Climate Science
Understand how carbon cycles through Earth's systems, what feedback loops amplify or dampen climate change, and examine the multiple independent lines of evidence that demonstrate Earth's climate is warming.
The Carbon Cycle
Carbon is constantly cycled between the atmosphere, oceans, living organisms, and rocks. Human activities have disrupted this natural balance by releasing stored carbon rapidly.
Carbon sinks (absorb CO2)
- • Photosynthesis: Plants and algae absorb CO2 and fix it into organic molecules: 6CO2 + 6H2O → C6H12O6 + 6O2
- • Oceans: Dissolve CO2 from the atmosphere (ocean acidification results)
- • Sediment: Carbon-rich organisms are buried and compressed over millions of years, forming fossil fuels
Carbon sources (release CO2)
- • Respiration: All living things release CO2 as they break down glucose for energy
- • Combustion: Burning fossil fuels and biomass releases stored carbon rapidly
- • Decomposition: Bacteria and fungi break down dead organisms, releasing CO2 and methane
- • Deforestation: Removes carbon sinks and releases stored carbon
The Greenhouse Effect
The greenhouse effect is a natural process that keeps Earth warm enough for life. Greenhouse gases (GHGs) in the atmosphere absorb and re-emit infrared radiation from Earth's surface, trapping heat. Human activities have enhanced this effect.
| Greenhouse Gas | Main Sources | Warming Potential (vs CO2) |
|---|---|---|
| CO2 | Fossil fuels, deforestation | 1 (reference) |
| CH4 (methane) | Livestock, landfill, rice paddies, gas leaks | ~86 (over 20 yrs) |
| N2O (nitrous oxide) | Agriculture (fertilisers), transport | ~273 |
| Water vapour (H2O) | Natural evaporation (amplified by warming) | Variable |
Climate Feedback Loops
A feedback loop occurs when a change in one part of the climate system causes further changes that either amplify (positive feedback) or reduce (negative feedback) the original change.
Positive feedback loops (amplifying)
Ice-albedo feedback:
Warming → ice melts → darker ocean/land exposed → more solar energy absorbed (less reflected) → further warming
Permafrost methane feedback:
Warming → Arctic permafrost thaws → organic matter decomposes → releases CH4 and CO2 → further warming
Water vapour feedback:
Warming → more evaporation → more water vapour (a greenhouse gas) → further warming
Negative feedback loops (stabilising)
Plant growth feedback:
Increased CO2 → plants grow faster → absorb more CO2 → (partially) reduces CO2 levels
Blackbody radiation:
Warming → Earth radiates more infrared energy to space (Stefan-Boltzmann law) → some heat escapes, limiting temperature rise
Evidence for Climate Change
Multiple independent lines of evidence from different scientific disciplines all point to the same conclusion: Earth's climate is warming, primarily due to human emissions of greenhouse gases.
Temperature records
Global average temperature has risen approximately 1.2 °C since the pre-industrial era. Australia has warmed by about 1.5 °C. The last decade (2014–2023) was the hottest on record globally.
Ice core records
Air bubbles trapped in Antarctic ice preserve ancient atmosphere samples up to 800,000 years old. They show CO2 and temperature have tracked together through many ice ages, and that current CO2 levels are unprecedented.
Sea level rise
Global mean sea level has risen about 20 cm since 1900, measured by tide gauges and satellites. This is caused by thermal expansion of seawater and melting of ice sheets and glaciers.
Coral bleaching (Great Barrier Reef)
The Great Barrier Reef has experienced mass bleaching events in 2016, 2017, 2020, 2022, and 2024 due to elevated ocean temperatures. Coral bleaching occurs when corals expel their symbiotic algae due to heat stress.
Key Vocabulary
| Term | Definition |
|---|---|
| Carbon sink | A natural reservoir that absorbs and stores more carbon than it releases (e.g., forests, oceans). |
| Albedo | The reflectivity of a surface. Snow and ice have high albedo (reflect most sunlight); dark ocean has low albedo (absorbs most). |
| Positive feedback | A process in which a change triggers further changes in the same direction, amplifying the original change. |
| Permafrost | Permanently frozen ground found in Arctic and sub-Arctic regions; contains vast stores of organic carbon that can be released as it thaws. |
Worked Examples
Describe the ice-albedo positive feedback loop and explain why it makes climate models complex.
The loop: Warming → sea ice melts → darker ocean surface exposed → ocean absorbs more solar energy (albedo decreases) → ocean warms further → more ice melts.
Why it makes modelling complex: Positive feedbacks are self-reinforcing and non-linear. A small initial warming can trigger large changes. The rate depends on the initial amount of ice, ocean temperature, cloud cover, and seasonal variations. These interactions must all be modelled simultaneously.
Observed evidence: Arctic sea ice extent has decreased by approximately 40% in summer since 1979, and the rate of loss is accelerating — consistent with positive feedback amplification.
Why is deforestation considered both a carbon source and a reduction of a carbon sink?
As a carbon source: When trees are burned or decompose after clearing, the carbon stored in their biomass is released as CO2 and CH4.
As a reduced sink: Once the forest is removed, that land can no longer absorb CO2 through photosynthesis. The photosynthetic capacity is lost permanently (unless regrowth occurs).
Net effect: Global deforestation accounts for approximately 10–15% of annual greenhouse gas emissions. Protecting and expanding forests is a key climate mitigation strategy.
A student argues that climate has always changed naturally, so current change is not a problem. Evaluate this argument using scientific evidence.
Valid point: Climate has indeed changed naturally throughout Earth's history due to orbital cycles, volcanic activity, and solar output variations.
However: 1. The current rate of warming (~0.2 °C per decade) is approximately 10 times faster than the fastest natural warming events in the past 800,000 years.
2. Isotopic analysis of atmospheric CO2 confirms the excess carbon is from fossil fuel burning (has the distinctive carbon-13/carbon-12 signature of ancient organic matter).
3. Natural forcings alone (solar output, volcanic activity) cannot account for observed warming; only models that include human emissions match actual temperature records.
Conclusion: While natural climate change exists, the current warming is unprecedented in rate and is attributable to human activity.
Knowledge Check
Select the correct answer for each question.
Question 1
Which process removes CO2 from the atmosphere and is therefore a carbon sink?
Question 2
The ice-albedo feedback is an example of a:
Question 3
Which greenhouse gas has a much higher global warming potential than CO2 over a 20-year period?
Question 4
What do Antarctic ice cores reveal about past climate?
Question 5
Thawing permafrost releasing methane and CO2 is an example of:
Key Concepts Summary
- •The carbon cycle involves natural exchanges between atmosphere, oceans, biosphere, and lithosphere; human activities have disrupted this balance by rapidly releasing stored carbon.
- •Greenhouse gases (CO2, CH4, N2O, H2O) trap heat in the atmosphere; human activities have increased their concentrations.
- •Positive feedbacks (ice-albedo, permafrost CH4, water vapour) amplify warming; negative feedbacks dampen it.
- •Multiple independent lines of evidence confirm current climate change: temperature records, ice cores, sea level rise, coral bleaching, and satellite data.
- •Australia is particularly vulnerable: the Great Barrier Reef, increased bushfire intensity, and changing rainfall patterns are documented impacts.