The Carbon Cycle and the Environment
AdvancedCarbon cycles between the atmosphere (CO₂), living things, the oceans and fossil fuels: photosynthesis absorbs CO₂; respiration and combustion release it. By burning fossil fuels massively, humans have upset the balance → atmospheric CO₂ rises → global warming.
- Uptake: photosynthesis; Release: respiration + combustion
- Stores: atmosphere / organisms / oceans / fossil fuels
- Fossil burning breaks the balance → warming; the fix is cutting emissions
§Text Passage
1Carbon is the core element of life, cycling endlessly between the atmosphere, living things, the oceans and underground. Plants absorb carbon dioxide through photosynthesis and make food; all living things release carbon dioxide back through respiration; human fuel-burning and natural wildfires send carbon back to the air as well. The cycle can be read as two 'pumps', one taking carbon up and one releasing it; sort each process to its side and the direction of the whole cycle becomes clear.
2Carbon is stored in several main reservoirs: carbon dioxide in the atmosphere, organic matter in living things, the oceans — which dissolve vast amounts of carbon dioxide — and fossil fuels buried underground for eons. For centuries the carbon dioxide taken up by photosynthesis roughly matched what respiration and natural fires released, so the atmospheric concentration stayed stable and the greenhouse effect remained at the gentle strength life needs.
3Since the Industrial Revolution began in the mid-eighteenth century, humans have burned coal and oil on a massive scale, releasing carbon buried for eons back into the air within a few generations, while large-scale deforestation has weakened the planet's carbon uptake. With release exceeding absorption, atmospheric carbon dioxide has climbed year by year, strengthening the greenhouse effect and becoming the main cause of global warming, with more frequent extreme weather and rising sea levels among the consequences.
4Responses run along two tracks: mitigation attacks the source — cutting emissions, developing renewable energy, planting trees and living low-carbon lives — while adaptation adjusts to changes that have already appeared, such as building flood defences and promoting heat-resilient designs. Hong Kong aims to achieve carbon neutrality by 2050. In addition, carbon dioxide dissolved in seawater forms carbonic acid, making the ocean more acidic and hindering corals and shellfish such as oysters from building their calcium carbonate shells — ocean acidification is the second crisis on the heels of warming.
1Summary 重點摘要
Carbon cycles between the atmosphere (CO₂), living things, the oceans and fossil fuels: photosynthesis absorbs CO₂; respiration and combustion release it. By burning fossil fuels massively, humans have upset the balance → atmospheric CO₂ rises → global warming.
2Vocabulary cards Vocabulary
The movement of carbon between living things, the atmosphere, oceans and rocks.
Organisms break down nutrients for energy, releasing CO₂.
Burning fuels (coal, oil) to release energy — and CO₂.
Atmospheric gases (CO₂, methane) trap heat. When excessive, it becomes global warming.
Plants absorb CO₂ and water to make food and release oxygen — the carbon cycle's main uptake.
Where carbon is held: the atmosphere, organisms, oceans and underground fossil fuels.
Clearing forests — less CO₂ absorbed, upsetting the carbon balance.
The level of CO₂ in the air — rising levels strengthen the greenhouse effect.
Emissions matched by removals — net zero; Hong Kong targets this by 2050.
3Step by step Steps
- Two 'pumps' in the diagramThe carbon cycle diagram can be read as two 'pumps': the uptake pump is photosynthesis, in which plants absorb carbon dioxide to make food; the release pumps are respiration and combustion — every living thing breathes out carbon dioxide day and night, and human fuel-burning and natural wildfires send carbon back to the air as well. Sort every arrow in the diagram into its pump first, and understanding the whole cycle becomes far easier.
- Carbon storesCarbon is stored in four main reservoirs: carbon dioxide gas in the atmosphere, organic matter in living organisms, the oceans — seawater dissolves vast amounts of carbon dioxide, making them among the largest active stores — and fossil fuels buried underground, a geological store accumulated over eons. The flows in and out of these stores decide whether atmospheric carbon dioxide rises or falls, forming the backbone of the whole carbon cycle.
- The original balanceFor centuries the carbon dioxide taken up by photosynthesis roughly matched what respiration and natural fires released, so the atmospheric concentration stayed stable and the greenhouse effect remained at the gentle strength life needs: without any greenhouse effect the Earth would be too cold to inhabit, while too strong a greenhouse warms the planet. The balance itself was the key.
- Humans break itSince the Industrial Revolution began in the mid-eighteenth century, humans have burned coal and oil on a massive scale, releasing carbon buried underground for eons back into the air within a few generations; at the same time, large-scale deforestation has weakened the planet's carbon uptake. With release exceeding absorption, atmospheric carbon dioxide has climbed year by year, strengthening the greenhouse effect and becoming the main cause of global warming.
- Consequences & responsesThe consequences include global warming, more frequent extreme weather and rising sea levels. Responses run along two tracks: mitigation attacks the source — cutting emissions, developing renewable energy, planting trees and living low-carbon lives — while adaptation adjusts to changes that have already appeared, such as building flood defences and promoting heat-resilient designs. Hong Kong aims to achieve carbon neutrality by 2050.
- The ocean: carbon store and acidificationThe ocean dissolves vast amounts of carbon dioxide from the air — one of the largest active carbon stores — slowing its build-up in the atmosphere. But dissolved carbon dioxide forms carbonic acid, making seawater more acidic as the pH falls, which hinders corals and shellfish such as oysters from building their calcium carbonate shells. Ocean acidification is the second crisis on the heels of warming, and both spring from the same excess carbon dioxide emissions.
4Diagrams Diagrams
5Worked example Example
Why does tree planting help slow global warming — but isn't a silver bullet?
- Trees absorb CO₂ via photosynthesis → short-term carbon storage
- But growth is slow and uptake limited
- If trees die or burn, the carbon returns to the atmosphere
- Fossil fuel emissions far exceed what new trees can absorb
- Answer: Tree planting helps: trees absorb CO2 by photosynthesis and store the carbon in their trunks for a time, and forests also protect soil and water. But it is no silver bullet — trees grow slowly and absorb only a limited amount; when they die or burn, the carbon returns to the air, and yearly fossil emissions far exceed what new trees can take up. So planting is an aid, while cutting emissions is the real fix; give both the benefits and the limits in your answer.
6Common mistakes Mistakes
7Quick check Quick check
Q1In the carbon cycle, which process absorbs CO₂?
Q2Name two processes that release CO₂.
Q3How do human activities upset the carbon balance?
Q4Is the greenhouse effect always a bad thing?
Q5What does 'carbon neutrality' mean?
8Recap Recap
- Uptake: photosynthesis; Release: respiration + combustion
- Stores: atmosphere / organisms / oceans / fossil fuels
- Fossil burning breaks the balance → warming; the fix is cutting emissions
- Photosynthesis is the main uptake — deforestation weakens it
- Mitigation: cut emissions, renewables, tree planting, low-carbon living