Of gray bodies and green houses...

[Editor]

It is very common in the to and fro of today’s enhanced greenhouse shouting matches for people to claim accepted calculations of the planet’s expected temperatures are invalid because Earth is not a black body but a gray one and that greenhouse effect is “mythical”.

Perhaps the simplest way of testing the existence of greenhouse effect is to ask: “does the Earth heat and cool as expected from insolation and radiation alone?”

Fortunately we have a local exemplar which is both gray and virtually devoid of greenhouse effect by virtue of being virtually devoid of atmosphere -- the Moon.

By day (that is, on the sun side) the Moon reaches surface temperatures of about 130 °C, by night it falls to about -150 °C (-233 °C in the permanently shaded South Polar Basin), so our gray body temperature increases about 280 °C from just before dawn to Lunar noon. Average Lunar albedo is just 12% so it reflects significantly less solar energy than Earth does. A meter below the surface the Moon is a near-constant -35 °C so we know the surface variation is a response to insolation and radiation alone.

Fortunately, Earth is very different and despite the fact Earth reflects almost one-third of incoming solar radiation even during ice ages the mean surface temperature is a relatively life-friendly 9-10 °C. Currently it’s a significantly more life-friendly 15 °C.

Isn’t it radioactive decay of crustal elements and volcanism that keeps it warm?

Actually not. Elemental decay is constant, Edited for some really sloppy statements:

As LarryOldtimer rightly calls me on, that statement is invalid as it stands. Radioactive elements have specific half-lives and the amount of radioactive materials declines over time, as of course, does the heat generated by this decay.

What I should have said is that isotopic decay and geothermal activity do not vary according to a nocturnal/diurnal cycle, which is not the same as "constant". Now we return to where I was going in the first place: End edit

... as is the pressure that keeps the planet’s core hot so we can cancel those from calculations and see that Earth’s equator should heat dramatically from dawn and cool equally dramatically overnight and yet there is a trivial few degrees variation between night and day.

So, why doesn’t the planet’s equatorial region alternately cook and freeze?

That is because it has something high altitude and desert regions lack -- an atmosphere so replete with greenhouse gases (mainly water vapor and droplets) as to be infrared opaque.

What’s that mean? It means the tropics do not cool by surface radiation.

Tomorrow I’ll be posting on why I’m unconcerned about enhanced greenhouse.

[LarryOldtimer]

Elemental decay is constant

Stupid me.  Here I was, taught physics in college back in the 1950s when I majored in physics, thinking that radioactive elements/isotopes had half-lives, and that there were diminishing amounts of all of these elements/isotopes over time, meaning that as time passes, there would be less heat produced, as there would be less mass of radioactive material to decay.  What wonderful news.  No longer will the fissionable materials in nuclear power plants have to be changed out.  Perpetual energy machines have arrived at last.

I would have to guess that everything I was taught in Thermodynamics was all wrong too.  Getting me feeling sorry for myself, you are.

[Editor]

Here I was, taught physics in college back in the 1950s when I majored in physics, thinking that radioactive elements/isotopes had half-lives, and that there were diminishing amounts of all of these elements/isotopes over time, meaning that as time passes, there would be less heat produced, as there would be less mass of radioactive material to decay.

And how right you are pick me up on that Larry

The thought (that didn't make it to paper) was that radio decay does not vary by time of day and so has no bearing on nocturnal/diurnal temperature differentials. Entirely my fault for paying too much attention to where I was going and not where I was at the time -- will return to edit and clarify such sloppy statements.

My very bad.

[LarryOldtimer]

Aw, it was late, and I had gotten a bit testy (getting to be an old geezer now).  I should simply have pointed it out without the snide remarks.  But . . . I do have to wonder at times whether the natural laws of physics and chemistry are even getting very much coverage in  the university system or today.

[Editor]

No offense taken Larry, the statement was invalid and errors should be called. I'm glad you did so rather than leaving room for misinterpretation and I'm happy to make corrections.

On a general note, this is an open forum and everyone should feel free to speak up if they disagree with or don't fully understand assertions made. Language is imperfect and users of it even more so -- contention and refinement can lead to clarity. The forum is here for all to use but none to abuse.