"What is the atmospheric greenhouse effect?
It is the warming of the surface and lower atmosphere caused by downward infrared emission by the atmosphere, primarily from water vapor, carbon dioxide, and clouds.
Greenhouse gases and clouds cause the lower atmosphere to be warmer, and the upper atmosphere to be cooler, than if they did not exist…
just as thermal insulation in a house causes the inside of a heated house to be warmer and the outside of the house to be cooler than if the insulation was not there.
While the greenhouse effect involves energy transfer by infrared radiation, and insulation involves conduction, the thermodynamic principle is the same.
Without greenhouse gases, the atmosphere would be unable to cool itself in response to solar heating.
But because an IR emitter is also an IR absorber, a greenhouse atmosphere results in warmer lower layers — and cooler upper layers — than if those greenhouse gases were not present.
As discussed by Lindzen (1990, “Some Coolness Concerning Global Warming”), most of the surface warming from the greenhouse effect is “short-circuited” by evaporation and convective heat transfer to the middle and upper troposphere.
Nevertheless, the surface is still warmer than if the greenhouse effect did not exist:
the Earth’s surface emits an average of around 390 W/m2 in the thermal infrared even though the Earth absorbs only 240 W/m2 of solar energy.
I have demonstrated before how you can directly measure the greenhouse effect with a handheld IR thermometer pointed at the sky. (Next article, below)
I say “directly measure” because an IR thermometer pointed at the sky measures the temperature change of a thermistor exposed to varying levels of IR radiation, just as the temperature of the Earth’s surface changes in response to varying levels of downwelling IR radiation.
I recently purchased a FLIR i7 thermal imager, which instead of measuring just one average temperature, uses a microbolometer sensor array (140 x 140 pixels) to make 19,600 temperature readings in an image format.
These are amazing little devices, originally developed for military applications such as night vision, and are very sensitive to small temperature differences, around 0.1 deg. C.
Because these handheld devices are meant to measure the temperature of objects, they are tuned to IR frequencies where atmospheric absorption/emission is minimized.
The FLIR i7 is sensitive to the broadband IR emission from about 7.5 to 13 microns.
While the atmosphere in this spectral region is relatively transparent, it also includes some absorption from water vapor, CO2, oxygen, and ozone.
The amount of atmospheric emission will be negligible when viewing objects only a few feet away, but is not negligible when pointed up at miles of atmosphere.
Everything around us has constantly changing temperatures in response to various mechanisms of energy gain and loss, things that are normally invisible to us, and these thermal imagers give us eyes to view this unseen world.
I’ve spent a few days getting used to the i7, which has a very intuitive user interface.
I’ve already used it to identify various features in the walls of my house; see which of my circuit breakers are carrying heavy loads; find a water leak in my wife’s car interior; and see how rain water flows on my too-flat back patio.
While the FLIR i7 is designed to not register temperatures below -40 deg. F/C (and is only calibrated to -4 deg. F) you can see that sky brightness temperatures well above this are measured
This is direct evidence of the greenhouse effect: the surface temperature of the microbolometer within the thermal imager is being affected by downwelling IR radiation from the sky and clouds, which is exactly what the greenhouse effect is.
If there was no downward emission, the sky temperature as measured by a perfectly designed thermal imager would read close to absolute zero (-460 deg. F), that is, it would measure the cosmic background radiation if the atmosphere was totally transparent to IR radiation.
Just so there is no confusion:
I am not talking about why the indicated temperatures are what they are…
I am talking about the fact that the surface temperature of the microbolometer is being changed by IR emission from the sky.
THAT IS the greenhouse effect.
"Help! Back Radiation has Invaded my Backyard!, by Roy Spencer, PhD
... Laypersons are no doubt confused by all of our recent esoteric discussions regarding radiative transfer, and whether global warming is even possible from a theoretical standpoint.
So, let’s take a break and return to the real world, and the experiments you can do yourself to see evidence of the “greenhouse effect”.
One of the claims of greenhouse and global warming theory that many people find hard to grasp is that there is a large flow of infrared radiation downward from the sky which keeps the surface warmer than it would otherwise be.
Particularly difficult to grasp is the concept of adding a greenhouse gas to a COLD atmosphere, and that causing a temperature increase at the surface of the Earth, which is already WARM.
This, of course, is what is expected to happen from adding more carbon dioixde to the atmosphere: “global warming”.
Well, it is one of the marvels of our electronic age that you can buy a very sensitive handheld IR thermometer for only $50 and observe the effect for yourself.
These devices use a thermopile, which is an electronic component that measures a voltage which is proportional to the temperature difference across the thermopile.
