Some facts about carbon dioxide (CO2)

(My starting point in lay discussions of climate)

  1. Under atmospheric conditions, CO2 never condenses or freezes into liquid CO2 or solid CO2. We never get clouds, rain, or snow made of CO2.

  2. CO2 is chemically very stable. In the atmosphere, other gases (such as methane and carbon monoxide) turn into CO2. But CO2 hardly ever turns into other gases.

  3. CO2 is a greenhouse gas. It absorbs and then re-emits heat radiation in such a way as to restrict its escape, making the Earth's surface warmer. CO2 is our atmosphere's most important non-condensing greenhouse gas.

  4. Water vapour is also a greenhouse gas. It too is important. However, because it can condense or freeze, its behaviour in the atmosphere is very different from that of CO2.

  5. Going back in time from 200 years ago to 800,000 years ago, atmospheric concentrations of CO2 varied within a range of values between about 180 and 290 ppmv (parts per million by volume), as the Earth went through about eight ice-age cycles. This is known from very careful measurements of air trapped in Antarctic ice. The measurements tell us that, whatever the complexities of the natural carbon cycle, the natural variation of atmospheric CO2 has been, in round numbers, within the range 180 to 290 ppmv for a very long time indeed.

  6. After the end of the last ice age around 10,000 years ago, and before industrial times, atmospheric CO2 concentrations stayed close to 280 ppmv, near the high end of the natural range. However, since then they have reached values close to 400 ppmv, taking them far outside the natural range.

  7. Most of this rise is due to human activity. It represents a large disturbance to the Earth's carbon cycle, by any natural measure, even though the actual CO2 concentrations appear small. It therefore represents a large disturbance to the climate system.

Some background notes:   The foregoing facts are all well known and well established, the first four from basic chemistry and physics. The importance of a greenhouse gas to the climate system depends mainly on three properties, first on whether in our atmosphere it condenses or not, second on its chemical stability, and third on how strongly a given amount of it warms the Earth's surface. Many discussions make the mistake of ignoring the first of these properties. A discussion avoiding that mistake is published in the peer-reviewed journal Science: Lacis, A. A., et al, Atmospheric CO2: principal control knob governing Earth's temperature, Science 330, 356-359, © 2010 American Association for the Advancement of Science. That discussion makes it clear why the non-condensing property of CO2 makes its role so different from that of water vapour.

The main point can be stated very simply. When CO2 is added to the atmosphere, this added CO2 acts like the input signal to an amplifier. Water vapour has no such action. The reason is that water can evaporate and water vapour can condense or freeze, depending on the temperature. The concentration of water vapour is therefore strongly influenced by temperature. This makes it act as a so-called feedback. The feedback is positive. Anything that makes the atmosphere warmer on average tends to make it moister on average, other things being equal, simply because warmer air can hold more water vapour before the vapour condenses. More water vapour means more greenhouse effect, hence still more warming. In other words, water vapour is part of the amplification mechanism.

Regarding the natural range of variation of atmospheric CO2, ca. 180 to 290 ppmv, a good recent description of the care with which CO2 is measured in ice cores can be found in another peer-reviewed journal, Philosophical Transactions of the Royal Society: Wolff, E. W., Greenhouse gases in the Earth system: a palaeoclimate perspective, Phil. Trans. R. Soc. A 369, 2133-2147, © 2011 The Royal Society. This paper also presents some of the measurement data showing the range of variation. See also this useful quick summary on the website of the Cambridge Centre for Climate Science.

Regarding `most of this rise is due to human activity' (taking CO2 from 280 to 390 ppmv), the evidence is strong, though subject to minor uncertainties. The most important feature is the steady rise over the past century or two -- not the transient, short-term fluctuations that are inevitably superposed on it, such as the regular seasonal cycle and other transients impacting the biosphere. These transients include effects from El Niño and volcanic eruptions. The timescales range up to several years, with associated rates of change of atmospheric CO2 that are relatively large, but temporary. The bottom line is that the total amount of fossil fuel burnt and cement produced since pre-industrial times is enough to account for the magnitude of the steady rise, even after allowing for oceanic absorption of CO2 and for the net absorption or emission from the natural and cultivated terrestrial biosphere, after smoothing out the transients. (This net terrestrial contribution appears to have been relatively small to date, though uncertain even as regards its sign.)

Regarding the disturbance to the carbon cycle -- and hence to the climate system -- the effects will be essentially permanent, from a human perspective, unless future generations decide to pull CO2 back out of the atmosphere. The natural carbon cycle has multiple timescales, including timescales far longer than a human lifetime. A readable account is given in a small book by carbon-cycle expert David Archer, The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate, ©2009 Princeton University Press. One of the most telling lines of evidence comes from recent work on the PETM (the Paleocene-Eocene thermal maximum, showing in ocean sediments the long-lasting effects of a large natural injection of carbon into the atmosphere that took place about 55-56 million years ago. The climate disturbance lasted for more than 100 millennia, producing severe ocean acidification and mass extinctions.

Here is a link to further discussion and references, including important recent work clarifying the relative timings of changes in temperature and atmospheric CO2 (Shakun et al (2012), Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Nature 484, 49-54, © Macmillan).

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This page first posted 21 July 2011; last updated 1 March 2014.
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