Greenhouse Effect Discussion

[PhillipS]

In recent days there has been an interesting discussion of the greenhouse effect and the role of CO2 and other GHGs in the Arctic Sea Ice Extent thread. Interesting, but off topic - so I've started this thread to avoid cluttering up that one.

In thinking about the comments on whether LW radiation emitted by GHGs warms the surface or slows the cooling of the surface I realized that we had only touched on half the issue. Does it make any difference if the surface is warming, not cooling? Here is my formulation of the issue that I ask you to think about and share your insight on:

Early in the day a surface is warming from the energy of the sunlight falling on it (161 W/m2). But it is also receiving energy from the GHGs in the atmosphere above it as those molecules emit LW.radiation (333 W/m2). The Sun is obviously hotter than the surface and most of its radiation is in the SW end of the spectrum; the air is cooler than the surface and its radiation is entirely LW. The energy values are from

Kiehl & Trenberth 1997

Questions:

1. Do the combined solar and GHG energy fluxes change the rate of warming? Would the surface warm as fast in an atmosphere without GHGs?
   
2. Do the combined solar and GHG energy fluxes change the maximum temperature the surface will heat to? If the surface warms until the energy it radiates is in equilibrium with the energy it receives, would the maximum temperature be higher or lower in an atmosphere without GHGs?
   
3. If you think that the GHG energy reaching the surface does not warm it, can you help us understand how the surface 'knows' what energy is coming from the sun and what is coming from the GHGs? Is there a Maxwell's Demon sitting on the surface ignoring the SW and LW photons from the Sun but excluding the LW photons from the GHGs?
   

I welcome all responses but ask that we keep the discussion civil.

[WeatherRusty]

In recent days there has been an interesting discussion of the greenhouse effect and the role of CO2 and other GHGs in the Arctic Sea Ice Extent thread. Interesting, but off topic - so I've started this thread to avoid cluttering up that one.

In thinking about the comments on whether LW radiation emitted by GHGs warms the surface or slows the cooling of the surface I realized that we had only touched on half the issue. Does it make any difference if the surface is warming, not cooling? Here is my formulation of the issue that I ask you to think about and share your insight on:

Early in the day a surface is warming from the energy of the sunlight falling on it (161 W/m2). But it is also receiving energy from the GHGs in the atmosphere above it as those molecules emit LW.radiation (333 W/m2). The Sun is obviously hotter than the surface and most of its radiation is in the SW end of the spectrum; the air is cooler than the surface and its radiation is entirely LW. The energy values are from

Kiehl & Trenberth 1997

Questions:

1. Do the combined solar and GHG energy fluxes change the rate of warming? Would the surface warm as fast in an atmosphere without GHGs?
   
2. Do the combined solar and GHG energy fluxes change the maximum temperature the surface will heat to? If the surface warms until the energy it radiates is in equilibrium with the energy it receives, would the maximum temperature be higher or lower in an atmosphere without GHGs?
   
3. If you think that the GHG energy reaching the surface does not warm it, can you help us understand how the surface 'knows' what energy is coming from the sun and what is coming from the GHGs? Is there a Maxwell's Demon sitting on the surface ignoring the SW and LW photons from the Sun but excluding the LW photons from the GHGs?
   

I welcome all responses but ask that we keep the discussion civil.

Thank you PhilipS.

I will touch on a couple things to get it started.

The energy flux values given in Dr. Trenberth's diagram are time and spatially integrated.

The 333W received from the atmosphere represents energy emitted by the entire sky dome. The Sun, while radiating much hotter, emits that energy to Earth's surface from a distended angle of 0.50 degrees. The Earth's surface receives nearly twice as much energy from it's own atmosphere as from the Sun because while the Sun is much hotter, the atmosphere is radiating from a much larger surface area and does so over the entire Earth's surface day and night, 24/7/365.

!. Over the diurnal cycle? The surface would warm faster in the absence of greenhouse gases. Water vapor in the atmosphere, the principle greenhouse gas, slows the rate of warming and limits it's range. It would also cool faster at night.

2. Answered by #1

3. The mean free path of IR radiation is incredibly short before being absorbed or emitted. IR radiated from high above does not reach the surface unimpeded as does SW radiation. The direct radiation "seen" by the surface in the infrared is emitted from a tiny fraction of an inch above the surface. This air near the surface has a temperature very nearly the same as the surface itself, and radiates a thermal radiation profile accordingly. Remember, the power emitted is proportional as the 4th power of the temperature according to the Stefan-Boltzmann equation.

[cmc0605]

The best way to view the system is to consider the top-of-atmosphere (TOA) fluxes and to characterize the surface-atmosphere column as a whole. Ultimately, the net solar flux absorbed by the atmosphere and at the surface must equal the net turbulent and IR fluxes going from the surface to the atmosphere. In many cases it doesn't really matter which term is doing the transferring, the bottom boundary becomes energetically closed, and the energy budget of the whole system can be characterized by the TOA terms. Instead, the surface budget determines the degree of disequilibrium between the surface and overlying air column.

Over the timescales of interest to climate, the surface also responds more strongly to TOA radiative perturbations than it does surface perturbations, since the atmosphere itself must adjust its outgoing longwave radiation to satisfy planetary energy balance; most of this energy emanates from the high atmosphere due to atmospheric opacity. Moreover, the troposphere and surface are well-coupled so any energy deposited in the atmosphere (e.g., by introducing a solar absorber in the troposphere) will propogate onto the surface temperature.

One of the erroneous pictures some people get with respect to the enhanced greenhouse effect (from simple cartoon sketches I imagine) is that adding CO2 simply increases the downward longwave emission to the surface and can keep the atmospheric temperature unchanged. It would be easy to create a synthetic case in which the downward IR didn't directly increase (e.g., if the lower atmosphere were already radiating like a blackbody at its temperature). Instead, adding CO2 reduces the flow of energy that escapes to space for a fixed T. The "fate" of the extra photons that are absorbed can vary, but viewed from space the whole system is now receiving more energy than it is losing. That means the whole troposphere-surface system must warm. The troposphere is pretty much yoked together by convection to stay on an adiabat, and that warming will be communicated to the surface. The surface in this case is a natual lower boundary, but the greenhouse effect could work just as well on a planet with no solid surface (a gas giant). In this case, once "equilibrium" has been established much of the extra downward radiation that a surface-based observer will see (looking up) is coming simply from a hotter atmsophere via T^4, not the direct emissivity change from CO2.

