Carbon Dioxide

 

Source: http://kauaian.net/blog/?p=286. Author: . Permission: Fair Use.

Background

Carbon dioxide (CO2) is a naturally occurring chemical compound composed of two carbon atoms covalently bonded to a carbon atom. CO2 exists as a trace gas in the atmosphere at standard temperature and pressure. CO2 is considered the most important greenhouse gas due to the wide degree of its emittance.  It is emitted from the burning of fossil fuels, land use changes, and cement production (among other sources). Since 1750, before the industrial revolution, global atmospheric concentrations of CO2 have increased from 280 parts per million by volume (ppm) to 391 ppm as of April 2010¹ which significantly exceeds atmospheric levels during any time during the past 650,000 years. CO2 is an especially dangerous long lived greenhouse gas because atmospheric CO2 is not destroyed chemically, and its removal from the atmosphere takes place through multiple processes that transiently store the carbon in the land and ocean reservoirs, and ultimately as mineral deposits. ²

Carbon Chemistry

Anthropogenic carbon dioxide is produced mainly by the combustion of carbon-containing fuels, such as wood, coal, petroleum, or natural gas. Combustion reactions produce energy in the form of heat, carbon dioxide, and usually water. For example, the chemical equation for the combustion of methane is given as: CH4 + 2 O2 → CO2 + 2 H2O.

Carbon Cycle

Carbon Cycle

Source: http://www.uwsp.edu/geo/faculty/ritt...bon_cycle.html. Author: NASA. Permission: Fair Use.

In nature, the levels of CO2 in the atmosphere vary over time. This is referred to as the “carbon cycle”, whereby carbon in the atmosphere is absorbed by the world’s oceans and plant growth (creating “carbon sinks”, or natural deposits of stored-CO2) and is eventually re-released into the atmosphere (turning sinks into “carbon sources”) when, for example, plant matter decomposes or water is evaporated. CO2 levels in the atmosphere are an inherent phenomenon of change between glacial and interglacial periods.³ There is also a natural seasonal cycle to atmospheric CO2 levels, coinciding with the Northern Hemisphere growing cycle. Generally speaking though, when in balance the amount of carbon that is absorbed and later released into the atmosphere during the carbon cycle is roughly even.⁴

CO2 and Greenhouse Effect

 Atmospheric CO2 absorbs long-wavelength radiation (heat), preventing it from escaping the atmosphere. Therefore, greater concentrations of atmospheric CO2 (and other greenhouse-gases) absorbs more and more long-wavelength radiation, further contributing to this warming effect. This process is colloquially referred to as the "greenhouse effect".

The majority of inorganic CO2 is trapped in the world’s oceans, amounting to some 93% of all carbon dioxide in the biosphere. CO2 in the ocean, unlike atmospheric CO2, does not absorb long-wavelength radiation and thus does not play a role in the greenhouse-gas effect.⁵

Carbon Feedbacks

One example of where human activity has a negative impact on the carbon cycle is so-called positive feedback loops. Artificial increases in atmospheric greenhouse-gases increase the rate at which, for instance, the Alaskan permafrost melts, releasing the huge sinks of carbon stored within them, thereby further increasing the rate at which the remaining permafrost melts. This has the effect of throwing the natural carbon cycle off-balance, which partly helps to explain significantly increasing atmospheric concentrations of CO2 in the post-industrial revolution era. While positive feedback loops have been studied in many areas (such as water vapor, which functions in much the same way as carbon dioxide and methane GHG’s) plenty of scientific uncertainty exists in terms of anthropogenic contributions to these feedbacks.⁶

Sources⁷

Respiration
Two-main sources of inorganic CO2 in the biosphere are autotrophic (plant-based) and heterotrophic (non plant-based) respiration. Autotrophic respiration accounts for roughly 60 billion tons of land-based CO2 and 58 billion tons of dissolved (sea-based) CO2 every year. Heterotrophic is responsible for 55 billion and 34 billion tons per year, respectively.

Volcanism

While volcanic activity is responsible for significant short-term increases in land, sea and atmospheric CO2 (and is widely mis-cited as being the primary driver of global CO2 emissions), globally, volcanic production of carbon dioxide is only responsible for roughly .02 to .05 billion tons per year.

Land-Use
Land-use patterns in human activity contribute somewhat significantly to CO2 emissions. For instance, clear cutting vegetation (“deforestation” or slash and burn agriculture) to make way for agricultural land leads to a sink of CO2 being released (via decomposing plant matter) and less overall plant-life to absorb atmospheric CO2. In the tropics alone, it is estimated land-use changes produce on-average 1.7 billion tons of CO2 per year. Overall, it is estimated over 200 billion tons of carbon dioxide have been released since the mid-nineteenth century due to human land-use activity.

The map shows the areas in the world that have experienced changes in forest cover. The red regions of the map show a net loss of forest, the dark green regions show a net gain of forest, and the light green regions represent the current forest cover.

Source: Millenium Ecosystem Assessment . Author: www.wri.org. Permission: n.

Energy
The generation of energy from stationary sources (most notably coal-power facilities) and the industrial combustion of fossil fuels results in roughly 39 billion tons of CO2 emissions per year. Mobile sources of energy-related CO2 emissions contribute a far greater 24% of the overall anthropogenic contribution to CO2 emissions. The majority of this stems from the combustion of petroleum fossil fuels in road transportation, while some comes from diesel and jet-fuel as well.

Industry (non-energy)
Sources such as the production of lime and concrete produce industrial CO2 emissions. Globally, these sources result in roughly .2 billion tons of CO2 emissions annually, about a factor of 10 more than that of volcanic emissions. Similarly, the production and use of industrial chemical feedstock’s contributes to an average of .25 billion tons of CO2 emissions per year.