Sunday, May 06, 2007

An Inconvenient Truth

I finally watched An Inconvenient Truth, the documentary about Global Warming as explained by Al Gore. I had expected all kinds of distortions of the truth after reading the popular press. However, I was pleasantly surprised by the movie's accuracy. He had the science straight. The few places where people can quibble with his science are minor and, I believe, might be due to oversimplifying the science for the lay audience.
  • The National Snow and Ice Center posted a Q&A about the movie that concurs with my assessment that the science facts in the movie are as right as can be explained in layman's terms.
  • Jeff Masters, chief meteorologist for Weather Underground, posted a excellent critique of the movie. He mostly agrees with Gore's assessment of the science, but has a quibble with the thickening the atmosphere remark. I was puzzled by the same remark, but I think I know what Gore was trying to say. More on that later. His closing paragraph is especially worth reading. "NSTA (National Science Teachers' Association) does not offer much content on climate change in their list of recommended materials. One of the recommended books, "Global Warming: Understanding the Debate" has no business being on their recommended reading list. This book is written by Kenneth Green, a fellow of the American Enterprise Institute (AEI). This fossil-fuel funded think tank recently offered $10,000 to any scientist willing to criticize the recent landmark 2007 Summary of Policy Makers climate change report issued by the United Nations sponsored Intergovernmental Panel on Climate Change (IPCC)."
  • National Geographic polled several scientists and they mostly concur with Gore. Like me, they think that the fault is with over simplification.
  • Read Gregg Easterbrook's piece in Slate and wonder what kind of ax he has to grind with Al Gore. "An Inconvenient Truth spends too little time on what audiences might do about global warming, too much time trying to impress us with the Ask Mr. Science side of Gore's personality." Then why does he devotes the first half of the article criticizing Gore and not the science? Then he zeroed in on the atmospheric thickening remark and tried to impress the readers with his own knowledge of quantum mechanics. "Carbon dioxide, the greenhouse gas produced by fossil-fuel combustion and forest fires, has molecular bonds that vibrate on the same wavelengths at which infrared energy radiates upward from Earth's surface; the vibration warms the CO2, trapping heat. The main atmospheric gas, nitrogen, does not absorb energy on those wavelengths. It is the chemistry of carbon dioxide, not its density, that matters." More on this later.
I was also initially puzzled by the remark about thickening of the atmosphere. Easterbrook and Masters interpreted the remark as meaning that the atmosphere would become denser or more viscous, which would not be true. However, meteorologists often use a prognostic called the 500 millibar (mb) height as a proxy for surface temperature. (Surface pressure at sea level is 1000 mb; 500 mb is the half height of the atmosphere. This is a convenient measure because the atmosphere has a soft boundary with no real cutoff point.)

Warm air expands and the earth's atmosphere actually grows in volume when the surface warms. Look at the 500 mb heights from the European Center for Medium Range Forecasting, ECMWF model at weather.unysis.com. See how the 500 mb height is about 5000 meters high in the colder regions and 5800 meters high in the warmer regions? I have heard meteorologists say in shorthand thickening of the atmosphere when referring to a warming trend.

OK, back to Gregg Easterbrook's own Mr. Science comment. First off, molecular vibrations are in the InfraRed (IR) part of the electromagnetic (EM) spectrum. Molecular rotations are mostly in the microwave part of the EM spectrum. The earth absorbs EM radiation in the UV and visible (higher energy) wavelengths and then reradiates them out in the IR and microwave (lower energy) wavelengths.

Molecular nitrogen comprises about 80% of our atmosphere and has two nitrogen atoms. Each atom has 3 degrees of freedom (DoF) for a total of 6 DoF. 3 of them are taken up by translation, 1 by vibration and 2 by rotation. Try this with a pencil, you can rotate it two ways.

Now consider O-C-O. It has 3 atoms, 9 DoF. It has 3 translational DoF, 4 vibrational DoF and 2 rotational DoF. See this cool applet of CO2 vibrations. The pull-down menu only had 3 types of modes, but the bending mode is degenerate, meaning it can happen in two different directions and count as 2 DoF.

4 vibrational modes can hold more energy than 1 vibrational mode. It is as simple as that. (oops, see correction * below) Methane, CH4, has 6 vibrational DoF (3 rotations because it is nonlinear) and has even more heat capacity. The frozen tundra contains methane. Melt them, release the methane, and we have moved to an all new regime. Yipes.

Energy transfer via molecular vibrations and rotations is not really chemistry. See, no molecular bonds were made or broken. It is the structure of CO2
. I think Mr. Easterbrook should get his own facts straight before casting any stones. ;-)

* Mark reminded me that N2 and O2 are homogeneous diatomic molecules. Their dipole moment does not change when they stretch. Hence, their vibrations are infrared inactive. Oops, I forgot about that. It has been a long time since I taught molecular spectroscopy. Luckily, he currently works as an infrared spectroscopist.

The careful reader will think, "Wait a minute, the dipole moment of CO2 does not change when you stretch it out." Yes, the symmetric stretches of CO2 and CH4 are also infrared inactive. But CO2 has a heck of a dipole.

1 comment:

  1. I thought the "thickening the atmosphere" comment was in reference to the larger molecular mass of CO2 (44) versus water vapor (18), N2 (28) and O2 (32).

    A friend with a pilots license once told me that this was part his curriculum. Contrary to expectations, the atmosphere is denser on dry days than wet days, but it sure doesn't feel like it.

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