Sign In | Sign Up | Help | Invite    
Advanced Search Ask A Question Community Recent Changes
My:             Contributions   
Contributors
{for ul in pageInfo.page}
${ul.nickName}
{var al = toBreakWord(ul.adUrl,18)} {if ul.adSentence !=''}${ul.adSentence}
{/if} {if ul.adUrl !=''}${al}
{/if}
 
{/for}
Earn Free Advertising   +   Earn Money By Writing What You Know at WISTEME.COM
Question Discussion History

Edit
    Question ID:   10868         Current Version: 1
Question: What is the carbon cycle?
Category: Science > Chemistry
Keywords: carbon cycle
Type: what
Rating:(0 ratings)    Views: 536    Discussions: 0   In Watch Lists: 1  

 
    Answer:

Carbon (C), the fourth most abundant element in the Universe, after hydrogen (H), helium (He), and oxygen (O), is the building block of life. It's the element that anchors all organic substances, from fossil fuels to DNA. On Earth, carbon cycles through the land, ocean, atmosphere, and the Earth's interior in a major biogeochemical cycle (the circulation of chemical components through the biosphere from or to the lithosphere, atmosphere, and hydrosphere). The global carbon cycle can be divided into two categories: the geological, which operates over large time scales (millions of years), and the biological/physical, which operates at shorter time scales (days to thousands of years).

Geological Carbon Cycle

Billions of years ago, as planetesimals (small bodies that formed from the solar nebula) and carbon-containing meteorites bombarded our planet's surface, the carbon content of the solid Earth steadily increased. 

Since those times, carbonic acid (a weak acid derived from the reaction between atmospheric carbon dioxide [CO2] and water) has slowly but continuously combined with calcium and magnesium in the Earth’s crust to form insoluble carbonates (carbon-containing chemical compounds) through a process called weathering. Then, through the process of erosion, the carbonates are washed into the ocean and eventually settle to the bottom. The cycle continues as these materials are drawn into Earth’s mantle by subduction (a process in which one lithospheric plate descends beneath another, often as a result of folding or faulting) at the edges of continental plates. The carbon is then returned to the atmosphere as carbon dioxide during volcanic eruptions.

The balance between weathering, subduction, and volcanism controls atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years. The oldest geologic sediments suggest that, before life evolved, the concentration of atmospheric carbon dioxide may have been one-hundred times that of the present, providing a substantial greenhouse effect during a time of low solar output. On the other hand, ice core samples taken in Antarctica and Greenland have led scientists to hypothesize that carbon dioxide concentrations during the last ice age (20,000 years ago) were only half of what they are today. 

Biological/Physical Carbon Cycle: Photosynthesis and Respiration 

Biology also plays an important role in the movement of carbon in and out of the land and ocean through the processes of photosynthesis and respiration. Nearly all forms of life on Earth depend on the production of sugars from solar energy and carbon dioxide (photosynthesis) and the metabolism (respiration) of those sugars to produce the chemical energy that facilitates growth and reproduction. 

Through the process of photosynthesis, green plants absorb solar energy and remove carbon dioxide from the atmosphere to produce carbohydrates (sugars). Plants and animals effectively “burn” these carbohydrates (and other products derived from them) through the process of respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism and renders the carbohydrate“fuel” back to carbon dioxide. Together, respiration and decomposition (respiration that consumes organic matter mostly by bacteria and fungi) return the biologically fixed carbon back to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis.

Photosynthesis and respiration also play an important role in the long-term geological cycling of carbon. The presence of land vegetation enhances the weathering of soil, leading to the long-term—but slow—uptake of carbon dioxide from the atmosphere. In the oceans, some of the carbon taken up by phytoplankton (microscopic marine plants that form the basis of the marine food chain) to make shells of calcium carbonate (CaCO3) settles to the bottom (after they die) to form sediments. During times when photosynthesis exceeded respiration, organic matter slowly built up over millions of years to form coal and oil deposits. All of these biologically mediated processes represent a removal of carbon dioxide from the atmosphere and storage of carbon in geologic sediments.

Carbon on the Land and in the Oceans: The modern carbon cycle

On land, the major exchange of carbon with the atmosphere results from photosynthesis and respiration. During the daytime in the growing season, leaves absorb sunlight and take up carbon dioxide from the atmosphere. In parallel, plants, animals and soil microbes consume the carbon in organic matter and return carbon dioxide to the atmosphere. When conditions are too cold or too dry, photosynthesis and respiration cease along with the movement of carbon between the atmosphere and the land surface. The amounts of carbon that move from the atmosphere through photosynthesis, respiration, and back to the atmosphere are large and produce oscillations in atmospheric carbon dioxide concentrations. Over the course of a year, these biological fluxes of carbon are over ten times greater than the amount of carbon introduced to the atmosphere by fossil fuel burning.

Carbon Dioxide in the atmosphere has been steadily rising since regular measurements began in 1958.

Fire also plays an important role in the transfer of carbon dioxide from the land to the atmosphere. Fires consume biomass and organic matter to produce carbon dioxide (along with methane, carbon monoxide, smoke), and the vegetation that is killed but not consumed by the fire decomposes over time adding further carbon dioxide to the atmosphere.

Over periods of years to decades, significant amounts of carbon can be stored or released on land. For example, when forests are cleared for agriculture the carbon contained in the living material and soil is released, causing atmospheric carbon dioxide concentrations to increase. When agricultural land is abandoned and forests are allowed to re-grow, carbon is stored in the accumulating living biomass and soils causing atmospheric carbon dioxide concentrations to decrease.

