Seasons and Global Warming
AdvancedRotation makes day and night; revolution plus the tilted axis makes the seasons. Global warming melts glaciers and raises seas — saving energy and planting trees help slow it.
- Rotation makes day and night; revolution plus the tilted axis makes the seasons.
- The axis tilts about 23.5 degrees; the hemispheres' seasons are opposite.
- More greenhouse gases mean global warming: melting glaciers and rising seas.
文體小知識：說明文把因果講透：自轉為何有日夜、地軸傾斜為何有四季、增溫從何而來、我們可以怎樣做。
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1Day follows night because the Earth keeps spinning. The Earth turns on its own axis like a top, taking about twenty-four hours for one turn. The half facing the Sun has day; the half facing away has night. That is why different parts of the world enter day and night at different times.
2The Earth's axis is not upright; it is tilted by about 23.5 degrees. As it spins, the Earth also travels around the Sun, taking about 365 days for one journey. Because of the tilt, different regions get sunlight at different angles and for different lengths of time as the Earth goes around — and that is how the four seasons come about.
3From June to August the northern hemisphere leans towards the Sun, gets more direct sunlight and longer days, and it is hot summer; at the same time the southern hemisphere leans away and it is winter. So while Hong Kong holds Christmas celebrations in December, midsummer is on in Australia and Santa has to wear a T-shirt! The seasons of the two hemispheres are exact opposites.
4Over the past hundred years and more, people have burned great amounts of coal and oil, sending out greenhouse gases such as carbon dioxide. The Earth is like being tucked under a thick blanket: its average temperature keeps rising. This is global warming. Warming melts glaciers and raises sea levels, so low-lying coastal areas may be flooded. Weather grows more extreme, with more rainstorms and heatwaves, and some land slowly turns to desert — desertification.
5Slowing global warming is everyone's job. Take buses and trains more, switch off lights when leaving, set the air-conditioner to twenty-six degrees — all of these cut carbon emissions. Plant more trees, which absorb carbon dioxide; waste less and reuse more. Small actions added together can truly help cool the Earth.
1Summary 重點摘要
Rotation makes day and night; revolution plus the tilted axis makes the seasons. Global warming melts glaciers and raises seas — saving energy and planting trees help slow it.
2Vocabulary cards Vocabulary
The Earth spinning on its own axis once about every 24 hours, causing day and night.
The Earth travelling around the Sun once about every 365 days; with the tilted axis it causes the seasons.
The imaginary line the Earth spins around, tilted about 23.5 degrees.
The two halves of the Earth divided by the equator; their seasons are opposite.
Spring, summer, autumn and winter, caused by revolution and the tilted axis.
Gases such as carbon dioxide that trap heat and warm the Earth.
The rise of the Earth's average temperature as greenhouse gases build up.
Huge, slow-moving ice in high mountains and polar regions; warming melts it.
Land slowly turning into desert through drought and damage.
The greenhouse gases such as carbon dioxide that human activities give off.
3Step by step Steps
- Step 1: Day and night come from rotationThe Earth turns on its own axis all the time; this is called rotation, and one turn takes about twenty-four hours. The Earth does not shine by itself; the Sun can only light up half of it. The half facing the Sun has day, the half facing away has night. As the Earth rotates, every place takes its turn on the sunlit side, so day and night keep swapping. When we are at day school in Hong Kong, children in America are mostly still asleep, because at that moment the two places face towards and away from the Sun.
- Step 2: The axis is tiltedThe axis the Earth spins around is called the Earth's axis, an imaginary line joining the North Pole and the South Pole. It does not stand upright but tilts by about 23.5 degrees, like a top leaning slightly. The tilt hardly changes, so as the Earth travels around the Sun, sometimes the North Pole leans towards the Sun and sometimes the South Pole does. Never look down on these 23.5 degrees: without the tilted axis, the Earth would have no clear four seasons.
