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Understanding the Carbon Cycle
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Understanding the Carbon Cycle

In its pure form carbon exists as diamond or graphite, the lead in pencils. Bound to oxygen, hydrogen and other carbon atoms, carbon forms essential compounds: sugars and fats that provide energy for plants and animals; petroleum, coal and natural gas that power human activity; and carbon dioxide and methane, atmospheric gases that trap heat from the sun and warm the Earth.

Plants, algae and some bacteria take up carbon dioxide from the atmosphere or oceans and convert it into sugars (carbon bound to other carbon, hydrogen and oxygen atoms), a process called photosynthesis. Animals eat sugar, a source of energy, and exhale carbon dioxide (carbon bound to two oxygen atoms) — respiration.

Animals and plants die and are buried in the earth, but their carbon compounds remain intact, a source of energy for microbes that feast on their remains and produce carbon dioxide and methane (carbon bound to four hydrogen atoms), some of which remains in the soil, some of which is released into the atmosphere.

Sometimes, plant and animal remains are buried in the earth or sink to the ocean floor and are protected from microbes. Over hundreds of millions of years animal remains are compressed deeper and deeper into the earth. Tissue and bone are destroyed but the carbon still remains, having formed compounds called hydrocarbons, long chains of carbon atoms bound to each other and to hydrogen atoms. Hydrocarbons are the main component of coal and petroleum — fossil fuels.

Humans use fossil fuels to produce heat and electricity, and in doing so the hydrocarbons in fossil fuels are converted into carbon dioxide and released into the atmosphere. Atmospheric carbon dissolves into the oceans or is taken up by plants and the cycle continues.

Rock in the Earth’s crust is composed of carbon, formed over millions of years when carbon binds to minerals. Carbon dioxide dissolved in the ocean forms bicarbonate, which combines with calcium to form limestone.

Weathering and erosion wash carbon compounds from rock in the Earth’s crust into the ocean. Carbon is also pulled beneath Earth’s crust — a process called subduction — and volcanoes, hot springs and geysers spew carbon dioxide and methane back into the atmosphere.

The geological components of the carbon cycle — weathering, erosion, subduction, the formation of fossil fuels — occur over millions of years. The biological components of the carbon cycle — photosynthesis, respiration, decomposition by microbes — occur over days to thousands of years.

On average, the amount of carbon that moves through biological components each year is 1,000 times greater than the amount of carbon that moves through geological components each year.

The problem now is that the carbon cycle is lopsided. It took hundreds of millions of years to sequester carbon deep in the earth and under the ocean floor, and humans have released much of that carbon into the atmosphere during the last century.

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