Water and Carbon Cycles
The Carbon Budget
Introduction
The carbon cycle regulates the movement of carbon between the atmosphere, oceans, biosphere, and lithosphere. The balance between inputs and outputs at a global scale is known as the carbon budget. If inputs and outputs are equal, the system is in equilibrium. If outputs exceed inputs (or vice versa), stores will increase or decrease, affecting the global climate, ecosystems, ocean chemistry, and weather patterns. Human activity is now altering this balance by rapidly transferring carbon from long-term geological stores into the atmosphere.
1. What is the Carbon Budget?
The carbon budget is the difference between the amount of carbon emitted into the atmosphere and the amount absorbed by sinks, such as vegetation, soils, and oceans.
Major sources (inputs to the atmosphere):
- Combustion of fossil fuels and biomass
- Cement production
- Wildfires
- Volcanic outgassing
- Respiration and decomposition
Major sinks (removal from the atmosphere):
- Photosynthesis and vegetation growth
- Carbon sequestration by oceans
- Soil formation and burial
- Formation of carbonate rocks and sediments
When sinks remove the same amount of carbon as sources emit, the budget is balanced. When sources exceed sinks, atmospheric CO₂ levels rise, which is the current situation.
Roughly 1/4 of human CO₂ emissions are absorbed by the oceans each year, while vegetation and soils absorb a slightly smaller proportion. The rest accumulates in the atmosphere, increasing the greenhouse effect.
2. Impact of the Carbon Cycle on the Land
Land acts as a major carbon store in vegetation and soils. Changes in the carbon budget have direct consequences for terrestrial ecosystems.
a) Vegetation and ecosystems
- Forests store vast amounts of carbon in biomass.
- If warming increases decomposition and respiration, more CO₂ is released back into the atmosphere.
- Deforestation reduces photosynthesis, lowering carbon uptake.
b) Soil health
- Soils contain more carbon globally than vegetation and the atmosphere combined.
- Warming increases microbial activity → faster decomposition → soil carbon loss.
- In cold or waterlogged areas (e.g. peatlands, permafrost), carbon can be trapped for thousands of years.
If permafrost thaws, methane and CO₂ are released, which is an example of a positive feedback loop.
Large areas of Siberian permafrost are thawing, turning once-frozen soils into sources of methane. This shifts parts of the Arctic from a carbon sink to a carbon source.
c) Desertification and drought
Changes to rainfall and temperature reduce vegetation cover and soil moisture, limiting photosynthesis and soil carbon storage.
3. Impact of the Carbon Cycle on the Oceans
The oceans are the largest active store of carbon. They absorb approximately 25% of human CO₂ emissions each year.
a) Carbon absorption
Carbon enters oceans through surface diffusion, photosynthesis by phytoplankton and dissolved ions from weathering.
b) Ocean acidification
Excess CO₂ dissolves in seawater, forming carbonic acid. This lowers pH and leads to:
- Weaker carbonate shells
- Coral bleaching
- Reduced biodiversity
- Disruption of marine food chains
Coral reef bleaching during marine heatwaves in the Indian and Pacific Oceans has reduced biodiversity and damaged carbonate-producing organisms that help lock away carbon in sediments.
c) Deep ocean sequestration
Sinking organic matter transfers carbon into the deep ocean for centuries. Some carbon becomes trapped in sediments and eventually forms carbonate rocks.
d) Warming oceans
Warm water holds less dissolved CO₂, so absorption decreases as oceans heat. This creates a positive feedback loop, adding more CO₂ to the atmosphere.
4. Impact of the Carbon Cycle on the Atmosphere
The atmosphere is the fastest-changing carbon store and the main driver of recent climate change.
a) Greenhouse effect
- CO₂, methane and other greenhouse gases trap heat.
- More greenhouse gases → more trapped heat → higher temperatures.
b) Rising CO₂ levels
- Atmospheric CO₂ has increased from ~280 ppm in the 1750s to over 410 ppm today, strengthening the greenhouse effect and warming the planet.
c) Feedback loops
- Thawing permafrost releases carbon
- Forest dieback reduces photosynthesis
- Wildfires release stored carbon
- Warmer oceans absorb less CO₂
Impact Summary Table
| Carbon Store | Change | Consequence |
|---|---|---|
| Oceans | More dissolved CO₂ → acidification | Coral bleaching, weaker shells, disrupted food chains |
| Land | Permafrost thaw | Methane release → stronger greenhouse effect |
| Vegetation | Deforestation | Lower photosynthesis → more CO₂ in atmosphere |
| Atmosphere | Rising CO₂ | Enhanced greenhouse effect → global warming |
5. Impact on Global Climate
A changing carbon budget affects climate systems worldwide:
Temperature: The enhanced greenhouse effect contributes to increased global temperatures.
Precipitation: More evaporation changes rainfall patterns.
Extreme weather: Heatwaves, storms, and droughts are becoming more frequent.
Cryosphere: Melting ice reduces albedo, leading to further warming.
Ocean circulation: Freshwater from melting ice may weaken thermohaline circulation.
Short exam-style explanation:
Increased CO₂ strengthens the greenhouse effect by trapping more outgoing longwave radiation. Higher temperatures warm the oceans, causing them to absorb less CO₂. This further increases atmospheric concentrations, forming a positive feedback loop that accelerates climate change.
Exam Tip
When answering A Level questions on systems:
- Link processes to changes in store size
- Use terms such as feedback loop, sequestration, ocean acidification, and enhanced greenhouse effect
- Include numbers: “Oceans absorb about 25% of human CO₂ emissions”
- Demonstrate systems thinking: land, ocean, atmosphere are interconnected