Understanding the Carbon Cycle

26 June 2009

Documents & Texts from America.gov

By Daniel A. Gorelick
Staff Writer

Washington — Carbon: the fourth most abundant element in the universe and the building block of life on Earth.

Carbon moves throughout the Earth — between the atmosphere, the oceans, sedimentary rock, soil and plants and animals — in what scientists call the carbon cycle.

Understanding how the carbon cycle works is critical to predicting Earth’s climate in the future.

“To predict the behavior of Earth’s climate system in the future, we must be able to understand the functioning of the carbon system and predict the evolution of atmospheric carbon dioxide,” wrote scientists Jorge Sarmiento, Steve Wofsy and their colleagues in the 1999 report “A U.S. Carbon Cycle Science Plan.”

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.

Global Carbon Budget

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.

Christine Goodale, a forest ecologist at Cornell University in New York state, characterizes it as “taking carbon that was locked away and putting it into a much more active form in the atmosphere.”

Humans are also destroying forests, releasing more carbon dioxide into the atmosphere and reducing the number of plants that absorb it from the atmosphere.

The atmosphere is overflowing with carbon, largely carbon dioxide. Some is absorbed by the oceans, some is absorbed by plants and soil, though how this happens is not well understood.

The carbon that remains in the atmosphere absorbs heat, preventing it from radiating out into space. Without this trapped heat the Earth would not be habitable. Too much heat, and the climate will change and become less habitable. The same applies to the oceans, where increased carbon changes the chemistry of sea water, making the oceans less habitable and killing sea life.

Atmospheric carbon is both good and bad, much like water: Humans need it to survive, but too much and you will drown.

According to a 2007 report by the United Nations Intergovernmental Panel on Climate Change, “about 50 percent of a carbon dioxide increase will be removed from the atmosphere within 30 years, and a further 30 percent will be removed within a few centuries. The remaining 20 percent may stay in the atmosphere for many thousands of years.”

The Earth and its atmosphere are a closed system, where carbon is neither created nor destroyed. The total amount of carbon does not change — carbon can be shuffled from pool to pool, from atmosphere to ocean, from soil to sediment, but cannot be added or removed. Carbon in the atmosphere, for example, cannot drift off into outer space. It has to go somewhere on Earth: taken up by plants, or dissolved back into oceans.