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Section 12.3 The role of carbon dioxide

Why is there such a strong focus on carbon dioxide in our discussions of climate change?

  • carbon dioxide has relatively high concentrations (0.04% in the atmosphere)

  • it is the second most important greenhouse component after water vapor

  • increasing concentrations clearly tied to human activity (carbon has provided energy basis for modern civilization)

  • this addition of anthropogenic carbon, although small relative to the total atmospheric inventory, is enough to enhance the greenhouse effect and cause warming

How do we know it is *our* carbon?

  • volcanic emissions are about ~0.3 Gt/yr whereas anthropogenic emissions of carbon dioxide are over 30 Gt/yr; about 100x more than volcanoes (source: USGS)

  • we can track how much carbon has been emitted based upon historical records. These emission statistics correlate with the observed carbon increases in the atmosphere and oceans

  • the carbon-13 composition of the air is decreasing with time. Why?

    • during photosynthesis, plants preferentially "fix" carbon-12 instead of carbon-13 (via RuBisCO or Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase)

    • this means plants are isotopically "light" (that is, plant-based carbon tends to demonstrate a depletion in carbon-13) compared to the atmosphere

    • biomatter is the source of fossil fuel, so CO\(_2\) released from fossil fuel combustion releases this isotopically light carbon into the atmosphere (which is becoming slightly more depleted in C-13)

    • the (biogenic) carbon that is being added to the atmosphere is free of carbon-14, indicating an ancient source

  • in summary: the addition of ancient, biogenic carbon to the atmosphere caused by the combustion of fossil fuels

How is carbon dioxide's impact quantified?

The impact of carbon dioxide is determined using a concept known as radiative forcing. In effect, this forcing is the change in energy absorbed by the atmosphere due to the increased concentration of greenhouse gases.

For example, the addition of carbon dioxide can be represented as an effective increase in energy (in W/m\(^2\)) added to the atmosphere: it describes changes in the difference between solar insolation and how much energy is radiated back into space:

  • added carbon dioxide concentration increases the greenhouse effect: more energy is trapped by selective absorption of longwave radiation

  • increasing carbon dioxide concentration is the largest forcing component

  • however, other greenhouse gases, in particular methane, nitrous oxide, and halocarbons are also playing a significant role

For carbon dioxide, atmospheric forcing can be described by the expression

\(F = 5.24 \ln(C/C_o)\)

where \(C_o = 278\) ppm (pre-industrial). This suggest that the increase to 400 ppm carbon dioxide has lead to a forcing of +2 W/m\(^2\text{.}\)

Climate Sensitivity

A closesly related concept to radiative forcing is climate sensitivity: that is, given some increase in greenhouce gas concentration (and/or some equivalent increase in radiative forcing) what is the observed temperature response? For carbon dioxide, the climate sensitivity refers to the temperature increase expected for a doubling of pre-industrial levels (i.e., from 280ppm to 560 ppm). Most modern studies suggest a CO2-doubling climate sensitivity of 1.5-4.5 C. The equation above suggests a climate sensitivity of 3.6 C.