The Carbon and Water Cycles

The carbon cycle is the movement of carbon between the atmosphere, living things, oceans, soil and rocks, and the water cycle (hydrologic cycle) is the continuous movement of water between the oceans, atmosphere and land through evaporation, condensation, precipitation and runoff. In brief: photosynthesis removes CO₂ from the air, and respiration, decomposition and combustion return it. In the water cycle, solar energy drives evaporation, water vapor condenses into clouds and falls as precipitation. Human activities, mainly the burning of fossil fuels, have raised the atmospheric CO₂ concentration from about 280 parts per million before the Industrial Revolution to over 420 ppm today.

Matter is recycled in ecosystems, unlike energy, which flows through them. This guide describes the stores and processes of the carbon cycle and the water cycle, shows how humans have changed them, compares the stores in a table, works through a calculation of carbon in the atmosphere and the residence time of water, and corrects common mistakes.

What are biogeochemical cycles?

A biogeochemical cycle describes the path of a chemical element or compound through the living (bio) and non-living (geo) parts of the environment. Each cycle has reservoirs (stores or pools), where the element is held for a time, and fluxes, the processes that move it between reservoirs. The time that an atom stays in a reservoir is the residence time: short for carbon in the atmosphere (a few years), long for carbon in limestone (millions of years).

How does the carbon cycle work?

Carbon is the basis of all organic molecules. The main reservoirs are the atmosphere (as CO₂ and methane), the oceans (dissolved CO₂, hydrogencarbonate ions and marine organisms), land plants and soils, fossil fuels, and sedimentary rocks (limestone and shale), which hold by far the most carbon. The main processes are:

• Photosynthesis: plants, algae and cyanobacteria take up CO₂ and fix it into organic compounds. About 120 billion tonnes of carbon per year on land and a similar amount in the oceans.
• Respiration: plants, animals and decomposers release CO₂.
• Feeding: carbon passes along food chains as organic compounds.
• Decomposition: decomposers break down dead matter, which returns CO₂ to the air and soil. When decomposition is slow, as in waterlogged conditions, peat forms.
• Combustion: burning of wood, wildfires and fossil fuels releases CO₂.
• Ocean exchange: CO₂ dissolves in surface water, and is released again, depending on temperature (cold water takes up more). Marine organisms use dissolved carbonate to build shells of calcium carbonate, and when they die, shells sink and form sediments that can become limestone.
• Fossilization: over millions of years, buried organic matter becomes coal, oil and natural gas.

How have humans changed the carbon cycle?

The combustion of fossil fuels and cement production release about 10 billion tonnes of carbon (roughly 37 billion tonnes of CO₂) per year at present, and deforestation and other changes in land use add more. Roughly half of the CO₂ emitted stays in the atmosphere, and the remainder is taken up by oceans and land vegetation. The ocean uptake makes the seawater more acidic (ocean acidification), as dissolved CO₂ forms carbonic acid. The increase of CO₂ and other greenhouse gases enhances the greenhouse effect and causes global warming, as explained in the articles on climate. Reducing emissions, protecting and restoring forests and peatlands, and developing carbon capture are ways of changing the balance.

How does the water cycle work?

The Earth has about 1.4 billion cubic kilometers of water, of which 97 percent is in the oceans, about 2 percent is frozen in ice caps and glaciers, and less than 1 percent is fresh liquid water in groundwater, lakes and rivers. The processes of the cycle are driven by solar energy and gravity:

• Evaporation: liquid water changes into vapor from the oceans, lakes and soil. Most of the evaporation (about 86 percent) occurs over the oceans.
• Transpiration: water vapor released by plants. Together with evaporation from land, it is called evapotranspiration.
• Sublimation: ice changes directly into vapor.
• Condensation: as moist air rises and cools, water vapor condenses into droplets, forming clouds.
• Precipitation: rain, snow, sleet and hail. Most of the precipitation (about 78 percent) falls over the oceans.
• Surface runoff: water flowing over the land into streams and rivers and eventually the sea.
• Infiltration and percolation: water soaks into soil and moves down to groundwater. It flows slowly to rivers and oceans.

