on the work of S. Rintoul), via EurekAlert.
  • CSIRO (n.d.). Latest greenhouse gas data. 50 years measuring the world’s cleanest air (1 April 2026). Expert commentary: El Niño declared (17 June 2026).
  • CSIRO and Bureau of Meteorology (2024). State of the Climate 2024.
  • Dawson, V., & Carson, K. (2013, 4 November). What do young people really know about climate change? The Conversation.
  • Dixon, M. (2020, 13 March). How to make the most of reciprocal reading. Tes.
  • Education Endowment Foundation (n.d.). Teaching and Learning Toolkit: reading comprehension strategies. Reciprocal Reading (project and evaluation, 2019).
  • ESA (2014, 10 December). Rosetta fuels debate on origin of Earth’s oceans.
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