Climate Change: Why Is Our Planet Getting Warmer?
Hot summer nights, melting Arctic ice, heavy rainstorms – all are linked to climate change. In this chapter you will learn how some gases keep the Earth warm, how human activities upset nature's balance, what the evidence shows and what we can do about it. This topic also links to junior secondary Geography.
1. Quick Recap: The Balance of CO2 and O2
As you learnt in the chapter Balance of carbon dioxide and oxygen, photosynthesis takes in carbon dioxide (CO2) and gives out oxygen (O2), while respiration does the opposite. For thousands of years these processes roughly balanced each other, so the CO2 level stayed fairly steady. CO2 is only a tiny part of the air (about 0.04% today), but it matters a lot.
Key takeaway: Photosynthesis removes CO2 and respiration adds it back, keeping a natural balance.
2. The Greenhouse Effect
Greenhouse gases
A greenhouse gas is a gas that absorbs heat and re-emits it, sending some of it back towards the Earth's surface. Carbon dioxide is one of the greenhouse gases. Others include methane (CH4), water vapour and nitrous oxide (N2O). Nitrogen and oxygen, which make up about 99% of the air, are not greenhouse gases.
How the greenhouse effect works
1. Sunlight passes through the atmosphere and warms the land and sea.
2. The warm surface gives out heat (invisible infrared radiation) towards space.
3. Greenhouse gases absorb some of this heat and re-emit it in all directions, so some goes back to the Earth's surface.
4. The Earth stays warmer than it would otherwise be. This is the greenhouse effect.
Analogy: Greenhouse gases act like a blanket on a cold night. The blanket makes no heat itself, but it slows down heat loss. Memory aid – "IN, OUT, BACK": sunlight comes IN, heat goes OUT, greenhouse gases send some BACK.
Did you know?
Without the natural greenhouse effect, the Earth's average surface temperature would be about −18 °C instead of about 15 °C – too cold for most life! The greenhouse effect itself is good; the problem is making it too strong.
Key takeaway: Greenhouse gases such as CO2 absorb heat from the Earth and re-emit some back to the surface, keeping our planet warm enough for life.
3. How Human Activities Upset the Balance
Since the Industrial Revolution began over 200 years ago, humans have added CO2 to the air faster than photosynthesis can remove it.
Burning fossil fuels
Fossil fuels (coal, oil and natural gas) formed from living things buried millions of years ago. Burning them to generate electricity, run vehicles and power factories quickly releases their stored carbon as CO2. In Hong Kong, much of our electricity comes from burning natural gas and coal, and electricity generation is our largest source of greenhouse gas emissions.
Deforestation
Cutting down or burning forests leaves fewer trees to take in CO2, and the carbon stored in trees is released as CO2 when they are burned or rot.
Some activities add methane too, such as cattle farming, rice fields and food waste rotting in landfills.
The enhanced greenhouse effect
More greenhouse gases → more heat re-emitted to the surface → higher average temperature → global warming. This stronger-than-normal greenhouse effect is called the enhanced greenhouse effect. It is like adding a thicker blanket.
Common mistake
Wrong: "The greenhouse effect is bad." Right: The natural greenhouse effect keeps the Earth habitable; the problem is the enhanced effect caused by human activities.
Wrong: "Global warming is caused by the hole in the ozone layer." Right: These are different problems. The ozone layer blocks harmful ultraviolet (UV) rays; global warming is mainly caused by greenhouse gases.
Key takeaway: Burning fossil fuels and deforestation add CO2 faster than nature can remove it, strengthening the greenhouse effect.
4. Science in Action: A Greenhouse Model
Setting up a fair test
Put a thermometer into each of two identical sealed bottles: bottle A with ordinary air (the control) and bottle B filled with carbon dioxide (made in a separate flask, so the reaction does not affect the temperature). Place both bottles at the same distance under the same lamp and record the temperature every minute for about 15 minutes.
Independent variable (changed): the amount of CO2
Dependent variable (measured): the rise in temperature
Control variables (kept the same): bottle size, lamp and distance, starting temperature, heating time
Results and limits
Bottle B usually warms up faster and higher, which supports the idea that CO2 absorbs heat. Repeat the test to make the results more reliable. But a bottle is only a simple model of the atmosphere and the difference is often small, so interpret the results carefully.
Key takeaway: In a fair test, change only the amount of CO2, measure the temperature and keep everything else the same.
5. The Evidence: Reading Climate Data
Weather is the condition of the air on a particular day. Climate is the average weather pattern of a place over a long time, usually 30 years or more.
What the data shows
Atmospheric CO2 concentration – measured since the 1950s (for example at Mauna Loa, Hawaii), it rises year after year.
Annual global surface temperature – up by more than 1 °C since the late 1800s, with most of the rise in recent decades.
