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Notes for the lecture on Wednesay November 28 (Guest lecturer: Professor Richard Gammon)
Human impact on the carbon cycle
We have already seen that the carbon cycle is a biogeochemical cycle describing the exchange of carbon atoms between various reservoirs within the earth system.  By burning fossil fuels we create a pertubation to the natural carbon cycle which results in accumulating levels of CO2 in the atmosphere.

Levels of CO2 in the atmosphere today
Today's levels of CO2 in the atmosphere are about 365 ppmv (parts per million per volume).  We can convert this volume mixing ratio to the total mass of carbon in the atmosphere by multiplying this number by the mass of the atmosphere weighted by the relative mass of carbon atoms compared to air molecules (for simplification we assume that air is composed of 80% N2 and 20% O2):

(365 x 10-6) x    12 / (0.8 x 28 + 0.2 x 32)  x  5 1018 =  760 x 1012 kgC
                                = 760 Gton C

where 12 g/mol is the mass of a unit of carbon, 28 g/mol is the mass of N2, and 32 the mass of O2; 5 1018 kg is the mass of the atmosphere.   1 ton = 1000 kg.  1 Gton (gigaton) = 1000 x 109 = 1012 kg. (you don't need to know how to do this conversion, but this simply shows how one goes from parts per million to gigatons)  In short one can also simply convert 365 ppm to gigatons by the following:  365 ppm x 2.08 GtonC/ppm = 760 Gton C.

CO2 levels today are about 30% higher than during preindustrial times, where CO2 levels were closer to 280 ppmv.  In comparison levels of atmospheric CO2 during glacial ages were even lower, at about 200 ppmv.  Based on CO2 measurements in the bubbles of ancient air trapped in ice cores, we know that atmospheric levels of CO2 today are higher than they have ever been over the last 500,000 years. 

From direct observations of atmospheric CO2 (remember the "Keeling Curve" Fig 1-2 in textbook) we see that the levels of CO2 are increasing at a rate of 1.5 ppmv each year.  In terms of gigatons, this corresponds to an additional 3 Gton of Carbon added to the atmosphere every year (1.5 ppmv/year x 2.08 GtonC/ppmv = 3 Gton C/year).

Anthropogenic sources of CO2
The dominant source of anthropogenic CO2 is through the combustion of fossil fuels (coal, oil, and natural gas).  The current burning rate of these fossil fuels is close to 6 Gton C each year. (see Table 13-1 in textbook).  We use these fossil fuels as sources of energy for electricity generation, transportation, industrial processes, commercial and residential use.  The U.S. is the leading consummer of fossil fuels, generating close to 1.5 Gtons of carbon every year. 
This source is more than a hundred times faster than the source of CO2 from volcanic activity or weathering.  Burning of fossil fuels can be viewed as a short-circuiting the long-term organic carbon cycle.  If we were to use all the fossil fuel reserves (containing 4000-6000 Gton C), it would take millions of years for the fossil fuels to form again through the slow processes of organic matter decomposition, and burial at great depth. 

Compared to the source of carbon from respiration of land biomass (about 60 Gton C/year), the burning of fossil fuels put out 10 times less CO2, and might appear to be a rather small source.  However, respiration is nearly balanced by photosynthesis such that it does not act a permanent source of CO2 to the atmosphere (we will see below that there is in fact a slight imbalance between respiration and photosynthesis resulting in plants being a temporary sink of CO2 today).

Another small source of anthropogenic CO2 comes from tropical deforestation, this amounts to about 1.5 Gton of carbon/year.

Sinks of anthropogenic CO2
Human activities emit 6 + 1.5 = 7.5 Gton of carbon every year, and yet the levels of CO2 in the atmosphere are only increasing at a rate of about 3 Gton of carbon each year.  Where does the missing carbon go?  Part of the CO2 resulting from fossil fuels is taken up by new forests growing in the Northern Hemisphere. These forests were cut down over the last couple of centuries and are not slowly regrowing, accounting for an estimated 0.5 Gton C/year sink.  It appears that existing forests are growing faster today than they were before. The exact cause is unknown but could involve the higher levels of atmospheric CO2, and/or possible fertilization effect from man-made nitrogen compounds (see textbook page 257).  This fertilization effect could account for an additional carbon sink of 2 Gton C/year.  It is unclear how forests will respond to further increases in CO2 levels and whether the fertilization effect wil persist or at some point actually reverse course such that forests might act as sources of CO2 to the atmosphere.

Another important player in the carbon cycle is the ocean.  CO2 can dissolve in the oceans, and today they take up about 2 Gtons of carbon each year.  However, the oceans have a limited capacity to store CO2.  When CO2 dissolves in the oceans, the oceans get more acidic making it harder for more CO2 to dissolve (the chemistry leading to the dissolution of CO2 in the oceans is beyond the scope of this class, but if you are interested you can take a look at page 258 in the textbook).  In addition, at any point in time only surface waters are in contact with the atmosphere, and the deep ocean does not see CO2 in the atmosphere. Only over long timescales (thousands of years) does the circulation in the oceans bring up more water to the surface which can incorporate more CO2 from the atmosphere.  Over even longer timescales, CO2 dissolved in sea water will form carbonates which will sink and form limestone returning CO2 to the long-term carbonate-silicate cycle over millions of years.

Here is a summary of the human perturbation to the carbon budget:
Sources of anthropogenic CO2
   Fossil fuel combustion  6.0 Gton C/year
   Tropical deforestation 1.5 Gton C/year
7.5 Gton C/year
Sinks of anthropogenic CO2
    Atmosphere 3.0 Gton C/year
    Forest regrowth 0.5 Gton C/year
    Fertilization effect (CO2 and nitrogen) 2.0 Gton C/year
    Oceanic uptake 2.0 Gton C/year
7.5 Gton C/year


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