Letter: A little carbon dioxide has a very big effect

Posted 6/14/18

In response to Dale Hennessy’s letter (“ People think climate science is clear — it isn’t ”), there is a good reason why carbon dioxide has such a large effect even at …

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Letter: A little carbon dioxide has a very big effect

Posted

In response to Dale Hennessy’s letter (“People think climate science is clear — it isn’t”), there is a good reason why carbon dioxide has such a large effect even at the low concentration of 400 parts per million, or 0.04 percent.

Sunlight lands on the Earth at an average power of 340 watts per square meter. About 30 percent of this is immediately reflected back into space, and about 70 percent is absorbed as heat, mostly on the surface of the Earth but partly in the atmosphere. The Earth stays in balance at its steady temperature by re-radiating an exactly equal amount of heat (or infrared radiation) back into space. The atmosphere absorbs most of this departing heat and then re-radiates it into space, so the infrared absorption of gases in the atmosphere is centrally important.

The atmosphere is made up of: nitrogen 78 percent; oxygen 21 percent; argon 0.9 percent; carbon dioxide 0.04 percent; and water vapor 0.001 percent, to 5%. Of these, nitrogen, oxygen and argon do not have any absorption to infrared radiation (heat), so the only two constituents that actively set the Earth’s temperature are water vapor and carbon dioxide.

Water vapor absorbs strongly over most of the infrared region, dominantly controlling Earth’s temperature, but it only absorbs weakly between 9 microns and 17 microns. In this region, heat can pass straight up through the atmosphere and escape into space, keeping us at the right temperature, but for one thing: carbon dioxide has absorption bands there. At 9.4, 10.5, 13.7 and 16.4 microns, carbon dioxide is king, with a disproportionate influence on the passage of heat through the atmosphere. Hence its strong effect at only 0.04 percent concentration.

Climate models that are run on large computers include every detail of these (precisely known) infrared absorptions and follow the flow of heat through the atmosphere. If carbon dioxide increases by just 130 parts per million, as it has done due to our combustion of oil, coal and etc. since pre-industrial times, then the excess warming becomes 2.3 watts per square meter. Methane and nitrous oxide add another 0.5 watts per square meter of warming.

So, small though the increase in carbon dioxide has been, the precise position of its infrared absorptions causes heat retention sufficient to cause exactly the ocean expansion and more than sufficient to cause the icecap melting that we have measured.

Malcolm McGeoch

Little Compton

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