Arctic sea ice – satellites show that the ice left at the end of summer has generally shrunk since the late 1970s.
Global sea level – rising.
Is the Sun to blame?
Satellites show that the Sun's energy reaching the Earth has stayed roughly the same since about 1980, while global temperature kept rising. So the best explanation for recent warming is the increase in greenhouse gases from human activities. Scientists around the world agree.
Tips for reading climate graphs
Look at the long-term trend, not single years – one cold winter does not mean global warming has stopped. Check the axes and units (e.g. ppm, parts per million, for CO2). Two things rising together does not by itself prove cause, so scientists also use experiments and rule out other causes.
Did you know?
The CO2 graph zigzags every year! CO2 dips in the northern spring and summer, when plants are busy photosynthesising, then rises again in autumn and winter – as if the planet is "breathing".
Key takeaway: Rising CO2, rising temperature, shrinking Arctic sea ice and rising sea level show that the climate is changing, mainly because of human activities.
6. Impacts of Climate Change
On the environment
Global warming – the Earth's average temperature rises and heatwaves become more common. The Hong Kong Observatory has recorded more very hot days and hot nights in Hong Kong than in the past.
Sea-level rise – ice on land (glaciers and ice sheets) melts, and seawater expands as it warms. Low-lying coasts face more flooding, and typhoon storm surges become more dangerous (Super Typhoon Mangkhut flooded some coastal areas of Hong Kong in 2018).
Extreme weather – warmer air holds more water vapour, so heavy rainstorms can become more common. Stronger typhoons, droughts and wildfires are also linked to warming.
On ecosystems
Warmer seas cause coral bleaching, and extra CO2 dissolving in seawater makes it more acidic, harming corals and shellfish. Polar bears lose the sea ice they hunt on. Some species may die out, reducing biodiversity.
On human health
Heat stroke and heat exhaustion during heatwaves, especially for the elderly and outdoor workers.
More mosquito-borne diseases such as dengue fever, as warm, wet weather helps mosquitoes breed.
Damage to food and water supplies from droughts and floods.
Air pollution from burning fossil fuels harms our lungs (check Hong Kong's AQHI).
Key takeaway: Climate change brings global warming, sea-level rise and extreme weather, and harms ecosystems and human health.
7. Low-carbon Living in Hong Kong
Low-carbon living means living in ways that release less CO2 and other greenhouse gases, that is, reducing your carbon footprint. Small actions by millions of people add up, and often save money too!
Easy low-carbon actions
Save electricity – set the air conditioner at about 25–26 °C, switch off at the plug, choose appliances with a Grade 1 energy label, and check your family's CLP or HK Electric bill.
Travel green – walk, cycle, or take the MTR, buses and trams instead of private cars.
Eat smart – eat more vegetables and less red meat (cattle produce methane), and choose local, seasonal food.
Reduce waste – follow the 3Rs (Reduce, Reuse, Recycle), bring your own bottle and bag, and finish your food, because food waste in landfills releases methane.
Did you know?
Hong Kong aims to become carbon neutral before 2050. This means the greenhouse gases it releases will be balanced by the amount removed.
Key takeaway: Saving energy, green travel, sensible eating and less waste cut our carbon footprint.
8. Planting Trees: Nature's Carbon Catchers
Trees take in CO2 for photosynthesis and store the carbon in their leaves, wood and roots for many years, while giving out oxygen. So planting trees and protecting forests helps restore the CO2–O2 balance and mitigates (reduces) climate change.
More benefits of planting
Cooling – shade and water evaporating from leaves cool the air, reducing the urban heat island effect and the need for air conditioning.
Protecting slopes – roots hold the soil, reducing erosion and landslides.
Wildlife – trees provide food and habitats, supporting biodiversity.
Cleaner air, less flooding – leaves trap dust, and plants and soil soak up rainwater.
But trees take years to grow and release their carbon if burned, so planting must go together with protecting existing green areas (such as Hong Kong's Country Parks, about 40% of our land, and the mangroves at Mai Po) and cutting emissions.
Key takeaway: Plants remove CO2 and store the carbon, so planting trees helps mitigate climate change and brings many other benefits.
Quick Review
1. Photosynthesis removes CO2 and respiration adds it back – a natural balance.
2. CO2 is a greenhouse gas: it absorbs heat and re-emits some of it back to the Earth's surface.
3. The natural greenhouse effect keeps the Earth at about 15 °C instead of −18 °C.
4. Burning fossil fuels and deforestation cause the enhanced greenhouse effect.
5. Evidence: rising CO2, rising temperature, shrinking Arctic sea ice and rising sea level, while the Sun's energy has not increased.
6. Impacts: global warming, sea-level rise, extreme weather, harm to ecosystems and health.
7. Low-carbon living and planting trees help mitigate climate change.
Well done! Every small low-carbon action helps our planet.