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The Carbon Cycle and the Greenhouse Effect — Answers

Answer sheet with model answers and success criteria for the 9th Grade earth, rocks & weather worksheet.

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Success TutoringWorksheetsAnswer sheet9th Grade · Science · Earth, Rocks & Weather

The Carbon Cycle and the Greenhouse Effect — Answers

Part A: The carbon cycle

Carbon is constantly recycled. Photosynthesis takes carbon dioxide from the air and turns it into glucose in plants. Carbon passes to animals when they eat plants. Respiration by all living things releases carbon dioxide. When organisms die, decomposers break them down and release carbon dioxide.

Some carbon is stored for millions of years in fossil fuels and carbonate rocks such as limestone. Combustion of fuels releases carbon dioxide back into the atmosphere. The oceans also absorb and release large amounts of carbon dioxide.

1.The process that removes carbon dioxide from the atmosphere and stores carbon in plants is (answer space).

Answer: photosynthesis

2.Name three processes that return carbon dioxide to the atmosphere.

Answer: Any three of: respiration (by plants, animals and microorganisms), decomposition by decomposers, combustion (burning fuels or forests), volcanic eruptions.

3.What role do decomposers such as bacteria and fungi play in the carbon cycle?
  1. AThey absorb carbon dioxide by photosynthesis
  2. BThey break down dead organisms and release carbon dioxide by respiration
  3. CThey turn carbon dioxide into oxygen
  4. DThey store carbon permanently in rocks

Answer: They break down dead organisms and release carbon dioxide by respiration

Without decomposers, carbon would stay locked in dead material.

4.How did fossil fuels such as coal form, and why is burning them a problem for the carbon cycle?

Answer: They formed from the remains of organisms buried and compressed over millions of years, locking away carbon. Burning them releases this stored carbon as carbon dioxide in a very short time, much faster than natural processes can remove it.

5.The oceans absorb some of the carbon dioxide released into the atmosphere.

Answer: True

Carbon dioxide dissolves in sea water and is used by marine algae. As the oceans absorb more, they become slightly more acidic.

Part B: The greenhouse effect

Greenhouse gases such as carbon dioxide, methane and water vapor absorb infrared radiation given off by Earth's surface and re-emit some of it back towards the surface. This natural greenhouse effect keeps Earth warm enough for life.

Human activities, especially burning fossil fuels and clearing forests, are increasing the amount of greenhouse gases in the atmosphere. This enhanced greenhouse effect traps more heat and is causing global warming and climate change.

6.Describe how the greenhouse effect keeps Earth warm.

Answer: Light from the Sun passes through the atmosphere and warms Earth's surface. The warm surface gives off infrared radiation. Greenhouse gases absorb some of this infrared and re-emit it in all directions, including back towards the surface, so heat is trapped and Earth stays warmer than it would otherwise be.

7.Without any natural greenhouse effect, Earth's average temperature would be about −18 °C instead of about 15 °C. What does this tell you?
  1. AThe greenhouse effect is always harmful
  2. BThe natural greenhouse effect is necessary for life as we know it
  3. CGreenhouse gases cool Earth down
  4. DThe Sun provides no heat

Answer: The natural greenhouse effect is necessary for life as we know it

The problem is not the greenhouse effect itself, but the extra warming caused by increasing greenhouse gas levels.

8.Name two greenhouse gases other than carbon dioxide.

Answer: Any two of: methane, water vapor, nitrous oxide, ozone, CFCs.

Part C: Interpreting the data

Carbon dioxide in the atmosphere is measured in parts per million (ppm). Approximate average values: before the industrial revolution 280 ppm; 1960 317 ppm; 2020 414 ppm.

9.By how much did the carbon dioxide concentration increase between 1960 and 2020?

Answer: 97 ppm

  1. 414 − 317 = 97 ppm
10.Calculate the average rate of increase in ppm per year between 1960 and 2020, to one decimal place.

Answer: 1.6 ppm per year

  1. 97 ÷ 60 = 1.616…
  2. ≈ 1.6 ppm per year
11.By what percentage has the concentration risen since pre-industrial times (280 ppm), up to 2020? Give your answer to one decimal place.

Answer: 47.9%

  1. Increase = 414 − 280 = 134 ppm
  2. 134 ÷ 280 × 100 = 47.86
  3. ≈ 47.9%
12.The carbon dioxide record goes up and down slightly every year, falling during the northern summer. Suggest why.

Answer: Most of the world's land and plants are in the Northern Hemisphere. In its summer, plants grow and photosynthesize more, removing carbon dioxide from the air; in winter, photosynthesis slows while respiration and decomposition continue, so the level rises again.

Part D: Reducing emissions

13.A car releases 0.2 kg of carbon dioxide for each kilometer it travels. How much carbon dioxide does it release in a year of 15,000 km, in kilograms and tonnes?

Answer: 3000 kg = 3 tonnes

  1. 0.2 × 15,000 = 3000 kg
  2. 1 tonne = 1000 kg, so 3 tonnes
14.A growing tree absorbs about 25 kg of carbon dioxide a year. How many trees would be needed to absorb the car's yearly emissions?

Answer: 120 trees

  1. 3000 ÷ 25 = 120 trees
15.Evaluate two different ways a community could reduce its carbon dioxide emissions. For each, explain how it affects the carbon cycle and give one limitation.[4 marks]

Model answer: 1. Generating electricity from solar and wind instead of coal or gas: this avoids releasing carbon stored in fossil fuels as carbon dioxide. A limitation is that output varies with the weather and time of day, so energy storage or backup is needed. 2. Planting trees and protecting existing forests: trees remove carbon dioxide from the air by photosynthesis and store carbon in their wood. A limitation is that trees take many years to grow, need land, and release their carbon again if they burn or are cut down. Other valid ideas include using public transport, cycling or electric vehicles, improving home insulation and reducing waste.

Success criteria:

  • First method explained, with its effect on the carbon cycle
  • Limitation of the first method
  • Second method explained, with its effect on the carbon cycle
  • Limitation of the second method
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