- Step 3: Revolution plus tilt makes the seasonsThe Earth spins and at the same time travels around the Sun; this journey is called revolution, and one lap takes about 365 days — a year. Because the axis is tilted, the angle of sunlight and the length of daylight for every region keep changing on the way round: when sunlight is direct and days are long, it is hot summer; when sunlight slants and days are short, it is cold winter; in between come spring and autumn. So the seasons need both revolution and the tilted axis working together — leave one out and there are no seasons.
- Step 4: Opposite seasons in the two hemispheresThe equator splits the Earth into the northern and southern hemispheres. From June to August the northern hemisphere leans towards the Sun with more direct sunlight — summer — while the southern hemisphere leans away, sunlight slants, and it is winter. From December to February it swaps around. So in December we wear coats for Christmas in Hong Kong, while children in Australia wear swimsuits to the beach for Christmas. Find the equator on a globe and picture the two hemispheres' opposite seasons, and you will see why seasons differ around the world at the same moment.
- Step 5: What causes global warmingFor over a hundred years, people have burned coal, oil and natural gas to make electricity, drive cars and run factories, giving off vast amounts of greenhouse gases such as carbon dioxide; cutting down great forests has reduced the plants that absorb carbon dioxide. Greenhouse gases are like a thick blanket wrapping the Earth, holding the Sun's heat at the surface, so the average temperature keeps rising — this is global warming. By watching temperatures, ice cores and sea temperatures over long years, scientists agree the Earth is warming, and human activity is the main cause.
- Step 6: The effects of warmingThe effects of global warming are already before our eyes. Rising heat melts glaciers and polar ice faster; meltwater flows into the sea and sea levels keep rising, so low-lying coastal cities and island nations face flooding. Warmer seas make typhoons stronger, and rainstorms and heatwaves more frequent. In some places long droughts slowly turn the land into desert — desertification — ruining crops and leaving animals and plants without homes. The shrinking sea ice of polar bears is one of the clearest examples.
- Step 7: Everyone can slow the warmingSlowing global warming starts with small daily acts. Save electricity: switch off lights when leaving a room, set the air-conditioner to twenty-six degrees, choose appliances with Grade 1 energy labels. Travel green: take buses and trains more, walk or ride a bicycle, use private cars less. Save paper and reuse things to cut waste. Join tree-planting days so trees can absorb carbon dioxide. One person's power is small, but a whole class, school and city acting together can truly help cool the Earth.
5Worked example Example
Looking at Hong Kong's hot, humid December, Grandma complains: December should be much cooler than this — why is it so hot now? Use what you know about the tilted axis and global warming to explain what part of Grandma's idea is right and what part needs adding.
- Hong Kong lies in the northern subtropics; in December sunlight falls directly on the southern hemisphere, so in Hong Kong it slants and days are short — that is winter with lower temperatures.
- But Hong Kong's subtropical winter has always been cool rather than snowy; snow is the winter climate of northern cities such as Harbin.
- Grandma is right that it feels hotter than before: global warming has raised average temperatures, and warm winters come more often.
- So cooler Decembers are the seasonal rule, and Decembers getting warmer is the extra change that warming lays on top of the rule.
- Answer: Grandma has the seasonal rule right: with the tilted axis and revolution, December is winter in the northern hemisphere and cooler. Yet Hong Kong's subtropical winters rarely saw snow anyway, and recent Decembers feel hotter because more greenhouse gases warm the whole globe. Cutting emissions, saving energy, green travel and planting trees are how we slow the warming.
6Common mistakes Mistakes
7Quick check Quick check
Q1What movement causes day and night?
Q2About how many degrees is the Earth's axis tilted?
Q3Why is Christmas in summer in Australia?
Q4What happens to the Earth as greenhouse gases build up?
Q5Name one way to slow global warming.
8Recap Recap
- Rotation makes day and night; revolution plus the tilted axis makes the seasons.
- The axis tilts about 23.5 degrees; the hemispheres' seasons are opposite.
- More greenhouse gases mean global warming: melting glaciers and rising seas.
- Warming brings extreme weather and desertification, harming people and wildlife.
- Save power, travel green and plant trees — everyone can help cool the Earth.