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Thursday, March 29, 2012

Long-term Trends in Atmospheric CO2


This article is the fourth post in a series about global CO2 trends and seasonal cycles.

1)  The Keeling Curve
2)  The Keeling Curve and Seasonal Carbon Cycles
3)   Seasonal Carbon Isotope Cycles
4)   Long-Term Trends in Atmospheric CO2
5)   Modeling Global CO2 Cycles

Long-Term Trends in Atmospheric CO2
Burning fossil fuels consumes oxygen and emits CO2.  This is simple chemistry that we learned in middle school, using a candle and a glass jar.  It is possible to calculate the quantity of CO2 released from the candle by the chemistry of the candle, and calculate the concentration of CO2 in the jar from the quantity of CO2 released by the candle.
We can also easily find the quantity of CO2 released into the atmosphere by the global consumption of coal, oil and natural gas.  Some respected organizations (the International Energy Agency, and the British Petroleum Statistical Review of World Energy) have collected the data on fossil fuels, and done the math on the quantity of CO2 released.
We can add the annual fossil fuel CO2 emissions to a baseline CO2 concentration, such as the 1970 global average, and compare the cumulative fossil fuel emissions to the observed change in world average CO2 concentration.
Only about 60% of the known CO2 emissions from burning fossil fuels winds up in the atmosphere.   We can infer that the other 40% is absorbed by various earth systems which act as carbon reservoirs or sinks.   Examples might include vegetation, the ocean, or precipitation of limestone.

In an earlier post, we explored the annual cycles of CO2 fluctuation, and how those cycles vary by latitude.
http://dougrobbins.blogspot.com/2012/03/keeling-curve-and-seasonal-carbon.html
We can also take the annual average for CO2 concentration in the Northern Hemisphere, and compare those readings to the annual average CO2 concentration in the Southern Hemisphere.  We can see a clear difference.  The Southern Hemisphere lags behind the Northern Hemisphere in terms of increasing CO2 by about 2.6 ppm.


Fossil fuel emissions are concentrated in the Northern Hemisphere; as we noted in the previous post, 90% of the world's population lives in the Northern Hemisphere, including the most industrialized economies.
It seems reasonable to conclude that the Northern Hemisphere leads the Southern Hemisphere in CO2 concentration gains, because the Northern Hemisphere is the source of the emissions.  The difference in CO2 concentration is very close to the quantity of global annual CO2 emissions.





The excess CO2 concentration of the Northern Hemisphere is closely matched, and easily explained by annual fossil fuel emissions of the Northern Hemisphere.  I allocated global CO2 emissions by hemisphere according to figures for national GDP.   In earlier posts, we have seen the strong correlation between GDP and energy use.  About 83% of global GDP, and by inference, CO2 emissions, occur in the Northern Hemisphere.  The excess CO2 delivered to the atmosphere in the Northern Hemisphere accounts very well for the difference in CO2 between the Northern and Southern Hemispheres.















The use of fossil fuels has grown exponentially since the industrial revolution ( M.K. Hubbert, 1956; D.H. Meadows, et al 2004).   Annual consumption has grown from essentially zero before the year 1800, to over 10.4 billion tonnes of oil equivalent in 2010.   

Air bubbles in ice cores from the Antarctic ice cap provide a record of historic and prehistoric concentrations of CO2 in the atmosphere.   The pre-industrial concentration of CO2 is generally reported around 280 ppm.   The record shows that CO2 levels were essentially constant from the year 1000 until the industrial revolution began, about 1800.

The increase in atmospheric CO2 fits well to an exponential function, reflecting the exponential growth of human population, and the exponential growth in the use of fossil fuels.  Trial and error produced this function, which ties pre-industrial CO2 concentration, and provides a good fit to the modern Keeling Curve.
CO2 concentration, ppm = e(n*0.001854) + 280,
where n = the number of months since Jan. 1800



Extrapolating the function forward, CO2 concentration would be expected to reach 450 ppm in the year 2031, and 500 ppm in the year 2042.

A carbon dioxide concentration of 450 ppm is sometimes cited as a theoretical "tipping point", beyond which climate change becomes irreversible, due to positive feedback mechanisms (i.e., release of greenhouse gasses from permafrost, dissolution of carbonate sediments due to ocean acidification, release of methane from gas hydrates, etc.).

Based on the consistency of the exponential increase in atmospheric CO2, the growth of world population, and industrialization of the world economy, it seems likely that atmospheric CO2 levels will continue to rise and likely exceed the 450 ppm and 500 ppm thresholds within the next 35 years.
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This is fourth of five posts in a series about global atmospheric CO2.
The Keeling Curve
http://dougrobbins.blogspot.com/2011/05/keeling-curve.html

The Keeling Curve and Seasonal Carbon Cycles
Seasonal Carbon Isotope Cycles
Long-Term Trends in Atmospheric CO2
Modeling Global CO2 Cycles

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Global CO2 data is available from Keeling et. al., on the Carbon Dioxide Information Analysis Center website.  http://cdiac.ornl.gov/trends/co2/

The population chart was prepared by "Radical Cartographer" Bill Rankin.  http://www.radicalcartography.net/
I used the version of the map posted here: http://www.geekosystem.com/world-population-latitude-longitude/

Data for CO2 released by fossil fuels is available from EIA CO2 Emissions from Fuel Consumption,
http://www.iea.org/co2highlights/co2highlights.pdf

And the BP Statistical Review of World Energy:
http://www.bp.com/sectionbodycopy.do?categoryId=7500&contentId=7068481

Historic CO2 levels:


Exponential Growth in Fossil Fuels



BP Statistical Review of World Energy, Global Fossil Fuel Consumption: 

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