Explaining greenhouse gases

Twinned bicycles

Over at ScienceBlogs, Paul Revere has written a three part primer (one, two, three) about the physics of climate change. It begins with the nature of electromagnetism and moves on to discuss the energy relationship between the Earth, the sun, and outer space. It is the sort of thing that feels very basic, but which is nonetheless important to understand through-and-through. In particular, the explanation of black bodies in the second portion is clear and informative.

The discussion of Wien’s Displacement Law is also quite informative. The law holds that every object in the universe emits electromagnetic radiation, and that the most common frequency exists in relation to that object’s temperature in degrees Kelvin. To go from one to the other, divide 2898 by the temperature in degrees Kelvin. The quotient is the peak wavelength, expressed in microns. Human body temperature is about 310 degrees Kelvin, so our peak electromagnetic wavelength is about 9.35 microns long – in the infrared portion of the electromagnetic (EM) spectrum. Since we are pretty similar in temperature to the surface of the Earth, the wavelengths radiated by the planet are in a nearby portion of the spectrum.

It is is ability of greenhouse gases to absorb this infrared energy that lets them prevent energy from returning to space. They are transparent to the dominant wavelengths emitted by the sun, but opaque to those radiating from the Earth. Increasing their concentrations in the atmosphere (through fossil fuel burning, deforestation, etc), causes more of the energy that comes to the Earth from the sun to remain in the atmosphere. As a result of the extra energy, the temperature rises. Incidentally, this is also why people sometimes mention using ground-based mirrors to fight climate change. They reflect light at the same peak wavelength as that of the sun (which passes relatively unimpeded through the atmosphere). By re-radiating at that visible wavelength, rather than the infrared one favoured by greenhouse gases, the energy can be made to escape again. Of course, it would take a massive number of mirrors to balance out the effect of increased greenhouse gas concentrations on the EM emissions from all non-mirrored areas.

One upshot of understanding the nature of these gases is the ability to appreciate how their increased concentration simply must add more energy to our planetary system. The scientific questions that remain are about precisely what changes that energy will generate, and at what rate. The three posts are well worth reading in their entirety.

[Update: 17 December 2009] See also: Greenhouse gases other than CO2

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

15 thoughts on “Explaining greenhouse gases”

  1. When students learn physics, they are taught about many simple systems that bow to the power of a few laws, yielding wonderfully precise answers: a page or so of equations and you’re done. Teachers rarely point out that these systems are plucked from a far larger set of systems that are mostly nowhere near so tractable. The one-dimensional atmospheric model can’t be solved with a page of mathematics. You have to divide the column of air into a set of levels, get out your pencil or computer, and calculate what happens at each level. Worse, carbon dioxide and water vapor (the two main greenhouse gases) absorb and scatter differently at different wavelengths. So you have to make the same long set of calculations repeatedly, once for each section of the radiation spectrum.

    It was not until the 1950s that scientists had both good data on the absorption of infrared radiation, and digital computers that could speed through the multitudinous calculations. Gilbert N. Plass used the data and computers to demonstrate that adding carbon dioxide to a column of air would raise the surface temperature. But nobody believed the precise number he calculated (2.5ºC of warming if the level of CO2 doubled). Critics pointed out that he had ignored a number of crucial effects. First of all, if global temperature started to rise, the atmosphere would contain more water vapor. Its own greenhouse effect would make for more warming. On the other hand, with more water vapor wouldn’t there be more clouds? And wouldn’t those shade the planet and make for less warming? Neither Plass nor anyone before him had tried to calculate changes in cloudiness. (For details and references see this history site.)

  2. In the case of the simplified earth-atmosphere system, the Earth’s surface warms from the sun’s incoming shortwave radiation. As it is now a warm body floating in cold space, Earth radiates long-wave energy back out at a rate that is dependent on its temperature. If that were the whole story, the earth would have balanced its incoming shortwave with its outgoing long-wave radiation at an average surface temperature of roughly -18°C and it would be a rather inhospitable place. As it is, the content of greenhouse gases in its atmosphere absorb some of that outgoing long-wave radiation and send it back down where we all live. The earth must balance this by warming enough so that it can radiate this additional energy back out again. The totality of this natural effect is around 33°C, bringing our average surface temperature to a comfortable +15°C.