It's also easy to imagine that the presence of radiative absorbers also changes the relaxation timescale to equilibrium, but it also depends on the atmospheric pressure level and heat capacity. On Titan or Venus, for example, there are only very weak seasonal/diurnal cycles in temperature in the atmsophere once you get to near surface pressure (though on Titan a low-inertia surface communicates with the atmosphere). The temperature gradients depend on the radiative timescale relative to the timescale for air to advect across the plane.t For climate change, WeatherRusty is correct that the water vapor feedback will make the outgoing radiation more sluggish than the blackbody Stefan-Boltzmann law, and that increases the timescale to equilibrium.

[WeatherRusty]

The best way to view the system is to consider the top-of-atmosphere (TOA) fluxes and to characterize the surface-atmosphere column as a whole. Ultimately, the net solar flux absorbed by the atmosphere and at the surface must equal the net turbulent and IR fluxes going from the surface to the atmosphere. In many cases it doesn't really matter which term is doing the transferring, the bottom boundary becomes energetically closed, and the energy budget of the whole system can be characterized by the TOA terms. Instead, the surface budget determines the degree of disequilibrium between the surface and overlying air column.

Over the timescales of interest to climate, the surface also responds more strongly to TOA radiative perturbations than it does surface perturbations, since the atmosphere itself must adjust its outgoing longwave radiation to satisfy planetary energy balance; most of this energy emanates from the high atmosphere due to atmospheric opacity. Moreover, the troposphere and surface are well-coupled so any energy deposited in the atmosphere (e.g., by introducing a solar absorber in the troposphere) will propogate onto the surface temperature.

One of the erroneous pictures some people get with respect to the enhanced greenhouse effect (from simple cartoon sketches I imagine) is that adding CO2 simply increases the downward longwave emission to the surface and can keep the atmospheric temperature unchanged. It would be easy to create a synthetic case in which the downward IR didn't directly increase (e.g., if the lower atmosphere were already radiating like a blackbody at its temperature). Instead, adding CO2 reduces the flow of energy that escapes to space for a fixed T. The "fate" of the extra photons that are absorbed can vary, but viewed from space the whole system is now receiving more energy than it is losing. That means the whole troposphere-surface system must warm. The troposphere is pretty much yoked together by convection to stay on an adiabat, and that warming will be communicated to the surface. The surface in this case is a natual lower boundary, but the greenhouse effect could work just as well on a planet with no solid surface (a gas giant). In this case, once "equilibrium" has been established much of the extra downward radiation that a surface-based observer will see (looking up) is coming simply from a hotter atmsophere via T^4, not the direct emissivity change from CO2.

It's also easy to imagine that the presence of radiative absorbers also changes the relaxation timescale to equilibrium, but it also depends on the atmospheric pressure level and heat capacity. On Titan or Venus, for example, there are only very weak seasonal/diurnal cycles in temperature in the atmsophere once you get to near surface pressure (though on Titan a low-inertia surface communicates with the atmosphere). The temperature gradients depend on the radiative timescale relative to the timescale for air to advect across the plane.t For climate change, WeatherRusty is correct that the water vapor feedback will make the outgoing radiation more sluggish than the blackbody Stefan-Boltzmann law, and that increases the timescale to equilibrium.

Right on!

The bolded represents another source of confusion. The atmosphere radiates 333W to the surface because of it's overall temperature. The radiation received back from the atmosphere isn't limited to the discreet wavelengths characterized by greenhouse gas molecules such as CO2. The atmosphere radiates a thermal profile (Planck curve) much the same as any bulk matter existing at a temperature above absolute zero. Viewed from space this 'radiation temperature' indicates the effective temperature of the Earth (255K) as given by the Stefan-Boltzmann Law.

Planck Curve:

The area below the various temperature curves is proportional to T^4 (Stephan-Boltzmann). The peak wavelength (X axis) is inversely proportional to the temperature.

[skierinvermont]

1. If you think that the GHG energy reaching the surface does not warm it, can you help us understand how the surface 'knows' what energy is coming from the sun and what is coming from the GHGs? Is there a Maxwell's Demon sitting on the surface ignoring the SW and LW photons from the Sun but excluding the LW photons from the GHGs?
   

I welcome all responses but ask that we keep the discussion civil.

I'll answer only this one because Rusty did a fine job on the other two.

Short answer:

The 'GHG energy' (IE LW radiation emitted by the atmosphere) that reaches the surface DOES warm it.

This is different from saying the atmosphere warms the surface, because that would indicate that the net energy flow is from atmosphere to surface, which it is not. The net energy flow is surface to atmosphere. Thus we say the surface warms the atmosphere because that is the direction of the energy flow. However, there are energy flows in both directions, so you could say that they warm each other, but this is not how we speak in normal english. Normally when we say X warms Y, we mean that the net energy flow is from X to Y. Saying the atmosphere warms the surface is like saying an ice cube warms a pot of boiling water. Technically, they do 'warm' each other in that there are energy flows in both directions (unless the ice cube is at a temperature of absolute 0K). All objects with a temperature above absolute zero emit energy to their surrounding environment.

[skierinvermont]

Right on!

The bolded represents another source of confusion. The atmosphere radiates 333W to the surface because of it's overall temperature. The radiation received back from the atmosphere isn't limited to the discreet wavelengths characterized by greenhouse gas molecules such as CO2. The atmosphere radiates a thermal profile (Planck curve) much the same as any bulk matter existing at a temperature above absolute zero. Viewed from space this 'radiation temperature' indicates the effective temperature of the Earth (255K) as given by the Stefan-Boltzmann Law.

Planck Curve:

The area below the various temperature curves is proportional to T^4 (Stephan-Boltzmann). The peak wavelength (X axis) is inversely proportional to the temperature.

