Monthly Archives: May 2015

why infrared absorbing gases in the atmosphere warm the surface.”

angech (Comment #136532)

SteveF ” I thought you had finally understood why infrared absorbing gases in the atmosphere warm the surface.”

We are talking about the temperature of the air at the surface, not the actual surface layer itself?
Green house gases absorb and re-emit infra red which other gases do to a much, much lesser extent.
The gas molecules therefor move faster causing more collisions and heat.
The heat works its way back upstairs by convection and emission.
Once the molecules are in steady state ie settled at the new average level the amount of heat going back out to space is equal to that coming in.
There is no continuous build up of heat in the atmosphere, it plateaus at the new CO2 level.
The air is warmer than it was previously so the surface will warm up.
Your point.

My problem the energy coming in must equal the energy going out at the new balance point.

Being hotter the object should emit more energy.
Hence the the outgoing long-wavelength flux should be increased.
Logic.

But the sun has not become hotter, The joules in are still the same.

The problem to me is that we are looking at an atmosphere layering problem.
In effect the addition of GHG is equivalent to lowering the albedo of the planet making it more of a blackbody than it is and thus increasing its ability to absorb heat.
But black or white body it still has to emit the same amount of radiation back out.
This might manifest as a change in the TOA level if the emitting body has warmer air.?
If one layer is hotter other layers above and below must be colder. The deep sea is still ice cold despite 4 billion years of sunshine
In any case the outgoing long-wavelength flux should not reduce but must be the same as previously.

My way of trying to say you are right but looking for any excuse.

SteveF (Comment #136533)

angech,
“My problem the energy coming in must equal the energy going out at the new balance point.”
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Sure, but you are ignoring the heat capacities involved. The atmosphere has a relatively small heat capacity (maybe equal to 4 meters of water). The ocean surface layer is about 60 meters on average, so about 15 times more thermal mass than the atmosphere. The deeper ocean has vast heat capacity, but warms only slowly to very slowly (decades to many centuries, depending on location and depth). Land areas have considerable thermal mass below the surface, but like the deep oceans, warm only very slowly. There is not going to be a “new equilibrium” established in anything less than a millennium, so it is useless to think about the process as an equilibrium.

Instead, you need to think of it as an energy balance, where rising GHG levels slightly restrict heat loss to space, and so warm the surface…. but at a rate of warming which changes at all time scales as the various heat sinks, with different capacities and different rates of heat uptake, move toward equilibrium. It is a simple accounting of joules: less heat going to space (due to more GHG’s) means heat must accumulate in the various sinks; the size and rates of those sinks limit the rate of surface warming. Yes, given enough time, the system would establish an equilibrium, but since GHG forcing is constantly changing (and will continue to!), there is no possibility of equilibrium being established. All we can do is make reasonable estimates of future warming based on projections of GHG levels, how those GHG levels will restrict heat loss, our understanding of internal system feedbacks due to gradually changing temperatures (atmospheric moisture, clouds, etc), and our understanding of the sizes and speeds of the multiple thermal sinks.

The best estimate for the rate of heat uptake (usually called the ‘TOA imbalance’) is in the range of 0.5 – 0.6 watts/M^2 averaged over the last decade or so, while the current best estimate of GHG forcing is ~2.29 watts/M^2 (~3.1 watts/M^2 GHG forcing less aerosol aerosol effects; IPCC AR5). Average surface temperature has changed by ~0.85 C since the mid 19th century (before significant man-made GHG influence). So the current best estimate of sensitivity based on energy balance is ~ 0.85 / (2.29 – 0.55) = 0.49 degree/Watt/M^2, or equivalent to 1.8C equilibrium increase for a doubling of CO2. People will argue about the details (exactly how much heat is being accumulated, exactly how much warming has taken place, exactly how much aerosol effects have reduced GHG forcing, etc), but the basic accounting is pretty clear. Based on these figures, the warming realized to date (~0.85C) represents only about (2.29 – 0.55)/(2.29) = ~76% or what it would have been save for heat accumulation, and only about (2.29 – 0.55)/(3.1) = ~56% of what it would have been in the absence of heat accumulation and aerosol effects.
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The greatest uncertainty lies in aerosol effects (~ -0.9 watt/M^2, IPCC, AR5), and (no surprise!) this is one of the most contentious factual issues remaining in climate science. Those alarmed by warming and committed to draconian action to reduce fossil fuel use will almost always argue that the estimated aerosol effects are larger than in AR5, implying greater sensitivity, while those more alarmed by draconian measures than by warming will argue that estimated aerosol effects are even smaller than in AR5, implying lower sensitivity.
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Nic Lewis used Bjorn Stephens’ revised (lower) estimates of aerosol effects to show that the best estimate of equilibrium sensitivity would be quite low (~1.5 – 1.6C per doubling), and that the probability of very high sensitivity (>3C per doubling, and a legitimate cause for alarm, if true) would be miniscule, based on the Stephens results. This naturally caused much gnashing of teeth and pulling of hair among the warming concerned, and lead to the odd public disclaimer from Stephens that while he stands by his aerosol estimates, he completely disavows Nic’s analysis based on those estimates; one of the strangest documents I have read from an actual scientist in a while.
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Of course, I don’t expect you will accept much of the above as correct. But I figured I would lay it out for you anyway.

angech (Comment #136534)

SteveF
“I don’t expect you will accept much of the above as correct”.

Very succinct, for the complexity of the matter. Covered a lot of the territory that I have had trouble in working out the figures involved and ideas
Thank you very much.