Guest Post by Willis Eschenbach
OK, a quick pop quiz. The average temperature of the planet is about 14°C (57°F). If the earth had no atmosphere, and if it were a blackbody at the same distance from the sun, how much cooler would it be than at present?
a) 33°C (59°F) cooler
b) 20°C (36°F) cooler
c) 8° C (15°F) cooler
The answer may come as a surprise. If the earth were a blackbody at its present distance from the sun, it would be only 8°C cooler than it is now. That is to say, the net gain from our entire complete system, including clouds, surface albedo, aerosols, evaporation losses, and all the rest, is only 8°C above blackbody no-atmosphere conditions.
Why is the temperature rise so small? Here’s a diagram of what is happening.
Figure 1. Global energy budget, adapted and expanded from Kiehl/Trenberth . Values are in Watts per square metre (W/m2). Note the top of atmosphere (TOA) emission of 147 W/m2. Tropopause is the altitude where temperature stops decreasing with altitude.
As you can see, the temperature doesn’t rise much because there are a variety of losses in the complete system. Some of the incoming solar radiation is absorbed by the atmosphere. Some is radiated into space through the “atmospheric window”. Some is lost through latent heat (evaporation/transpiration), and some is lost as sensible heat (conduction/convection). Finally, some of this loss is due to the surface albedo.
The surface reflects about 29 W/m2 back into space. This means that the surface albedo is about 0.15 (15% of the solar radiation hitting the ground is reflected by the surface back to space). So let’s take that into account. If the earth had no atmosphere and had an average albedo like the present earth of 0.15, it would be about 20°C cooler than it is at present.
This means that the warming due to the complete atmospheric system (greenhouse gases, clouds, aerosols, latent and sensible heat losses, and all the rest) is about 20°C over no-atmosphere earth albedo conditions.
Why is this important? Because it allows us to determine the overall net climate sensitivity of the entire system. Climate sensitivity is defined by the UN IPCC as “the climate system response to sustained radiative forcing.” It is measured as the change in temperature from a given change in TOA atmospheric forcing.
As is shown in the diagram above, the TOA radiation is about 150W/m2. This 150 W/m2 TOA radiation is responsible for the 20°C warming. So the net climate sensitivity is 20°C/150W-m2, or a temperature rise 0.13°C per W/m2. If we assume the UN IPCC canonical value of 3.7 W/m2 for a doubling of CO2, this would mean that a doubling of CO2 would lead to a temperature rise of about half a degree.
The UN IPCC Fourth Assessment Report gives a much higher value for climate sensitivity. They say it is from 2°C to 4.5°C for a CO2 doubling, or from four to nine times higher than what we see in the real climate system. Why is their number so much higher? Inter alia, the reasons are:
1. The climate models assume that there is a large positive feedback as the earth warms. This feedback has never been demonstrated, only assumed.
2. The climate models underestimate the increase in evaporation with temperature.
3. The climate models do not include the effect of thunderstorms, which act to cool the earth in a host of ways .
4. The climate models overestimate the effect of CO2. This is because they are tuned to a historical temperature record which contains a large UHI (urban heat island) component. Since the historical temperature rise is overestimated, the effect of CO2 is overestimated as well.
5. The sensitivity of the climate models depend on the assumed value of the aerosol forcing. This is not measured, but assumed. As in point 4 above, the assumed size depends on the historical record, which is contaminated by UHI. See Kiehl for a full discussion.
6. Wind increases with differential temperature. Increasing wind increases evaporation, ocean albedo, conductive/convective loss, ocean surface area, total evaporative area, and airborne dust and aerosols, all of which cool the system. But thunderstorm winds are not included in any of the models, and many models ignore one or more of the effects of wind.
