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
Natural carbon flows between air, land and ocean are far larger than human emissions. Why does carbon in the air still keep rising?
- Because forests have become carbon sources.
- Natural flows roughly balance; human emissions tip it.
- Because natural flows are getting much bigger.
- It is not rising, because nature absorbs it.
Show the answer
Natural flows roughly balance; human emissions tip it.
Natural sources and sinks nearly cancel out. Human emissions are an extra net addition, and land and oceans absorb only part of it.
Burning fossil fuels and weakening carbon sinks add carbon to the air faster than land and oceans remove it.
The global carbon cycle moves carbon between the air, oceans, living things, soils and rocks. A store is carbon held in one place. A flux is a transfer over time. A sink takes in more carbon than it releases. A source releases more than it takes in. A large store is not automatically a sink today.
Plants take carbon from the air through photosynthesis. Breathing, rotting and fire return it. Oceans absorb carbon dioxide and release some back. Sea creatures pass carbon through food chains, and some is buried in mud. These exchanges work over very different timescales.
Burning fossil fuels quickly moves carbon from rocks deep underground into the air. Making cement also releases carbon dioxide. Cattle, rice fields, waste and gas leaks add methane. Fertilisers add nitrous oxide. Cement releases carbon dioxide both from the fuel used and from the chemical breakdown of limestone.
Cutting down forests releases carbon and leaves fewer trees to absorb it. Draining peat lets it rot, releasing more carbon. So land use can add sources and weaken sinks at the same time.
Land and oceans absorb part of human emissions, but not all. So the carbon in the air keeps rising. Natural exchanges are much larger than human emissions but stay roughly balanced, so comparing gross natural flows with net human emissions is misleading.
- Store vs flux
- Carbon held vs carbon moving.
- Sink vs source
- Net uptake vs net release.
- Net balance
- Air carbon rises while inputs exceed removals.
Step by step
Find the store
Fossil fuel, trees, soil, ocean or air.
Change a transfer
Burning, rotting or reduced uptake.
Check the balance
Compare what is added with what is removed.
Real examples to use
Carbon dioxide levels
Carbon dioxide in the air has risen from about 280 ppm before the Industrial Revolution to over 420 ppm today.
Worked example: A simple carbon budget
In a made-up system, natural inputs and removals are each 100 units a year. People add 6 units, and extra uptake removes 3. So the air gains 3 units a year, even though natural flows are much bigger.
Watch out for this
Forests are always carbon sinks because they hold a lot of carbon.
A forest is a store. Whether it is a sink or a source depends on growth, rotting and fires over time.
Check your understanding
A drained peatland releases more carbon while plant uptake falls. What changes?
- A source grows and a sink weakens, so air carbon rises.
- The air's carbon must fall because peat holds carbon.
- The land darkens, so it reflects less sunlight.
Show the answer
A source grows and a sink weakens, so air carbon rises.
Yes. Both changes add to atmospheric carbon.