Common Fallacies: Radiation and Greenhouse Gases

Radiation-Related

Fallacy:

Greenhouse gases can't raise the tempature.

The basic idea behind this view is that energy leaving the system will equal the energy entering the system when the climate establishes an equilibrium, and therefore greenhouse gases can't have any long-term effects upon the temperature. From this the arguer will typically conclude that the only force that can change the long-term climate of the earth is the sun.

Basic Response:

While a new equilibrium will result in the rate of energy leaving the system being equal to the amount of energy entering the system, the level of energy within the system may be higher or lower. Higher greenhouse gas levels will generally raise the level of energy within the system.

Different Responses:

  1. Without greenhouse gases the earth would be much cooler. With this response, one calculates the temperature of the earth in the absence of the atmosphere.
  2. The bathtub analogy In this response, one uses an analogy involving a bathtub with water pouring into it at a constant rate. Higher level of greenhouse gases correspond to a smaller drain.
  3. The Pinball Machine Analogy The climate is viewed as a pinball machine, pinballs correspond to photons, and higher levels of greenhouse gases are represented by more bumpers being added to the pinball machine.
  4. We know that greenhouse gases raise temperatures as the result of demonstrable physics. Our knowledge of the effects of greenhouse gases flows from our understanding of branches of physics which deal with radiative transfer and spectroscopy.

Fallacy:

Paleoclimate records demonstrate that it is temperature changes which drive changes in the level of carbon dioxide, not the other way around.

I saw the chart used by Al Gore in "An Inconvenient Truth" and the chart showed rises in carbon dioxide following rises in temperature.

Basic Response:

In the case of greenhouse gases, we are not dealing with a simple cause-effect relationship, but two things which are interdepedent. Changes on either the temperature side or the carbon dioxide side will lead to changes on the other side of the equation.

Additional Points:

  1. Positive feedback exists between the temperature level and the level of carbon dioxide. If the temperature rises, this will tend to result in more carbon dioxide entering the atmosphere as the result of a diminished capacity of the ocean to act as a sink for carbon dioxide. However, raising the level of carbon dioxide in the atmosphere will raise reduce the ability of the global climate system to emit thermal infrared radiation into space below the rate at which thermal radiation is entering the system, thus resulting in higher temperatures until the system reaches the point at which the amount of thermal radiation leaving the system equals the amount of thermal radiation leaving the system.
  2. There are periods in the past where carbon dioxide rose first. A good example of this is would be the Paleocene-Eocene Thermal Maximum (PETM), but there are others including the Quaternary ice ages and the Cretaceous.

Fallacy:

Water vapor is more important than carbon dioxide

The proponent of this argument will typically say something to the effect that, "People are neglecting carbon dioxide when it is obvious that water vapor is more important."

Basic Response:

Water vapor has a stronger greenhouse effect than carbon dioxide, but it is carbon dioxide which largely regulates the amount of water vapor which is in the atmosphere.

Additional Points:

  1. Carbon dioxide stays in the atmosphere longer. Left to itself, water vapor won't remain in the atmosphere for very long. Perhaps about two weeks. While there is no one figure for how long carbon dioxide will stay in the atmosphere due to there being different sinks which absorb carbon dioxide at different rates, a substantial percentage will remain in the atmosphere for over a century, and some of what we emit will stay in the atmosphere for millenia.
  2. By itself, water vapor won't raise the temperature except by a small amount and only over a very short amount of time. Because water vapor stays in the atmosphere for a few weeks before falling out as rain, what we put into the atmosphere tends not to stay there long enough to significantly raise the temperature.

Fallacy:

The greenhouse effect only occurs near the ground.

Long wave radiation gets absorbed before it gets a chance to go any higher.

Basic Response:

There is a kernel of truth to this: namely, that the infrared radiation which enters the system is principally the result of sunlight being absorbed at the surface and re-emitted as long wave radiation which is first absorbed near the surface. However, whatever energy enters the system has to get out somehow. This implies that the greenhouse effect isn't simply at ground level.

Additional Points:

  1. Long wave radiation will be absorbed first near the ground, but then it will tend to be re-emitted. When it is re-emitted where ever it is re-emitted, it could be re-emitted in either direction. At some point, it has to make a random walk out of the atmosphere and into space.
  2. This isn't the only way that such radiation is able to ascend. In part it will also be carried by means of moist air convection. But this will get it only part way before it is re-emitted. And in the stratosphere it no longer has the luxury of hitching a ride on moist air convection since the air is very dry. And at that point the only effective way for the radiation to escape will be by means of a random walk of absorption and re-emission.

Fallacy:

The effects of water vapor swamp those of carbon dioxide.

If you look at the troposphere, nearly all long wave radiation will get absorbed before carbon dioxide has even a small chance of absorbing it.

Basic Response:

Only in the lower atmosphere. The greenhouse effect which occurs as the direct result of carbon dioxide is something which occurs principally in the stratosphere.

Additional Points:

  1. Water vapor is limited to approximately the lower 5 km of atmosphere.
  2. Carbon dioxide is fairly evenly distributed through out the atmosphere.
  3. Thermal radiation has to leave the system at some point. The long wave radiation which reaches the level where water vapor is absent is therefore subject to the greenhouse effect where carbon dioxide dominates.
  4. The effects of carbon dioxide are amplified by water vapor. When long wave radiation reaches the level at which carbon dioxide dominates, it may then either head up or head down. If this radiation reaches the surface or otherwise heats the surface, it can result in more water vapor feedback and a stronger greenhouse effect.

Fallacy:

Even in the stratosphere, the amount of carbon dioxide is too small to have any effect.

After all, the parts per million of carbon dioxide in the stratosphere is so much smaller than water vapor in the lower atmosphere.

Basic Response:

In the case of greenhouse gases, it isn't so much a matter of how much, but how many times the quantity of a given gas gets doubled.

Additional Points:

  1. The increase in temperature is roughly proportional to the logarithm of the CO2 concentration minus the logarithm of the original CO2 concentration.
  2. If there isn't that much carbon dioxide, then it doesn't take that much more carbon dioxide to double the amount.
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License