On the other hand, correct me if I am wrong, if we instantaneously double CO2 downward LW radiation in the CO2 spectrum would immediately increase 3.7W/m2 would it not? Given the rest of the atmosphere has not warmed yet and would not for quite some time due to convective processes with the surface, there could not be an increase in the LW radiation at any other spectrum could there? Once the rest of the atmosphere warmed, wouldn't downward LW radiation increased further than 3.7W/m2? (call this scenario #1)

Or perhaps the downward LW radiation would not increased by 3.7W/m2 until the atmosphere had returned to thermal equilibrium. In that case the initial increase in downward LW radiation would be very small - perhaps .5W/m2 - but would gradually increased as the surface and thus atmosphere warmed. (call this scenario #2)

So which is it, # 1 or #2?? I think it is #2.

[cmc0605]

On the other hand, correct me if I am wrong, if we instantaneously double CO2 downward LW radiation in the CO2 spectrum would immediately increase 3.7W/m2 would it not? Given the rest of the atmosphere has not warmed yet and would not for quite some time due to convective processes with the surface, there could not be an increase in the LW radiation at any other spectrum could there? Once the rest of the atmosphere warmed, wouldn't downward LW radiation increased further than 3.7W/m2? (call this scenario #1)

Or perhaps the downward LW radiation would not increased by 3.7W/m2 until the atmosphere had returned to thermal equilibrium. In that case the initial increase in downward LW radiation would be very small - perhaps .5W/m2 - but would gradually increased as the surface and thus atmosphere warmed. (call this scenario #2)

So which is it, # 1 or #2?? I think it is #2.

The radiative forcing for a doubling of CO2 (3.7 W/m2) refers to a reduction of energy looking down at the tropopause, not the gain at the surface (and they generally won't be of equal magnitude except in idealized cases). The immediate surface increase (with no change in T) is smaller, but depends very much on location.

[skierinvermont]

The radiative forcing for a doubling of CO2 (3.7 W/m2) refers to a reduction of energy looking down at the tropopause, not the gain at the surface (and they generally won't be of equal magnitude except in idealized cases). The immediate surface increase (with no change in T) is smaller, but depends very much on location.

Aha makes sense.. pretty similar to what I described in scenario 2 except for the incorrect definition of radiative forcing. What's your background? You seem pretty knowledgeable on the subject.

This leads to another question also. If the outgoing LW radiation is immediately reduced by 3.7W/m2, isn't the surface temperature going to rise enough to increase its radiation by a lot more than 3.7W/m2 before equilibrium is restored? Or is the fact that the whole atmosphere including the tropopause is warming mean that only enough warming to increase radiation by 3.7W/m2 will be necessary before equilibrium is restored? I think I may have answered my own question.

[dabize]

This thread is actually very helpful to some of us who are semi-literate re the quantitative details of energy transfer physics.

A lot of this does indeed seem to me to be hair splitting, but there are times when the details rule with an iron hand, and one must know that they exist before dismissing their importance in any given context.

Thanks!

[cmc0605]

Aha makes sense.. pretty similar to what I described in scenario 2 except for the incorrect definition of radiative forcing. What's your background? You seem pretty knowledgeable on the subject.

I'm an atmospheric science graduate student working in climate/paleoclimate

[skierinvermont]

I'll answer only this one because Rusty did a fine job on the other two.

Short answer:

The 'GHG energy' (IE LW radiation emitted by the atmosphere) that reaches the surface DOES warm it.

This is different from saying the atmosphere warms the surface, because that would indicate that the net energy flow is from atmosphere to surface, which it is not. The net energy flow is surface to atmosphere. Thus we say the surface warms the atmosphere because that is the direction of the energy flow. However, there are energy flows in both directions, so you could say that they warm each other, but this is not how we speak in normal english. Normally when we say X warms Y, we mean that the net energy flow is from X to Y. Saying the atmosphere warms the surface is like saying an ice cube warms a pot of boiling water. Technically, they do 'warm' each other in that there are energy flows in both directions (unless the ice cube is at a temperature of absolute 0K). All objects with a temperature above absolute zero emit energy to their surrounding environment.

Coming back to this example, a more analogous example than saying 'an ice cube warms a pot of boiling water' would be saying that a towel wrapped around a pot of boiling water warms the pot. We probably wouldn't say that the towel warms the pot. More likely we would say that the stove (the sun) warms the pot (the earth), the pot (the earth) warms the towel (the atmosphere) and that the towel (the atmosphere) slows the cooling of the pot (the earth).

[cmc0605]

This leads to another question also. If the outgoing LW radiation is immediately reduced by 3.7W/m2, isn't the surface temperature going to rise enough to increase its radiation by a lot more than 3.7W/m2 before equilibrium is restored? Or is the fact that the whole atmosphere including the tropopause is warming mean that only enough warming to increase radiation by 3.7W/m2 will be necessary before equilibrium is restored? I think I may have answered my own question.

This is why it's important to consider the whole unit. The right way to define climate sensitivity in this case (ignoring the other feedbacks like water vapor, etc) is to take the derivative of the Stefan Boltzmann function, 4σT^3 (in units of Watts per aquare meter per Kelvin). That gives you a measure of the radiative-restoring strength, but the question then becomes what temperature do you use? Set up like this, it's the planetary emission temperature ~255 K), not the surface temperature. With the planetary emission temperature, you get ~3.7 W/m2/K restoring and so ~1 K increase in temperature for 2xCO2, again ignoring other feedbacks.

[WeatherRusty]

On the other hand, correct me if I am wrong, if we instantaneously double CO2 downward LW radiation in the CO2 spectrum would immediately increase 3.7W/m2 would it not? Given the rest of the atmosphere has not warmed yet and would not for quite some time due to convective processes with the surface, there could not be an increase in the LW radiation at any other spectrum could there? Once the rest of the atmosphere warmed, wouldn't downward LW radiation increased further than 3.7W/m2? (call this scenario #1)

Or perhaps the downward LW radiation would not increased by 3.7W/m2 until the atmosphere had returned to thermal equilibrium. In that case the initial increase in downward LW radiation would be very small - perhaps .5W/m2 - but would gradually increased as the surface and thus atmosphere warmed. (call this scenario #2)

So which is it, # 1 or #2?? I think it is #2.

Radiative forcing is defined simply as the change in energy flux as measured at the tropopause. You are correct in scenario #1 that a change in forcing (3.7W/m^2) is immediate, while the response to the forcing takes time. The longwave radiation absorbed by the surface does not come from high up in the troposphere, that energy is radiated, absorbed, passed on by molecular collision etc. quadzillions of times before it has any chance of reaching the surface.

