Friday, October 18, 2013

A primer on the greenhouse effect

Looking back at my first posts, I realized that I had neglected to explain the greenhouse effect.  This post is intended to rectify that omission.

First a basic principle: All energy that enters or leaves the Earth's atmosphere must be in the form of radiation.  And yes, that includes heat.  There's no atmosphere in space so heat cannot be lost from the planet via conduction and convection.  The general process is as follows.

1) Energy from the sun (including ultraviolet and visible radiation) enters the atmosphere.

2) About 30% of the sun's radiation is reflected back into space by aerosols in the air (produced by volcanoes and coal-fired power plants) or via snow, ice, and other light-colored surfaces.  Of the 70% that reaches the ground, most is visible light as most ultraviolet is absorbed by the ozone layer.

3) When visible light reaches the ground, the energy is absorbed by the surfaces on the ground.  That absorbed energy is reradiated as infrared radiation (also known as heat).  This is how your car heats up on a sunny day, especially in the summer, as the dashboard, steering wheel, and seats all absorb visible light and reradiates that energy as heat.

4) The rate at which the absorbed energy is reradiated is determined by concentration gradients.  The greater the gradient, the faster heat is lost, as we all have experienced.  You lose far more heat when you go outside without a jacket at 10ºC (50ºF) than you do at 25ºC (77ºF) because the gradient between your skin temperature and the air is far greater at an air temperature of 10ºC than at 25ºC.

5) Infrared is absorbed by tri-atomic molecules (H2O, CO2, NO2, O3) or higher (CH4) in the atmosphere, which then reradiate that energy in all directions, including back toward the Earth's surface.  Diatomic molecules (N2, O2) are invisible to infrared radiation and so do not have any role in the greenhouse effect.

6) The infrared that is reradiated back to the surface warms the Earth by decreasing the concentration gradient between the surface of the Earth (including the surface of the oceans) and the air just above the surface.  This slows the rate at which heat is lost from the surface, causing the surface to retain more of the heat and raising the temperature.

7) Eventually, the infrared makes it back out of the atmosphere into space.

While the term "greenhouse effect" is better known, I personally think a better analogy is "blanket effect" as in reality the greenhouse effect keeps the Earth warm in a similar way to the way a blanket keeps a person warm.  A blanket does not produce any heat on its own but merely absorbs a person's radiated heat and then reradiates that heat in all directions, including back toward the skin.  That decreases the temperature gradient between the skin and the surrounding air, causing less heat to escape from the skin and more of that heat to be retained inside the body.

For the Earth to maintain thermodynamic equilibrium, the amount of infrared lost must equal the amount of solar energy that reaches the surface.  If there is an imbalance, the Earth will either warm if the imbalance is positive or cool if the imbalance is negative.

Detailed diagram of the greenhouse effect from Trenberth et al. 2009

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