  3. This too is straightforward physics; if an object’s temperature increases, it takes no time at all for it to radiate away energy at a higher rate, according to the Stephan-Boltzmann radiation equation. But all the other terms do take time for their impact to be felt. If we raise earth’s temperature, it takes time (weeks to months) for more water vapor to accumulate in the atmosphere due to extra evaporation from more heat. If we raise earth’s temperature, it takes time for snow and ice to decrease due to increased melting from more heat. Water vapor feedback, albedo feedback, in fact all the feedbacks except the default value, take time to show themselves. For some of them, like water vapor feedback, it doesn’t take very much time, only weeks to months. For others, like albedo feedback, it takes longer; extra heat can take decades or even longer to melt large ice masses and bring about albedo feedback. It can even take centuries for increased temperature to cause the release of CO2 from the warming oceans, so “carbon-cycle feedback” can be even more slow-moving. But the default feedback, due to the straightforward fact that warmer objects radiate more, is truly instantaneous. It’s the only one.

  4. “So there it is: the solution to global warming is as easy to describe as it is difficult to put into practice. Emissions of the six kinds of air pollutants causing the problem – CO2, methane, black carbon, halocarbons, nitrous oxide, and carbon monoxide, plus VOCs – must all be reduced dramatically. And we must simultaneously increase the rate at which they are removed from the air and reabsorbed by the earth’s oceans and biosphere.”

    Gore, Al. Our Choice: A Plan to Solve the Climate Crisis. (p. 49 paperback)

  5. “Charney was seeking the equilibrium global warming, the warming after the atmosphere and ocean have come to a new final temperature in response to increased carbon dioxide. The immediate effect of doubling carbon dioxide, if everything else were fixed, would be a decrease of about 4 watts (per square metre) in the heat radiation from Earth to space. This is simply physics… The added carbon dioxide increases the opacity (opaqueness) of the atmosphere for heat radiation, so radiation to space arises from a higher level, where it is colder, thus reducing emission to space.

    Any physicist worth his salt can immediately tell you the answer to Charney’s problem if everything except temperature is fixed… We can use Planck’s law to calculate how much Earth must warm up to radiate 4 more watts and restore the planet’s energy balance. The answer we find is 1.2 degrees Celsius. So the climate sensitivity in this simple case of Planck radiation is 0.3 degrees Celsius per watt of climate forcing.

    This simple Planck’s law climate sensitivity, 0.3 degrees Celsius per watt of climate forcing, is called the no-feedback climate sensitivity. Feedbacks occur in response to variations in temperature and can cause further global temperature change, either magnifying or diminishing the no-feedback, or blackbody, response. Feedbacks are the guts of the climate problem. Forcings drive climate change. Feedbacks determine the magnitude of climate change.

    Hansen, James. Storms of My Grandchildren. p.42 (hardcover)

  6. “The amount of atmospheric carbon dioxide during the Cenozoic varied from as little as 170 ppm in recent ice ages to 1,000 to 2,000 ppm in the early Cenozoic. Thus the largest carbon dioxide amount was probably close to three doublings of the smallest amount (170 -> 340 -> 680 -> 13,60). Large carbon dioxide change is usefully expressed as the number of doublings, because the infrared absorption bands become saturated as carbon dioxide increases. Additional absorption occurs in weak bands and at the edges of strong absorption bands, but it takes more and more carbon dioxide to yield a given increment of climate forcing. The result is that forcing increases by about 4 watts [per square metre] with each doubling.”

    Hansem, James. Storms of My Grandchildren.. (p. 156-7 hardcover)

  7. Not So Clear
    H. Jesse Smith
    Water vapor is the greenhouse gas with the greatest effect on the radiative balance of Earth’s atmosphere, and it amplifies climate warming through positive feedback. Therefore, knowing precisely how much radiative forcing water vapor provides is of great importance for understanding atmospheric physics and climate change. Ptashnik et al. report laboratory measurements of the absorption of radiation by water in the near-infrared, performed at a range of temperatures and pressures. The data show that spectrally broad continuum absorption (as distinct from the better-characterized series of sharper, higher-cross-section resonances) is actually much higher than commonly assumed in atmospheric models. The result of this difference amounts to a globally averaged value of about 0.75 W/m 2 of additional radiative forcing, roughly 0.2% of the total solar input at the top of the atmosphere and about 1% of the global mean clear-sky atmospheric absorption. The authors speculate that this extra absorption could be due to the effect of water dimers.

    J. Geophys. Res. 116, D16305 (2011).


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