In the oceans, carbon dioxide exchange is largely controlled by sea surface temperatures, circulating currents, and by the biological processes of photosynthesis and respiration. carbon dioxide can dissolve easily into the ocean and the amount of carbon dioxide that the ocean can hold depends on ocean temperature and the amount of carbon dioxide already present. Cold ocean temperatures favor the uptake of carbon dioxide from the atmosphere whereas warm temperatures can cause the ocean surface to release carbon dioxide. Cold, downward moving currents such as those that occur over the North Atlantic absorb carbon dioxide and transfer it to the deep ocean. Upward moving currents such as those in the tropics bring carbon dioxide up from depth and release it to the atmosphere.

Life in the ocean consumes and releases huge quantities of carbon dioxide. But in contrast to land, carbon cycles between photosynthesis and respiration vary rapidly; i.e., there is virtually no storage of carbon as there is on land (i.e., tree trunks and soil). Photosynthetic microscopic phytoplankton are consumed by respiring zooplankton (microscopic marine animals) within a matter of days to weeks. Only small amounts of residual carbon from these plankton settle out to the ocean bottom and over long periods of time represent a significant removal of carbon from the atmosphere.

The Human Role

In addition to the natural fluxes of carbon through the Earth system, anthropogenic (human) activities, particularly fossil fuel burning and deforestation, are also releasing carbon dioxide into the atmosphere. When we mine coal and extract oil from the Earth's crust, and then burn these fossil fuels for transportation, heating, cooking, electricity, and manufacturing, we are effectively moving carbon more rapidly into the atmosphere than is being removed naturally through the sedimentation of carbon, ultimately causing atmospheric carbon dioxide concentrations to increase. Also, by clearing forests to support agriculture, we are transferring carbon from living biomass into the atmosphere (dry wood is about 50 percent carbon). The result is that humans are adding ever-increasing amounts of extra carbon dioxide into the atmosphere. Because of this, atmospheric carbon dioxide concentrations are higher today than they have been over the last half-million years or longer. 

Not all of the carbon dioxide that has been emitted by human activities remains in the atmosphere. The oceans have absorbed some of it because as the carbon dioxide in the atmosphere increases it drives diffusion of carbon dioxide into the oceans. However, when we try to account for sources and sinks for carbon dioxide in the atmosphere we uncover some mysteries. For example, fossil fuel burning releases roughly 5.5 gigatons of carbon (GtC [giga=1 billion]) per year into the atmosphere and that land-use changes such as deforestation contribute roughly 1.6 GtC per year. Measurements of atmospheric carbon dioxide levels (going on since 1957) suggest that of the approximate total amount of 7.1 GtC released per year by human activities, approximately 3.2 GtC remain in the atmosphere, resulting in an increase in atmospheric carbon dioxide. In addition, approximately 2 GtC diffuses into the world's oceans, thus leaving 1.9 GtC unaccounted for. 

What happens to the leftover 1.9 GtC? Scientists don't know for sure, but evidence points to the land surface. However, at this time, scientists do not agree on which processes dominate, or in what regions of the Earth this missing carbon flux occurs. Several scenarios could cause the land to take up more carbon dioxide than is released each year. For example, re-growth of forests since the massive deforestation in the Northern Hemisphere over the last century could account for some of the missing carbon while changing climate could also contribute to greater uptake than release. The missing carbon problem illustrates the complexity of biogeochemical cycles, especially those in which living organisms play an important role. It is critically important that we understand the processes that control these sources and sinks so that we can predict their behavior in the future. Will these sinks continue to help soak up the carbon dioxide that we are producing? Or will they stop or even reverse and aggravate the atmospheric increases? With the use of satellites and field studies, NASA scientists will help to obtain crucial information on the carbon cycle. 

Source: NASA

Read more questions from WISTEME through
     Add to MSN Add to My AOL
 Rate this Question
   Add to Groups   Add to Watch Lists   Share Question
                          
 
    More Readings :
[QID:10186]    What is phosphorus?  
[QID:8940]    What is Mole Day?  
[QID:6494]    What is hydrogen?  
[QID:4911]    Can all the fires be put out with water?  
[QID:4877]    Why do soap bubbles show so many colors?  
[QID:4702]    What are the properties of acids and bases?  
[QID:4685]    What are aqueous reactions?  
[QID:4684]    What is the difference between elements, atoms, molecules and compounds?  
[QID:4446]    What do I need to know about nitrogen oxides?  
[QID:4426]    What do I need to know about lead?  
     Question ID:  ${question.id}         Current Version:  ${question.version}

{for qh in questionHistory} {if qh.status == 'r'} {else} {/if} {/for}
Version Contributor Date (ET) Voting
${qh.version} ${qh.nickName} ${qh.date} Rejected
${qh.version} ${qh.nickName} ${qh.date} {if qh.status != 'c'} {if qh.status == 'a'} Approved {else} {if qh.rstatus == 'c'} On-going {else} Pending {/if} {/if} {else}   {/if}
Start a New Topic
ID Topics Replies Latest Post (ET)
{if dlist!=null} {for d in dlist} {/for}
${parseInt(d_index)+1} ${d.sentence} ${d.replyNum} ${d.lastestDate}
{else}
No discussion topic.
{/if}
Label Name:
 
Group Name:
 
 
{else}
     You have no group.
{/if}
Advertisements
{if advertisements.length > 0} {else} {/if}
{for ad in advertisements}
${ad.adTitle}
${ad.adSentence}
${ad.adUrl}
{/for}

Home | About Us | Terms of Use | Privacy Policy | Browse Questions | RSS Feed

Copyright ©2010 WISTEME LLC. All Rights Reserved.