Earth’s Life Support Systems
The Distribution and Size of the Major Stores in the Carbon and Water Systems
Water and carbon are essential components of Earth’s life support systems. Both exist in a range of stores (or reservoirs) distributed across the atmosphere, oceans, land, and cryosphere. The size of these stores and the rate at which water and carbon move between them determine how the Earth’s systems function and respond to change.
1. The Major Stores in the Global Water System
Water exists in three states – solid (ice), liquid (water), and gas (water vapour) – and moves between stores through the global hydrological cycle.
| Store | Approximate size (as % of total global water) | Key Features |
|---|---|---|
| Oceans | ~96.5% | The largest store of water on Earth. Saline water that regulates global temperature and climate through heat transfer. |
| Cryosphere (ice caps, glaciers, permafrost) | ~1.7% | Stores freshwater as ice, mainly in Antarctica and Greenland. Sensitive to climate change and a key regulator of sea level. |
| Groundwater | ~1.7% | Includes deep groundwater and soil moisture. A vital source of freshwater for ecosystems and human use. |
| Surface water (rivers, lakes) | ~0.013% | Small but dynamic stores that are crucial for human settlement, agriculture, and biodiversity. |
| Atmosphere | ~0.001% | Water vapour and clouds that drive weather systems and transfer latent heat. Despite its small volume, this store is vital for energy balance. |
| Biosphere | Trace amount | Water contained within living organisms; essential for photosynthesis and metabolic processes. |
Key point:
Although the atmosphere and surface water contain only a tiny fraction of global water, they play a disproportionately large role in the hydrological cycle, influencing climate, weather patterns, and ecosystem health.
2. The Major Stores in the Global Carbon System
Carbon is stored in both organic and inorganic forms across four main spheres: the atmosphere, hydrosphere, lithosphere, and biosphere. The majority of Earth’s carbon is locked up in rocks and sediments, with much smaller amounts circulating between other stores.
| Store | Approximate size (gigatonnes of carbon, GtC) | Key Features |
|---|---|---|
| Lithosphere (sedimentary rocks and fossil fuels) | ~100,000,000 GtC | The largest carbon store, held in carbonate rocks such as limestone and fossil fuels (coal, oil, gas). Exchange with the atmosphere occurs over geological timescales. |
| Oceans | ~38,000 GtC | The second largest store. Carbon is stored as dissolved CO₂, bicarbonates, and marine biomass. The ocean acts as both a source and sink of carbon, exchanging with the atmosphere through diffusion. |
| Biosphere | ~2,000 GtC | Includes all living organisms and dead organic matter. Carbon is cycled rapidly through photosynthesis, respiration, and decomposition. |
| Soil and peat | ~1,500 GtC | Organic carbon stored in decomposed plant material. Soils are an important medium-term carbon store, influenced by land use and climate. |
| Atmosphere | ~830 GtC | Present mainly as carbon dioxide (CO₂) and methane (CH₄). This store regulates Earth’s temperature through the greenhouse effect. |
| Cryosphere | ~400 GtC (approx.) | Frozen organic matter trapped in permafrost. Warming temperatures risk releasing this carbon as methane and CO₂. |
Key point:
The lithosphere and oceans dominate carbon storage, but smaller, more dynamic stores — especially the atmosphere and biosphere — are critical in regulating short-term climate and ecosystem processes.
3. Distribution and Global Patterns
1. Latitudinal variation
- Water and carbon stores are unevenly distributed due to differences in climate, vegetation, and geology.
- Tropical regions have larger biological carbon stores due to dense vegetation and rapid carbon cycling.
- Polar regions hold vast cryospheric water and carbon stores locked in ice and permafrost.
2. Oceanic dominance
- The oceans are the largest active store for both water and carbon.
- Warm tropical waters release CO₂, while cooler polar waters absorb it — helping to regulate atmospheric carbon levels.
3. Atmospheric importance
- Although relatively small, the atmosphere is a key transfer store linking the water and carbon systems.
- Changes in atmospheric concentration (e.g. rising CO₂ levels) drive feedbacks that influence both cycles and global climate.
4. Human impact on distribution
- Deforestation, urbanisation, and fossil fuel combustion are shifting the balance between stores, particularly increasing the size of the atmospheric carbon store.
- Groundwater extraction, reservoir construction, and climate change alter the distribution of water stores across continents.
4. Links Between the Water and Carbon Systems
- Photosynthesis: Requires water and CO₂, linking the biosphere’s carbon and water stores.
- Respiration and transpiration: Release CO₂ and water vapour back into the atmosphere.
- Oceans: Exchange CO₂ with the atmosphere and play a major role in both the global carbon and water cycles.
- Cryosphere changes: Melting ice releases trapped carbon and freshwater, influencing global feedback loops.