SCIENTIFIC ANIMATION: GLOBAL WARMING: carbon dioxide and the greenhouse effect

GLOBAL WARMING: CARBON DIOXIDE & THE GREENHOUSE EFFECT: this animation shows a carbon dioxide molecule at upper right. At the start of the animation, a photon (shown as a whitish elongated ovoid) from the sun strikes the land. The land warms and emits an infrared photon (shown as a pink elongated ovoid). This IR photon strikes the carbon dioxide molecule and, as it is captured, sets up a flexing motion in the molecule. Soon afterwards, the molecule emits an infrared photon that is shown travelling to the ground. After emission, the molecule stops flexing and returns to its original state.

Carbon Dioxide consists of two oxygen atoms covalently joined to a central carbon atom. The three atoms lie in a line (it is a linear molecule). The central carbon atom is shown in black and the two oxygen atoms in red.

Infrared Radiation is electromagnetic radiation with wavelengths longer than visible light and shorter than microwaves. It is felt as heat.

Absorption of Infrared Radiation by carbon dioxide happens because the carbon-oxygen bonds can stretch or flex at a frequency that allows them to absorb an infrared photon. There are a number of ways in which the molecule can oscillate. One type involves the bonds stretching and contracting like springs. The molecule can absorb an IR photon when this stretching is asymmetric. The alternative is for the bonds to flex as shown in this animation. The flexing movement allows absorption at a longer wavelength. Because the CO₂ molecule can vibrate in different ways it can absorb different frequencies of IR. This means that the infrared absorption spectrum of carbon dioxide shows two main peaks.

Emission of Infrared Radiation by carbon dioxide the oscillating molecule can then re-emit the infrared energy as a photon travelling in a random direction. This will happen repeatedly over time as the particular molecule absorbs and then re-emits IR photons. Consequently, roughly half of the emitted photons from a given molecule will travel back towards the surface.

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GREENHOUSE EFFECT this repeated absorption and re-emission by carbon dioxide effectively scatters the infrared radiation and so causes the heating known as the greenhouse effect. Without an atmosphere containing greenhouse gases this heat would be radiated back into space. However, carbon dioxide and other greenhouse gases absorb and re-emit this infrared in various directions. Some IR will ultimately escape back into space after taking a tortuous route from molecule to molecule but the overall effect is to heat the atmosphere.

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LINKS TO MAJOR GREENHOUSE GAS MOLECULAR IMAGES:

• Carbon Dioxide, CO₂
• Methane, CH₄
• Hydrofluorocarbons (HFCS)
• Perfluorocarbons (PFCS)
• Chlorofluorocarbons (CFCS)
• Nitrous oxide, N₂O
• Nitrogen trifluoride, NF₃
• Sulfur hexafluoride, SF₆

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