The energy radiated by the Sun warmed surface is likewise absorbed almost immediately by greenhouse gases in very close proximity to the surface. The energy gets bounced around quadzillions of times on the way up. And all that happens in a faction of a second! The scale and speed at which these processes play out is not within the realm of our common experience, so people have a difficult time conceptualizing what is actually taking place.

There are two basic types of radiative processes going on. The discreet emission given by the vibrational modes of the greenhouse gas molecules as one, then the other being the bulk thermal energy given off by the atmosphere as two.

Scenario #1 deals with discreet molecular emisson.

Scenario #2 deals with the bulk matter radiation.

They are both occurring simultaneously! I hope that makes sense.

[skierinvermont]

This is why it's important to consider the whole unit. The right way to define climate sensitivity in this case (ignoring the other feedbacks like water vapor, etc) is to take the derivative of the Stefan Boltzmann function, 4σT^3 (in units of Watts per aquare meter per Kelvin). That gives you a measure of the radiative-restoring strength, but the question then becomes what temperature do you use? Set up like this, it's the planetary emission temperature ~255 K), not the surface temperature. With the planetary emission temperature, you get ~3.7 W/m2/K restoring and so ~1 K increase in temperature for 2xCO2, again ignoring other feedbacks.

Ok so just treat the whole earth and atmosphere as one blackbody you are saying?

If doubling CO2 reduces energy emitted by 3.7W/m2, then equilibrium will only be restored when the body warms enough to increase emissions by 3.7W/m2, which comes out to roughly 1C. But then the question is will the warming be distributed evenly? My guess is not exactly but pretty close.

[cmc0605]

Ok so just treat the whole earth and atmosphere as one blackbody you are saying?

If doubling CO2 reduces energy emitted by 3.7W/m2, then equilibrium will only be restored when the body warms enough to increase emissions by 3.7W/m2, which comes out to roughly 1C. But then the question is will the warming be distributed evenly? My guess is not exactly but pretty close.

While we're ignoring feedbacks, I'd expect it to be pretty close, though you'd expect some deviations because of the different radiative restoring efficiencies between a hot region (like the tropics) and colder regions (like the poles) since Stefan Boltzmann is non-linear. It would also depend somewhat on how well-coupled the surface is to the troposphere.

Keep in mind that the definition of what is a "no feedback" case is rather arbitrary. In my example, I took it to be a uniform warming of the whole troposphere (this is the default convention in the climate community because it's pretty easy to calculate, and then you can define any other feedbacks relative to that reference system).

One of those feedbacks is the lapse rate feedback, whereby regions that maintain a moist adiabat typically result in more temperature change aloft than at the surface. Albedo feedbacks can shift the response somewhat to regions where albedo is expected to change most significantly.

[WeatherRusty]

Ok so just treat the whole earth and atmosphere as one blackbody you are saying?

If doubling CO2 reduces energy emitted by 3.7W/m2, then equilibrium will only be restored when the body warms enough to increase emissions by 3.7W/m2, which comes out to roughly 1C. But then the question is will the warming be distributed evenly? My guess is not exactly but pretty close.

Thus we get back to a system thrown out of thermodynamic equilibrium becoming more chaotic and turbulent up until the point equilibrium is restored. The system is not warmed evenly, that is the very point of being pushed further into a non-equilibrium state.

[WeatherRusty]

While we're ignoring feedbacks, I'd expect it to be pretty close, though you'd expect some deviations because of the different radiative restoring efficiencies between a hot region (like the tropics) and colder regions (like the poles) since Stefan Boltzmann is non-linear. It would also depend somewhat on how well-coupled the surface is to the troposphere.

Keep in mind that the definition of what is a "no feedback" case is rather arbitrary. In my example, I took it to be a uniform warming of the whole troposphere (this is the default convention in the climate community because it's pretty easy to calculate, and then you can define any other feedbacks relative to that reference system).

One of those feedbacks is the lapse rate feedback, whereby regions that maintain a moist adiabat typically result in more temperature change aloft than at the surface. Albedo feedbacks can shift the response somewhat to regions where albedo is expected to change most significantly.

Speaking of the lapse rate feedback in the context of the greenhouse effect, that feedback is negative serving to reduce the overall positive radiative feedback produced by an increase in atmospheric water vapor. The greenhouse effect wants to cool the upper air while the lapse rate feedback warms the upper air.

[physicsguy21]

Hi all, I'm the new guy, so I hope I'm not butting in

At least as I've been educated to believe, the rate of energy-in/energy-out is key in determining the 'temperature' of a recieving body. About 99.75% of the atmosphere can only attain 'heat' by conduction via the solar heated surface or other molecules in the atmosphere. Oxygen and Nitrogen molecules attain their temperature via conduction from the solar heated surface during the day, but cannot emit the 'heat' attained on their own, nor can they absorb LW. They can only conduct it back to the surface at night or with other molecules, kind of like an insulating mechanism to the surface, slowing the cooling rate. 'GHGes' do the same thing though re-radiating photons directly without help of direct physical contact, increasing the height of emission and slowing radiative release.

The oceans are like a greenhouse fluid, and have a huge thermal capacity and radiate 'heat' very slowly relative to the rate they attain it from solar, and are hence able to warm to a higher extent than would be expected with a typical blackbody formula...so contrary to the globally avged 240Wm2, (or 480Wm2 arriving over the lit portion of the globe), the oceans retain thermal energy at night, warming further towards the SW noontime vector peak which is usually incident to at least 700Wm2, often close to 1000Wm2. There is little to no diurnal variation in SSTs.

Question is what would the rate of energy-in/energy-out be without 'GHGes' in the atmosphere? And can the 'GHE' warm the oceans?

I contend that it would not be anywhere close to equal, LW photon firing frequency from the atmosphere actually cannot penetrate the oceans deeper than the ~ width of a human hair, at most, (the 'evaporative sheet'). The ocean surface is also on avg ~3C warmer than the air above it.

