During most of earth's history, the climate was much warmer than it is today.  Yet the younger sun put out 30% less light than it does today???  How is that possible?  The answer is carbon!  You have all heard about the greenhouse effect and global warming.  These are issues that deal with the way humans have been shifting carbon around in our environment (by burning fossil fuels).  It turns out, nature has been doing the same thing all by itself from the start.

Carbon can take many forms, and the carbon we are referring to is carbon dioxide (CO₂) in our atmosphere.  Presently our atmosphere contains very little CO₂ ... only .03%.  The atmospheres of both Venus and Mars are almost completely composed of carbon dioxide.

Carbon (in different forms) is constantly moving between the atmosphere, oceans, and land in a cycle know as the carbon cycle.  It is the focus of much attention because CO₂ (carbon dioxide) is an important "greenhouse" gas which effect global temperatures.  In unit 4 we will discuss the greenhouse effect in detail.  For now, all you need to know is this - the more CO₂  in the atmosphere ... the warmer we get.  By burning fossil fuels, we are releasing CO₂  into the atmosphere and making things warmer.

How does carbon enter the atmosphere? 

• Volcanic outgassing
• Respiration (animals exhale CO₂ )
• Decay of biomass
• Burning of fossil fuels
• Weathering of carbonate rocks (limestone)

How does carbon leave the atmosphere? 

• Plants convert atmospheric CO₂  to biomass (producing carbohydrates and oxygen) via photosynthesis.  If the plant decays, it simply puts the CO₂  back in the air (using decomposer bacteria).  If it gets buried (in a bog for example), it can not decay and the CO₂  is tied up as biomass which may eventually get converted to a fossil fuel (coal, oil, or natural gas)

   
• When we eat the plants, we take in carbon as biomass.  This means you are a part of the carbon cycle.  Every breath you take you exhale carbon and every time you eat something, you are taking carbon in.
   
• The chart (below) shows that forests act as a major CO₂  reserve.  The clearing of the tropical rain forests not only releases this storehouse of carbon dioxide when it is burned, but the loss of the trees itself means we've lost another tool for removing additional CO₂  for tomorrow.

• Rain absorbs CO₂  and converts it to carbonic acid (H₂CO₃).  Over geological time, this acid corrodes rocks made of granite. In the first unit we talked about the weathering of granite - and if you recall, the continents are mostly granite. This reaction releases the highly resistant SiO₂ as sand, also produces calcium and magnesium as ions in solution. The CO₂  ends up as bicarbonate in solution (CO₃ ).  The calcium, magnesium, and CO₃ then collect in the oceans and build up. In the oceans they reach saturation levels (near the surface) and begin to precipitate as calcite, which is calcium carbonate. Ocean-dwelling life-forms convert the saturated calcium carbonate into body parts such as shells, pearls, bony structures, and coral. When they die, the animals' parts end up on the ocean floor as carbonate (limestone).

All this "buried" CO₂  is then stored as thick layers of ocean floor sediments.  If this layer of the crust is later subducted under by another plate, the CO₂  may be released back into the atmosphere as volcanic gas.
 

The Carbon Cycle

Note that carbon dioxide migrates in many cycles ... each working on vastly different time scales.
 

Where is all the carbon now (and how much is there)?

GT = gigatons  A gigaton is a billion (109) metric tons, 1012 kg, or about 2200 billion pounds.

You can see that most of the carbon reserve is tied up in the deep ocean but as bicarbonate in solution.  Marine organisms can not make use of it because they thrive near the surface and because minerals such as calcite (CaCO₃) are unsaturated at depths below 4 to 5 km so it becomes difficult for organisms to use it for making hard parts.  This is because the solubility of CO₂  increases as the temperature drops and pressure increases (and why your soda fizzes when you release the pressure or warm it up).  At unsaturated levels, these hard parts would dissolve ... leaving the organism defenseless.
 

The carbon cycle is a thermostat which helps the earth regulate the climate.

• Too much CO₂  in the atmosphere and the earth gets warm!
• Too little CO₂  in the atmosphere and the earth cools off!

• The rate of CO₂  reaction to form carbonate (via chemical weathering of rocks) is strongly dependent on the temperature. If the temperature goes up, the rate of chemical reaction goes up too, sucking CO₂  from the atmosphere very fast. The reduced CO₂  levels in the atmosphere then helps cool the atmosphere.
• If the temperature drops, the rate of chemical reaction slows. If less CO₂  is removed from the atmosphere, the temperature rises.
• This positive feedback will ensure that the planet's temperature will stay in a range where liquid water can exist, keeping Earth habitable.
• The carbon cycle helps explain how the earth could be much warmer in the past despite the fact that the sun produced 30% less light.  There was a lot more carbon dioxide in the atmosphere when compared to the levels we see today.

Taken from Christopher P McKay, NASA Ames Research  Center, Astronomy Magazine, September 99 page 92
 

• Venus is an example of a planet with a run-away greenhouse effect.  Since it has no water, it has no mechanism to remove CO₂  from its atmosphere.  Venus is hot (482 degrees Celsius) because it's closer to the sun and has a thick atmosphere which is 96% carbon dioxide!

 
• The sun (as it ages) slowly gets hotter and hotter.  In time, this will continue to remove more CO₂  from the atmosphere.  Eventually the CO₂  levels may reach levels so low that plants may not be able to exist.  No plants, no us.  Another possibility is that once the CO₂  levels drop to zero, the feedback mechanism fails and we burn up.  Either way, bad news!

 

On February 20, 2000 a team of astronomers from Penn State painted a pretty dismal picture concerning the fate of the earth in the next billion years.  Read their press release here.
 
 

• In the short run, we are experiencing global warming.  Is this natural or a result of the burning of fossil fuels (and clear-cutting the tropical forests)?  The data confirms that both are rising.

 


from NOAA

Click here to see another chart of global temperature change.
 

Carbon Dioxide levels in the Atmosphere

Permission from http://www.bsi.vt.edu/biol_4684/Cycles/Cprobs.html

Globally, human activities are adding about 26 gigatons (26,000 million tons) of carbon dioxide into the atmosphere each year, this is only about 3% of the natural flux between atmosphere and oceans or land.

Carbon Dioxide and You

Do you think you only contribute only a very small amount to this increase in atmospheric carbon dioxide?  Guess again!  Read this article and then you will see that you are putting tons of CO₂ into the air each year.  For example, you emit 25 pounds of CO₂  into the air for every  gallon of gas you use ... and a ton of CO₂  just to run your refrigerator for a year!  It shows that (on average) each person is responsible for adding over 8 tons of  CO₂  into our atmosphere each year!  This article was a real reality check for me when I read it.

Links to the Carbon Cycle:

http://www.gcrio.org/CONSEQUENCES/vol4no1/carbcycle.html

©Jim Mihal 2004, 2006 - all rights reserved