The carbon cycle
Carbon is the backbone of life, and it constantly moves between Earth’s systems in the carbon cycle.
Carbon reservoirs (approximate)
| Reservoir | Carbon stored (billion tonnes) |
|---|---|
| Rocks (limestone, sediments) | Tens of millions |
| Deep ocean | About 37 000 |
| Fossil fuels (coal, oil, gas) | Several thousand |
| Soils and permafrost | About 3000 or more |
| Atmosphere | About 875 and rising |
| Land plants | About 450–650 |
Carbon fluxes
| Process | Direction |
|---|---|
| Photosynthesis | Atmosphere → plants |
| Respiration and decay | Living things → atmosphere |
| Ocean absorption and release | Atmosphere ↔ ocean |
| Volcanic eruptions | Rock → atmosphere (small, about 0.3–0.4 billion tonnes CO₂/year) |
| Burning fossil fuels | Fossil carbon → atmosphere (about 37 billion tonnes CO₂/year) |
| Deforestation | Plants and soil → atmosphere |
| Rock weathering and sedimentation | Atmosphere → rock (very slow) |
Before industrialization, these flows were nearly balanced. Humans now release far more carbon than natural carbon sinks (forests, oceans and soils) can absorb. Sinks absorb roughly half of human emissions; the rest accumulates in the atmosphere.
The greenhouse effect
Greenhouse gases let incoming sunlight pass but absorb outgoing infrared radiation and re-emit some back toward Earth’s surface.
| Gas | Main human sources | Warming power per molecule vs. CO₂ (over 100 years) |
|---|---|---|
| Carbon dioxide (CO₂) | Fossil fuels, deforestation, cement | 1 |
| Methane (CH₄) | Livestock, landfills, oil and gas leaks, rice paddies | About 28–30 |
| Nitrous oxide (N₂O) | Fertilizers, industry | About 265–273 |
| Fluorinated gases | Refrigerants, industry | Hundreds to thousands |
Water vapour is the most abundant greenhouse gas, but it acts mainly as a feedback: warmer air holds more water vapour.
The evidence for climate change
| Evidence | Observation |
|---|---|
| Atmospheric CO₂ | Measured at Mauna Loa since 1958: from about 315 ppm to over 420 ppm today; ice cores show today’s level is the highest in at least 800 000 years |
| Global temperature | About 1.2–1.3 °C warmer than the late 1800s; the past decade was the warmest on record, with 2024 the warmest year |
| Isotopes | Carbon added to the air has the chemical fingerprint of fossil fuels |
| Sea level | Risen about 20 cm since 1900, and the rate is speeding up |
| Ice | Arctic summer sea ice has shrunk dramatically; Greenland and Antarctic ice sheets are losing mass |
| Oceans | Absorbing over 90% of excess heat; becoming more acidic |
| Ecosystems | Earlier springs, species shifting toward the poles |
The Intergovernmental Panel on Climate Change (IPCC) concluded in 2021 that it is "unequivocal" that human influence has warmed the atmosphere, ocean and land.
Natural vs. human causes
Earth’s climate has changed before, driven by variations in Earth’s orbit (Milankovitch cycles), solar output, volcanic eruptions and plate tectonics. But these natural factors can’t explain today’s rapid warming. Solar output has been flat or slightly declining since the 1980s while temperatures have risen.
Feedback loops
A feedback loop can amplify or dampen change.
| Feedback | Type | How it works |
|---|---|---|
| Ice–albedo | Positive (amplifying) | Melting ice exposes darker water and land, which absorb more heat, causing more melting |
| Permafrost thaw | Positive | Thawing soils release CO₂ and methane |
| Water vapour | Positive | Warmer air holds more water vapour, a greenhouse gas |
| Plant growth (CO₂ fertilization) | Negative (dampening) | More CO₂ can increase plant growth, absorbing some carbon |
Impacts in Canada
- Canada has warmed about twice the global average rate; the North about three times.
- Longer and more severe wildfire seasons; 2023 was Canada’s worst wildfire season on record.
- Melting permafrost damages northern roads and buildings.
- Coastal flooding and erosion threaten communities in the Maritimes and the North.
- Changes to fisheries, agriculture and Indigenous ways of life on the land and ice.
Responses
Mitigation reduces emissions:
- switching to renewable energy (solar, wind, hydro) and nuclear power
- electrifying transportation and heating (electric vehicles, heat pumps)
- improving energy efficiency
- protecting and restoring forests and wetlands
- carbon pricing and capture technologies
Adaptation prepares for changes already underway: flood barriers, heat-wave plans, wildfire management and drought-resistant crops.
The 2015 Paris Agreement set a goal of keeping warming well below 2 °C and pursuing efforts to limit it to 1.5 °C. Canada has committed to net-zero emissions by 2050.
Common mistakes
- Confusing weather and climate.
- Thinking natural climate change in the past means current change is natural. The cause today is mainly human activity.
- Confusing the ozone hole with global warming.
Try it yourself
- Find the Mauna Loa CO₂ graph (the Keeling Curve). Why does it zigzag each year?
- Calculate your household’s approximate carbon footprint with an online calculator. Identify two ways to reduce it.
- Explain the ice–albedo feedback using a diagram.
(Hint for 1: Northern Hemisphere plants absorb CO₂ in summer and release it in winter.)
Quick recap
- Carbon moves among the atmosphere, oceans, life, soils and rocks.
- Greenhouse gases trap heat; humans have increased them sharply.
- Multiple independent lines of evidence show recent warming is human-caused.
- Feedback loops can amplify warming.
- Mitigation reduces emissions; adaptation prepares for impacts.