Note that the climate sensitivity figure of half a degree per W/m2 is an average. It is not the equilibrium sensitivity. The equilibrium sensitivity has to be lower, since losses increase faster than TOA radiation. This is because both parasitic losses and albedo are temperature dependent, and rise faster than the increase in temperature:
a) Evaporation increases roughly exponentially with temperature, and linearly with wind speed.
b) Tropical cumulus clouds increase rapidly with increasing temperature, cutting down the incoming radiation.
c) Tropical thunderstorms also increase rapidly with increasing temperature, cooling the earth.
d) Sensible heat losses increase with the surface temperature.
e) Radiation losses increases proportional to the fourth power of temperature. This means that each additional degree of warming requires more and more input energy to achieve. To warm the earth from 13°C to 14°C requires 20% more energy than to warm it from minus 6°C (the current temperature less 20°C) to minus 5°C.
This means that as the temperature rises, each additional W/m2 added to the system will result in a smaller and smaller temperature increase. As a result, the equilibrium value of the climate sensitivity (as defined by the IPCC) is certain to be smaller, and likely to be much smaller, than the half a degree per CO2 doubling as calculated above.
Joel Shore (08:48:35)
Joel, thanks immensely for that very clear exposition of the greenhouse effect, it’s in my permanent bookmarks.
I am embarrassed to confess, however, that I laughed out loud at the part you recommended, where he says:
The problem with this is that decreasing water vapor decreases clouds, as does decreasing temperature. With decreasing temperature and water vapor, very rapidly, few clouds would form in the tropics. Any reduction in tropical clouds makes a huge difference in the amount of solar radiation striking the earth, one that would swamp any reduction in GHG forcing.
Chris puts the ∆T from CO2 at about 10°C. If we take the midrange IPCC sensitivity value of 3°C per 3.7Wm-2, this represents a forcing of some 11 W/m2.
Now let’s look at clouds. Instantaneous cloud forcing in the tropics is on the order of one to two hundred! W/m2 (net of SW and LW), so a small change in clouds will wipe out the effect of the loss of CO2. That huge difference between CO2 forcing and cloud forcing is the reason I call clouds the elephant in the room … because its forcing dwarfs that of CO2.
The fact that Chris doesn’t mention any effect from that reduction in clouds speaks volumes. Clouds are the elephant in the room, and when someone starts making claims about “what would happen if” and doesn’t mention clouds, they’re not talking science, they are engaged in advocacy. Which is a shame, because the rest of the post was so good and so clear on the science.
Thanks again for the link,
w.
Okay…Well, I will have to think more about this. It is not vital to understanding what is wrong with your argument…but it is useful for understanding why your argument doesn’t just give something close to the no-feedback sensitivity, which is what I think it should give by its construction if calculated correctly.
I keep trying to tell you what you are missing: You are missing the fact that what you are calling “forcing” is a combination of forcing and feedbacks, so if the feedbacks are positive then you are overestimating the radiative forcing. You have just created a circular argument that essentially says “If the feedbacks are not positive then the climate sensitivity isn’t very high.” We already knew that.
Willis, you are losing focus. I don’t personally care whether or not you believe in Chris’s positive feedback scenario. The point is that even if you don’t, you can’t argue for a low climate sensitivity with a circular argument that shows that if positive feedbacks are not allowed then the climate sensitivity is not very large.
Chris’s proposed positive feedback mechanism is just useful because it makes the circularity of your argument clear: If Chris’s description of what happens is correct, it is clear that your argument would not diagnose the correct climate sensitivity. After all, both of us would agree that the majority of the radiative greenhouse effect is due to water vapor and clouds, not CO2. Yet, in Chris’s scenario, all you have to do is remove the radiative forcing due to CO2 and you lose pretty much all of the water vapor and (I think) clouds too and their radiative effects. (Furthermore, you will likely also significantly increase the earth’s surface albedo by freezing over large expanses of both land and water, so even though cloud albedo would be less, surface albedo would be higher.)
