Notes: Mon. 30 Sept

The Greenhouse Effect

We started last time to say that the Greenhouse effect is a natural process in which "Greenhouse gases" absorb and reradiate long wavelength energy, heating the Earth to a livable temperature.

Some of these greenhouse gases are H2O, CO2, methane, and nitrogen oxides. There are lots of greenhouse gases, but the main one you hear about is carbon dioxide. Why? Because humans are adding a lot of CO2 to the atmosphere by burning fossil fuels. Why does it matter?

  • CO2 has a long residence life in the atmosphere. That is, once a CO2 molecule is put into the atmosphere, it is not removed quickly.
  • Not surprisingly, there has been an observed increase in atmospheric CO2 over time.
  • And as atmospheric CO2 has increased, so have global temperatures. This effect has been termed global warming.

    As you saw in lab, not everyone is convinced that increasing greenhouse gas emissions causes global warming. There is still scientific controversy because:

  • the observed amount of global warming is much less than originally predicted. We measure changes in temperature in tenths of degrees per year, which is barely statistically significant.
  • North America, home of the most influential lawmakers, does not follow the global trend of warming. Instead, it has cooled.

    These two facts make people skeptical about the reality of global warming. However, there is an explanation for both: global climate models are not refined enough to predict the enormously complex climate system. Some factors which may change our expected outcome are:

  • ***the role of sulfate aerosols*** (described in more detail below)
  • natural variations in the climate system
  • variations in solar input can cause temperature variations: models assume the Earth constantly receives the same amount of energy from the Sun.
  • absorbency of clouds

    The Role of Aerosols

    An aerosol is any particle, liquid or solid, suspended in the air. Aerosols may be responsible for weakening the effect of global warming through two mechanisms:
  • the particles themselves reflect away incoming energy
  • the particles act as nuclei around which clouds condense. Then the clouds also reflect away incoming radiation.

    There has been a recorded increase in amount of certain aerosols in the atmosphere. We must examine where these aerosols are coming from.

    Remember that this topic is still quite controversial today because models are not yet refined enough to be able to predict climate change perfectly. There are many factors that are not being taken into account, including feedback loops. An example of a feedback loop: colder temperatures cause an increase in glaciers which leads to higher albedo and more energy reflected away from Earth which causes colder temperatures. This particular feedback loop is a positive feedback loop (where cold temperatures lead to colder temperatures). The point is that processes like these are very hard to model, so opponents of global warming theories simply say that you can't trust the models.

    In summary, if you believe the models, there are two competing effects here:
    Atmospheric greenhouse gases, notably carbon dioxide, are increasing due to the burning of fossil fuels. This acts to increase temperatures overall and leads to global warming. On the other hand, the same industries and automobiles which are emitting carbon dioxide are also creating sulfate haze. This cools the Earth by blocking incoming radiation. So the temperature increase expected from greenhouse gases is not as big.

    NOTE: Ozone depletion is a completely different thing. That deals with the destruction of ozone by CFC's. The mechanism for ozone depletion takes place in the stratosphere (remember "polar stratospheric clouds, or PSC's"?). When talking about greenhouse gases and sulfate haze, we are mostly dealing with the troposphere.

    Atmospheric Stability

    In order to understand the movement of the atmosphere, we must understand why it moves at all. Atmospheric stability is the tendency of a parcel of air to remain where it is, whereas instability is the tendency of the parcel of air to change its position. Here we are talking about vertical motions (ascending or descending).

    AS AIR RISES, IT COOLS. AS AIR DESCENDS, IT HEATS UP. How much it heats or cools at it moves is governed by either the dry adiabatic lapse rate (DAR) or the saturated adiabatic lapse rate (SAR). "Adiabatic" simply means that the parcel neither gains nor loses heat as it moves.

  • The DAR = 10oC/1000 meters. This means that as the air mass goes up 1000 m, it cools down 10 degrees, and as it descends 1000 m, it heats up 10 degrees. The DAR is used so long as the air mass is not saturated, that is, as long as it is not at 100% relative humidity.
  • The SAR = 6oC/1000 meters. It is used when the air mass has reached saturation.

    Notice that saturated air changes temperature more slowly than dry air. In moist air, some latent energy is released during condensation or absorbed during evaporation. This extra energy helps moderate the temperature.

    A third rate, called the environmental lapse rate, or ELR defines whether or not the air mass will be stable. The ELR describes the temperature of the surrounding air that our moving air parcel must travel through. The average ELR is 6.5 oC/1000 meters, but it can vary.

    These three rates determine stability as follows:

  • If the ELR < SAR < DAR, then any rising air parcel - saturated or not - will want to sink back down because it will be colder than the surrounding air. The atmosphere is stable.
  • If the SAR < DAR < ELR, then any parcel of rising air will be warmer than the surrounding air. Since warmer air wants to rise, the parcel will continue rising, and the air is unstable.
  • If the SAR < ELR < DAR, then the rising air parcel will be stable if it is dry; it will be unstable if it is saturated. This is called "conditionally stable."

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