[cmc0605]

Hi all, I'm the new guy, so I hope I'm not butting in

At least as I've been educated to believe, the rate of energy-in/energy-out is key in determining the 'temperature' of a recieving body. About 99.75% of the atmosphere can only attain 'heat' by conduction via the solar heated surface or other molecules in the atmosphere. Oxygen and Nitrogen molecules attain their temperature via conduction from the solar heated surface during the day, but cannot emit the 'heat' attained on their own, nor can they absorb LW. They can only conduct it back to the surface at night or with other molecules, kind of like an insulating mechanism to the surface, slowing the cooling rate. 'GHGes' do the same thing though re-radiating photons directly without help of direct physical contact, increasing the height of emission and slowing radiative release.

The oceans are like a greenhouse fluid, and have a huge thermal capacity and radiate 'heat' very slowly relative to the rate they attain it from solar, and are hence able to warm to a higher extent than would be expected with a typical blackbody formula...so contrary to the globally avged 240Wm2, (or 480Wm2 arriving over the lit portion of the globe), the oceans retain thermal energy at night, warming further towards the SW noontime vector peak which is usually incident to at least 700Wm2, often close to 1000Wm2. There is little to no diurnal variation in SSTs.

Question is what would the rate of energy-in/energy-out be without 'GHGes' in the atmosphere? And can the 'GHE' warm the oceans?

I contend that it would not be anywhere close to equal, LW photon firing frequency from the atmosphere actually cannot penetrate the oceans deeper than the ~ width of a human hair, at most, (the 'evaporative sheet'). The ocean surface is also on avg ~3C warmer than the air above it.

With respect, it's difficult to make sense of a lot of this. The specific heat of water is a rather distinct concept from the physics of the greenhouse effect (the former just tells you how much energy you need to raise the temperature of, say a kilogram of substance, by a certain amount). The high specific heat and the high mass of ocean water smooth out temperature variations rather well, even if you replaced the warming from the greenhouse effect by an equivalent increase in solar irradiance for example. I also can't understand what you mean by "...able to warm to a higher extent than would be expected with a typical blackbody formula." The emission of radiation only cares about the emissivity and temperature of an object, not how long it took to get to that temperature. In fact, it's because it takes so long for the ocean temperature to respond to perturbations that you end up with "committed warming in the future" (i.e., warming left in the pipeline if the forcing stays fixed). The slow increase in temperature means the emission to space doesn't immediately increase to restore planetary energy balance.

I've seen a lot of people on the internet have a hard time with the concept of infrared radiation heating ocean water due to the extremely short distance a photon travels before it is absorbed. That energy can be mixed downward quite rapidly due to turbulent motions, but even for a perfectly still fluid the top layer of the water would still heat up until the incoming flux equaled the outgoing emission (and evaporation, sensible heating, etc). What's more, as I've tried to explain above, it's the whole troposphere that is warming because the greenhouse effect influences what's going on higher in the atmosphere, and this warming is coupled to the surface by both radiative and non-radiative terms.

There's a few studies that have looked at what happens if you could remove the atmospheric greenhouse effect. In all the studies that I know of, the oceans pretty much freeze over. There is little thermal inertia in a snowball state, so you'd expect larger temperature changes.

[WeatherRusty]

I am tired and about to head off for the night, but I wish to quickly address a common misconception. It is often stated that oxygen and nitrogen do not absorb or radiate infrared radiation, in contrast to greenhouse gases which do. This is true in the sense that an individual molecule of O2 or N2 can not absorb or radiate in the infrared but that does not mean that the bulk atmosphere made up predominantly by those gases does not radiate in the infrared. It does.

The bulk atmosphere radiates away energy in the same manner as any other form of composite matter with a temperature above absolute zero. Emitted thermal radiation is the consequence of molecular motion taking place within the mass of an object. Molecules are in constant collision with one another which produces friction much the same way as you feel heat when you rub your to hands together vigorously. The more rapid the molecular motion (temperature), the more energetic is the emitted thermal radiation. This is the radiation (thermal radiation) which the Earth radiates away to space.

The excitation of a greenhouse gas molecule when it interacts with infrared radiation at a precise wavelength of light is a fundamentally different process.

To be continued..................

[The_Global_Warmer]

While we're ignoring feedbacks, I'd expect it to be pretty close, though you'd expect some deviations because of the different radiative restoring efficiencies between a hot region (like the tropics) and colder regions (like the poles) since Stefan Boltzmann is non-linear. It would also depend somewhat on how well-coupled the surface is to the troposphere.

Keep in mind that the definition of what is a "no feedback" case is rather arbitrary. In my example, I took it to be a uniform warming of the whole troposphere (this is the default convention in the climate community because it's pretty easy to calculate, and then you can define any other feedbacks relative to that reference system).

One of those feedbacks is the lapse rate feedback, whereby regions that maintain a moist adiabat typically result in more temperature change aloft than at the surface. Albedo feedbacks can shift the response somewhat to regions where albedo is expected to change most significantly.

This is definitely a great case study in the Arctic region with the year to year fluctuations now in portions of the arctic in terms of how of the suns energy makes it to the surface. Largely a region in the past would have ice year round with snow most of the year. The ice there would absorb most of the radiation, the rest reflected back. So almost no net gain in energy at the surface. If we replace that ice and snow with open water for half of the sunny season. We now go from almost no net gain in surface absorption to a very low albedo of .07 to .08. this means in some cases we increase by 100W/M2 to 150W/M2 over millions of square kilometers of area. This seems to be having a much much much larger impact in the arctic vs global.

[physicsguy21]

Thank you for the responses.

I am tired and about to head off for the night, but I wish to quickly address a common misconception. It is often stated that oxygen and nitrogen do not radiate infrared radiation, in contrast to greenhouse gases which do. This is true in the sense that an individual molecule of O2 or N2 can not absorb or radiate in the infrared but that does not mean that the bulk atmosphere made up predominantly by those gases does not radiate in the infrared. It does.

The bulk atmosphere radiates away energy in the same manner as any other form of composite matter with a temperature above absolute zero. Emitted thermal radiation is the consequence of molecular motion taking place within the mass of an object. Molecules are in constant collision with one another which produces friction much the same way as you feel heat when you rub your to hands together vigorously. The more rapid the molecular motion (temperature), the more energetic is the emitted thermal radiation. This is the radiation which the Earth radiates away to space.