So, Chris is saying, “Reduce the radiative forcing just by removing CO2 and you will get rid of all this other radiative effects.” However, in your calculation, you take the radiative forcing to be the entire amount of the radiative effects, including all of that due to water vapor. Ergo, if Chris’s (or any other positive feedback scenario) is correct, your calculation of the climate sensitivity will not correctly diagnose the climate sensitivity in the system.
Having said this, and I hesitate to add this because I worry that it will distract you, I have to say that I don’t really buy your argument against Chris’s picture. For one thing, you talk about clouds having this huge effect in the tropics but you yourself admit that even if you remove all effects of cloud albedo, there would still be ~20 C of warming to explain by greenhouse gases. And, as I noted, this doesn’t even touch the issue of the additional earth surface albedo that you get as you start to ice over more of the planet. Furthermore, I think a lot of the dramatic effects you talk about for clouds cooling in the tropics are really redistributions of heat within the troposphere. This is not to dispute that the net effect of those clouds on the earth’s radiation budget is cooling, but some of the effect that seems so dramatic to you is redistribution of heat up into the troposphere…which the models already have in there by more-or-less pegging the temperature structure to the moist adiabatic lapse rate.
But again, let’s take one issue at a time: Independent of whether the net feedbacks are negative or positive, your argument claiming to show that you can estimate the climate sensitivity by looking at the radiative effects and the resulting temperature change is wrong. We know the relation between radiative effects and temperature change: It is governed by the Steffan-Boltzmann Equation. (The only slight hitch is that one does have to worry about the temperature structure in the atmosphere; e.g., I think the negative lapse rate feedback is not really a radiative effect between the earth / sun / space but is instead an issue of how the temperature rise at the surface changes for a given temperature rise at the effective radiating level.) What we don’t know is what part of the current radiative effects that we see is a radiative forcing and what is a feedback. Just assuming that it is all forcing makes for a circular argument.
By the way, just to let you know: I am not just hypothesizing that your argument is wrong in this way. I am quite sure of it…much more sure than I actually am regarding whether the feedbacks really are positive. Even if you turn out to be right on that issue, your argument in this post is still wrong, wrong, wrong. I really hope you will be willing to listen closely to what I am saying and think about. So far, you seem to have a block that is preventing you from comprehending what I am saying.
Joel Shore (18:16:30),
Hate to be a buttinsky here [not really], but this is too funny:
I think maybe one more “wrong” would have been the tipping point, especially if Joel stamped his foot when he said it.
To be a little more serious, what is the unit of measurement for a “forcing?” How about for a “feedback”?
We have quantifiable units of ohms, newtons, volts, henrys, Watts, etc. Without a unit of forcing and feedback, climate science is winging it.
So I propose the symbols ⇥ for a forcing, and ↩ for a feedback. The unit of forcing can be known as a Willis, the unit of feedback as a Smokey.
Let’s let Joel assign the quantities, since he’s never wrong.
Smokey,
Forcings are measured in Watts per meter-squared. Feedbacks are measured in Watts per meter-squared per degree Kelvin. So, for example, Dessler and Sherwood in their 2009 Science perspectives piece (http://www.sciencemag.org/cgi/content/summary/323/5917/1020) say that most estimates of the water vapor feedback are in the range of 1.5 to 2.0 W/(m^2)/K, which means that each 1 K rise in temperature would result in 1.5 to 2.0 W/(m^2) effect on the radiative balance due to the increase in water vapor in the atmosphere.
The Stefan-Boltzmann Equation itself gives a negative feedback of about -3.8 W/(m^2) per degree Kelvin. So long as the feedbacks due to everything else besides Stefan-Boltzmann are positive, then you get a magnification of the climate sensitivity due to the S-B Equation alone (i.e., the approximately 1. 1 K per CO2 doubling). If they are negative, you get a diminishing of this sensitivity. [If you imagine “tuning” the feedbacks, as the magnitude of all the other feedbacks approach 3.8 W/(m^2) per degree Kelvin, the sensitivity gets larger and larger, with feedbacks at or above this value leading to a true “runaway” effect.]