The “Greenhouse Effect”
What it is and its relationship to global warming:
First and foremost, you can’t even talk about global warming without referencing the greenhouse effect. Likewise, you can’t discuss the greenhouse effect and its influence on the climate without first defining the climate itself. Simply enough, climate is the long-term average of sensible weather elements that characterize the general conditions of the atmosphere over a given period of time at some particular point or region of the Earth's surface. These elements would typically include solar radiation, temperature, humidity, clouds and precipitation (what type, how often and when, and amount), atmospheric pressure, and wind speed and direction. “Long-term” in this case is often rather ambiguous; however The World Meteorological Organization (WMO) “officially” requires that this averaging take place over a period of 30 consecutive years, which was selected as a length of time long enough “to eliminate year-to-year variations.” Even this is rather arbitrary, as you will see “official” reports talking about or referencing climates that are based on everything from one month to million year averages. For our purposes, remember that the definition of climate is based upon the sum of the daily weather.
The case for advocating anthropogenic global warming (AGW) is based almost entirely upon the presence of carbon dioxide (CO2) in the atmosphere, CO2’s contribution to the “greenhouse effect”, historically rising global averaged temperatures, climate model predictions which show that increasing CO2 leads to increasing the Earth's average temperature (cause and effect), and a supposed “consensus” among scientists that all of this is so. While we will be tackling all those various points concerning AGW, for now all we need to keep in mind is that AGW is a function of the “greenhouse effect.”
The so-called “greenhouse effect” works by allowing much of the incoming solar radiation (shortwave radiation) to pass through the Earth's atmosphere, warming the surface and lower atmosphere (transmittance), but preventing most of the outgoing infrared radiation (longwave radiation) from escaping into outer space through chemical bond absorption of the energy followed by reemission (emissivity) of the radiation. The Earth maintains its atmospheric energy balance through this process which is not so much “trapping” as it is redistributing (see diagram below). Put this altogether and you have the so-called “greenhouse analogy,” a process that occurs naturally and has the effect of keeping the Earth's average temperature at a tolerable level for life – about 59-60 degrees Fahrenheit (15 degrees Celsius).
(Figure 1 – NASA – public domain).
There are, however, major differences between a greenhouse atmosphere and the earth’s atmosphere – and SIZE does matter! In a greenhouse, except for a very short period of time at the beginning of the first heating cycle (sunrise) and on a very small scale, the convective process is virtually eliminated because of the relatively small volume to heat and because it is physically cut off from the outside atmosphere by the glass itself. The literal greenhouse is working because it is small, contained, limits convection, and heats (mostly) by conduction - not because it traps heat through absorption and re-radiation - as can be proved if you replace the “glass” with certain clear plastics which do not absorb longwave or shortwave radiation. We meteorologists call this convective interuptus (a highly scientific term). Ironically, in order to keep the temperatures moderate enough for plants, the greenhouse operator must “artificially” induce convection by opening the windows to the atmosphere. In the real atmosphere, except under strong thermal inversion situations (where the temperature actually warms as you ascend), convection is encouraged. We therefore live in an open greenhouse.
The quantitative evaluation of the greenhouse effect of the atmosphere (Ga) is given by the equation: Ga = E – Fc, where E is the emission from the Earth’s surface
(E = σ Ts4 with σ = 5.67 10-8 W/m2 K-4 and Ts is the surface temperature) and Fc is outgoing longwave radiation (OLR) for clear skies.[1] Total greenhouse effect (G) equals the combined effect of cloud and the atmosphere on the greenhouse effect and is given by: G = Ga + C1, where C1 = Fc – F and F is the OLR for average cloudy conditions. More on these when we take a look at climate modeling in the next chapter.
[1] Ramanathan, V. and A. Inamdar, 2006: “The Radiative Forcing due to Clouds and Water Vapor” in Frontiers of Climate Modeling, J. T. Kiehl and V. Ramanthan, Editors, (Cambridge University Press 2006), pp. 119-151.