The excitation of a greenhouse molecule when it interact with infrared radiation at a precise wavelength of light is a fundamentally different process.

To be continued..................

Thanks a bunch, I understand (the vibrational modes/excitement levels determining thermal presence within individual molecules), but when discussing the GHE the effect I was taught that it must(?) be disected on a molecular scale because it is relating to the specific interception of photons, and the differential between gain/loss rates being affected through the recieving body as a whole.

I'm fairly sure SWR contains a clear vector while LWR contains no vector until TOA, I do not believe the presence of "greenhouse gases" changes this? I'm not denying the obvious existance of the radiative greenhouse effect, but it truly appears to be more than just radiative in nature. Or so it appears?

With respect, it's difficult to make sense of a lot of this. The specific heat of water is a rather distinct concept from the physics of the greenhouse effect (the former just tells you how much energy you need to raise the temperature of, say a kilogram of substance, by a certain amount). The high specific heat and the high mass of ocean water smooth out temperature variations rather well, even if you replaced the warming from the greenhouse effect by an equivalent increase in solar irradiance for example.

Thankyou. I'm probably mis-interpreting what you're trying to say, I'm just trying to determine how much of the GHE is radiative in nature. It sounds like you're stating that a change in backradiation of 1Wm2 will have the same impact on the oceans as a change in solar radiation of 1Wm2?

Are you accounting for wind-driven evaporation+general evaporation rate at the ocean surface? Plus the coinciding cloud cover and precipitation? Evaporation only occurs on the ocean surface.

I was taught that DWLR initally cannot penetrate deeper than ~ 0.25mm deep into the oceans, and that whatever is attainable for 'mixing' is un-efficient after evaporative activity is taken into account with surface wind & precipitation. How can you compare solar heating to backradiation from greenhouse gases?

I also can't understand what you mean by "...able to warm to a higher extent than would be expected with a typical blackbody formula." The emission of radiation only cares about the emissivity and temperature of an object, not how long it took to get to that temperature. In fact, it's because it takes so long for the ocean temperature to respond to perturbations that you end up with "committed warming in the future" (i.e., warming left in the pipeline if the forcing stays fixed). The slow increase in temperature means the emission to space doesn't immediately increase to restore planetary energy balance.

I'm sorry, I meant the S-B constant. Isn't the specific rate of diurnal cooling (and how it is slowed) the cornerstone of the greenhouse effect on Earth?

As for the freezing of the oceans without a GHE, I'll try to find some of the papers I recall on this. I cannot see why the freezing of the oceans without the radiative aspect of the GHE (GHGes) has any reality unless you'e assuming they were frozen to start with? If we could remove all GHGes from the atmosphere now, the 480Wm2 insolation impacting 1/2 of the Earth's surface at a time would on it's own be enough to maintain a liquid ocean state given the thermal capacity involved. Avging 240Wm2 (incident to -18C) across all areas of the surface may work for a planet that either does not have an atmosphere-ocean system or is not rotating but in Earth's case I don't see why it makes any sense.

The allowance for liquid water could very well have been atmospheric pressure early on in the Earth's lifetime impacting evaporation rate, or the slow process of the oceans' formation. The boiling point of water varies with altitude as gravity compresses gas, hence converts potential energy into kinetic energy closer to the surface, -80C in the upper levels to ~15C at the surface.

When trying to account for the entire energy budget, it'd be nice to know if it requires additional energy to maintain a kinetic circulatory mode against gravity. Shouldn't we account for the total planetary energy budget when calculating additions in radiative form? Energy is always being converted from one form to another in our climate system. Thermal, electric, magnetic, kinetic, so on.

[WeatherRusty]

Thank you for the responses.

Thanks a bunch, I understand (the vibrational modes/excitement levels determining thermal presence within individual molecules), but when discussing the GHE the effect I was taught that it must(?) be disected on a molecular scale because it is relating to the specific interception of photons, and the differential between gain/loss rates being affected through the recieving body as a whole.

I'm fairly sure SWR contains a clear vector while LWR contains no vector until TOA, I do not believe the presence of "greenhouse gases" changes this? I'm not denying the obvious existance of the radiative greenhouse effect, but it truly appears to be more than just radiative in nature. Or so it appears?

Thankyou. I'm probably mis-interpreting what you're trying to say, I'm just trying to determine how much of the GHE is radiative in nature. It sounds like you're stating that a change in backradiation of 1Wm2 will have the same impact on the oceans as a change in solar radiation of 1Wm2?

Are you accounting for wind-driven evaporation+general evaporation rate at the ocean surface? Plus the coinciding cloud cover and precipitation? Evaporation only occurs on the ocean surface.

I was taught that DWLR initally cannot penetrate deeper than ~ 0.25mm deep into the oceans, and that whatever is attainable for 'mixing' is un-efficient after evaporative activity is taken into account with surface wind & precipitation. How can you compare solar heating to backradiation from greenhouse gases?

I'm sorry, I meant the S-B constant. Isn't the specific rate of diurnal cooling (and how it is slowed) the cornerstone of the greenhouse effect on Earth?

As for the freezing of the oceans without a GHE, I'll try to find some of the papers I recall on this. I cannot see why the freezing of the oceans without the radiative aspect of the GHE (GHGes) has any reality unless you'e assuming they were frozen to start with? If we could remove all GHGes from the atmosphere now, the 480Wm2 insolation impacting 1/2 of the Earth's surface at a time would on it's own be enough to maintain a liquid ocean state given the thermal capacity involved. Avging 240Wm2 (incident to -18C) across all areas of the surface may work for a planet that either does not have an atmosphere-ocean system or is not rotating but in Earth's case I don't see why it makes any sense.

The allowance for liquid water could very well have been atmospheric pressure early on in the Earth's lifetime impacting evaporation rate, or the slow process of the oceans' formation. The boiling point of water varies with altitude as gravity compresses gas, hence converts potential energy into kinetic energy closer to the surface, -80C in the upper levels to ~15C at the surface.

When trying to account for the entire energy budget, it'd be nice to know if it requires additional energy to maintain a kinetic circulatory mode against gravity. Shouldn't we account for the total planetary energy budget when calculating additions in radiative form? Energy is always being converted from one form to another in our climate system. Thermal, electric, magnetic, kinetic, so on.

