What this lesson teaches
I can explain the natural greenhouse effect and how human activity has enhanced it.
Syllabus K229, Topic 2.2, Key Question 2. How do anthropogenic factors contribute to climate change?
Make a guess
A forest is cleared and the trees are burned. Besides the smoke, is there a second way this raises carbon dioxide in the air?
- No. New crops on the land absorb just as much carbon.
- Yes. Fewer trees are left to take carbon dioxide in.
- Yes. The ash left behind keeps giving off carbon dioxide for years.
Show the answer
Yes. Fewer trees are left to take carbon dioxide in.
Trees absorb carbon dioxide as they grow. Clearing them removes a sink, so the air keeps carbon dioxide that the forest would have taken up.
People add greenhouse gases by burning fuel, farming and clearing forests, and this has made recent decades steadily warmer.
More people and more industry use more energy, transport and food. But emissions also depend on lifestyle, technology and energy sources. So people and countries do not add equal amounts.
Burning coal, oil and gas releases carbon dioxide. Power stations, transport and factories are the main sources. Methane comes from gas leaks, cattle, rice fields and rotting waste. Fertilisers release nitrous oxide.
Clearing forests does two things. Burning trees and disturbing soil release stored carbon. And fewer trees are left to take in carbon dioxide. Regrowth can absorb carbon again, but slowly, and it may not replace all that was lost.
The evidence is clear. Each of the four decades up to 2020 was warmer than the one before, in records going back to 1850. From 2011 to 2020, Earth was about 1.1 degrees C warmer than in 1850 to 1900. This is a global average; local changes differ.
- Carbon dioxide
- From burning coal, oil and gas.
- Methane
- From leaks, cattle, rice fields and waste.
- 2011-2020
- About 1.1 degrees C warmer than 1850-1900.
Step by step
Add gases
Fuel, industry, farming and waste release greenhouse gases.
Remove sinks
Cleared forests release carbon and take in less.
Warm the planet
More greenhouse gases strengthen the greenhouse effect.
Real examples to use
Carbon dioxide levels
Before the Industrial Revolution, the air held about 280 parts per million (ppm) of carbon dioxide. It now holds over 420 ppm, mainly from burning coal, oil and gas.
Worked example: A two-part forest answer
Burning cleared trees releases carbon dioxide. The smaller forest then takes in less carbon dioxide as it grows. Both raise carbon dioxide in the air. Counting only the smoke misses the lost sink.
Try it step by step
Problem 1
Carbon dioxide in the air rose from about 280 parts per million before industry to about 420 parts per million in the early 2020s.
Show the working
- Change: Change = 420 - 280 = 140 parts per million
- Percentage change: Percentage change = 140 / 280 x 100 = 50%. Carbon dioxide is half again as high as before industry.
Problem 2
A weather station counted 20 very hot days a year (above 34 degrees C) in the 1980s and 25 a year in the 2010s.
Show the working
- Change: Change = 25 - 20 = 5 days
- Percentage change: Percentage change = 5 / 20 x 100 = 25%. Always divide by the starting value.
Problem 3
The same station's annual rainfall fell from 2,400 mm in the 1980s to 2,160 mm in the 2010s.
Show the working
- Change: Change = 2,160 - 2,400 = -240 mm. A fall gives a negative change.
- Percentage change: Percentage change = -240 / 2,400 x 100 = -10%
Watch out for this
The country with the most people must emit the most per person.
Total and per-person emissions are different. Energy use and lifestyle matter as well as population.
Check your understanding
How does clearing a forest add to warming?
- Stored carbon is released, and fewer trees take it in.
- New fields and crops take in the same carbon as the old forest.
- The land changes colour and so reflects more sunlight.
Show the answer
Stored carbon is released, and fewer trees take it in.
Yes. There are two effects.