EASC 1000 - Topic 11

The carbon cycle and its importance

• Carbon is the 4th most abundant element in the universe (but not on Earth) and — to the best of our knowledge — it is essential to life.

• Just like water, carbon is ‘cycled’ through the 4 major subsystems of the Earth System; however, the carbon cycle is more complicated and not entirely understood.

• Atmospheric carbon (as Carbon Dioxide — CO2) is a critical factor in the regulation of the climate of the planet, so the carbon cycle is a subject of much interest to scientists in the context of Global Change (not just climate change)

• Human activity has altered some aspects of carbon cycling, and it is generally accepted that these are affecting global climates by increasing the levels of atmospheric CO2.

Positive and negative feedback mechanisms

In the case of the carbon cycle, the concept of positive a negative feedback from changes in the system becomes important, as does the time scale involved in some of the movement of carbon.

• Negative Feedback - A change in one system results in a change elsewhere that balances the results over time. Example: a domestic heating system in your house, as temperatures fall, the thermostat starts the system, and the house warms up again to the desired temperature.

• Positive Feedback - A change in one system results in a change elsewhere that speeds up the process, over time. Example: ice melting on lakes in spring and summer. As ice begins to melt, it exposes the lake water which absorbs solar radiation for more than the white ice, which reflects it. The warming of the water melts the ice more quickly, which increases the rate of temperature increase, which melts more ice and so on.

The fixed amount of carbon on Earth (not including the small amounts added by meteorite impacts)

800 000 billion tonnes.

Carbon is present is different forms of reservoirs

• Lithosphere - carbon-rich minerals of the mantle, crust, sediments, fossil fuels.

• Biosphere - Cellular structure of plants and animals on land and in the ocean.

• Hydrosphere - Dissolved and particulate carbon of the oceans.

• Atmosphere - Gases (primarily CO2 and CH4) and aerosols (dust particles).

Carbon is exchanged (or transferred) between these various reservoirs by a variety of natural and anthropogenic (human) processes.

Where is most of Earth’s carbon tied up?

Most of Earth’s carbon is tied up in the solid Earth (rocks) but the relatively small amounts of carbon in the biosphere allows life to exist, and carbon in the oceans and atmosphere control important greenhouse gases that affect climate on the planet.

The major fluxes of carbon in the modern earth

• Most significant movements are between the atmosphere, biosphere, and hydrosphere. This is via the process of photosynthesis, driven by solar energy. Respiration and decomposition within the biosphere return broadly equivalent amounts to the atmosphere. Direct exchange is mostly what occurs as well.

• Human activity contribute in both directions, but the use of fossil fuels results in a net input of carbon into the atmosphere. Currently estimated at about 10 billion tonnes per year.

• Balancing this has been vital over earth history— it prevents too much CO2 building up in the atmosphere, or too much being removed to geological storage. But the maintenance of climate for 4 billion years also involves the Lithosphere.

Sedimentary rocks and carbon

Sedimentary rocks of biogenic or chemical origin may contain significant carbon that is removed from the other reservoirs. This removal process is a longer time scale of tens of millions of years.

Contributions to the Carbon cycle from the lithosphere

Volcanism and subduction processes are both part of the Carbon cycle that link the deep earth (lithosphere and asthenosphere) to the atmosphere, hydrosphere, and biosphere at timescales operating over millions of years.

• Some of the carbon dioxide produced above the subduction zones has a ‘recycled’ component that comes from the oceans or from sedimentary rocks.

The fast and the slow carbon cycles

• Fast carbon cycle - links the atmosphere and the oceans and the biosphere, and which regulates climate and other things on short time scales.

• Slow carbon cycle - involves geological processes such as volcanism, sedimentation, and plate subduction, but it operates on a much larger time scale,

The ocean as a sink for accumulating CO2

The world’s oceans also contain a lot of carbon dioxide, its kept largely in solution as bicarbonate ions (HCO3)