Very nice follow up post. Good probing questions. I will respond to the points addressed to me.

First, yes we must look to the greenhouse gases for the absorption of the Earth's thermal radiation as it passes through the atmosphere. Emission of energy in the specific resonant wavelengths at which the greenhouse gases absorb and emit IR is also particular to these same greenhouse gases.

Frequently, an excited greenhouse gas molecule will pass on it's acquired energy by collision with N2 or O2. The energy has now been transfered from the vibrating greenhouse gas molecule to the kinetic energy of the N2 or O2. Kinetic energy, or molecular motion within the gas will have increased somewhat. The greenhouse gas molecule will no longer be in an energy state whereby it can randomly emit a photon of IR.

When you say that SWR contains a clear vector to space, the reason is that most of the visible light spectrum does not interact with the individual molecules of gas along the path.

Most IR contains no vector simply because it is absorbed by greenhouse gases along

every path or vector, the energy being either transfered by collision with the atmosphere's constitute molecules or emitted again as the greenhouse molecule spontaneously decays to a lower energy state. So, the presence of greenhouse gases is what changes things. The emitted energy is emitted along a random vector, on average half up and half back down. This occurs zillions of times with zillions of greenhouse gas molecules until by chance the energy is released to space at the atmospheric layer of emissivity high in the troposphere where the mean free path of IR becomes much longer. All that bouncing around slows the loss of energy to space and we have a greenhouse effect as a result.

Best estimates attribute about 50% of the greenhouse effect to water vapor, 25% to clouds, 20% to carbon dioxide and 5% to everything else.

The greenhouse effect on present day Earth raises the surface temperature to 33K above the effective temperature of 255K, or to 288K. The freezing point is 273K. So, due to the atmospheric greenhouse effect the surface is 15K above the freezing point of water. If all greenhouse effect were removed, in less than 50 years the global ocean would freeze over everywhere except maybe near the equator.

Also a watt is a watt is a watt. A watt is a measure of power or the ability to do work. A watt of energy provided by SW radiation is equal to a watt of energy provided by long wave radiation. The SW will penetrate water to deeper depth before it's energy is fully absorbed, whereas the long wave IR will penetrate the 'skin' of the water before being dissipated. In either case the energy is absorbed by the water. Woops...I promised to stick to the area directed to me.

The end.

[cmc0605]

The greenhouse effect on present day Earth raises the surface temperature to 33K above the effective temperature of 255K, or to 288K. The freezing point is 273K. So, due to the atmospheric greenhouse effect the surface is 15K above the freezing point of water. If all greenhouse effect were removed, in less than 50 years the global ocean would freeze over everywhere except maybe near the equator.

It's even worse than that because freezing large portions of the planet over would increase the albedo well beyond the current ~30%, and thus lower the "effective temperature" to well below 255 K. Actually, the situation you describe (where only the equator stays above freezing) is very hard to sustain as a stable structure, though it's been proposed as a plausible solution to super slow-rotating exoplanets (where one side pretty much always faces the host star).

[physicsguy21]

Very nice follow up post. Good probing questions. I will respond to the points addressed to me.

First, yes we must look to the greenhouse gases for the absorption of the Earth's thermal radiation as it passes through the atmosphere. Emission of energy in the specific resonant wavelengths at which the greenhouse gases absorb and emit IR is also particular to these same greenhouse gases.

Thanks. You sound very knowledgable in this.

Frequently, an excited greenhouse gas molecule will pass on it's acquired energy by collision with N2 or O2. The energy has now been transfered from the vibrating greenhouse gas molecule to the kinetic energy of the N2 or O2. Kinetic energy, or molecular motion within the gas will have increased somewhat. The greenhouse gas molecule will no longer be in an energy state whereby it can randomly emit a photon of IR.

But how much is transferred? Isn't is usually sort of 'making up for the difference' between the vibrational rates of the two molecules that collide? Isn't this why GHG molecules are ~ the same temperature as molecules around them?

When you say that SWR contains a clear vector to space, the reason is that most of the visible light spectrum does not interact with the individual molecules of gas along the path.

Most IR contains no vector simply because it is absorbed by greenhouse gases along

every path or vector, the energy being either transfered by collision with the atmosphere's constitute molecules or emitted again as the greenhouse molecule spontaneously decays to a lower energy state. So, the presence of greenhouse gases is what changes things. The emitted energy is emitted along a random vector, on average half up and half back down. This occurs zillions of times with zillions of greenhouse gas molecules until by chance the energy is released to space at the atmospheric layer of emissivity high in the troposphere where the mean free path of IR becomes much longer. All that bouncing around slows the loss of energy to space and we have a greenhouse effect as a result.

Thanks. A few questions nagging in my head.

1. How do you account for the 'heat' contained in gases such as Nitrogen and Oxygen, which as a whole unit, emit very slowly?

2. Knowing Nitrogen & Oxygen do insulate the surface somewhat at night through conduction (as does most of the atmosphere), how do you seperate that (conductive) effect from the radiative greenhouse effect of IR backradiation through GHGes and clouds?

3. How do you account for atmospheric pressure with altitude in terms of the GHE, and the manner in which it'd affect energy loss rates closer to the surface? The boiling point of water varies with altitude due to air pressure around the pot of water varying. Is it a coincidence that below the upper-atmospheric inversion (UV absorbed by ozone), that the contrast of the thermal profile equates to the varying boiling point of water, relatively?

Best estimates attribute about 50% of the greenhouse effect to water vapor, 25% to clouds, 20% to carbon dioxide and 5% to everything else.

I assume this is the radiative portion of the GHE? Since clouds involve droplets with various gases intertwined, wouldn't clouds emit in the full spectrum as a greybody?

I am also curious, shouldn't we take into account the Earth's totel net energy budget when determining the impact of GHGes? Energy is constantly being converted from one form to another, (main forms of thermal, magnetic, kinetic, EM, and electric). When this is done the budget increases dramatically, I believe it increases by ~360% (?) but I could be wrong with that number. So how is this accounted for?