The atmosphere holds only about 0.001 percent of the water, but the average residence time of a water molecule there is only about 9 days, so water is moved quickly.

Stores and fluxes of carbon and water

Gt C means gigatonnes (billions of tonnes) of carbon. The values are approximate, and different sources give somewhat different numbers.

How have humans changed the water cycle?

• Land use: deforestation reduces transpiration and rainfall in some regions, and increases runoff. Urban areas with paved surfaces cut infiltration and cause flash floods.
• Extraction: irrigation uses about 70 percent of freshwater withdrawals. Groundwater is pumped faster than it recharges in places such as the Ogallala aquifer in the United States and in northern India.
• Dams and reservoirs: store water but increase evaporation.
• Climate change: a warmer atmosphere holds more water vapor (roughly 7 percent more per degree Celsius), which makes rainfall more intense in many regions, and shifts patterns, with more droughts in others. Glaciers are melting, which raises sea levels.
• Pollution: contaminates water supplies.

Worked example: CO₂ in the air and residence time of water

Carbon dioxide

The mass of the atmosphere is about 5.15 × 10¹⁸ kg. A concentration of 420 ppm by volume of CO₂ corresponds, by mass, to roughly 420 × (44 ÷ 29) ppm = 637 ppm, because CO₂ (44 g/mol) is heavier than the average air molecule (29 g/mol).

1. Mass of CO₂ in the air: 5.15 × 10¹⁸ kg × 637 × 10⁻⁶ = 3.28 × 10¹⁵ kg = 3.28 × 10¹² tonnes, i.e. 3 280 Gt of CO₂.
2. As carbon: carbon is 12/44 of the mass of CO₂, so 3 280 × 12 ÷ 44 ≈ 895 Gt of carbon. This is consistent with the value of about 870 to 900 Gt C in the table (the difference arises from the CO₂ concentration used).
3. Emissions: if humans emit about 10 Gt C per year and about half stays in the air, the atmosphere gains roughly 5 Gt C per year, i.e. 5 ÷ 895 ≈ 0.56 percent per year. For 420 ppm, that is about 2.3 ppm per year, which is close to the observed recent increase of roughly 2 to 2.5 ppm per year.

Residence time of water

Residence time = amount in reservoir ÷ flux through it. The atmosphere holds about 12 900 km³ of water, and the total precipitation (equal to the evaporation) is about 505 000 km³ per year.

1. 12 900 ÷ 505 000 = 0.0255 years.
2. 0.0255 × 365 = 9.3 days.

Water in the oceans (about 1 335 000 000 km³) with an inflow from rivers and rain of about 505 000 km³ per year gives 1 335 000 000 ÷ 505 000 ≈ 2 640 years, which is close to the value of 3 000 years in the table.

How were the cycles discovered?

Jan van Helmont’s willow experiment (1640s), the work of Joseph Priestley and Antoine Lavoisier in the 1770s and 1780s on gases, and Jean-Baptiste Boussingault’s studies of plants in the nineteenth century established how plants use carbon from air. Svante Arrhenius calculated in 1896 that CO₂ affects temperature. Charles David Keeling began measuring atmospheric CO₂ at Mauna Loa, Hawaii, in 1958, and the “Keeling curve” shows a continuous rise with an annual cycle caused by the seasons. Pierre Perrault measured rainfall and river flow in the seventeenth century, and showed that rainfall is enough to supply rivers.

Which mistakes are common?

Mistake 1: Saying that photosynthesis alone controls CO₂ in the air

Respiration, decomposition, ocean exchange, volcanoes and fossil fuel combustion also have large effects.

Mistake 2: Confusing fossil fuel carbon with the short-term cycle

Fossil fuels are carbon stored for millions of years. Burning them moves carbon quickly into the atmosphere.

Mistake 3: Thinking that water is used up

Water is recycled. The problem is its distribution and quality, not the amount in the planet.

Mistake 4: Mixing up evaporation and transpiration

Evaporation is from surfaces of water or soil. Transpiration is from plants.