Human Effect On The Carbon Cycle

Picture Above: The global carbon cycle. Numbers represent the mass of carbon, in gigatonnes of carbon (Gt C). (A gigatonne is a thousand million tonnes.)                              Source: Wheeling Jesuit University/NASA (2000)

   Currently each year, during the combustion of fossil fuels around 6.5 billion tonnes of carbon in the form of carbon dioxide are released into the atmosphere.  Another 1-2 billion tonnes is emitted per year from land clearing.  About 3 billion tonnes of the carbon stays in the atmosphere.  The ocean takes up just over 2 billion tonnes.  The remaining 2-3 tonnes of carbon is taken from the air and stored by terrestrial sinks such as growing forests.  

   The Earth’s surface is warmed as sunlight passes through the atmosphere.  Heat, or infrared radiation, is released back into incoming energy.  Water vapour, carbon dioxide and some other trace gases can absorb part of this radiation, allowing it to warm the lower atmosphere, while the remainder is emitted to space.  Greenhouse gases are also known as atmospheric trace gases that trap heat.  Water vapour is the cause of about three-quarters of the natural greenhouse effect.  Carbon dioxide is the next most significant greenhouse gas.  Increased trapping of infrared radiation is due to higher concentrations of carbon dioxide and other greenhouse gases in the atmosphere.  The lower atmosphere is likely to warm up, changing the weather and climate.

   For 800 years before the Industrial Revolution the global cycle remained fairly unchanged, but in the last 200 years the change in the cycle has been rapid and very noticeable.  Human activity is responsible for the rapid increase in atmospheric carbon dioxide concentrations.  Proof of this is:

• Between the northern and southern hemisphere measured variations of atmospheric concentrations match calculations of carbon dioxide released from fossil fuel combustion.  Regions of peak fossil fuel use are the biggest global sources of carbon dioxide.
• There is a slight decline in atmospheric oxygen concentrations as carbon in fossil fuels and plants combines with oxygen during combustion to form carbon dioxide.