What this lesson teaches
I can weigh human influence through the carbon cycle and greenhouse effect against natural causes.
Syllabus 8834, Topic 1.2, 1. The Science of Climate Change. How significant is the influence of human activities on Earth's climate compared to natural factors?
Make a guess
Scientists want to know whether recent warming is mainly human-caused. Which test is strongest?
- Measure how bright the Sun is today.
- Record the temperature on the hottest day this year.
- Compare models with and without human causes.
- Point out that climate changed naturally before.
Show the answer
Compare models with and without human causes.
Attribution compares observed warming with models run on natural causes alone and on natural plus human causes. Only the second matches what happened.
The physics of greenhouse gases, observations and attribution studies all show that humans are the main cause of today's warming.
Earth receives energy mainly as shortwave sunlight. The surface warms and gives off longwave infrared heat. Greenhouse gases absorb and re-emit some of this heat. Naturally, this keeps Earth warmer than it would otherwise be.
Adding more long-lived greenhouse gases means less heat escapes to space at first. So the surface and lower air warm until the energy flows balance again. This is the enhanced greenhouse effect. It is about outgoing heat, not a layer reflecting sunlight.
Contemporary climate change means a lasting recent change, not a single weather event. Attribution asks what caused it. Scientists combine measured gas increases, physics, warming patterns and models. Models compare natural causes alone with natural plus human causes. One hot day proves nothing.
The IPCC, the UN's climate science panel, brings this evidence together. Its 2023 report concludes humans caused global warming. It found about 1.1 degrees C of warming in 2011-2020, compared with 1850-1900, which is used as a rough pre-industrial baseline.
Natural factors, like the Sun and volcanoes, affect single years. But they cannot explain the steady warming. Pollution particles have cooled climate a little, but humans still cause net warming.
- Enhanced greenhouse effect
- More gases, less heat escaping, more warming.
- Attribution
- Testing which causes explain a change.
- IPCC 2023
- About 1.1 degrees C warmer in 2011-2020 than 1850-1900.
Step by step
Find a physical cause
More greenhouse gas reduces heat escaping.
Compare evidence
Observations, patterns and model tests.
Test alternatives
Can natural causes alone explain it?
Real examples to use
IPCC, 2021
The Intergovernmental Panel on Climate Change stated that it is "unequivocal" that human influence has warmed the atmosphere, ocean and land.
Worked example: A fair attribution test
A short warm spell could be weather. But decades of warming over land and sea, plus rising greenhouse gases and their known effects, is much stronger evidence. Attribution needs several lines of evidence to agree.
Watch out for this
Past natural warming proves today's warming is natural.
Each period's cause must be tested. Different causes can warm the planet at different times.
Check your understanding
Which comparison best shows the cause of warming, not just that it happened?
- Showing one city had a hot afternoon.
- Testing observed patterns against models with and without human causes.
- Comparing temperatures before and after industry without testing other causes.
Show the answer
Testing observed patterns against models with and without human causes.
Yes. This tests which causes can explain the pattern.