If you point the device at something hot, the higher-intensity IR radiation heats up the hot-viewing side of the thermopile, and the IR thermometer displays the temperature it is radiating at (assuming some emissivity…my inexpensive unit is fixed at e=0.95).
If you instead point it at the cold sky, the sky-viewing side of the thermopile loses IR radiation, cooling it to a lower temperature than the inside of the thermopile.
For instance, last night I drove around pointing this thing straight up though my sunroof at a cloud-free sky.
I live in hilly territory, the ambient air temperature was about 81 F, and at my house (an elevation of 1,000 feet), I was reading about 34 deg. F for an effective sky temperature.
If the device was perfectly calibrated, and there was NO greenhouse effect, it would measure an effective sky temperature near absolute zero (-460 deg. F) rather than +34 deg. F,
and nighttime cooling of the surface would have been so strong that everything would be frozen by morning.
Not very likely in Alabama in August.
What was amazing was that driving down in elevation from my house caused the sky temperature reading to increase by about 3 deg. F for a 300 foot drop in elevation.
My car thermometer was showing virtually no change.
This pattern was repeated as I went up and down hills.
The IR thermometer was measuring different strengths of the greenhouse effect, by definition the warming of a surface by downward IR emission by greenhouse gases in the sky.
This reduces the rate of cooling of the Earth’s surface (and lower atmosphere) to space, and makes the surface warmer than it otherwise would be.
If you have a day where there are patches of blue and clouds, you can point the thermometer at the clouds and pick up a warmer reading than the surrounding blue sky.
I did it this morning.
When I moved from a view of the blue sky to the patch of clouds, the sky-viewing side of the thermopile became warmer…even though the thermopile is already at a higher temperature than the sky.
The display would read a few degrees warmer than the reading looking at blue sky.
If you perform this experiment yourself, you need to be careful about the elevation angle above the horizon you are pointing being about the same.
Even in a clear sky, as you move from the zenith (overhead), down toward the horizon the path length of sky the IR thermometer sees increases, and so you measure radiation from lower altitudes, which are warmer.
This makes the effective sky temperature goes up.
(This is ALSO evidence of the greenhouse effect, since looking at the sky above the horizon is like adding greenhouse gases to the atmosphere overhead.
The (apparent) concentration of greenhouse gases in the lower atmosphere goes up, and so does the intensity of the back radiation.)
Even earlier in the morning, about 5:30, the middle-level clouds were thicker, and I measured a sky temperature in the 50’s F. We will see more evidence of that using air temperatures, below.
This shows that the addition of an IR absorber/emitter, even at a cold temperature (the middle level clouds were probably somewhere around 30 deg. F), causes a warm object (the thermopile) to warm even more! This is the effect that some people claim is impossible.
Remember, the IR thermometer calibrated temperature output is based upon real temperatures, the temperatures on either side of the thermopile.
And if you think this is just an effect of some sunlight reflecting off the cloud….read on.
Evidence from The Box
I have been seeing the same effect in “The Box”, which is my attempt to use the greenhouse effect to warm and cool a thin aluminum plate coated with high-emissivity paint,
that is heavily insulated from its surroundings in order to isolate just the radiative transfers of energy between the sky and the plate.
This can be considered a clumsy, inefficient version of the IR thermometer.
But now, *I* am making actual temperature measurements.
The following plot shows data from the last 2 days, up through this morning’s events.
In the lower right, I have also circled where thin middle-level clouds came over, emitting more IR radiation downward than the clear sky, and causing a warming of the plate.
Since the plate is mostly isolated from heat exchanges with the surrounding air and warm ground, it responds faster than the ambient air temperature to the intensity of “back radiation” downwelling from the sky.
When I woke this morning before sunrise, around 5:30, I saw these mid-level clouds (I used to be a certified aviation weather observer), I measured about 50 deg. F from the handheld IR thermometer.
This supports what people already experience…cloudy nights are, on average, warmer than clear nights.
The main reason is that clouds emit more IR downward, change the (im)balance between upwelling and downwelling IR, and if you change the balance between energy flows in and out of an object, its temperature will change.
Conservation of Energy, they call it.
What this Means for the Miskolczi “Aa=Ed” Controversy
Except for relatively rare special cases, the total amount of IR energy downwelling from the sky (Ed) will ALWAYS remain less than the amount upwelling from below and absorbed by the sky (Aa).
As long as (1) the atmosphere has some transparency to IR radiation (which it does), and
(2) the atmosphere is colder than the surface (which it is), then Ed will be less than Aa…
even though they are usually close to one another, since temperatures are always adjusting to minimize IR flux divergences and convergences.
But it is those small differences that continuously “drive” the greenhouse effect."