Guest post by Ben Herman and Roger Pielke Sr.
We have received a further question on our post:“The Greenhouse Effect” by Ben Herman and Roger Pielke Sr.
The question is summarized by the following text
Anyway my question refers to the common example of taking away the atmosphere and observing a cold surface. But as I understand it, the mean daytime surface temperature on the moon is over 100 C, with no greenhouse effect. The mean nighttime temp drops to -150 C. http://www.solarviews.com/eng/moon.htm
This is important to note, because encouraging a popular picture in which the presence of the atmosphere only warms the surface takes all the convection and fluid dynamics out of the discussion, and that’s where all the important complexities are.
Isn’t it more the case that the atmosphere both warms and cools the surface, depending on circumstances? The IR absorption of H2O and other GHG’s warms the surface relative to what it would otherwise be, but as the lunar case shows, convection and turbulent mixing cools the surface relative to what would happen without an atmosphere. Take away the atmosphere and you take away both warming and cooling mechanisms.
We have reproduced the substance of our follow up answer below.
Predicting the surface temperature indeed involves the interaction of the atmospheric and ocean turbulent sensible and latent fluxes, long- and short- wave radiative fluxes and interfacial fluxes between the surface and the atmosphere. I have been urging for years to move away from the surface temperature to characterize global warming and cooling (and replace with ocean heat content changes in Joules) because the surface temperature is such a limited sample of the heat content changes of the climate system as well as involving these complicated feedbacks.
On the Moon, there is, of course, no atmosphere, so its surface temperature results from the difference between the surface long wave radiative emissions, the amount of solar radiation absorbed and reflected, and the conduction of heat into and out of the surface. The effect of the atmosphere on Earth is to mute the diurnal (and seasonal) temperature range as a result of the turbulent fluxes, and other effects (such as clouds and precipitation). These atmospheric effects, for example, result in lower daytime and higher nighttime temperatures from what they otherwise would be. I presume this is the cooling and warming effects that you refer to. However, even with these effects, the surface is clearly warmer than it would be without the CO2 and water vapor IR absorption bands.
But the reasons are that the atmosphere scatters back to space some sunlight, and takes up some of the surface heating through conduction, and mixes it it by convection and turbulence. Also, the relatively rapid rotation of the earth on its axis does not permit the daytime side to reach equilibrium before it starts nighttime cooling. As a result, daytime temperatures on earth are cooler than they would be with no atmosphere, and warmer at night than with no atmosphere.
Of course, the Moon, with no atmosphere, still has to have basically the same effective radiating temperature as does the Earth. This should be
[sigma *Tmd**4 + sigma* Tmn**4]/2 = sigma*Te**4 where Tmd is the daytime temperature of of the Moon, Tmn is the night time temperature of the Moon, and Te is the effective radiating temperature of the Earth.
The fact that the daytime time temperature is warmer than the Earth’s temp is simply a result of the fact that the Moon is not in an equilibrium state – it warms up during the daytime and cools down at night, just as does the Earth. However the warming during day and cooling at night must balance each other or the Moon ( and the Earth) would be steadily heating up or cooling down over time. The daytime warming occurs because the outgoing IR cannot balance the absorbed solar during the day. The nighttime cooling occurs because the outgoing IR is greater than the non-existing solar at night. The existence of a partially absorbing atmosphere does, as you stated, keep days cooler and nights warmer.
Also, the length of a day on the Moon is 29.5 earth days, almost a full Earth month. Therefore the daylight side of the Moon heats due to solar radiation, for half a month. Then when it’s night, it cools for another half month. Thus the daytime and nighttime temperatures are much more extreme. There is no greenhouse effect on the Moon, of course, and if the Moon’s day was the same 24 hours as an Earth day, its day and night temperatures would not vary as much but its radiative equilibrium temperature would be the same.
Update #2 John Nielsen-Gamon has alerted us to more information on the Moon’s radiative temperature. John e-mailed
I read your blog post on Greenhouse Part 2. I also recently came across the Science of Doom web site; it seems to be of very high quality. You might want to link to http://scienceofdoom.com/2010/06/03/lunar-madness-and-physics-basics/ [on] your post to direct the reader to further details on the radiative temperature of the Moon.
Update – corrected text (underlined) h/t to Gerald E. Quindry