A short primer: The Greenhouse Effect Explained

The greenhouse effect is real.  If there were no greenhouse gases in the atmosphere, earth would be a cold place.   Compare Mars versus Venus – Mars has minimal greenhouse gas molecules in its’ atmosphere due to low atmospheric pressure, and is cold.  By contrast, Venus has a lot of greenhouse gas molecules in its’ atmosphere, and is very hot.  Temperature increases as greenhouse gas concentration increases.  These are undisputed facts.

outgoing radiation = incoming radiation – changes in oceanic heat content

The image below from AER Research explains the radiative balance.

http://www.aer.com/scienceResearch/rc/rc.html

About 30% of the incoming shortwave radiation (SW) is reflected by clouds and from the earth’s surface.  20% is absorbed by clouds and re-emitted back into space as longwave (LW) radiation.  The other 50% reaches the earth’s surface and warms us.  All of that 50% eventually makes it back out into space as LW radiation, through intermediate processes of convection, conduction or radiation.  As greenhouse gas concentration increases, the total number of collisions with GHG molecules increases.  This makes it more difficult for LW radiation to escape.  In order to maintain equilibrium, the temperature has to increase.  Higher temperatures mean higher energies, which in turn increase the frequency of emission events.  Thus the incoming/outgoing balance is maintained.

Click for larger image

http://www.aer.com/scienceResearch/rc/m-proj/lbl_clrt_mls.html

The important greenhouse gases are: H2O, CO2, O3, N2O, CO and CH4.  The reason why the desert can get very cold at night is because of a lack of water vapor.  The same is true for Antarctica.  The extreme cold in Antarctica is due to high albedo and a lack of water vapor and clouds in the atmosphere, which results in almost all of the incoming radiation returning immediately to space.

An earth with no CO2 would be very cold.  The first few tens of PPM produce a strong warming effect, and increases after that are incremental.  It is widely agreed that a doubling of CO2 will increase atmospheric temperatures by about 1.2C, before feedbacks.  So the debate is not about the greenhouse effect, it is about the feedbacks.

Suppose that the amount of reflected SW from clouds increases from 20% to 21%?  That would cause a significant cooling effect.  Thus the ability of GCM models to model future temperatures is largely dependent on the ability to model future clouds.  Cloud modeling is acknowledged to be currently one of the weakest links in the GCMs.  Given the sensitivity to clouds, it is perhaps surprising that some high profile climate scientists are willing to claim that 6C+ temperature rises are established science.

So the bottom line is that the greenhouse effect is real.  Increasing CO2 will increase temperatures.  If you want to make a knowledgeable argument, learn about the feedbacks.  That is where the disagreement lies.