The greenhouse effect on present day Earth raises the surface temperature to 33K above the effective temperature of 255K, or to 288K. The freezing point is 273K. So, due to the atmospheric greenhouse effect the surface is 15K above the freezing point of water. If all greenhouse effect were removed, in less than 50 years the global ocean would freeze over everywhere except maybe near the equator.

I don't understand this, admittedly. The 480Wm2 insolation covers 1/2 of the globe, rather than 240Wm2 spanning the entire globe, right? 480Wm2 is well above the freezing threshold with values between 700Wm2-1100Wm2 in most areas near noontime, and there is basically no diurnal variation in SSTs since the thermal capacity is so high. So how would the oceans freeze if we removed the radiative portion of the GHE? Removing H2O from the atmosphere means you have to remove cloud cover and precipitation plus any ice/snow that accumulated via precipitation.

I'm also curious how DLWR can warm the oceans significantly when it can only penetrate ~ 0.25mm down? That is less than the evaporative surface/transpiration layer. The temperature of the ocean surface impact evaporation rate as does wind, so how can the radiative GHE warm the oceans to a measuable extent?

Also a watt is a watt is a watt. A watt is a measure of power or the ability to do work. A watt of energy provided by SW radiation is equal to a watt of energy provided by long wave radiation. The SW will penetrate water to deeper depth before it's energy is fully absorbed, whereas the long wave IR will penetrate the 'skin' of the water before being dissipated. In either case the energy is absorbed by the water. Woops...I promised to stick to the area directed to me.

The end.

Thank you for the response.

[WeatherRusty]

Thanks. You sound very knowledgable in this.

But how much is transferred? Isn't is usually sort of 'making up for the difference' between the vibrational rates of the two molecules that collide? Isn't this why GHG molecules are ~ the same temperature as molecules around them?

Thanks. A few questions nagging in my head.

1. How do you account for the 'heat' contained in gases such as Nitrogen and Oxygen, which as a whole unit, emit very slowly?

2. Knowing Nitrogen & Oxygen do insulate the surface somewhat at night through conduction (as does most of the atmosphere), how do you seperate that (conductive) effect from the radiative greenhouse effect of IR backradiation through GHGes and clouds?

3. How do you account for atmospheric pressure with altitude in terms of the GHE, and the manner in which it'd affect energy loss rates closer to the surface? The boiling point of water varies with altitude due to air pressure around the pot of water varying. Is it a coincidence that below the upper-atmospheric inversion (UV absorbed by ozone), that the contrast of the thermal profile equates to the varying boiling point of water, relatively?

I assume this is the radiative portion of the GHE? Since clouds involve droplets with various gases intertwined, wouldn't clouds emit in the full spectrum as a greybody?

I am also curious, shouldn't we take into account the Earth's totel net energy budget when determining the impact of GHGes? Energy is constantly being converted from one form to another, (main forms of thermal, magnetic, kinetic, EM, and electric). When this is done the budget increases dramatically, I believe it increases by ~360% (?) but I could be wrong with that number. So how is this accounted for?

I don't understand this, admittedly. The 480Wm2 insolation covers 1/2 of the globe, rather than 240Wm2 spanning the entire globe, right? 480Wm2 is well above the freezing threshold with values between 700Wm2-1100Wm2 in most areas near noontime, and there is basically no diurnal variation in SSTs since the thermal capacity is so high. So how would the oceans freeze if we removed the radiative portion of the GHE? Removing H2O from the atmosphere means you have to remove cloud cover and precipitation plus any ice/snow that accumulated via precipitation.

I'm also curious how DLWR can warm the oceans significantly when it can only penetrate ~ 0.25mm down? That is less than the evaporative surface/transpiration layer. The temperature of the ocean surface impact evaporation rate as does wind, so how can the radiative GHE warm the oceans to a measuable extent?

Thank you for the response.

This reads rather Bethesdaesk if you know what I mean. No offense, but if you are sufficiently intelligent and knowledgeable to ask such questions, you should be able to research on your own if interested in the answers.

The answer to the first question is all of it. The energy absorbed and the energy emitted are exactly the same since the energy involved is quantized at very specific wavelengths or frequencies.

Since the energy absorbed and emitted by atoms and molecules is limited to specific quanta or energies, the energy re-radiated equals the energy absorbed.

Individual molecules in the gas do not have a "temperature" per se. They have a whole range of velocities or kinetic energies, the average of which we measure as the temperature of the gas.

[TerryM]

This reads rather Bethesdaesk if you know what I mean. No offense, but if you are sufficiently intelligent and knowledgeable to ask such questions, you should be able to research on your own if interested in the answers.

The answer to the first question is all of it. The energy absorbed and the energy emitted are exactly the same since the energy involved is quantized at very specific wavelengths.

Since the energy absorbed and emitted by atoms and molecules is limited to specific quanta or energies, the energy re-radiated equals the energy absorbed.

I too had noticed the similarity to Beth, I think your right answering as you have, one point at a time.

When posters advance multiple, disparate arguments it does little but muddy the thread. If you have a number of points to make, make them one at a time, and pursue each discussion until it is resolved.

Not any kind of 'rule' - just a suggestion if you are really interested in debating new information.

Terry

[skierinvermont]

Yes this discussion has definitely lost me. Asking too much at once I think. Need to focus on one particular detail at a time.

[TerryM]

Yes this discussion has definitely lost me. Asking too much at once I think. Need to focus on one particular detail at a time.

Amen

[physicsguy21]

This reads rather Bethesdaesk if you know what I mean. No offense, but if you are sufficiently intelligent and knowledgeable to ask such questions, you should be able to research on your own if interested in the answers.

The answer to the first question is all of it. The energy absorbed and the energy emitted are exactly the same since the energy involved is

quantized at very specific wavelengths or frequencies.

Since the energy absorbed and emitted by atoms and molecules is limited to specific quanta

or energies, the energy re-radiated equals the energy absorbed.

Individual molecules in the gas do not have a "temperature" per se. They have a whole range

of velocities or kinetic energies, the average of which we measure as the temperature of the gas.

I'm not sure who you're referring to but what I am trying to ask is why you assume the greenhouse effect is solely radiative in nature. There are physics that suggest otherwise.

I've done a fair amount of research and came up with a different conclusion, hence my questioning. Conservation of energy only applies to net incoming radiation and outgoing radiation at TOA.