Atmosphere Level

Canopy Level

Leaf Level

Soil Level

Mass spec.

References

Atmosphere

Ten million years prior to the Industrial Revolution, mean atmospheric CO₂ averaged 0.02% of Earth's atmosphere. Two million years prior to industrialization concentrations ranged from 200 ppm to 280. One thousand years prior to 1850, levels remained steady at 280 ppm. By 1996, roughly 150 years post-industrial revolution, that percentage had risen to 358 ppm. Today, carbon dioxide constitutes 0.037% of the atmosphere and is increasing at an annually increasing rate of 1.8 ppm. This translates into a 30% increase in atmospheric CO₂ since the beginning of the Industrial Revolution. Anthropogenic forcing such as coal, oil and gas combustion and deforestation are responsible for this unprecedented increase (Fig. 1). Since CO₂ is a greenhouse gas, its excess in the atmosphere has been strongly implicated in global warming.

Figure 1: CO2 levels as function of time and latitude

Annual per capita emission rates vary from country to country according to climate, degree of industrialization, energy taxes, population size and the manner in which electricity is generated. With these factors taken into consideration, the United States emits the greatest amount of CO₂ at 1350 million metric tons per year. Russia by contrast, the worlds largest country yet not as densely populated as the U.S., emits approximately 950 million metric tons annually.country yet not densely populated emits approximately 950 million metric tons annually. (Fig 2)

The global disparity in annual emissions is most apparent between developed countries and developing ones. In 1985 for example, per capita emissions for the developed world measured more than five times those of the developing world. (Bongaarts). While an increase in global population over the next century may decrease per capita emissions rates, the gross annual emissions are projected to increase. In his study on the link between population growth and global warming, John Bongaarts puts forth a model predicting a 35% increase in emissions from 1985 to 2100.

Of particular interest is the impact of these emissions on the global carbon cycle.
The cycle can be described in terms of carbon sources and sinks that when summed together yield an increase in atmospheric carbon dioxide levels. Sources of carbon dioxide include deforestation and fossil fuels while sinks include the ocean and a currently unidentified terrestrial sink. (Schlesinger 1991).

Atmospheric increase = Fossil fuel emissions + Net emissions from land use - Oceanic uptake - Missing terrestrial C sink

Figure 3: The global Carbon cycle

Of the 7 billion tons of carbon annually forced around the globe, approximately 3 billion tons perpetuate in the atmosphere while the ocean sucks down another 2 billion tons. Scientists believe vegetation incorporate the remaining carbon, ultimately passing it to soil in modified forms. However, the exact amount absorbed and location of the sink are yet to be determined. Currently, scientists are studying the North American Boreal forest above of 40° N latitude.

That carbon exists in two stable isotopic forms ¹²C, ¹³C, is particularly useful in studying this cycling of carbon throughout the atmosphere. Seasonal fluctations in atmospheric CO₂ can be tracked using ¹³C to study the effects of anthropogenic forcing. Research has shown that plants sequester CO₂ at a greater rate under elevated atmospheric CO2. This translates into atmospheric enrichment in the heavier ¹³C while plants exhibit 13C depletion.

It's important to note that atmospheric CO₂ levels over land fluctuate on diurnal, seasonal, and latitudinal scales. The 358 ppm mentioned above represents an annual average. At the diurnal level, local atmospheric CO₂ drops during the day when vegetation actively photosynthesizes. The reverse occurs at night as vegetation undergoes respiration instead. d¹³C, notation used to describe 13C/12C, also fluctuates diurnally, yielding information about relative 13C levels. (The leaf level section explains) this in further detail.

Seasonally, vegetation, and hence photosynthesis, abounds in summer hemispheres. During this time, atmospheric CO₂ levels decrease yielding a greater relative concentration of atmospheric 13C. The opposite is true in winter hemispheres. Seasonal levels are a strong function of latitude as more vegetation exists in warmer latitudes.

But what about the increased atmospheric carbon dioxide from fossil fuels? How does this change the 13C/12C ratio? Roeloffzen et. al 1990 analyzed air samples from 7 different stations situated in the Pacific area in an attempt to answer these questions. For all of the sites in question, the researchers noticed an overall decrease iin d¹³C, meaning higher 12C. This results from the distinct d¹³C signature of fossil fuels of -27.28 (See leaf level for an explanation)

Figure 4: Trends and seasonal components of d¹³C at Mauna Loa and Line Islands (Roeloffzen et. al 1990)

From the data from the seven stations, Roeloffzen et. al 1990 constructed time evolutions of global CO2 and d¹³C levels from 1981-1989 across latitudes. Since 95% of fossil fuel emissions originate in the Northern hemisphere, higher C values and more negative d¹³C were observed there: